Infectious Hypodermal and Hematopoietic Necrosis Virus

Overview and Taxonomy of Infectious Hypodermal and Hematopoietic Necrosis Virus

Infectious hypodermal and hematopoietic necrosis virus (IHHNV) stands as one of the most economically impactful and epidemiologically complex pathogens affecting global penaeid shrimp aquaculture. Classified as a notifiable crustacean disease agent by the World Organisation for Animal Health (WOAH), IHHNV has been responsible for catastrophic mortality events in Penaeus stylirostris, chronic growth retardation and severe deformities in Penaeus vannamei and Penaeus monodon, and significant annual losses estimated in the hundreds of millions of dollars [1, 11, 22, 24]. The virus was first recognized in the late 1970s and early 1980s during epizootics in farmed shrimp in the Americas, and it has since disseminated globally through the movement of live animals, frozen commodity products, and possibly via wild crustacean vectors [1, 7, 11]. Its unique biology, broad host range, and evolving relationship with its hosts make IHHNV a compelling subject for taxonomic, ecological, and pathogenic investigation.

Taxonomic Classification and Nomenclature

IHHNV is a non-enveloped, linear, single-stranded DNA (ssDNA) virus belonging to the family Parvoviridae, subfamily Densovirinae, and genus Penstyldensovirus [1, 12]. The current officially recognized species name under the International Committee on Taxonomy of Viruses (ICTV) is Decapod penstylhamaparvovirus 1 (DpHV-1), although the historical and widely used acronym IHHNV remains entrenched in the scientific literature and regulatory frameworks [1, 5]. Within the parvovirid classification, IHHNV is further distinguished by its ability to replicate autonomously in crustacean cells without the need for a helper virus, unlike many vertebrate parvoviruses [11]. The virus is also referred to as Penaeus stylirostris penstyldensovirus 1 (PstDV1) in some earlier genomic reports, particularly those from the Americas [16].

The virion is among the smallest known for shrimp viruses, measuring approximately 22–24 nm in diameter, with an icosahedral capsid composed of a single major capsid protein (CP) of about 37 kDa [4, 17, 23, 25]. The genome is approximately 3.9–4.1 kb in length, containing three major open reading frames (ORFs): ORF1 encodes a non-structural protein (NS1) involved in replication, ORF2 encodes a smaller non-structural protein (NS2) of uncertain function, and ORF3 encodes the capsid protein (CP) [2, 4, 8, 14]. The genomic organization and replication strategy (rolling-circle replication via a hairpin telomere) are characteristic of the parvoviruses, although IHHNV exhibits a notably high evolutionary rate for a DNA virus, estimated at 1.39 × 10⁻⁴ substitutions/site/year, which is comparable to that of many RNA viruses and contributes to its rapid adaptation and genetic diversification [27].

Genotypic Diversity and Phylogenetic Structure

Phylogenetic analyses of IHHNV isolates from disparate geographic regions have consistently identified three major genotypes or lineages, designated Type I, Type II, and Type III [6, 10, 12, 15, 27]. Type I isolates are predominantly found in the Americas (Ecuador, Peru, Mexico, the United States, and Brazil) and are considered the original epidemic lineage that caused the devastating outbreaks in P. stylirostris in the 1980s and 1990s [10, 12, 27]. Type II isolates are widely distributed across Asia (China, Vietnam, Philippines, Thailand, Indonesia, Korea) and have also been detected in Latin America, suggesting recent transboundary movement [6, 9, 15, 19, 20]. Type III isolates are less common and have been reported from the Western Pacific and Australia, though their pathogenic significance is still under investigation [19]. Importantly, the classification into infectious (Type I and II) and non-infectious (Type III) forms has been hypothesized, but recent evidence demonstrates that both Type I and II isolates can cause active infection, while Type III sequences may represent endogenous viral elements integrated into shrimp genomes [5, 10].

