Transmissible Gastroenteritis Virus

Overview and Taxonomy of Transmissible Gastroenteritis Virus

Taxonomic Classification and Phylogenetic Position

Transmissible gastroenteritis virus (TGEV) is a highly pathogenic, enveloped, single-stranded positive-sense RNA virus belonging to the family Coronaviridae, subfamily Orthocoronavirinae, genus Alphacoronavirus [1, 3]. Within the alphacoronavirus genus, TGEV is classified as a member of species Alphacoronavirus 1, which also includes the closely related porcine respiratory coronavirus (PRCV), feline coronavirus (FCoV), and canine coronavirus (CCoV) [1, 3]. This taxonomic grouping reflects a shared evolutionary ancestry and significant genetic homology, particularly in the conserved replicase genes, while the spike (S) protein gene exhibits substantial divergence that dictates host range and tissue tropism [1, 3]. The International Committee on Taxonomy of Viruses (ICTV) recognizes TGEV as a distinct virus within this species complex, distinguished from PRCV primarily by the presence of a full-length S gene and the ability to cause severe enteric disease [1, 23].

Phylogenetic analyses of complete TGEV genomes have consistently resolved two major genotypic clusters, designated genotype I (GI) and genotype II (GII) [8, 26]. Genotype I is further subdivided into subtypes Ia and Ib, with most classical vaccine strains and historical Chinese isolates clustering within GI [8, 26]. In contrast, genotype II predominantly comprises more recently isolated strains from the United States, which exhibit distinct genetic signatures, including unique deletions and amino acid substitutions that may influence their biological characteristics [7, 26]. This bipartite phylogenetic structure underscores the ongoing evolutionary divergence of TGEV across different geographic regions and highlights the importance of continuous molecular surveillance [1, 26].

Virion Structure and Genomic Organization

TGEV virions are pleomorphic, spherical particles approximately 80–120 nm in diameter, characterized by the presence of prominent club-shaped spike (S) glycoprotein projections extending from the viral envelope, which confer the characteristic "corona" or crown-like appearance observed by electron microscopy [3, 12]. The viral envelope is derived from the host cell membrane and incorporates three additional structural proteins: the membrane (M) glycoprotein, which is the most abundant envelope component and plays a critical role in virion assembly; the small envelope (E) protein, involved in virus budding and morphogenesis; and the nucleocapsid (N) protein, which encapsidates the viral RNA genome [3, 12].

The TGEV genome is a single-stranded, positive-sense RNA molecule of approximately 28.5 kilobases (kb) in length, making it one of the largest RNA viral genomes known [3, 12]. The genomic organization follows the canonical coronavirus architecture: a 5′ cap structure and a 3′ polyadenylated tail flank a series of open reading frames (ORFs). The 5′-proximal two-thirds of the genome encode the large replicase polyproteins, pp1a and pp1ab, which are proteolytically processed into 16 nonstructural proteins (nsps) that form the viral replication-transcription complex [3, 15, 17]. These nsps include key enzymes such as the papain-like proteases (PL1pro and PL2pro), the main protease (Mpro or 3CLpro), the RNA-dependent RNA polymerase (RdRp), the helicase, and various accessory proteins involved in host immune evasion [15, 17, 22].

The 3′-proximal one-third of the genome encodes the four canonical structural proteins in the order: spike (S), envelope (E), membrane (M), and nucleocapsid (N) [3, 12]. Interspersed among these structural genes are several accessory ORFs, including ORF3a, ORF3b, and ORF7, which encode non-essential proteins that modulate host cell responses and viral pathogenesis [3, 21, 28]. Notably, ORF3a has been implicated in virulence, and deletions within this region have been observed in some field strains, although its precise role in enteric pathogenesis remains a subject of active investigation [21, 28].

The Spike Glycoprotein: A Determinant of Tropism and Pathogenicity

The spike (S) protein is the most extensively studied structural component of TGEV, owing to its critical functions in receptor binding, membrane fusion, and induction of neutralizing antibodies [3, 13, 14]. The S protein is a large, type I transmembrane glycoprotein that forms homotrimeric spikes on the virion surface. It is composed of two functional subunits: the N-terminal S1 subunit, which contains the receptor-binding domain (RBD), and the C-terminal S2 subunit, which mediates membrane fusion [3, 13]. The S1 subunit of TGEV harbors four major antigenic sites (A, B, C, and D), with sites A and D being the primary targets of virus-neutralizing antibodies [4, 27].

The primary cellular receptor for TGEV is porcine aminopeptidase N (pAPN, also known as CD13), a 150-kDa transmembrane metalloprotease expressed on the brush border membrane of intestinal epithelial cells [10, 11]. The interaction between the TGEV S1 domain and pAPN is a prerequisite for viral attachment and subsequent entry into susceptible cells [10, 11]. This receptor usage is shared with other alphacoronaviruses, including PRCV and, intriguingly, the more distantly related porcine deltacoronavirus (PDCoV), which also utilizes pAPN for cellular entry, representing a cross-genus receptor usage [10]. The critical importance of pAPN for TGEV infection has been definitively demonstrated in vivo using pAPN-knockout pigs generated via CRISPR/Cas9 technology; these animals are completely resistant to TGEV challenge, exhibiting no clinical signs, no viral antigen in intestinal tissues, and normal intestinal histopathology [11].

Beyond pAPN, TGEV employs additional co-receptors and attachment factors that facilitate viral entry. The epidermal growth factor receptor (EGFR) has been identified as a crucial co-factor that interacts with the TGEV S protein and synergizes with pAPN to promote viral internalization [16, 24]. TGEV binding to EGFR activates downstream signaling cascades, including the PI3K/AKT and MEK/ERK1/2 pathways, which are essential for clathrin- and caveolae-mediated endocytosis [9, 16]. Furthermore, transferrin receptor 1 (TfR1) has been characterized as a supplementary receptor that assists TGEV entry into porcine intestinal epithelial cells, with the TGEV S1 protein directly interacting with the extracellular region of TfR1 [20]. This multi-receptor engagement strategy underscores the sophisticated molecular mechanisms TGEV has evolved to efficiently invade its target cells.

The N-terminal domain of the S protein (S-NTD) has historically been considered a key determinant of enteric tropism and virulence [13, 14]. However, recent studies using reverse genetics have challenged this paradigm. Recombinant TGEV viruses engineered with a 224-amino-acid deletion in the S-NTD, analogous to the deletion found in PRCV, retained the ability to cause clinical disease and mortality in piglets, indicating that the S-NTD is not the sole determinant of enteric tropism [13]. Instead, a single nucleotide change (U655>G) resulting in an S219A substitution in the S protein, along with a 6-nucleotide insertion at position 1124 (Y374-T375insND), has been shown to be sufficient to confer enteric tropism to a respiratory TGEV isolate [14]. These findings highlight the complex, multifactorial nature of TGEV tissue tropism, which likely involves interactions between multiple S protein domains and host cell factors.

The Emergence of Porcine Respiratory Coronavirus (PRCV)

A seminal event in the evolutionary history of TGEV was the emergence of porcine respiratory coronavirus (PRCV) in the 1980s, first detected in Europe and subsequently in the United States [1, 23]. PRCV is a naturally occurring deletion mutant of TGEV, characterized by a large deletion in the 5′ end of the S gene, typically ranging from 621 to 681 nucleotides, which results in the loss of the N-terminal domain of the S protein [1, 2, 23]. This deletion abrogates the virus's ability to bind to sialic acid receptors on intestinal epithelial cells and drastically reduces its affinity for pAPN, thereby shifting its tropism from the enteric tract to the respiratory tract [1, 14]. Consequently, PRCV causes mild or subclinical respiratory infections in pigs, in stark contrast to the severe, often fatal enteric disease caused by TGEV [1, 23].

The emergence of PRCV has had profound implications for TGEV epidemiology and diagnosis. Because PRCV is antigenically cross-reactive with TGEV, serological surveys using traditional assays cannot distinguish between antibodies induced by TGEV infection and those induced by PRCV infection [1, 2]. This cross-reactivity has complicated the interpretation of seroprevalence studies and has likely led to an overestimation of TGEV prevalence in regions where PRCV is endemic [1, 2]. Indeed, recent studies in Poland and the United States have demonstrated that PRCV is far more prevalent than TGEV in domestic swine populations, with PRCV seroprevalence rates reaching 12.2% in Poland compared to only 2.2% for TGEV [2]. In the United States, the prevalence of TGEV declined dramatically after March 2013, falling from 3.8–6.8% to less than 0.1%, a phenomenon attributed in part to the widespread circulation of PRCV and the resulting herd immunity [7].

Genetic analyses of PRCV strains have revealed considerable heterogeneity in the size and location of the S gene deletion. While most European and Korean PRCV strains possess a 672-nucleotide deletion at a conserved position at the 5′ end of the S gene, some Polish PRCV strains have been found to harbor a 690-nucleotide deletion that differs in both size and location from any previously described PRCV strain, suggesting a possible independent evolutionary origin [2]. Furthermore, phylogenetic analyses indicate that European PRCV strains form a distinct cluster separate from US PRCV strains, with the latter grouping more closely with TGEV strains, implying that PRCV may have emerged from different TGEV precursors in different geographic regions [23].

Genetic Diversity, Recombination, and Evolution

TGEV, like all coronaviruses, possesses a large RNA genome and a relatively error-prone RNA-dependent RNA polymerase, which together contribute to a high mutation rate and substantial genetic diversity [3, 8]. In addition to point mutations, TGEV evolution is profoundly shaped by homologous recombination, a process that occurs with high frequency during co-infection of a single cell with two or more viral strains [3, 6, 18]. Recombination events have been documented between TGEV strains belonging to different phylogenetic clusters, as well as between TGEV and PRCV, generating novel chimeric viruses with altered pathogenic potential [6, 7, 18].

Several natural recombinant TGEV strains have been characterized in China. The JS2012 strain, isolated from piglets in Jiangsu Province, was identified as a recombinant between the Miller M6 and Purdue 115 strains, with breakpoints located in ORF1a and the S gene [6]. This recombinant virus caused 100% mortality in newborn piglets, demonstrating that recombination can generate highly virulent strains [6]. Similarly, the AHHF strain from Anhui Province was shown to be a recombinant between the Purdue and Miller clusters, with two breakpoints identified in ORF1a and the S gene [18]. These findings underscore the role of recombination as a major driving force in TGEV evolution and the emergence of new pathogenic variants [3, 26].

Codon usage bias analysis has provided additional insights into TGEV evolution and host adaptation. The TGEV genome is characterized by a high A/U nucleotide content, particularly at the synonymous third codon position, and exhibits a moderate codon usage bias [8]. Natural selection, rather than mutation pressure, appears to be the dominant evolutionary force shaping codon usage patterns in TGEV, particularly for genotype I strains [8]. The codon adaptation index (CAI) analysis suggests that genotype I strains may be better adapted to the porcine host than genotype II strains, which may partially explain the predominance of genotype I strains in global swine populations [8].

Geographic Distribution and Epidemiological Significance

TGEV has a worldwide distribution, with documented outbreaks in Europe, Asia, and the Americas [3, 5, 25]. The virus is a significant cause of economic losses in the swine industry, particularly in regions with intensive pig production [3, 25]. A comprehensive meta-analysis of TGEV prevalence in China from 1983 to 2022, encompassing data from 50,403 pigs across 36 studies, estimated an overall seroprevalence of 10% [25]. However, significant regional variation was observed, with prevalence rates as high as 38% in northeast China, likely reflecting the colder climate, which favors virus survival and transmission [25]. Seasonal patterns are also evident, with TGEV outbreaks occurring predominantly during the winter months, when low temperatures and humidity enhance viral stability in the environment [7, 25].

In the United States, TGEV prevalence declined markedly after 2013, with rates falling from 3.8–6.8% to less than 0.1% [7]. This decline has been attributed to a combination of factors, including the widespread adoption of improved biosecurity measures, the emergence of PRCV-induced herd immunity, and the possible displacement of TGEV by the more recently emerged porcine epidemic diarrhea virus (PEDV), which causes similar clinical signs and may have outcompeted TGEV for susceptible hosts [7, 12]. Despite this decline, TGEV remains a notifiable disease to the World Organisation for Animal Health (WOAH), and continued surveillance is essential to detect potential re-emergence or the introduction of novel variants [3, 19].

Zoonotic Potential and Public Health Considerations

Historically, TGEV was not considered a zoonotic pathogen, and no human infections had been documented. However, recent discoveries have prompted a reassessment of this view. The detection of a novel canine-feline recombinant alphacoronavirus, designated CCoV-HuPn-2018, in the nasopharyngeal swabs of hospitalized humans in Malaysia has raised the possibility that alphacoronaviruses, including TGEV-related viruses, may have a broader host range than previously appreciated [3]. While CCoV-HuPn-2018 is most closely related to canine coronavirus, its detection in humans underscores the potential for cross-species transmission events within the Alphacoronavirus 1 species complex [3]. Furthermore, the ability of TGEV to replicate in human intestinal cell lines in vitro and the widespread distribution of pAPN homologs across mammalian species suggest that the species barrier may not be absolute [3]. Although there is currently no evidence of TGEV transmission to humans, the continuous evolution and recombination of coronaviruses, as highlighted by the COVID-19 pandemic, necessitate ongoing vigilance and enhanced surveillance at the animal-human interface [1, 3]. The Food and Agriculture Organization (FAO) and the World Health Organization (WHO) have emphasized the importance of a One Health approach to monitor emerging coronaviruses in animal populations to mitigate potential public health risks.