Whole-genome sequencing of IHHNV strains from Peru and Ecuador revealed that South American Type II isolates form a distinct subcluster, diverging from Asian Type II strains by approximately 3–5% nucleotide identity [10, 16, 21]. This phylogeographic structure indicates that IHHNV has been repeatedly introduced into new regions, often through the trade of live broodstock or frozen commodity shrimp, and that local viral populations can become genetically isolated over time [12, 27]. The high mutation rate and the emergence of novel genotypes underscore the need for continuous genomic surveillance, as the emergence of more virulent or transmissible variants remains a credible threat to global shrimp farming [11, 27].

Host Range and Susceptibility

IHHNV exhibits one of the broadest host ranges among penaeid pathogens, infecting not only multiple species of penaeid shrimp but also non-penaeid crustaceans, including crabs and freshwater crayfish [1, 11]. The list of confirmed susceptible species under WOAH criteria (Chapter 1.5 of the Aquatic Animal Health Code) includes the major commercial penaeids: Penaeus vannamei (Pacific white shrimp), Penaeus monodon (black tiger shrimp), Penaeus stylirostris (blue shrimp), Penaeus japonicus (kuruma shrimp), and Penaeus semisulcatus (green tiger shrimp) [1, 5, 11, 18]. In recent years, experimental and field studies have expanded the known host spectrum to include the crab Helice tientsinensis, a common inhabitant of shrimp ponds in northern China, which supports IHHNV replication and can transmit the virus back to P. vannamei [3]. Similarly, red claw crayfish (Cherax quadricarinatus) imported as commodity into South Korea were found harboring infectious Type II IHHNV, with viral DNA detected in muscle, hepatopancreas, and gills, confirming their role as potential carriers [19]. The red swamp crayfish (Procambarus clarkii) farmed extensively in China also shows a high prevalence of IHHNV (up to 56.7% in some provinces), and the virus can be transmitted horizontally through waterborne routes [9, 11]. Furthermore, wild crustaceans in the Andaman and Nicobar Archipelago, including P. monodon, Scylla serrata (mud crab), and various other shrimp species, exhibited co-infection with IHHNV and white spot syndrome virus (WSSV), highlighting the role of wild reservoirs in viral persistence and spread [13].

The susceptibility of Macrobrachium rosenbergii (giant freshwater prawn) to IHHNV has been documented, with proteomic changes in hemocytes (downregulation of arginine kinase, upregulation of prophenoloxidase) indicative of active infection [26]. This expands the host range beyond marine penaeids into freshwater palaemonids, which may serve as bridging hosts between aquaculture systems and natural waterways.

Tissue Tropism and Pathological Hallmarks

IHHNV exhibits a marked tropism for tissues of ectodermal and mesodermal origin, including the cuticular epithelium, gills, connective tissue, hematopoietic organs, and antennal gland [1, 8]. However, the classic dogma that endodermal tissues (e.g., hepatopancreas) are not susceptible has been challenged by recent findings. Hou et al. (2023) demonstrated via feeding challenge experiments in P. vannamei that the hepatopancreas not only becomes infected but harbors the highest viral load (up to 19.4 copies/mg) among tested organs, with 100% positivity by PCR [8]. In situ hybridization and histology confirmed the presence of Cowdry type A eosinophilic intranuclear inclusion bodies in hepatopancreatic tubule cells [8, 13]. These inclusions are pathognomonic for IHHNV infection and represent sites of viral replication and capsid assembly within hypertrophied nuclei [5, 13, 24]. The absence of an inflammatory response in infected shrimp, even in the presence of high viral loads, suggests that IHHNV may subvert host immune signaling, consistent with the concept of the virus evolving toward a more tolerant equilibrium with its host in many endemic regions [5, 10].

Evolutionary Dynamics and Ecological Implications

The high nucleotide substitution rate of IHHNV, comparable to that of RNA viruses, drives rapid genetic turnover and facilitates adaptation to new hosts and environments [27]. Bayesian skyline plot analyses of isolates from the Gulf of California (Mexico) revealed that effective viral population size expanded dramatically during the early 1990s, coinciding with the initial epizootic in wild shrimp populations, and then stabilized as host–virus coevolution progressed [27]. The detection of putative endogenous viral elements (EVEs) in the P. vannamei genome, particularly sequences homologous to non-infectious Type III IHHNV, lends credence to the hypothesis that ancient integrations conferred a degree of adaptive tolerance, possibly through RNA interference or other innate immune mechanisms [5, 10]. This phenomenon may explain why contemporary IHHNV isolates from Peru and Ecuador, while still infectious to specific-pathogen-free (SPF) shrimp, fail to induce clinical signs or mortality in the currently farmed P. vannamei lines [10]. Nevertheless, in less adapted hosts like P. monodon or P. stylirostris, IHHNV can still cause severe runting, deformities, and mortality, emphasizing the importance of host genetic background and viral genotype in disease outcome [22, 24].

As WOAH-listed pathogen, IHHNV remains a compulsory notification agent in many countries, and its detection in imported commodity shrimp, wild populations, and alternative crustacean hosts necessitates rigorous biosecurity protocols at hatcheries, grow-out farms, and border inspection points [1, 6, 20]. The virus’s ability to persist in the environment and to infect a wide array of decapod species, combined with its high evolutionary rate, poses a continuous challenge for the sustainable expansion of the global shrimp aquaculture industry.

Molecular Pathogenesis: Host-Virus Protein Interactions

The molecular pathogenesis of Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) is fundamentally governed by an intricate network of interactions between viral proteins and host cellular machinery. As a non-enveloped, linear single-stranded DNA virus belonging to the family Parvoviridae [12], IHHNV encodes three primary proteins, the capsid protein (CP), non-structural protein 1 (NS1), and non-structural protein 2 (NS2), which orchestrate viral entry, replication, and evasion of host defenses. The World Organization for Animal Health (WOAH) classifies IHHNV as a notifiable crustacean pathogen, underscoring the global economic significance of understanding these molecular interactions [12, 28, 29]. Recent advances in yeast two-hybrid screening, virus overlay protein blot assays (VOPBA), and proteomic profiling have begun to unravel the specific host protein partners that IHHNV exploits, revealing a pathogenesis strategy that involves metabolic reprogramming, cytoskeletal manipulation, and subversion of innate immune signaling.

The Capsid Protein (CP) and Host Cell Entry Mechanisms

The capsid protein serves as the primary interface between IHHNV and the host cell surface, mediating attachment and internalization. Comprehensive yeast two-hybrid screening using CP as bait identified 14 unique interacting host proteins from Litopenaeus vannamei, with subsequent backcross verification confirming that fatty acid binding protein 1-B.1-like (FABP1-B.1-like), elongation factor 2-like isoform X2 (EF2L-X2), and insulin-like growth factor-binding protein-related protein 1 (IGFBP-rP1) directly bind to CP [2]. The interaction with FABP1-B.1-like is particularly intriguing, as fatty acid binding proteins are implicated in lipid transport and cellular signaling, suggesting that IHHNV may hijack lipid metabolism pathways to facilitate membrane fusion or endosomal escape. EF2L-X2, a translation elongation factor, indicates a potential mechanism for viral hijacking of the host translational machinery to prioritize viral protein synthesis over cellular proteins, a strategy common among parvoviruses.

More specifically, NM23 (nucleoside diphosphate kinase) was identified as a putative host cell receptor for IHHNV CP in gill membranes of L. vannamei through a combination of 2-dimensional VOPBA and mass spectrometry [14]. The direct interaction between NM23 and CP was confirmed via glutathione-S-transferase (GST) pulldown assays, providing strong evidence for NM23 as a critical entry mediator. Phage display library screening revealed that CP binds to dodecapeptides containing three characteristic motifs, SWY, SKWV, and PQR, with the SWY motif also identified within the NM23 sequence [14]. This structural mimicry suggests that IHHNV CP utilizes a conserved binding interface to engage NM23, potentially triggering downstream signaling cascades that facilitate viral trafficking to the nucleus. The importance of this interaction is underscored by the observation that NM23 is ubiquitously expressed in shrimp tissues, particularly in gills and hematopoietic organs, which aligns with IHHNV’s known tissue tropism.