Molecular Virology and Genomic Organization of TGEV

Transmissible Gastroenteritis Virus (TGEV) is a member of the genus Alphacoronavirus within the family Coronaviridae, order Nidovirales [1, 3, 12]. As an enveloped, single-stranded positive-sense RNA virus, TGEV possesses one of the largest known RNA genomes, approximately 28.5 kilobases in length, which is organized in a characteristic coronavirus layout: a 5′ untranslated region (UTR), a large replicase gene comprising open reading frames 1a and 1b (ORF1a/ORF1b), followed by genes encoding structural and accessory proteins, and a 3′ UTR with a poly(A) tail [5, 12, 41]. The genomic organization is highly conserved among coronaviruses, with a canonical gene order of 5′-replicase-S-E-M-N-3′, interspersed with various accessory genes that are critical for viral pathogenesis and host interaction [3, 26]. The viral genome functions directly as mRNA for the translation of the replicase polyproteins, a hallmark of the Nidovirales order that also includes the production of a nested set of subgenomic mRNAs (sg mRNAs) via discontinuous transcription during negative-strand synthesis [12].

Genomic Architecture and Replicase Gene Organization

The replicase gene, occupying roughly two-thirds of the genome, is translated from the genomic RNA to yield two large polyproteins, pp1a and pp1ab, with the latter produced via a -1 ribosomal frameshift mechanism at the junction of ORF1a and ORF1b [3, 12]. These polyproteins are extensively processed by virus-encoded proteases to generate 16 non-structural proteins (Nsps 1-16), which assemble into the replication-transcription complex (RTC) [15, 22]. Among these, the papain-like proteases (PL1 and PL2) within Nsp3 are responsible for cleaving the N-terminal region of the polyprotein, while the main protease (Mpro, also known as 3CLpro) encoded within Nsp5 processes the remainder [22]. The nsp3 protein of TGEV, in particular, has been shown to function as a potent antagonist of the host innate immune response, inhibiting NF-κB activation through its deubiquitinase (DUB) activity, which suppresses IκBα degradation and prevents p65 nuclear translocation [15]. This DUB activity is specifically associated with the PL2 domain within the 590–1215 amino acid region of nsp3 [15]. Conversely, nsp14, a bifunctional enzyme with 3′-to-5′ exoribonuclease (ExoN) and N7-methyltransferase activities, has been identified as a potent inducer of interferon-β (IFN-β) production, mainly through activation of NF-κB and interaction with the host DEAD-box RNA helicase DDX1 [35]. The opposing actions of nsp3 (anti-IFN) and nsp14 (pro-IFN) highlight the sophisticated counterbalancing mechanisms employed by TGEV to modulate the host antiviral state, a theme that runs deep through its molecular virology [22, 35]. TGEV nsp2 has also been implicated in inflammation, with the first 120 amino acids being critical for activating NF-κB and upregulating pro-inflammatory cytokines, directly contributing to the enteritis characteristic of infection [17].

Structural Proteins and Their Functional Determinants

The spike (S) glycoprotein is the most extensively studied structural protein due to its pivotal roles in receptor binding, host cell tropism, and induction of neutralizing antibodies [4, 29, 32]. The TGEV S protein is a large type I transmembrane protein that forms characteristic club-shaped peplomers on the virion surface [5, 12]. It is cleaved by host proteases into S1 (receptor-binding) and S2 (membrane fusion) subunits, though the cleavage is not as stringent as in some other coronaviruses [3]. The S1 subunit contains the N-terminal domain (S-NTD) and the C-domain, with the latter harboring the primary receptor-binding site for porcine aminopeptidase N (pAPN, CD13), which is the established functional receptor for TGEV [10, 11, 45]. The interaction between the S1 domain and pAPN is essential for viral entry; knockout of pAPN in pigs via CRISPR/Cas9 technology confers complete resistance to TGEV infection, validating pAPN as an indispensable entry determinant in vivo [11]. Additionally, the S1 domain of TGEV has been shown to bind sialic acid residues on glycoconjugates, a secondary receptor-binding activity that may facilitate initial attachment to the intestinal mucus layer and enhance viral entry [14, 45]. However, the relative importance of sialic acid binding appears to be less critical than pAPN engagement, as soluble TGEV S1 protein shows no detectable sialic acid binding in certain assays, contrasting sharply with the spike of infectious bronchitis virus (IBV) [45].

Beyond pAPN, a co-receptor network has been identified. Epidermal growth factor receptor (EGFR) has been shown to interact directly with the TGEV spike protein, and its activation synergizes with pAPN to promote clathrin- and caveolin-mediated endocytosis, stimulating downstream PI3K/AKT and MEK/ERK1/2 signaling pathways [16, 24]. Transferrin receptor 1 (TfR1), which is highly expressed on rapidly dividing intestinal epithelial cells, also serves as a supplementary receptor; the S1 protein binds to the extracellular region of TfR1, and overexpression of porcine TfR1 enhances TGEV infection [20]. The N-terminal 224 amino acids of the S protein (S_NTD) have historically been considered a determinant of enteric tropism, as this region is deleted in porcine respiratory coronavirus (PRCV), a naturally occurring deletion mutant of TGEV that exhibits a switch from enteric to respiratory tropism [1, 13]. However, precise reverse genetics studies using recombinant TGEVs with an engineered deletion of this NTD (S_NTD224) demonstrated that such viruses remain fully enteropathogenic in neonatal piglets, causing severe diarrhea and mortality, thus directly refuting the long-held dogma that this domain is the sole determinant of enteric tropism [13]. Subsequent work identified that a single amino acid substitution (S219A, corresponding to a U655>G change in the S gene) is necessary, though not sufficient, to confer enteric tropism to a respiratory virus, and that an additional 6-nucleotide insertion (Y374-T375insND) in the S gene further enhances viral replication in the gut by approximately 1,000-fold [14]. These findings underscore the multifactorial and complex molecular basis of TGEV tissue tropism, which involves multiple domains of the S protein, as well as host proteases and cellular signaling [14, 44].

The membrane (M) and envelope (E) proteins are critical for virion morphogenesis and assembly. The M protein, the most abundant component of the viral envelope, adopts a triple-spanning transmembrane topology and plays a central role in the budding process at the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) [5, 12]. The E protein, a small pentameric ion channel (viroporin), is essential for efficient virus production and release [3]. The nucleocapsid (N) protein, which encapsidates the genomic RNA, is a multifunctional phosphoprotein involved in RNA replication, transcription, and translation, and is highly immunogenic [31, 33]. The TGEV N protein localizes to the cytoplasm, induces endoplasmic reticulum (ER) stress, and activates NF-κB, leading to upregulation of interleukin-8 (IL-8) and Bcl-2 [37]. Furthermore, the N protein upregulates the expression of the neonatal Fc receptor (FcRn) both through direct activation of the NF-κB pathway (with the central region, aa 128-252, being essential) and through induction of TGF-β secretion, which in turn enhances FcRn expression via JNK signaling [31]. This N-mediated FcRn upregulation promotes IgG transcytosis and may play a role in mucosal immunity [31]. Antigenic cross-reactivity between TGEV and porcine epidemic diarrhea virus (PEDV) N proteins has been mapped to two critical N-terminal epitopes (aa 58-68 and aa 78-85 in the PEDV N protein), complicating serological diagnostics [33].

Accessory Proteins and Immune Evasion Strategies

TGEV encodes several accessory proteins, including ORF3a, ORF3b, and ORF7, which are not essential for viral replication in vitro but are implicated in virulence and host range [21, 28]. The ORF3a protein is of particular interest due to its role in the TGEV-PRCV evolutionary relationship. PRCV strains, which are naturally occurring deletion mutants of TGEV, typically possess a large deletion in the 5′ end of the S gene (typically 672–690 nucleotides) and also frequently harbor deletions or truncations in ORF3a [2, 21, 23]. However, a virulent TGEV strain from Britain with a large deletion in ORF3a but an intact S gene demonstrated that ORF3a is not essential for enteric virulence [28]. The presence of ORF3a deletions in Chinese field strains of TGEV, alongside the documented emergence of natural recombinants between the classic Purdue and Miller genotypes, highlights the ongoing genetic plasticity of TGEV [6, 18, 21]. The ORF7 protein is a small hydrophobic protein whose function is less well defined, but its transcription has been shown to be regulated by host microRNAs, specifically miR-4331, via targeting of the cellular protein CDCA7 [42].

The molecular virology of TGEV is deeply intertwined with the manipulation of host cell stress and cell death pathways. TGEV infection induces a robust ER stress response, activating all three branches of the unfolded protein response (UPR): ATF6, IRE1, and PERK [30, 34, 36]. The PERK-eIF2α axis serves as a potent antiviral host response, inhibiting viral protein translation and promoting type I interferon production [34]. TGEV, in turn, has evolved countermeasures. The virus exploits the IRE1α-Xbp1s axis to downregulate the antiviral microRNA miR-27b, which otherwise targets SOCS6 to suppress viral replication [30]. Similarly, TGEV uses IRE1α to reduce miR-30a-5p levels, leading to upregulation of SOCS1 and SOCS3, which then inhibit JAK-STAT signaling and dampen the IFN-I response [36]. This virus-driven manipulation of the host microRNA landscape is a recurring theme; TGEV infection profoundly alters the expression of numerous cellular miRNAs, which in turn regulate viral replication, mitochondrial function, and inflammatory signaling [38, 39, 42, 43]. For instance, miR-222 attenuates TGEV-induced mitochondrial dysfunction by targeting thrombospondin-1 (THBS1) and CD47 [38], while miR-4331 promotes mitochondrial damage by targeting RB1 and activating the p38 MAPK pathway [39]. The virus also induces a complete autophagic response, which functions as a host restriction mechanism to inhibit viral replication; pharmacological inhibition of autophagy enhances TGEV yield, while activation by rapamycin suppresses it [40].

Recombination is a major driving force in TGEV evolution and genetic diversity. Multiple natural recombinant strains have been characterized, such as the Chinese isolates JS2012 and AHHF, which are recombinants between the Miller and Purdue clusters, with breakpoints located within ORF1a and the S gene [6, 18]. The US “variant” genotype of TGEV, which became dominant after 2013, shares unique deletions and amino acid changes with PRCV, strongly suggesting a recombination event between a variant TGEV and PRCV [7]. Phylogenetic analyses of full-length genomes consistently delineate two major genotypes, GI and GII, with GI being predominantly represented by Chinese and historical vaccine strains, while GII comprises more recently isolated US strains and some European PRCVs [8, 26]. This genomic diversity, driven by both point mutations and recombination, underscores the continuous evolutionary pressure on TGEV and its close relative PRCV, and has direct implications for vaccine efficacy and cross-protection [1, 7, 26]. The codon usage bias analysis of TGEV reveals that the genome is A/U-rich at third codon positions, and that natural selection, rather than mutation pressure, is the dominant force shaping codon usage patterns, particularly in the GI genotype which appears more adapted to the porcine host [8].

Molecular Pathogenesis and Virulence Mechanisms of TGEV

Transmissible gastroenteritis virus (TGEV), an enveloped, single-stranded positive-sense RNA virus belonging to the genus Alphacoronavirus within the family Coronaviridae [1, 3], remains one of the most formidable enteric pathogens of swine, capable of inducing catastrophic, near-complete mortality in neonatal piglets [3]. The virulence of TGEV is a multifaceted phenotype, arising from a sophisticated interplay between viral entry mechanisms, exploitation of host cellular machinery, subversion of innate immune defenses, and the induction of severe immunopathology. The molecular pathogenesis is deeply rooted in the virus's capacity to invade and destroy the intestinal epithelium, primarily through the coordinated actions of the spike (S) glycoprotein, an array of nonstructural proteins (nsps), and accessory proteins that collectively orchestrate a devastating enteritis.

Viral Entry and Receptor Engagement: The Gateway to Pathogenesis

The initial step in TGEV pathogenesis, infection of the differentiated enterocytes lining the small intestinal villi, is governed by the viral S protein. The S1 subunit of the spike protein binds with high affinity to its primary functional receptor, porcine aminopeptidase N (pAPN) [10, 11]. The critical nature of this interaction is unequivocally demonstrated by the fact that APN-knockout pigs, generated via CRISPR/Cas9 technology, are completely resistant to TGEV infection, exhibiting no clinical signs, villous atrophy, or viral antigen upon challenge [11]. This establishes pAPN as an indispensable entry factor in vivo. However, TGEV pathogenesis is not mediated by a simple receptor-ligand interaction alone. The virus employs a sophisticated, multi-receptor entry strategy. The epidermal growth factor receptor (EGFR) serves as a critical co-factor, with its extracellular receptor binding domain 1 physically interacting with the TGEV spike protein. APN and EGFR synergistically promote viral invasion by clustering early in infection and co-stimulating downstream PI3K/AKT and MEK/ERK1/2 endocytosis signaling pathways [16]. This signaling cascade, initiated by EGFR activation, leads to the phosphorylation and activation of cofilin via Rac1/Cdc42 GTPases, which in turn drives F-actin polymerization and reorganization at the cell membrane, a prerequisite for efficient viral entry [24]. Furthermore, transferrin receptor 1 (TfR1) has been identified as a supplementary receptor that assists TGEV entry, with the S1 protein directly interacting with its extracellular region [20]. The internalization process itself is a dynamic and rapid event, occurring within approximately two minutes after attachment. Single-virus tracking has revealed that TGEV enters cells via both clathrin-mediated and caveolae-mediated endocytosis, processes that are actin- and dynamin 2-dependent [9]. This multi-receptor, multi-pathway entry mechanism underscores the virus's evolutionary adaptation to efficiently colonize the intestinal epithelium, ensuring robust infection even under variable host conditions.