The structural biology of CP further illuminates its pathogenic role. Cryo-electron microscopy reconstruction of IHHNV virus-like particles (VLPs) revealed that DNA encapsulation induces significant capsid conformational changes, including a 50 Å expansion in diameter, a 1.5-fold thickening of the capsid interior due to DNA–capsid interactions, and counter-clockwise twisting of the “tripod” structure at the five-fold axes [17]. These structural rearrangements are hypothesized to represent capsid maturation events that occur upon genome packaging in host cells, potentially exposing previously buried domains that mediate endosomal escape or nuclear localization. The disassembly and reassembly of IHHNV VLPs are controlled by disulfide bridging and calcium ions, as demonstrated by sensitivity to dithiothreitol (DTT) and calcium chelators (EGTA) [25], indicating that redox conditions within endosomes likely trigger genome release, a mechanism paralleling that of mammalian parvoviruses.

Non-Structural Protein 1 (NS1) and Cellular Metabolic Hijacking

NS1, the major replication initiator protein, exhibits a remarkably broad interactome, with yeast two-hybrid screening identifying 90 positive clones corresponding to 14 unique host proteins [2]. Three of these interacting partners were validated through backcross verification: glyceraldehyde 3-phosphate dehydrogenase (GAPDH), phosphopyruvate hydrolase (PEP), and alpha-glucosidase (α-Glu) [2]. The interaction with GAPDH is mechanistically significant because this glycolytic enzyme is known to moonlight in nuclear functions, including transcriptional regulation and DNA repair. Parvovirus NS1 proteins in mammalian systems often localize to nuclear replication centers where they recruit cellular replication factors; the binding of IHHNV NS1 to GAPDH may therefore facilitate the formation of viral replication compartments by tethering metabolic enzymes that provide nucleotide precursors or ATP required for viral DNA synthesis.

Similarly, the interaction with PEP, a key enzyme in gluconeogenesis/glycolysis, suggests that IHHNV NS1 redirects carbon flux toward anabolic pathways that support viral genome replication. This metabolic reprogramming is a hallmark of many viral infections, where infected cells shift from oxidative phosphorylation to aerobic glycolysis (the Warburg effect) to generate biosynthetic intermediates. The binding of α-Glu, involved in glycogen breakdown, further supports a model in which IHHNV NS1 orchestrates a coordinated upregulation of glucose metabolism to fuel the high energy demands of parvoviral replication. These findings align with proteomic data from IHHNV-infected Macrobrachium rosenbergii hemocytes, where arginine kinase was significantly downregulated [26], suggesting disruption of cellular energy homeostasis that may suppress host antiviral responses.

Non-Structural Protein 2 (NS2) and Subversion of Host Trafficking Pathways

NS2, though less characterized than NS1, interacts with an equally diverse set of host proteins. Yeast two-hybrid screening using NS2 as bait yielded 35 positive clones representing 11 unique proteins, with validated interactors including FABP1-B.1-like, α-Glu, importin subunit alpha-1 (IMPα1), 40S ribosomal protein S20-like isoform X1 (RPS20-X1), and NPC intracellular cholesterol transporter 2-like (NPC2-like) [2]. The identification of IMPα1 is particularly compelling, as importin α proteins mediate nuclear import of cargo proteins by recognizing nuclear localization signals (NLS). Parvovirus NS proteins often contain functional NLS sequences that allow them to enter the nucleus and modulate host transcription or splicing. Binding of IHHNV NS2 to IMPα1 may therefore facilitate the nuclear translocation of NS2 itself or of viral replication complexes, ensuring efficient access to the host replication machinery.

The interaction with NPC2-like, a cholesterol transporter involved in lysosomal cholesterol efflux, suggests that IHHNV NS2 manipulates cellular lipid homeostasis. Cholesterol is essential for membrane fluidity and the formation of lipid rafts, which serve as platforms for viral entry and assembly for many enveloped and non-enveloped viruses. By binding NPC2-like, NS2 may alter cholesterol distribution within endolysosomal compartments, potentially enhancing viral uncoating or egress. The concurrent interaction with RPS20-X1, a component of the small ribosomal subunit, reinforces the theme of translational hijacking observed with CP–EF2L-X2 binding, indicating that IHHNV employs multiple strategies to redirect the host translational apparatus toward viral protein synthesis.

Host Proteomic Responses and Immune Evasion

Proteomic analysis of hemocytes from IHHNV-infected M. rosenbergii revealed a coordinated dysregulation of several key immune and metabolic proteins [26]. Arginine kinase and sarcoplasmic calcium-binding protein were specifically downregulated at both the protein and mRNA levels, while prophenoloxidase 1 (proPO1) and hemocyanin isoforms exhibited bidirectional regulation, some isoforms upregulated, others downregulated [26]. Arginine kinase is critical for maintaining ATP levels in crustaceans, and its downregulation likely reflects the energetic stress imposed by viral replication. More importantly, prophenoloxidase is a central component of the shrimp innate immune system, catalyzing melanin synthesis that encapsulates and kills pathogens. The dysregulation of proPO1 suggests that IHHNV actively suppresses melanization, a key antiviral defense, while the modulation of hemocyanin, a multifunctional protein involved in oxygen transport and phenoloxidase-like activity, indicates a broader disruption of humoral immunity.

Interestingly, IHHNV infection in Penaeus vannamei is characterized by the absence of an inflammatory response. Histopathological examination of naturally infected shrimp from Peruvian farms showed Cowdry type A eosinophilic intranuclear inclusions but no evidence of hemocytic infiltration or tissue necrosis [5]. This finding aligns with the observation that IHHNV establishes persistent, non-cytopathic infections in many modern shrimp lines, suggesting that the virus has evolved mechanisms to avoid triggering programmed cell death or inflammatory signaling. The absence of inflammation may be partly attributable to the upregulation of peroxiredoxin (Prx) in response to IHHNV VLP administration [30]. Peroxiredoxins are antioxidant enzymes that scavenge reactive oxygen species (ROS), which are typically produced during antiviral responses. By inducing Prx expression, IHHNV may neutralize ROS-mediated signaling, thereby dampening the inflammatory cascade and allowing viral persistence.

Tissue Tropism and Molecular Determinants of Pathogenesis

The molecular interactions described above provide a mechanistic basis for IHHNV tissue tropism. Quantitative PCR and histological analyses have demonstrated that IHHNV targets tissues of both ectodermal/mesodermal origin (cuticular epithelium, connective tissue, hematopoietic organs) and, controversially, endodermal tissues such as the hepatopancreas [8]. Hepatopancreas exhibited the highest viral loads among tested organs in feeding challenge experiments, with 100% positivity and 19.4 copies/mg [8]. The broad tropism is explained by the ubiquitous expression of NM23 and other CP-interacting proteins, while the particularly high replication in hepatopancreas likely reflects the abundance of metabolic enzymes (GAPDH, PEP, α-Glu) that NS1 and NS2 hijack for viral genome replication.