Determinants of Enteric Tropism and Virulence

A defining feature of TGEV pathogenesis is its profound enteric tropism, which directly correlates with virulence. The molecular basis for this tropism has been meticulously mapped to the S protein, particularly its N-terminal domain (S_NTD). Historically, the S_NTD was considered a primary virulence and enteric tropism determinant. However, studies using reverse genetics have refined this understanding. A recombinant TGEV with a 224-amino-acid deletion in the S_NTD (S_NTD224), a deletion analogous to that found in the naturally attenuated porcine respiratory coronavirus (PRCV), exhibited reduced growth kinetics in cell culture but remained fully virulent and enteropathogenic in two-day-old piglets, directly demonstrating that the NTD is not the sole enteric tropism determinant [13]. This finding challenges a long-held dogma and focuses attention on other regions of the S protein. Indeed, comprehensive studies using chimeric recombinant viruses have pinpointed the minimum determinants of enteric tropism to specific residues in the N-terminus. A single U655>G nucleotide change in the S gene, resulting in an S219A amino acid substitution, was sufficient to confer enteric tropism to a respiratory virus isolate. Furthermore, an additional 6-nt insertion at position 1124, leading to a Y374-T375insND modification, dramatically increased viral titers in the gut by 1000-fold [14]. These subtle changes likely influence glycosaminoglycan binding and modulate the overall conformation of the S protein, affecting its ability to bind host receptors or co-factors specifically in the intestinal environment. The functional importance of the S protein is further underscored by the emergence of PRCV, a naturally occurring deletion mutant of TGEV that lost a 621–681 nucleotide segment at the 5' end of the S gene. This deletion is the primary molecular event that shifts tissue tropism from the enteric to the respiratory tract, resulting in a mild or subclinical phenotype [1, 2, 23]. The dynamic evolution of TGEV through recombination and point mutations in the S gene, as seen in natural recombinant strains like JS2012 and AHHF, can generate highly virulent viruses that cause 100% mortality, highlighting the ongoing plasticity of virulence determinants [6, 18].

Subversion of Host Innate Immunity: Viral Arsenal of Antagonists

A hallmark of TGEV pathogenesis is its ability to establish a robust infection despite triggering a significant host interferon (IFN) response. While TGEV infection activates pattern recognition receptors (PRRs) such as RIG-I and MDA5, leading to the induction of type I IFNs (IFN-α/β) [53, 54], the virus has evolved a sophisticated arsenal of nonstructural proteins to counteract this antiviral state. This balance between induction and evasion is a critical determinant of virulence.

Nonstructural protein 3 (Nsp3) is a potent antagonist of the NF-κB signaling pathway. TGEV infection paradoxically inhibits NF-κB activity in a dose-dependent manner, an effect mediated primarily by Nsp3. This inhibition is achieved through the suppression of IκBα ubiquitination and degradation, thereby preventing p65 phosphorylation and nuclear translocation. The deubiquitinating (DUB) activity of the papain-like protease 2 (PLP2) domain within Nsp3 (amino acids 590-1215) is essential for this immune evasion, leading to reduced transcription of pro-inflammatory cytokines and chemokines [15]. Similarly, the papain-like protease 1 (PL1) of TGEV antagonizes the production of IFN-β. By exerting its DUB activity, PL1 specifically inhibits RIG-I- and STING-dependent signaling pathways, preventing the nuclear translocation of IRF3 and ultimately suppressing IFN-β expression [22].

Complementing these direct signaling inhibitors, TGEV manipulates the host's endogenous microRNA (miRNA) regulatory networks. The virus exploits the IRE1α arm of the endoplasmic reticulum (ER) stress response to degrade host miRNAs that would otherwise be antiviral. For example, TGEV-induced ER stress activates the RNase activity of IRE1α, which unconventionally splices Xbp1 mRNA. The resulting transcription factor, Xbp1s, then inhibits the transcription of miR-27 and miR-30a-5p [30, 36]. The downregulation of miR-27b relieves suppression on its target, SOCS6, which may have proviral effects. More critically, the suppression of miR-30a-5p leads to elevated expression of its targets, SOCS1 and SOCS3, which are potent negative regulators of the JAK-STAT signaling cascade. By upregulating SOCS1/3, TGEV effectively blunts the downstream antiviral effects of the very IFN-β it induces, creating a permissive environment for its own replication [36]. This intricate "Trojan horse" strategy, where the host's stress response is hijacked to dismantle its own antiviral defenses, is a formidable virulence mechanism.

Induction of Inflammatory Responses and Cell Death: The Path to Enteritis

The clinical manifestation of TGEV, severe watery diarrhea, vomiting, and dehydration, is a direct result of extensive enteritis and villous atrophy. This pathology is initiated by a potent inflammatory response and the induction of programmed cell death. TGEV infection is a profound trigger of inflammation, primarily through the activation of the RIG-I/NF-κB/HIF-1α/glycolysis axis. Using advanced porcine intestinal organoid models, research has shown that TGEV binding to RIG-I activates NF-κB, which then upregulates the transcription factor HIF-1α. HIF-1α, in turn, drives a metabolic shift toward aerobic glycolysis, a process that is essential for the full expression of pro-inflammatory cytokines [46]. This metabolic reprogramming represents a central nexus linking viral sensing to immunopathology.

Concurrently, TGEV triggers multiple inflammasome pathways, amplifying the inflammatory cascade. The NLRP3 inflammasome is activated upon TGEV infection, leading to caspase-1 activation, the maturation and release of IL-1β, and the induction of pyroptosis, a highly inflammatory form of lytic cell death. Importantly, inhibition of NLRP3 enhances TGEV replication, indicating that this inflammasome acts as a critical host defense mechanism, albeit one that contributes to tissue damage [51]. More recently, the NLRP1 inflammasome has been identified as a key sensor in TGEV infection, particularly in intestinal epithelial cells (IECs). TGEV infection disrupts the normal inhibitory interaction between porcine NLRP1 (pNLRP1) and dipeptidyl peptidase 9 (pDPP9), leading to inflammasome assembly and the release of IL-1β and IL-18. Interestingly, pNLRP1 itself acts as an interferon-stimulated gene (ISG), suggesting a feedback loop where the host attempts to bolster the inflammasome response while the virus drives its activation [55].

In addition to pyroptosis, TGEV induces apoptosis and mitochondrial dysfunction. The virus triggers both the death receptor (extrinsic) pathway (Fas/Caspase-8) and the mitochondrial (intrinsic) pathway (Bax/Bcl-2/Caspase-9), a process exacerbated by oxidative stress (ROS) and the activation of the P38MAPK signaling pathway [49]. The N protein directly contributes to this by inducing ER stress, which activates NF-κB and upregulates IL-8 expression [37]. Furthermore, TGEV infection dysregulates the host's circular RNA (circRNA) landscape. For instance, downregulation of circBIRC6-2 reduces the expression of its encoded protein, BIRC6-236aa, which normally stabilizes the mitochondrial permeability transition pore (mPTP) by interacting with VDAC1. The loss of this regulation promotes mPTP opening, loss of mitochondrial membrane potential, and mitochondrial dysfunction [50]. Other miRNAs, such as miR-222 and miR-4331, also modulate this mitochondrial damage by targeting pathways involving CD47/THBS1 and RB1/IL1RAP, respectively, further contributing to cellular energy crisis and apoptosis [38, 39].

Impact on Intestinal Stem Cells and Epithelial Barrier Integrity

The profound villous atrophy observed in TGEV infection is a consequence of not only direct enterocyte destruction but also a disruption of the regenerative capacity of the intestinal crypts. TGEV targets Paneth cells, the guardians of the intestinal stem cell (ISC) niche, through the APN receptor. Infection induces mitochondrial damage and ROS generation in these cells, leading to their loss and a subsequent reduction in the production of essential Notch ligands, such as DII4. The resulting inactivation of Notch signaling in Lgr5+ ISCs inhibits their self-renewal and differentiation, particularly into absorptive enterocytes. Paradoxically, this loss of Notch signaling drives a compensatory differentiation of goblet cells, which secrete mucins that can actually enhance TGEV replication, creating a vicious cycle that accelerates disease [52]. This "bottom-up" pathogenesis, where infection originates in the crypt and destroys the regenerative unit before the villi are fully compromised, reveals a novel and devastating virulence strategy.

In stark contrast to this pathological mechanism, other studies have observed that TGEV infection can simultaneously activate a compensatory regenerative response by promoting ISC self-renewal via the Wnt/β-catenin pathway. TGEV infection has been shown to induce the accumulation and nuclear translocation of β-catenin, driving the expression of Wnt target genes like Lgr5, Sox9, and Cyclin D1 [47]. This duality suggests that the outcome of infection, whether it leads to catastrophic crypt collapse or attempted regeneration, may be a dynamic balance between the virus's cytopathic effects and the host's reparative signals. Ultimately, these damages are compounded by the loss of epithelial barrier integrity. TGEV infection disrupts tight junction proteins (ZO-1, Occludin) and microfilaments, increasing intestinal permeability [48, 56]. This allows for the paracellular leakage of fluids and contributes to the characteristic diarrhea, while also enabling the systemic absorption of luminal antigens, further fueling the inflammatory response.

Epidemiology, Transmission, and Global Impact of TGEV

Global Prevalence and Geographic Distribution

Transmissible gastroenteritis virus (TGEV) remains a pathogen of substantial concern to global swine health, though its epidemiological landscape has shifted dramatically over the past three decades. Historically recognized as a major cause of neonatal diarrhea worldwide, TGEV circulates endemically with varying prevalence across continents. Comprehensive meta-analyses have revealed that the overall estimated seroprevalence of TGEV infection in Chinese pig populations from 1983 to 2022 stands at approximately 10% (3,887 positive out of 50,403 pigs sampled across 36 studies), with pronounced regional and seasonal variation [25]. Crucially, northeastern China exhibits a markedly higher prevalence of 38% (2,582 out of 3,078,700 pigs), a finding that underscores the influence of cold climate on viral transmission dynamics [25].

In Europe, the epidemiological picture is more nuanced due to the widespread circulation of porcine respiratory coronavirus (PRCV), a naturally occurring deletion mutant of TGEV. A recent serosurvey in Poland, the first of its kind for that nation, detected anti-TGEV antibodies in only 2.2% of serum samples (95% CI 1.2–3.2), while anti-PRCV antibodies were present in 12.2% of samples (95% CI 9.8–14.4) [2]. Notably, TGEV RNA was undetectable in any of the 498 nasal swab and stool samples tested, whereas PRCV RNA was identified in 6.22% of samples [2]. These data indicate that PRCV has largely supplanted TGEV in European swine herds, a trend likely driven by the respiratory virus's altered tropism and its ability to induce cross-protective immunity against TGEV. Similarly, serological surveillance in Hungary has confirmed that TGEV seropositivity is generally low and distributed evenly across the country, though positive titers do not necessarily correlate with protective immunity [66].

The situation in the Americas provides a stark contrast and a compelling case study in viral evolution. In the United States, systematic surveillance between January 2008 and November 2016 demonstrated that TGEV prevalence ranged from 3.8% to 6.8%, with a pronounced seasonal peak during colder months until March 2013 [7]. Remarkably, after March 2013, prevalence plummeted to less than 0.1%, a decline that coincided with the emergence and rapid spread of porcine epidemic diarrhea virus (PEDV) in North America [7]. This dramatic shift suggests a complex ecological interplay between enteric coronaviruses, wherein PEDV may have outcompeted TGEV for susceptible hosts or induced cross-protective immune responses. In South America, retrospective analyses have confirmed the presence of TGEV in Argentina as early as 2010–2015, with immunohistochemistry and in situ hybridization techniques confirming active viral replication in neonatal diarrhea cases [19]. These findings highlight that TGEV continues to circulate in regions where diagnostic capacity may have previously underreported its presence.

Clinical sample testing from China using advanced molecular diagnostics reveals contemporary detection rates consistent with the meta-analytical findings. A duplex colloidal gold immunochromatographic strip assay applied to 121 clinical samples identified a TGEV-positive rate of 9.9%, with a co-infection rate with PEDV of 5.8% [58]. More sensitive multiplex quantitative RT-PCR assays have reported TGEV-positive rates of 1.88% among 160 diarrheic samples [59] and 0.87% among 462 clinical specimens collected across multiple Chinese provinces in 2022–2023 [60]. These relatively low but persistent detection rates indicate that TGEV remains endemic in Chinese swine herds, often circulating at subclinical levels or as part of polymicrobial enteric infections.

Transmission Dynamics and Host Susceptibility

TGEV is transmitted primarily via the fecal-oral route, with infected pigs shedding enormous quantities of virus in their feces [5, 12]. The virus exhibits remarkable environmental stability, particularly at low temperatures, which facilitates its persistence on farms and its spread via fomites, contaminated feed, and transport vehicles [25]. Aerosol transmission is considered minimal for enteric TGEV strains, though PRCV, by virtue of its respiratory tropism, readily spreads through the airborne route, further complicating control strategies in regions where both viruses coexist [1, 23].

The age-dependent nature of TGEV pathogenesis is a defining feature of its epidemiology. Neonatal piglets under two weeks of age experience nearly 100% mortality following infection, a consequence of their immature intestinal epithelium and underdeveloped immune system [3, 12]. In contrast, older weaned pigs and adult swine typically develop milder or subclinical disease, serving as important reservoirs for viral maintenance within herds [12]. This age-related resistance is attributable to multiple factors, including the maturation of intestinal stem cell populations and the development of robust mucosal immune responses. Interestingly, TGEV has been shown to selectively target Paneth cells, specialized cells located at the base of intestinal crypts that are critical for stem cell niche maintenance, through the aminopeptidase N (APN) receptor [52]. This initial invasion induces mitochondrial damage and reactive oxygen species generation in Paneth cells, ultimately leading to their loss and the consequent disruption of Notch signaling that is essential for Lgr5⁺ intestinal stem cell self-renewal and differentiation [52]. This "bottom-up" pathogenesis explains the rapid villous atrophy observed within 48 hours of infection and underscores the vulnerability of neonatal pigs, whose Paneth cell populations are still developing.

The molecular basis of TGEV host range and tissue tropism is intimately linked to its receptor usage. Porcine aminopeptidase N (pAPN, also known as CD13) is the primary functional receptor for TGEV, mediating viral attachment and entry into intestinal epithelial cells [10, 11]. The critical importance of pAPN has been unequivocally demonstrated through gene editing: APN-knockout neonatal piglets generated via CRISPR/Cas9 technology are completely resistant to TGEV challenge, exhibiting no clinical signs, normal intestinal villi, and undetectable viral antigen in target tissues [11]. However, pAPN is also used by porcine deltacoronavirus (PDCoV), indicating that this receptor serves as a cross-genus entry gateway for enteric coronaviruses [10]. Beyond pAPN, TGEV employs a suite of co-receptors and accessory factors that facilitate its entry into host cells. The epidermal growth factor receptor (EGFR) acts as a co-factor, with the TGEV spike protein binding to the extracellular domain of EGFR to promote viral entry via clathrin- and caveolae-mediated endocytosis [16, 24]. EGFR signaling triggers downstream activation of PI3K/AKT and MEK/ERK1/2 pathways, as well as cofilin-mediated actin reorganization, all of which are necessary for efficient internalization [24]. Additionally, transferrin receptor 1 (TfR1) serves as a supplementary receptor that enhances TGEV invasion, particularly in cells with high TfR1 expression such as those from anemic piglets [20]. The exploitation of multiple entry receptors explains the broad tropism of TGEV within the intestinal epithelium and its ability to infect a variety of porcine cell types in vitro.

Evolutionary Epidemiology and the Emergence of PRCV

The evolutionary relationship between TGEV and PRCV is one of the most fascinating chapters in coronavirus biology. PRCV arose spontaneously from TGEV through a large deletion in the spike (S) gene, specifically within the N-terminal domain (S_NTD), which resulted in the loss of sialic acid binding capacity and a dramatic shift in tissue tropism from the enteric to the respiratory tract [1, 13]. This deletion event occurred independently in Europe and the United States, giving rise to distinct PRCV lineages that cluster phylogenetically with their respective geographic TGEV ancestors [23]. European PRCV strains form a separate phylogenetic cluster, whereas US PRCV strains group closely with US TGEV isolates, reflecting divergent evolutionary trajectories [23].

The epidemiological consequences of PRCV emergence have been profound. Because PRCV replicates primarily in the respiratory tract and causes mild or subclinical disease, it has spread rapidly and extensively through swine populations, particularly in Europe, where seroprevalence rates often exceed 50% [1, 2]. Critically, PRCV infection induces cross-reactive antibodies that neutralize TGEV, thereby providing a degree of herd immunity that has contributed to the decline in TGEV-related disease in regions where PRCV is endemic [1]. This phenomenon is likely responsible for the low TGEV seroprevalence observed in Poland (2.2%) compared to the relatively high PRCV seroprevalence (12.2%), as well as the absence of detectable TGEV RNA in that study [2]. In the United States, the situation is more complex: recent genomic analyses have identified a "variant" TGEV genotype that shares unique deletions and amino acid changes with contemporary PRCV strains, suggesting that recombination between TGEV and PRCV has occurred, potentially generating viruses with altered biological characteristics [7].

Recombination events are a major driver of TGEV genetic diversity and have been documented extensively in China. The JS2012 strain, isolated from piglets in Jiangsu Province, was identified as a natural recombinant between the Miller M6 and Purdue 115 strains, with breakpoints located in the replicase gene [6]. This recombinant virus maintained the genetic integrity of virulent TGEV strains and caused 100% mortality in experimentally infected newborn piglets [6]. Similarly, the AHHF strain from Anhui Province represents another natural recombinant that clusters phylogenetically between the Purdue and Miller clusters, with recombination breakpoints identified in ORF1a and the S gene [18]. These findings indicate that co-circulation of multiple TGEV lineages in Chinese swine herds facilitates recombination, contributing to ongoing viral evolution and the potential emergence of novel variants with altered virulence or antigenic profiles [26]. Deletion events in accessory genes further expand the evolutionary repertoire: field strains from China have been found to carry ORF3a deletions previously associated only with PRCV, suggesting that these deletions do not abrogate enteric virulence [21, 28]. Indeed, a virulent British TGEV isolate retained its enteric pathogenicity despite having a large ORF3a deletion, challenging earlier assumptions about the necessity of this gene for intestinal tropism [28].

Global Impact and Economic Burden

The economic impact of TGEV on the global swine industry is staggering, though quantifying it precisely is challenging due to underreporting, co-infections, and the shifting epidemiological landscape. In China alone, the economic losses attributed to TGEV have been described as "huge threats and losses" to pig husbandry, with mortality rates approaching 100% in infected piglets [3]. The systematic review and meta-analysis covering 1983 to 2022 estimated that TGEV continues to cause significant morbidity across Chinese provinces, with the highest burden concentrated in colder northeastern regions [25]. The World Organisation for Animal Health (WOAH) recognizes TGEV as a notifiable disease in many countries, reflecting its potential to cause rapid, widespread outbreaks in naive populations [5]. In the United States, prior to the PEDV-driven decline, TGEV was estimated to cause annual losses of tens of millions of dollars due to piglet mortality, reduced growth rates, and the costs of biosecurity and vaccination [7].

The global impact of TGEV is compounded by its frequent co-infection with other enteric pathogens, particularly PEDV and PDCoV. Clinical surveys using multiplex molecular diagnostics have documented substantial co-infection rates, including PEDV/TGEV dual infections (1.25–3.25% of samples) and even triple infections with PDCoV (0.63–11.90%) [59, 60]. These mixed infections complicate clinical diagnosis, exacerbate disease severity, and impede efforts to control individual pathogens. The Food and Agriculture Organization of the United Nations (FAO) has highlighted the importance of differential diagnosis for swine enteric coronaviruses, as the clinical signs of TGEV, PEDV, and PDCoV are virtually indistinguishable [58].

The zoonotic potential of TGEV remains a topic of ongoing investigation and concern. While TGEV is not currently considered a human pathogen, the detection of a canine-feline recombinant alphacoronavirus (CCoV-HuPn-2018) in a human patient with pneumonia has raised the possibility that alphacoronaviruses, including TGEV or its relatives, could cross species barriers [3]. The fact that TGEV uses pAPN as its primary receptor and that human APN shares structural homology with the porcine protein lends biological plausibility to this concern, though no evidence of human TGEV infection has yet been documented [1]. The Centers for Disease Control and Prevention (CDC) maintains surveillance for emerging coronaviruses, and the continuous evolution of TGEV through recombination and mutation underscores the need for vigilant monitoring.

Vaccination remains the cornerstone of TGEV control, and a wide array of vaccine platforms have been developed, including inactivated whole-virus vaccines, live attenuated vaccines, subunit vaccines based on the spike protein, virus-like particle (VLP) vaccines, recombinant Lactobacillus-based oral vaccines, and next-generation nucleic acid vaccines such as circRNA constructs [4, 29, 32, 57, 61-65, 67]. However, the efficacy of commercial vaccines has been variable, particularly in the face of emerging variant strains and immunosuppressive co-infections [12, 29]. The emergence of PRCV and its widespread circulation in Europe and the United States has paradoxically provided a form of natural immunization against TGEV, but it has also complicated vaccination strategies, as PRCV-induced antibodies can interfere with serological diagnosis and vaccine efficacy assessment [1, 7].

Biosecurity measures remain critical for preventing TGEV introduction and spread, particularly in regions where PRCV is absent or where high-value breeding herds are maintained. The virus's ability to persist in the environment and its transmission via contaminated fomites necessitate strict protocols for personnel movement, equipment disinfection, and all-in/all-out pig flow [12]. The seasonal nature of TGEV outbreaks, with peaks during cold months, aligns with the virus's enhanced stability at low temperatures and the increased concentration of susceptible animals in indoor facilities during winter [25]. Understanding these epidemiological drivers is essential for implementing targeted control measures, such as intensified biosecurity during high-risk seasons and strategic vaccination of sows to enhance lactogenic immunity in piglets.

Clinical Manifestations and Pathological Features

Transmissible gastroenteritis virus (TGEV) induces a clinical syndrome of extraordinary severity in neonatal swine, characterized by a pathognomonic triad of profuse watery diarrhea, frequent vomiting, and rapid dehydration that culminates in mortality rates approaching 100% in piglets under two weeks of age [3, 12, 29]. The disease exhibits a pronounced age-dependent gradient of susceptibility, with clinical severity inversely proportional to the age of the affected animal [12]. In neonatal piglets, the incubation period is remarkably short, typically ranging from 18 to 24 hours post-exposure, after which the clinical cascade unfolds with devastating rapidity [3, 6]. The initial clinical signs often include listlessness, anorexia, and vomiting, followed within hours by the onset of voluminous, watery, yellowish-green diarrhea that may contain undigested milk curds [72]. The fecal material is characteristically devoid of blood or mucus, reflecting the non-hemorrhagic nature of the enteropathy. The relentless fluid loss through both vomiting and diarrhea precipitates profound dehydration, metabolic acidosis, and electrolyte imbalances, leading to sunken eyes, dry mucous membranes, and diminished skin turgor. Affected piglets huddle together, exhibit piloerection, and rapidly succumb to hypovolemic shock, typically within 24 to 48 hours of clinical onset [6, 72]. The mortality in piglets less than one week old is virtually absolute, whereas weaned pigs and adult swine experience a milder, often self-limiting diarrheal illness characterized by anorexia, transient fever, and diarrhea lasting 3–7 days, with mortality being exceptionally rare in this cohort [12]. This age-related resistance is attributed to the maturation of the intestinal epithelium, the development of a more robust adaptive immune system, and the progressive replacement of fetal-type enterocytes with adult-type cells that are less permissive to TGEV replication.

Macroscopic Pathological Findings

At necropsy, the most striking and consistent macroscopic lesions are confined to the gastrointestinal tract, with the small intestine bearing the brunt of the pathological insult. The stomach is frequently distended with undigested milk curds, reflecting the profound impairment of gastric emptying and intestinal motility induced by the infection [19]. The small intestine, particularly the jejunum and ileum, appears thin-walled, translucent, and distended with copious quantities of watery, yellow-to-green fluid containing gas bubbles [19, 72]. The intestinal wall is markedly attenuated, a direct consequence of the extensive villous atrophy that characterizes the disease. The mesenteric lymphatic vessels are often engorged with chyle, and the mesenteric lymph nodes may be swollen and edematous. In contrast, the large intestine typically contains scant, watery contents and may appear grossly normal, although the cecum and colon can exhibit mild congestion in protracted cases [19]. The kidneys may appear pale and swollen due to dehydration-induced prerenal azotemia, and the urinary bladder is typically empty, reflecting the severe fluid depletion. No significant macroscopic lesions are observed in the respiratory tract, heart, liver, or spleen, underscoring the primary enterotropism of TGEV [19, 72]. However, it is critical to note that co-infections with other enteric pathogens, particularly porcine epidemic diarrhea virus (PEDV) and porcine deltacoronavirus (PDCoV), are exceedingly common in field settings and can substantially confound the gross pathological picture, leading to more extensive and severe lesions [58-60].