Cross-species transmission studies have further elucidated the molecular basis of host range. The Chinese mitten crab Helice tientsinensis was confirmed as a susceptible host meeting WOAH criteria, with IHHNV detection in gills, hepatopancreas, and stomach [3]. The hepatopancreas again harbored the highest viral loads, and Cowdry type A inclusion bodies were observed in hemal sinuses and hepatopancreatic tubules [3]. The ability of IHHNV to replicate in crab hepatopancreas indicates that the host protein interactors identified in penaeid shrimp are evolutionarily conserved across decapod crustaceans, at least for the core replication machinery. This conservation poses significant risks for aquaculture systems, as foraging behavior of wild crabs may facilitate viral spillover into shrimp ponds [3].

Genetic Variation and Coevolutionary Dynamics

The molecular pathogenesis of IHHNV is further complicated by significant genetic diversity among circulating strains. Whole-genome sequencing of isolates from Ecuador and Peru revealed that South American genotypes form a distinct cluster within Type II genotypes, divergent from Asian, African, and Australian isolates [10]. Experimental bioassays using these representative isolates demonstrated that while the virus remains infectious, it caused no mortality or clinical signs in specific pathogen-free P. vannamei, P. monodon, or P. stylirostris [10]. This tolerance correlates with the presence of endogenous viral elements (EVE) in the shrimp genome, which are hypothesized to provide adaptive immunity through RNA interference or other mechanisms [5]. The evolutionary rate of IHHNV is unexpectedly high (1.39 × 10⁻⁴ substitutions/site/year), comparable to RNA viruses, and Bayesian skyline plots correlate with epizootiological records [27], indicating that viral evolution is shaped by host population dynamics and selective pressures from the host immune system.

Viral interference between IHHNV and White Spot Syndrome Virus (WSSV) adds another layer of complexity to pathogenesis. Co-infection studies have documented that IHHNV can reduce WSSV replication and mortality, a phenomenon attributed to competition for cellular resources or activation of antiviral pathways [11]. The molecular basis of this interference may involve the upregulation of peroxiredoxin by IHHNV VLPs, which subsequently reduces WSSV copy numbers [30]. Administration of IHHNV VLPs encapsulating VP28 dsRNA silenced WSSV challenge more effectively than naked dsRNA, and also stimulated immune-related gene expression within 6 hours post-administration [31]. These findings suggest that IHHNV CP engages pattern recognition receptors that trigger a broad antiviral state, potentially through the JAK-STAT or Toll pathways, which cross-protect against heterologous viruses.

Epidemiology and Global Distribution of IHHNV

Infectious hypodermal and hematopoietic necrosis virus (IHHNV), taxonomically classified as Decapod Penstylhamaparvovirus 1, represents one of the most globally pervasive and economically consequential viral pathogens affecting penaeid shrimp aquaculture and wild crustacean populations. As a notifiable crustacean disease listed by the World Organisation for Animal Health (WOAH), the epidemiological profile of IHHNV has evolved dramatically since its initial emergence, transitioning from an acute, high-mortality pathogen in specific hosts to a persistent, often subclinical agent that exhibits complex interactions with its hosts and the broader ecosystem [1, 11]. The global distribution of IHHNV now encompasses virtually all major shrimp-producing regions across Asia, the Americas, the Pacific Islands, the Middle East, and select areas of Africa and the Atlantic, with prevalence rates fluctuating dramatically based on host species, geographical location, farming practices, and the genetic lineage of the circulating viral strains. Understanding the nuanced epidemiological landscape of IHHNV requires a multifaceted examination of its historical epizootics, contemporary surveillance data, phylogenetic diversity, host range expansion, and the emerging recognition of viral tolerance and endogenous viral elements that have fundamentally altered the host-pathogen dynamic.