Histopathological Hallmarks and Cellular Pathogenesis

The histopathological signature of TGEV infection is the dramatic and rapid atrophy of the small intestinal villi, which is the anatomical correlate of the malabsorptive diarrhea that defines the clinical syndrome [19, 52]. Within 24 to 48 hours of infection, the tall, finger-like villi of the jejunum and ileum undergo progressive shortening and blunting, often to the point of near-total effacement, while the crypts undergo compensatory hyperplasia and elongation [52, 71]. The villus-to-crypt ratio, normally approximately 7:1 in the neonatal piglet jejunum, can plummet to 1:1 or less in severely affected animals [71]. This architectural collapse is driven by the lytic infection and subsequent exfoliation of mature, absorptive enterocytes lining the villous tips, which are the primary cellular targets of TGEV [47, 52]. The virus gains entry into these cells via the functional receptor porcine aminopeptidase N (pAPN), a metalloprotease highly expressed on the brush border membrane of differentiated enterocytes [10, 11]. The critical role of pAPN has been unequivocally demonstrated in APN-knockout piglets, which are completely resistant to TGEV infection and exhibit no villous atrophy or viral antigen upon challenge, confirming that pAPN is the indispensable gateway for TGEV entry in vivo [11].

The loss of absorptive enterocytes results in a dramatic reduction in the functional absorptive surface area of the small intestine, leading to profound malabsorption of water, electrolytes, and nutrients. Concurrently, the hyperplastic crypts, which are populated by undifferentiated, secretory progenitor cells, continue to secrete fluid and electrolytes into the intestinal lumen, exacerbating the net fluid loss [52]. The absorptive deficit is compounded by the downregulation of key nutrient transporters, including the sodium-dependent glucose transporter 1 (SGLT1) and the facilitative glucose transporter 2 (GLUT2), which are critical for glucose and sodium absorption [70]. Paradoxically, TGEV infection has been shown to enhance the expression of these transporters in the early stages of infection, potentially as a viral strategy to increase glucose uptake and fuel viral replication, but this effect is ultimately overwhelmed by the catastrophic loss of enterocytes [70].

The "Bottom-Up" Scenario: Paneth Cells and Intestinal Stem Cells

A paradigm-shifting insight into the pathogenesis of TGEV-induced villous atrophy has emerged from studies elucidating the virus's interaction with the intestinal crypt microenvironment. Contrary to the traditional "top-down" model, wherein the virus directly infects and destroys villous enterocytes, a more nuanced "bottom-up" scenario has been proposed [52]. In this model, TGEV initially targets Paneth cells, specialized secretory cells located at the base of the intestinal crypts that express high levels of pAPN and are essential for maintaining the intestinal stem cell (ISC) niche [52]. TGEV infection of Paneth cells induces mitochondrial damage and reactive oxygen species (ROS) generation, leading to Paneth cell loss and a consequent reduction in the secretion of Notch ligands, particularly DLL4 [52]. The Notch signaling pathway is a master regulator of ISC self-renewal and differentiation; its inactivation skews the differentiation program away from absorptive enterocytes and toward secretory lineages, particularly goblet cells [52]. This lineage shift results in a relative increase in mucin-secreting goblet cells, which paradoxically facilitates TGEV replication, as the virus can exploit the mucin-rich environment for enhanced spread [52]. Furthermore, the loss of Notch signaling inhibits the self-renewal of Lgr5+ ISCs, impairing the regenerative capacity of the epithelium and exacerbating villous atrophy [52]. This intricate interplay between viral tropism for Paneth cells, Notch signaling disruption, and ISC dysfunction provides a mechanistic explanation for the rapid and profound villous atrophy observed within 48 hours of infection.

Inflammatory Responses and Inflammasome Activation

TGEV infection elicits a robust and complex inflammatory response that contributes significantly to the pathogenesis of the disease. The virus activates multiple pattern recognition receptor (PRR) pathways, including retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) and Toll-like receptors (TLRs), which converge on the nuclear factor-kappa B (NF-κB) signaling cascade [46, 53]. RIG-I and melanoma differentiation-associated protein 5 (MDA5) are the primary sensors of TGEV RNA, and their activation leads to the phosphorylation and nuclear translocation of NF-κB p65, driving the transcription of a battery of pro-inflammatory cytokines, including interleukin-1β (IL-1β), IL-6, IL-8, and tumor necrosis factor-alpha (TNF-α) [46, 53, 68]. The NF-κB pathway is further modulated by viral proteins; non-structural protein 2 (Nsp2) acts as a potent activator of NF-κB, while non-structural protein 3 (Nsp3) exerts an inhibitory effect via its deubiquitinase activity, highlighting the dynamic and counterbalanced nature of the host-virus interaction [15, 17].

A critical component of the inflammatory response is the activation of the inflammasome, a multiprotein complex that mediates the cleavage and secretion of IL-1β and IL-18 and induces a lytic form of cell death known as pyroptosis. TGEV infection activates both the NLRP3 and NLRP1 inflammasomes in porcine intestinal epithelial cells [51, 55]. NLRP3 activation is dependent on potassium efflux and leads to caspase-1-dependent processing of pro-IL-1β and gasdermin D (GSDMD), the executioner of pyroptosis [51]. The release of IL-1β and IL-18 amplifies the inflammatory cascade, recruiting neutrophils and other immune cells to the site of infection, which contributes to tissue damage [55]. Notably, inhibition of NLRP3 enhances TGEV replication, indicating that the inflammasome serves a critical antiviral function [51]. The NLRP1 inflammasome, which is highly expressed in epithelial barrier tissues, is also activated by TGEV infection through a mechanism involving the disruption of its interaction with dipeptidyl peptidase 9 (DPP9) [55]. Furthermore, TGEV-induced type I interferon (IFN) upregulates NLRP1 expression, establishing a positive feedback loop that enhances antiviral immunity [55]. The inflammatory milieu is further shaped by the induction of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). TGEV infection activates all three branches of the UPR, activating transcription factor 6 (ATF6), inositol-requiring enzyme 1 (IRE1), and protein kinase R-like ER kinase (PERK), which have divergent effects on viral replication and inflammation [30, 34, 36]. The IRE1 pathway, for instance, mediates the splicing of X-box-binding protein 1 (Xbp1s), which in turn suppresses the expression of anti-coronaviral microRNAs such as miR-27b and miR-30a-5p, thereby promoting viral replication and dampening the IFN response [30, 36].

Disruption of Epithelial Barrier Integrity

The integrity of the intestinal epithelial barrier is severely compromised during TGEV infection, contributing to the pathophysiology of diarrhea and facilitating secondary bacterial translocation. The tight junction proteins, including zonula occludens-1 (ZO-1), occludin, and claudins, which form the paracellular seal between adjacent enterocytes, are markedly downregulated in TGEV-infected intestinal epithelium [48, 56, 68]. This disruption is mediated, at least in part, by the activation of the NF-κB signaling pathway, which directly suppresses the transcription of tight junction genes [48]. The loss of tight junction integrity leads to increased paracellular permeability, or "leaky gut," allowing the passive movement of water, ions, and small molecules into the intestinal lumen, further exacerbating diarrhea. The actin cytoskeleton, which provides structural support to the tight junction complex, also undergoes dramatic remodeling during TGEV infection. The virus induces the polymerization of F-actin at the cell membrane, a process that is dependent on the epidermal growth factor receptor (EGFR) and the downstream activation of Rac1/Cdc42 GTPases and cofilin [24]. This cytoskeletal reorganization is essential for viral entry but also contributes to the destabilization of the apical junctional complex [56]. The disruption of the epithelial barrier is further compounded by the loss of goblet cells and the reduction in mucin secretion, which compromises the protective mucus layer that normally prevents direct contact between luminal contents and the epithelial surface [47].

Mitochondrial Dysfunction and Apoptosis

TGEV infection induces profound mitochondrial dysfunction in infected enterocytes, which is a central driver of cellular injury and apoptosis. The virus triggers the opening of the mitochondrial permeability transition pore (mPTP), leading to a collapse of the mitochondrial membrane potential (MMP), the release of cytochrome c into the cytosol, and the activation of the intrinsic apoptotic cascade [38, 39, 50]. The mPTP opening is regulated by the interaction between voltage-dependent anion-selective channel protein 1 (VDAC1) and cyclophilin D (CypD). TGEV infection downregulates a circular RNA, circBIRC6-2, which encodes a novel protein, BIRC6-236aa, that normally stabilizes the VDAC1-CypD complex and prevents mPTP opening [50]. The loss of this protective mechanism sensitizes cells to mitochondrial dysfunction. Concurrently, TGEV infection upregulates miR-4331, which targets retinoblastoma 1 (RB1), leading to increased expression of interleukin-1 receptor accessory protein (IL1RAP) and activation of the p38 MAPK pathway, further promoting mitochondrial damage [39]. Conversely, miR-222 is upregulated as a compensatory response and attenuates mitochondrial dysfunction by targeting thrombospondin-1 (THBS1) and CD47 [38]. The resulting mitochondrial dysfunction is accompanied by a surge in ROS production, which overwhelms the cellular antioxidant defenses and induces oxidative stress [49, 69]. The ROS-mediated damage to lipids, proteins, and DNA triggers both the intrinsic (mitochondrial) and extrinsic (death receptor) apoptotic pathways, as evidenced by the activation of caspase-3, caspase-8, and caspase-9, and the upregulation of Bax and Fas [49]. The extensive apoptosis of enterocytes is a major contributor to the villous atrophy and barrier dysfunction that characterize the disease.

Diagnostic Approaches for TGEV Infection

The accurate and timely diagnosis of transmissible gastroenteritis virus (TGEV) infection is paramount for implementing effective control measures, mitigating economic losses, and differentiating this highly pathogenic alphacoronavirus from other swine enteric pathogens that present with nearly identical clinical signs. The diagnostic landscape for TGEV has evolved considerably, moving from classical virological and histopathological techniques to highly sensitive molecular assays and rapid, field-deployable point-of-care tests. Given the high mortality rate approaching 100% in neonatal piglets and the clinical similarity to infections caused by porcine epidemic diarrhea virus (PEDV), porcine deltacoronavirus (PDCoV), and rotavirus, a definitive laboratory diagnosis is not merely beneficial but essential [3, 12]. The World Organisation for Animal Health (WOAH) recognizes TGEV as a listed disease, underscoring its economic significance and the need for robust surveillance and diagnostic capabilities globally. This section provides an exhaustive analysis of the diagnostic approaches available for TGEV, from traditional methods to cutting-edge molecular and immunological platforms, with a critical evaluation of their applications, limitations, and interpretive nuances.

Clinical and Pathological Assessment as a Diagnostic Prelude

While clinical signs alone are insufficient for a definitive diagnosis, they provide the critical initial impetus for laboratory investigation. TGEV infection is characterized by an acute onset of profuse, watery diarrhea, often described as "milky" or "yellowish," accompanied by vomiting, severe dehydration, and rapid weight loss, primarily in piglets under two weeks of age [3, 72]. The morbidity within a naive herd can approach 100%, with mortality rates in neonates reaching 100% within days. In older pigs, the disease is typically milder, presenting as transient diarrhea and anorexia. A key pathological hallmark observed at necropsy is a thin, transparent intestinal wall due to severe villous atrophy, particularly in the jejunum and ileum [52, 71]. Histopathological examination reveals the fusion and shortening of villi, crypt hyperplasia, and vacuolation of enterocytes [19]. However, these gross and microscopic lesions are pathognomonic for a severe enteric coronavirus infection but are not specific to TGEV, as PEDV and PDCoV induce nearly identical pathology [19, 60]. Therefore, histopathology serves as a supportive diagnostic tool that must be corroborated by specific virological or molecular testing.

Molecular Diagnostics: The Gold Standard for Detection and Differentiation

Nucleic acid amplification tests (NAATs), particularly real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR), have become the cornerstone of TGEV diagnosis due to their unparalleled sensitivity, specificity, and ability to differentiate TGEV from other co-circulating enteric coronaviruses.

Multiplex Real-Time RT-PCR and qPCR Assays

The frequent co-circulation and co-infection of TGEV with PEDV and PDCoV in swine populations necessitates the use of multiplex assays. Several highly validated multiplex qRT-PCR and quantitative PCR (qPCR) platforms have been developed. Li et al. [59] designed a TaqMan probe-based multiplex qRT-PCR targeting the M gene of PEDV, the S gene of TGEV, and the M gene of PDCoV. This assay demonstrated exceptional analytical sensitivity, with a detection limit of 2.95 × 10⁰ copies/μL for each virus, and showed no cross-reactivity with other common porcine viruses. In a field evaluation of 160 clinical samples, this method revealed positive rates of 38.13% for PEDV, 1.88% for TGEV, and 5.00% for PDCoV, with a co-infection rate of PEDV+TGEV at 1.25% [59]. Similarly, Chen et al. [60] developed a multiplex qPCR assay targeting the PEDV M gene, TGEV S gene, and PDCoV N gene, achieving a limit of detection of 10 copies/μL. Their large-scale surveillance of 462 clinical samples from five Chinese provinces yielded discrete positive rates of 19.70% for PEDV, 0.87% for TGEV, and 10.17% for PDCoV, with a notable mixed infection rate of PEDV/PDCoV at 23.16% [60]. These data underscore the critical need for multiplexing, as single-target assays would miss a substantial proportion of mixed infections, leading to an incomplete epidemiological picture.

Conventional and Nanoparticle-Assisted PCR

Before the widespread adoption of real-time platforms, conventional RT-PCR was the standard. However, its lower sensitivity and requirement for post-amplification processing (e.g., gel electrophoresis) have relegated it to a secondary role. A significant advancement was the development of a duplex nanoparticle-assisted PCR (nanoPCR) assay by Zhu et al. [74]. This method, which incorporates nanoparticles to enhance thermal conductivity and reaction efficiency, demonstrated a detection limit of 8.5 × 10¹ copies/μL for TGEV, which was ten times more sensitive than conventional PCR [74]. In a screening of 114 clinical samples, the nanoPCR assay identified a TGEV positive rate of 3.5%, demonstrating its utility for sensitive detection in resource-limited settings where real-time equipment may be unavailable.

Reverse Transcription Loop-Mediated Isothermal Amplification (RT-LAMP)

For true point-of-care or on-farm diagnosis, RT-LAMP offers a compelling alternative to PCR, as it requires only a simple heat source (e.g., a water bath or heat block) and provides results in under 30 minutes. El-Tholoth et al. [73] engineered a portable, 3D-printed microfluidic device that integrates sample distribution, self-sealing, and real-time RT-LAMP for the simultaneous detection of PEDV, TGEV, and PDCoV. This device achieved limits of detection of 10 genomic copies per reaction for PEDV and PDCoV and 100 genomic copies for TGEV, with performance comparable to benchtop qRT-PCR [73]. The rapid turnaround time (approximately 30 minutes) and minimal equipment requirements make this technology exceptionally valuable for outbreak investigations in the field, enabling immediate implementation of quarantine and biosecurity measures.

Serological Diagnostics: Detecting Past Exposure and Herd Immunity

Serological assays are indispensable for surveillance, determining herd-level prevalence, and assessing vaccine-induced immunity. They are less useful for diagnosing acute infections in neonates, as maternal antibodies may be present, and the rapid disease course often leads to death before a detectable adaptive immune response is mounted.

Enzyme-Linked Immunosorbent Assay (ELISA)

ELISA is the most widely used serological tool. Commercial and in-house ELISAs typically use whole virus or recombinant spike (S) or nucleocapsid (N) proteins as antigens. A critical challenge in TGEV serology is the antigenic cross-reactivity with porcine respiratory coronavirus (PRCV), a naturally occurring deletion mutant of TGEV that lacks a significant portion of the S protein's N-terminus [1, 2]. Differentiating between TGEV and PRCV antibodies is essential for accurate epidemiological studies. Antas and Olech [2] utilized a TGEV/PRCV differential immunoassay to screen 828 serum samples from Polish pigs, revealing a seroprevalence of 2.2% for TGEV and 12.2% for PRCV. This differential capability is often achieved using ELISAs based on the S protein region deleted in PRCV (e.g., the SAD epitopes), allowing for specific detection of TGEV antibodies. Furthermore, phage-based ELISAs have been explored as an alternative. Suo et al. [27] identified a peptide (TLNMHLFPFHTG) via phage display that binds specifically to the TGEV S-AD region and developed a phage-mediated ELISA. While this assay was more sensitive than conventional antibody-based ELISA, it did not surpass the sensitivity of semi-quantitative RT-PCR, though it offered advantages in speed and simplicity [27].

Virus Neutralization Test (VNT)

The VNT remains the gold standard for detecting functional, neutralizing antibodies. It is highly specific and can differentiate TGEV from PRCV, as PRCV typically induces lower or undetectable neutralizing antibody titers due to its altered S protein. However, the VNT is labor-intensive, requires live virus and cell culture facilities, and takes several days to complete, making it unsuitable for high-throughput screening or rapid diagnosis. It is primarily used for confirmatory testing and research purposes.

Antigen Detection Methods

Direct detection of viral antigens in feces or intestinal tissue provides a rapid diagnosis of active infection.

Immunohistochemistry (IHC) and Immunofluorescence (IFA)

IHC on formalin-fixed, paraffin-embedded intestinal tissues is a highly specific method for visualizing TGEV antigen within infected enterocytes. Piñeyro et al. [19] used monoclonal antibody-based IHC to confirm the presence of TGEV antigen in archived intestinal samples from Argentina, providing the first etiological confirmation of TGEV in the country. IFA can be performed on frozen tissue sections or cell cultures inoculated with clinical samples. While these methods are valuable for research and confirmatory diagnosis, they are time-consuming, require specialized expertise, and are not practical for large-scale screening.

Colloidal Gold Immunochromatographic Assay (GICA)

For rapid, on-site preliminary diagnosis, the immunochromatographic strip (lateral flow assay) is an invaluable tool. Zhou et al. [58] developed a duplex colloidal gold immunochromatography assay (GICA) strip for the simultaneous detection of PEDV and TGEV. The assay showed no cross-reactivity with five other common porcine viruses and had a visual detection limit of approximately 10⁴ TCID₅₀/mL for TGEV. The strips were stable for 12 months at 4°C or 25°C. When validated against 121 clinical samples, the GICA strip identified a TGEV positive rate of 9.9% and a co-infection rate with PEDV of 5.8% [58]. The simplicity, speed (results in 10-15 minutes), and lack of equipment requirements make GICA strips ideal for use by farm personnel and veterinarians for initial triage, although their lower sensitivity compared to PCR means that negative results from suspect cases should be confirmed by molecular methods.

Advanced and Emerging Diagnostic Technologies

In Situ Hybridization (ISH)

ISH allows for the detection of viral nucleic acid within the context of intact tissue architecture, providing spatial information about viral replication. Piñeyro et al. [19] employed ISH using a probe specific for TGEV mRNA encoding the spike protein to confirm the presence of metabolically active virus in intestinal sections. This technique is particularly useful for retrospective studies on archived formalin-fixed tissues and for differentiating active infection from passive antigen presence.

Next-Generation Sequencing (NGS) and Metagenomics

NGS is not a routine diagnostic tool but is indispensable for characterizing emerging strains, identifying recombination events, and understanding viral evolution. Whole-genome sequencing of TGEV isolates has revealed the existence of distinct genotypes (GI and GII) and the emergence of natural recombinant strains, such as the JS2012 strain, which is a recombinant between the Purdue and Miller clusters [6, 26]. NGS-based metagenomics can also be used to detect TGEV in clinical samples without prior knowledge of the pathogen, making it a powerful tool for investigating outbreaks of unknown etiology.

Intestinal Organoid and Precision-Cut Intestinal Slice (PCIS) Models

While primarily research tools, these ex vivo models are being used to study TGEV pathogenesis and could be adapted for diagnostic applications, such as assessing the virulence of field isolates or testing antiviral compounds. Zhang et al. [46] demonstrated that apical-out porcine intestinal organoids are highly susceptible to TGEV and faithfully recapitulate the inflammatory responses seen in vivo. Similarly, Krimmling et al. [44] showed that porcine precision-cut intestinal slices can be used to analyze the enterotropism of different TGEV strains. These models provide a more physiologically relevant platform than traditional cell lines for studying host-virus interactions.

Diagnostic Challenges and Interpretive Caveats

Several factors complicate the diagnosis of TGEV. First, the clinical and pathological similarity to PEDV and PDCoV necessitates the use of differential diagnostic assays [59, 60]. Second, the widespread circulation of PRCV, which is antigenically related but causes mild or subclinical respiratory disease, can lead to false-positive serological results if non-differential ELISAs are used [1, 2]. Third, the rapid disease course in neonatal piglets means that by the time samples are collected, the virus may have already been cleared from the feces, leading to false-negative PCR results. In such cases, IHC or ISH on intestinal tissues may be more reliable. Fourth, the genetic diversity of TGEV, including the emergence of recombinant strains with deletions in the S gene or ORF3a, can affect the performance of molecular assays if primers and probes are not designed against highly conserved regions [6, 21, 28]. Finally, the presence of PCR inhibitors in fecal samples can lead to false negatives, necessitating the use of appropriate internal controls. A comprehensive diagnostic strategy should therefore integrate clinical history, pathological examination, and a combination of molecular and serological tests, interpreted in the context of the herd's vaccination status and the local epidemiological situation.

Host Immune Response and Vaccine Development Strategies

The interplay between transmissible gastroenteritis virus (TGEV) and the porcine host immune system constitutes a complex, multifaceted battleground, the outcome of which determines the trajectory from acute, often fatal, enteritis to survival and potential immunity. A comprehensive understanding of these molecular and cellular dynamics is not merely an academic exercise; it is the essential bedrock upon which rational and efficacious vaccine strategies are built. This section dissects the host immune response to TGEV, from the initial innate sensing mechanisms to the adaptive memory responses, and subsequently examines how this knowledge is being harnessed and translated into a diverse arsenal of vaccine platforms, ranging from traditional inactivated preparations to cutting-edge circular RNA and vectored constructs.

Innate Immune Sensing and Signaling Cascades

The porcine intestinal epithelium, the primary target of TGEV, is equipped with an array of pattern recognition receptors (PRRs) that serve as the first line of defense. Upon infection, TGEV is sensed by several key PRR families, triggering a cascade of signaling events that shape the subsequent innate response. Retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5), members of the RIG-I-like receptor (RLR) family, have been identified as critical sensors for TGEV. Infection activates the RLR signaling pathway, leading to the phosphorylation and nuclear translocation of transcription factors including nuclear factor-kappa B (NF-κB), interferon regulatory factor 3 (IRF3), and activator protein 1 (AP-1) [53]. This activation is dependent on the adaptor molecules MAVS and STING, while the Toll-like receptor (TLR) adaptors MyD88 and TRIF play a more minor role [53]. The engagement of RLRs is a potent trigger for the production of pro-inflammatory cytokines and type I interferons (IFN-I). Studies using porcine intestinal organoids have provided physiologically relevant confirmation of this pathway, demonstrating that TGEV-induced inflammatory responses are regulated via the RIG-I/NF-κB/hypoxia-inducible factor-1α (HIF-1α)/glycolysis axis, both ex vivo and in vivo [46]. This axis highlights a metabolic component to the inflammatory response, where HIF-1α positively regulates inflammation by activating glycolysis, a key energy source for activated immune cells [46].

Beyond RLRs, the inflammasome pathway represents another crucial arm of the innate response. TGEV infection has been demonstrated to activate the NOD-like receptor protein 3 (NLRP3) inflammasome in porcine intestinal epithelial cells, leading to the activation of caspase-1, the maturation and release of interleukin-1β (IL-1β), and the induction of pyroptosis, a highly inflammatory form of programmed cell death mediated by gasdermin D (GSDMD) [51]. This NLRP3-dependent response is a significant contributor to the intestinal inflammation characteristic of TGE. More recently, the NLRP1 inflammasome has also been implicated, with TGEV infection increasing IL-1β and IL-18 levels in intestinal epithelial cells and tissues. Importantly, the porcine NLRP1 (pNLRP1) was shown to act as an interferon-stimulated gene (ISG), possessing antiviral capabilities against TGEV infection, highlighting a direct role for this inflammasome in viral restriction [55]. The activation of NLRP1 by TGEV was found to impede its interaction with dipeptidyl peptidase 9 (pDPP9), but through a unique mechanism involving its ZU5 domain rather than the function-to-find domain (FIIND) reported in humans [55]. This intricate dialogue between the virus and the host's inflammatory machinery is a critical determinant of disease pathogenesis.

The Interferon Response and Viral Countermeasures

Type I interferons (IFN-α/β) are central to the antiviral state. TGEV infection is a potent inducer of IFN-β production, and this induction is mediated, in part, by the viral non-structural protein 14 (nsp14). TGEV nsp14 has been identified as the most potent IFN-β inducer among the viral proteins, acting mainly through the activation of NF-κB. This process involves an interaction with the host cellular RNA helicase DDX1, which serves as a coactivator for IFN-β production [35]. The induction of IFN-β is further supported by TGEV infection activating all three branches of the unfolded protein response (UPR), ATF6, IRE1, and PERK, in response to ER stress. The PERK-eIF2α axis plays a dominant role in suppressing TGEV replication by globally attenuating protein translation, which directly inhibits viral protein synthesis, and by promoting type I IFN production through NF-κB activation [34]. Interferon-gamma (IFN-γ), primarily produced by immune cells such as natural killer cells and T cells, also exerts direct antiviral activity against TGEV. This effect is mediated through the induction of porcine interferon regulatory factor 1 (poIRF1) signaling, and IFN-γ displays a synergistic antiviral effect when combined with IFN-α [75].

In a classic evolutionary arms race, TGEV has evolved sophisticated strategies to subvert the host interferon response and its downstream effectors. The viral papain-like protease 1 (PL1) functions as a potent antagonist of IFN-β production. PL1 employs its deubiquitinase (DUB) activity to inhibit RIG-I- and STING-dependent signaling, thereby suppressing IFN-β expression and blocking the nuclear translocation of IRF3 [22]. Furthermore, TGEV utilizes the ER stress sensor IRE1α to manipulate the host microRNA (miRNA) landscape to its advantage. TGEV infection activates IRE1α, which, through its RNase activity, leads to the splicing of Xbp1 mRNA. The resulting transcription factor, Xbp1s, then inhibits the transcription of miR-27, ultimately reducing the levels of miR-27b-3p, a specific microRNA that normally suppresses TGEV replication by targeting SOCS6 [30]. In a parallel evasion strategy, TGEV uses IRE1α to downregulate miR-30a-5p. This reduction in miR-30a-5p leads to the increased expression of its targets, SOCS1 and SOCS3, which are negative regulators of the JAK-STAT signaling pathway. By boosting SOCS1 and SOCS3 levels, TGEV effectively dampens the IFN-I antiviral response, despite the presence of elevated endogenous IFN-I [36]. These intricate mechanisms underscore the virus's ability to finely tune the host environment for its own propagation.

Adaptive Immunity and Mucosal Dynamics

The adaptive immune response, encompassing both humoral and cellular arms, is essential for clearing TGEV infection and providing long-term protection, particularly at the mucosal surface. The spike (S) protein, particularly the S1 domain containing antigenic sites A and D, is the primary target for neutralizing antibodies [4, 27]. Mucosal immunity, mediated by secretory immunoglobulin A (sIgA), is paramount for protection at the intestinal epithelium, the primary portal of entry. TGEV infection has been shown to upregulate the expression of the neonatal Fc receptor (FcRn) via its nucleocapsid (N) protein and through the secretion of TGF-β, a mechanism that likely enhances the transcytosis of IgG across the intestinal epithelium and potentiates mucosal immunity [31]. The cellular immune response, involving both CD4+ helper T cells and CD8+ cytotoxic T lymphocytes (CTLs), is critical for eliminating infected cells. Infection with virulent TGEV strains, however, can impair the function of dendritic cells (DCs), which are the key antigen-presenting cells required to bridge the innate and adaptive responses. Virulent TGEV has been shown to inhibit the ability of monocyte-derived and intestinal DCs to sample antigen, migrate, and activate T-cell proliferation, whereas attenuated strains have the opposite, enhancing effect [77]. This DC modulation is a crucial factor in the pathogenesis of virulent strains. At the crypt level, TGEV infection has a profound effect on intestinal stem cells (ISCs). Interestingly, TGEV infection induces the proliferation of Lgr5+ ISCs via activation of the Wnt/β-catenin pathway, promoting epithelium regeneration and tissue repair [47]. However, simultaneously, the virus targets Paneth cells, which provide essential niche factors like Notch ligands (Dll4) to ISCs. TGEV-induced mitochondrial damage and ROS generation in Paneth cells leads to their loss and a reduction in Notch signaling, inhibiting ISC self-renewal and driving differentiation towards goblet cells, which ironically may accelerate viral replication [52]. This dual effect on the crypt epithelium reveals a complex pathology where the host attempts repair while the virus manipulates cell fate for its own benefit.

Vaccine Development Strategies: A Multi-Platform Approach

Harnessing the knowledge of these immune mechanisms has led to the development of a wide array of TGEV vaccine candidates, each with distinct strengths and weaknesses. The goal is to induce robust, long-lasting, and protective immunity, particularly the mucosal sIgA response in neonatal piglets.

Conventional and Recombinant Inactivated Vaccines

Traditional vaccine approaches remain a mainstay. Inactivated whole-virus vaccines are safe but often require adjuvants and multiple doses to induce effective immunity. The choice of inactivating reagent is critical, as demonstrated by a comparative study of formaldehyde (FA), β-propiolactone (BPL), and binaryethylenimine (BEI). While all three induced humoral and cellular immunity in mice, the FA group elicited the strongest humoral response (specific IgG), whereas the BEI group induced a superior cellular immune response, indicated by higher CD4+ and CD8+ T cell counts and stronger lymphocyte proliferation [61]. This highlights that even the formulation of a "traditional" vaccine can be optimized to skew the immune response in a desired direction. Furthermore, supplementation of inactivated vaccines with retinoic acid (RA) has shown promise. Subcutaneous immunization of pigs with a whole inactivated TGEV vaccine (WI-TGEV) combined with RA significantly enhanced intestinal mucosal immunity. RA promoted the induction of gut-homing CD8+ T cells expressing α4β7 and CCR9 molecules and recruited them to the small intestine, while also enhancing both IgA and IgG antibody titers [76]. This strategy offers a practical approach to improve the efficacy of parenterally-administered inactivated vaccines against a mucosal pathogen.

Subunit, Virus-Like Particle, and DNA Vaccines

Subunit vaccines, focusing on the immunodominant S protein, offer a high safety profile. A combined subunit vaccine incorporating the S1 domains of both TGEV and PEDV has demonstrated promising immunogenicity in a mouse model, representing a step towards bivalent protection [29]. The use of virus-like particles (VLPs) provides a more native antigen presentation. An innovative VLP platform using the self-assembling ADDomer backbone has been developed to display specific neutralizing epitopes (sites A and D) from the TGEV S protein. This recombinant ADDomer-VLP vaccine, produced in a baculovirus expression system, self-assembled into particles and, when administered to piglets, induced strong humoral and cellular immune responses, including high levels of neutralizing antibodies, IFN-γ, IL-2, IL-4, and enhanced CTL activity, with no adverse reactions [4]. Recombinant DNA vaccines have also been explored. Plasmids encoding the TGEV S1 protein, alone or in combination with PEDV S proteins, elicited specific antibodies, neutralizing activity, and CTL responses in mice [67]. While safe and stable, the immunogenicity of DNA vaccines in large animals often requires optimization.

Live Attenuated and Chimeric Virus Vaccines

Live attenuated vaccines (LAVs) generally induce the most robust and durable immune responses, closely mimicking natural infection. However, safety concerns regarding reversion to virulence remain. A rationally designed LAV was constructed using a reverse genetics system to engineer a chimeric virus based on the attenuated TGEV genome but expressing the spike protein from a virulent US PEDV strain (rTGEV-RS-SPEDV). This virus was fully attenuated in highly susceptible five-day-old piglets, causing no mortality or weight loss. When tested in older pigs, it provided solid protection against a virulent PEDV challenge, significantly reducing challenge virus titers in the jejunum and rendering viral RNA undetectable in feces. It also induced a strong PEDV-specific humoral immune response including neutralizing antibodies [32]. This chimeric approach elegantly combines the safety profile of an attenuated TGEV backbone with the protective immunogenicity of a heterologous PEDV spike. The N-terminal domain of the TGEV S protein, which is deleted in PRCV, has historically been considered an enteric tropism determinant. However, using a reverse genetics platform with CRISPR/Cas9, it was demonstrated that a 224-amino-acid deletion in this domain (S_NTD224) did not ablate enteric tropism or virulence in piglets, though it did affect growth kinetics in vitro. This finding challenges a long-held paradigm and provides important insights for the rational design of LAVs, suggesting that deletion of this region alone is insufficient for attenuation [13].

Bacterial Vector-Based Oral Vaccines

Given the enteric nature of TGEV, oral vaccination is a highly attractive route for inducing local mucosal immunity. Genetically engineered Lactobacillus species are promising vectors for the delivery of TGEV antigens. A recombinant L. acidophilus expressing the SAD antigenic epitopes of TGEV was developed as an oral vaccine. This candidate induced significantly higher levels of TGEV-specific sIgA in mice compared to a commercial inactivated vaccine, along with strong IgG and IFN-γ responses, indicating robust induction of both mucosal and humoral/cellular immunity [62]. Similarly, L. plantarum engineered to express the S protein fused with a dendritic cell-targeting peptide (DCpep) effectively enhanced antigen uptake and presentation, leading to increased percentages of B cells and T cells in the ileum lamina propria and elevated specific sIgA and IgG titers in piglets [65]. A bivalent L. casei strain was also constructed to co-express the D antigenic site of TGEV and the core neutralizing epitope (COE) of PEDV. This antibiotic resistance-free recombinant strain, when given orally to mice, induced significant serum IgG and mucosal sIgA responses against both viruses, with neutralizing activity, demonstrating the potential for a multivalent oral vaccine delivered by a safe food-grade bacterium [63].

Bivalent Circular RNA Vaccines

The success of mRNA vaccines against COVID-19 has paved the way for next-generation RNA-based vaccines for veterinary applications. A recent study developed a bivalent circular RNA (circRNA) vaccine against both PEDV and TGEV. This platform offers advantages in stability and prolonged antigen expression compared to linear mRNA. The vaccine encoded the S1 domains of TGEV and PEDV. The authors encountered a key challenge of antigenic dominance, where the TGEV component (TS1) was more immunogenic than the PEDV component (PS1). To balance the response, they conjugated the PEDV S1 to a porcine Fc region (PS1F) to enhance its immunogenicity. After dose optimization of the individual circRNAs, they achieved comparable antibody and T-cell responses against both antigens. Furthermore, a sequential vaccination regimen, priming with the bivalent circRNA vaccine and boosting with commercial inactivated vaccines, elicited a predominantly Th1-driven antibody response and effectively neutralized both viruses [57]. This work highlights the potential of circRNA vaccines for swine and underscores the importance of strategic formulation and dosing schedules to overcome immunogenicity imbalances in multivalent vaccines.

The field of TGEV vaccinology is a vibrant and rapidly evolving landscape. The constant emergence of new variants and the related porcine respiratory coronavirus (PRCV) [1, 2] necessitate continuous monitoring and adaptation of vaccine strains, as acknowledged by the World Organisation for Animal Health (WOAH). The most successful future strategies will likely involve a combination of approaches, leveraging the potency of live or RNA-based platforms with the safety of subunit or vectored systems, and will be meticulously tailored to induce the potent and durable mucosal immunity required to halt this devastating enteric pathogen.

Prevention, Control, and One Health Implications

The prevention and control of transmissible gastroenteritis virus (TGEV) represent a formidable challenge in contemporary swine medicine, demanding a multifaceted strategy that integrates rigorous biosecurity protocols, advanced vaccination technologies, strategic antiviral interventions, and a comprehensive understanding of the virus’s evolutionary dynamics. The economic toll exacted by TGEV, particularly the near-100% mortality in neonatal piglets, necessitates a proactive and scientifically grounded approach. Furthermore, the close genetic and ecological relationships between TGEV, its deletion mutant porcine respiratory coronavirus (PRCV), and other coronaviruses raise critical One Health considerations that extend beyond the farm gate, touching upon zoonotic potential and the broader ecology of emerging infectious diseases.

Biosecurity, Surveillance, and Diagnostic Strategies

The cornerstone of TGEV prevention is the implementation of stringent biosecurity measures designed to prevent the introduction and dissemination of the virus within and between swine herds. Given that TGEV is highly contagious and shed in high concentrations in feces, all-in/all-out production systems, rigorous cleaning and disinfection protocols, and strict control of personnel, equipment, and vehicle movements are non-negotiable. The virus’s ability to persist in the environment, particularly in cold conditions, further underscores the need for thorough sanitation. The seasonal nature of TGEV, with peak prevalence during cold months as documented in both the United States [7] and China [25], aligns with the increased stability of enveloped viruses at lower temperatures and the potential for enhanced airborne transmission in enclosed, poorly ventilated winter housing. This seasonality should inform the timing of heightened surveillance and prophylactic vaccination campaigns.

Effective control is predicated on rapid and accurate diagnosis. The clinical similarity of TGEV to other swine enteric coronaviruses, such as porcine epidemic diarrhea virus (PEDV) and porcine deltacoronavirus (PDCoV), which often co-circulate and cause indistinguishable clinical signs, makes differential diagnosis essential [58-60]. Advanced molecular diagnostics have become the gold standard. Multiplex quantitative reverse transcription-polymerase chain reaction (qRT-PCR) assays, capable of simultaneously detecting and differentiating TGEV, PEDV, and PDCoV with high sensitivity (detection limits as low as 10 copies/μL), are now indispensable tools for clinical monitoring and epidemiological surveillance [59, 60]. These assays enable the rapid identification of mixed infections, which are increasingly recognized as common and can complicate clinical management and vaccine efficacy assessment [60]. For on-site, point-of-need diagnosis, particularly in resource-limited settings, innovative technologies have been developed. A portable, 3D-printed microfluidic device utilizing reverse transcription-loop-mediated isothermal amplification (RT-LAMP) can co-detect TGEV, PEDV, and PDCoV within 30 minutes, with analytical performance comparable to benchtop qRT-PCR [73]. Similarly, colloidal gold immunochromatographic assay (GICA) strips offer a rapid, specific, and stable tool for preliminary on-farm diagnosis, capable of detecting both PEDV and TGEV simultaneously without cross-reactivity with other common porcine viruses [58]. The deployment of such field-deployable diagnostics is critical for enabling prompt implementation of control measures, such as immediate isolation of affected litters and enhanced sanitation.

Serological surveillance, while less useful for acute outbreak management, provides valuable insights into herd-level exposure and the prevalence of TGEV and PRCV. Studies in Poland, using a TGEV/PRCV immunoassay, revealed a seroprevalence of 2.2% for TGEV and 12.2% for PRCV, with PRCV RNA detected in 6.22% of nasal swabs [2]. This highlights the widespread circulation of PRCV, which, due to its antigenic similarity to TGEV, can complicate serological interpretation and may contribute to herd immunity against TGEV, albeit incompletely. In Hungary, seropositivity was found to be generally low and evenly distributed, but it was noted that positive samples do not necessarily correlate with an efficient level of protective immunity [66]. This underscores the need for quantitative assays that measure neutralizing antibody titers rather than mere seropositivity. The World Organisation for Animal Health (WOAH) recognizes TGEV as a listed disease, and reporting of outbreaks is mandatory in many countries, facilitating international surveillance and coordination of control efforts.

Vaccination: Current Modalities and Next-Generation Platforms

Vaccination remains the most effective and economically viable strategy for controlling TGEV, particularly for protecting the most vulnerable demographic: neonatal piglets. The goal of vaccination is to induce robust lactogenic immunity in sows, which is passively transferred to piglets via colostrum and milk, providing critical protection during the first weeks of life. However, the development of safe and broadly effective vaccines has been hampered by the genetic diversity of TGEV, the emergence of recombinant strains, and the inherent challenges of inducing potent mucosal immunity in the gut.

Conventional Vaccines and Inactivation Methods: Historically, both modified-live virus (MLV) and inactivated vaccines have been employed. The choice of inactivating reagent significantly influences the immunogenicity of the final product. A comparative study evaluating formaldehyde (FA), β-propiolactone (BPL), and binaryethylenimine (BEI) for inactivating the TGEV HN-2012 strain found that FA-inactivated vaccines induced the earliest and strongest humoral (IgG) response, while BEI-inactivated vaccines elicited superior cellular immunity, as evidenced by higher CD4+ and CD8+ T lymphocyte proliferation and subset frequencies [61]. This suggests that the inactivant can be strategically selected to tailor the immune response profile. However, conventional inactivated vaccines often fail to induce robust mucosal IgA responses, which are critical for neutralizing the virus at the intestinal epithelium. To address this, novel adjuvant strategies are being explored. For instance, retinoic acid (RA), a metabolite of vitamin A, has been shown to enhance the ability of parenterally administered inactivated TGEV vaccine to induce gut-homing CD8+ T cells expressing α4β7 and CCR9, thereby recruiting them to the small intestine and boosting both mucosal IgA and systemic IgG titers [76]. This approach represents a promising strategy to bridge the gap between systemic vaccination and mucosal protection.

Subunit and Virus-Like Particle (VLP) Vaccines: The spike (S) protein of TGEV, particularly its S1 subunit containing the major neutralizing epitopes (sites A and D), is the primary target for subunit vaccine development. Recombinant subunit vaccines offer a superior safety profile compared to live-attenuated vaccines, as they eliminate any risk of reversion to virulence. A study developing novel subunit vaccines incorporating PEDV S1 and TGEV S1 proteins demonstrated promising immunogenicity in a mouse model, suggesting a viable path toward bivalent or multivalent protection against co-circulating enteric coronaviruses [29]. However, the immunogenicity of individual subunits can be suboptimal. To overcome this, virus-like particle (VLP) platforms have emerged as a highly effective strategy. VLPs mimic the native viral structure, presenting antigens in a repetitive, ordered array that potently stimulates both B-cell and T-cell responses. A groundbreaking study utilized the ADDomer platform, a self-assembling protein scaffold derived from the adenovirus, to incorporate specific neutralizing epitopes from the TGEV S protein (sites A and D) and PEDV S protein (SS2 and 2C10 regions) [4]. The resulting recombinant ADDomer-VLPs, produced in a baculovirus expression system, self-assembled into immunogenic particles that, upon administration to piglets, induced strong neutralizing antibody responses against both TGEV and PEDV, elevated levels of IFN-γ, IL-2, and IL-4, and enhanced cytotoxic T lymphocyte (CTL) activity [4]. This platform demonstrates the potential for rapid, simplified development of safe and efficacious epitope-based vaccines.

Live Bacterial Vector Vaccines: Oral vaccination is the most direct route to induce intestinal mucosal immunity. Genetically engineered probiotic bacteria, such as Lactobacillus species, represent an attractive delivery vehicle for TGEV antigens. These vectors are non-pathogenic, can be administered orally, and have intrinsic adjuvant properties. Recombinant Lactobacillus acidophilus expressing the SAD epitope of the TGEV S protein was shown to induce significantly higher levels of specific secretory IgA (SIgA) in mice compared to a commercial inactivated vaccine, along with robust serum IgG and IFN-γ responses [62]. Similarly, Lactobacillus plantarum strains engineered to display the TGEV S protein, particularly when fused with dendritic cell-targeting peptides (DCpep), have demonstrated remarkable efficacy in piglets, significantly increasing the percentages of MHC-II+CD80+ B cells and CD3+CD4+ T cells in the ileal lamina propria, and elevating fecal SIgA and serum IgG titers [64, 65]. A bivalent Lactobacillus casei strain co-expressing the TGEV S D antigenic site and the PEDV S COE (core neutralizing epitope) has also been developed, inducing neutralizing antibodies against both viruses in mice [63]. These live bacterial vectors offer a safe, cost-effective, and scalable platform for oral vaccination, capable of eliciting the full spectrum of humoral, mucosal, and cellular immune responses.

Nucleic Acid and Circular RNA Vaccines: The COVID-19 pandemic has catalyzed the rapid advancement of mRNA-based vaccine technologies, which are now being applied to veterinary medicine. Circular RNA (circRNA) vaccines offer advantages over linear mRNA, including enhanced stability and prolonged antigen expression. A recent study developed bivalent circRNA vaccines encoding the PEDV S1 and TGEV S1 antigens [57]. Initial challenges with the lower immunogenicity of the PEDV S1 antigen were addressed by conjugating it to a porcine Fc region (PS1F) to enhance antigen presentation. After dose optimization, the bivalent circRNA vaccine induced comparable levels of antigen-specific antibodies and T-cell immunity against both viruses. Furthermore, a sequential vaccination regimen combining the bivalent circRNA vaccine with a commercial inactivated vaccine elicited a predominantly Th1-driven antibody response and effectively neutralized both PEDV and TGEV [57]. This highlights the potential of heterologous prime-boost strategies to optimize immune responses. DNA vaccines, encoding the TGEV S1 protein alone or in combination with PEDV S proteins, have also been shown to induce specific antibody responses, lymphocyte proliferation, and CTL activity in mice [67].

Chimeric and Reverse Genetics-Based Vaccines: The deep understanding of TGEV molecular biology has enabled the rational design of live-attenuated vaccines through reverse genetics. A landmark study engineered a chimeric virus based on an attenuated TGEV genome, replacing its spike protein with that of a virulent US PEDV strain (rTGEV-RS-SPEDV) [32]. This chimeric virus was fully attenuated in highly sensitive five-day-old piglets, causing no mortality or weight loss, and induced only minor tissue damage. Crucially, it protected three-week-old pigs against challenge with virulent PEDV, reducing challenge virus titers in the jejunum to undetectable levels in feces and inducing robust PEDV-specific neutralizing antibodies [32]. This approach elegantly leverages the safety of an attenuated TGEV backbone to deliver protective antigens from a heterologous, highly virulent virus. Similarly, the identification of the minimum determinants of TGEV enteric tropism, specifically a single amino acid change (S219A) in the spike protein, provides a molecular target for the rational attenuation of virulent strains [14]. Deletion of the N-terminal domain of the spike protein (S_NTD224), analogous to the deletion found in PRCV, was shown to mildly influence TGEV virulence but was not the sole determinant of enteric tropism, providing new insights for vaccine design [13].

Antiviral and Adjunctive Therapeutic Strategies

While vaccination is the primary preventive tool, the development of effective antiviral therapies is critical for outbreak management and treatment of infected animals, particularly in the absence of a perfect vaccine. The search for anti-TGEV agents has identified several promising candidates targeting various stages of the viral life cycle and host-pathogen interactions.

Direct-Acting Antivirals: Thapsigargin (TG), an inducer of endoplasmic reticulum (ER) stress, has demonstrated potent antiviral activity against TGEV in cell lines, porcine intestinal organoids, and neonatal piglets [78]. Oral administration of TG inhibited TGEV infection and alleviated associated tissue injury. Mechanistically, TG improved the expression of ER-associated protein degradation (ERAD) components, suggesting it may enhance the host’s ability to clear viral proteins [78]. Curcumin, a natural polyphenol, exhibits both direct virucidal activity and inhibition of TGEV adsorption, primarily acting in the early phase of replication [81]. Cardenolides, which are Na+/K+-ATPase inhibitors, have also been identified as a novel class of anti-TGEV agents, diminishing viral protein expression and blocking virus-induced apoptosis in a dose-dependent manner [84].

Host-Targeted Immunomodulators: Modulating the host immune response to limit TGEV-induced pathology is a promising adjunctive strategy. All-trans retinoic acid (ATRA), the active metabolite of vitamin A, has been shown to attenuate TGEV-induced inflammation in IPEC-J2 cells by suppressing the RLRs/NF-κB signaling pathway, reducing pro-inflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α), and restoring tight junction protein expression (ZO-1, Occludin) [68]. ATRA also mitigates TGEV-induced apoptosis by inhibiting ROS-mediated P38MAPK signaling [49]. Eugenol, a phenolic compound found in clove oil, similarly alleviates TGEV-induced intestinal injury by inhibiting the NF-κB signaling pathway, reducing inflammatory cytokines, and restoring tight junction integrity [48]. Furthermore, eugenol protects against TGEV-induced oxidative stress and apoptosis by activating the ROS-NRF2-ARE signaling pathway [69]. High-dose dietary zinc oxide has also been shown to mitigate TGEV infection in piglets, preventing villus atrophy, reducing caspase-3-mediated apoptosis, and stimulating an earlier and higher systemic TGEV-specific antibody response [71]. Interferon-gamma (IFN-γ) has direct antiviral activity against TGEV, mediated by an IRF1 signaling pathway, and acts synergistically with IFN-α [75]. The identification of host restriction factors, such as interferon-induced transmembrane protein 3 (IFITM3), which inhibits TGEV entry and replication, opens avenues for developing therapeutics that upregulate these innate antiviral effectors [79, 80].

Probiotics and Biologicals: Probiotic bacteria offer a safe and natural approach to prevent TGEV infection. Bacillus subtilis and its secreted lipopeptide, surfactin, have been shown to effectively inhibit TGEV entry into intestinal epithelial cells by competing for viral entry receptors (EGFR and APN) and by directly attaching to viral particles [83]. Recombinant single-chain fragment variable (scFv) antibodies, generated from the B cells of TGEV-infected piglets, have demonstrated potent neutralizing activity against TGEV in vitro, representing a potential therapeutic for passive immunization [82].

One Health Implications: Zoonotic Potential and Evolutionary Dynamics

The study of TGEV extends far beyond swine health, offering critical insights into coronavirus evolution, cross-species transmission, and pandemic preparedness, core tenets of the One Health framework. The emergence of PRCV, a naturally occurring deletion mutant of TGEV that shifted tropism from the enteric to the respiratory tract, serves as a powerful model for understanding how minor genetic changes can dramatically alter viral pathogenesis and host range [1, 7, 23]. The continuous evolution of TGEV through mutation, recombination, and deletion is well-documented. Natural recombinant strains between the Purdue and Miller clusters have been identified in China, exhibiting high virulence and 100% mortality in piglets [6, 18]. The emergence of “variant” TGEV genotypes in the United States, sharing unique deletions with PRCV, suggests ongoing recombination events that blur the lines between these two viruses [7]. The detection of PRCV strains in Poland with novel deletions of varying sizes (672 nt and 690 nt) in the S gene further underscores the genetic plasticity of this viral group [2]. These evolutionary dynamics have direct implications for vaccine efficacy, as circulating strains may diverge antigenically from vaccine strains, leading to breakthrough infections.

Perhaps the most pressing One Health concern is the potential zoonotic transmission of TGEV or its derivatives. The detection of a canine-feline recombinant alphacoronavirus, CCoV-HuPn-2018, in a human patient with pneumonia provides alarming evidence that alphacoronaviruses can cross the species barrier into humans [3]. This finding, coupled with the fact that TGEV uses the same functional receptor, aminopeptidase N (pAPN), as the human coronavirus NL63 (HCoV-NL63), raises the theoretical possibility that TGEV could adapt to use human APN. While current evidence suggests TGEV is not a human pathogen, the high mutation rate of RNA viruses, the selective pressure of widespread vaccination, and the ever-present risk of recombination with other coronaviruses in animal reservoirs (including pigs, dogs, cats, and potentially wildlife) create a precarious situation. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have emphasized the importance of surveillance at the human-animal interface for emerging coronaviruses. The swine population, given its large size, global distribution, and close contact with humans, represents a significant reservoir for potential future pandemic threats. Therefore, robust surveillance of TGEV and PRCV in swine herds, coupled with genomic characterization of circulating strains, is not merely a veterinary concern but a critical component of global health security. The development of broadly protective vaccines that can anticipate viral evolution, and the continued exploration of pan-coronavirus antiviral strategies, are essential investments for mitigating the risk of future zoonotic spillover events.

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