Historical Emergence and Global Dissemination

The first documented epizootics of IHHNV occurred in the late 1970s and early 1980s, initially reported in Penaeus stylirostris (blue shrimp) farming operations in Hawaii and later in other regions of the Americas and the Pacific. These initial outbreaks were characterized by catastrophic mortality rates, often exceeding 90% within affected populations, which precipitated a global awareness of the pathogen and led to its inclusion on international notifiable disease lists. The rapid and devastating spread of IHHNV during this period is consistent with the introduction of a highly virulent pathogen into a naive host population with no prior evolutionary history of exposure. Molecular clock analyses, utilizing Bayesian coalescent approaches on capsid protein gene sequences, have estimated a mean rate of nucleotide substitution for IHHNV of approximately 1.39×10⁻⁴ substitutions per site per year, a rate unexpectedly high for a single-stranded DNA virus and more comparable to that of RNA viruses [27]. This high evolutionary rate explains the virus’s capacity for rapid adaptation and its ability to establish persistent infections across diverse geographical and host environments. The genetic signature of this rapid expansion is particularly evident in wild Penaeus shrimp populations along the northwestern Pacific coast of Mexico, where historical epizootiological records closely correspond with Bayesian skyline plot inferences of changes in effective population size of infections [27].

From its initial foci, IHHNV disseminated globally through multiple anthropogenic pathways, including the international trade of live postlarvae, broodstock, and frozen commodity shrimp. The virus has been detected repeatedly in imported commodity crustaceans, underscoring the role of global seafood supply chains in its transboundary spread. For instance, screening of red claw crayfish (Cherax quadricarinatus) imported into South Korea from Indonesia revealed IHHNV DNA in nine out of ten batches, with the phylogenetic clustering of these sequences alongside infectious type II strains commonly circulating in Southeast Asia [19]. Similarly, whiteleg shrimp (Penaeus vannamei) imported from Vietnam into South Korean markets exhibited an IHHNV detection rate of 39.7% (23 out of 58 samples), representing the first documented detection of the virus in imported shrimp in that nation [20]. The presence of IHHNV in frozen P. vannamei destined for export from Colombia further illustrates that even processed products can serve as vectors for viral dissemination, albeit without the risk of live-virus transmission through properly processed frozen material [6]. The repeated translocation of IHHNV into previously unaffected regions has been supported by phylogenetic evidence; for example, isolates from the Gulf of Mexico and the Northwestern Atlantic Ocean, first reported in wild-caught Penaeus monodon, have been shown to be paraphyletic, suggesting multiple independent introductions from different source localities rather than a single, recent common ancestor [7].

Geographic Prevalence and Endemicity by Region

The Americas: A Spectrum from Endemic Tolerance to Active Surveillance

The epidemiological landscape of IHHNV in the Americas is characterized by stark contrasts between regions of high endemicity with low clinical impact and areas where the virus remains under active surveillance with variable prevalence. In Peru and Ecuador, which together constitute a significant proportion of global P. vannamei production, IHHNV is now considered endemic, with surveillance data revealing a prevalence range of 3.3% to 100% across shrimp farms in major producing regions such as Guayas, El Oro, and Esmeralda in Ecuador, and Tumbes and Piura in Peru [5, 10]. Critically, whole genome sequencing of representative circulating genotypes from these nations has demonstrated that the strains form a distinct phylogenetic cluster within the Type II genotype, divergent from Asian, African, Australian, and Brazilian isolates [10]. Experimental bioassays using specific pathogen-free (SPF) P. vannamei, P. monodon, and P. stylirostris with representative Ecuadorian and Peruvian isolates revealed no mortality or clinical signs in any of the three penaeid species, despite confirmation of infection through quantitative PCR and, in some species, histological detection of Cowdry type A inclusion bodies [10]. This finding suggests that the currently farmed P. vannamei lines in Ecuador have developed tolerance to circulating IHHNV genotypes, indicative of an evolved virus-host equilibrium. Further molecular diagnostic surveys on the northern coast of Peru reported an overall IHHNV prevalence of 88.66% across six epidemiological units, with all positive cases exhibiting only grade 1 histological lesions characterized by mild nuclear alterations and the presence of eosinophilic intranuclear inclusions in target tissues such as cuticular, epicardial, and connective tissue [5]. Notably, no evidence of an inflammatory response was observed, and no mortalities were attributed to the pathogen in the sampled units, consistent with the hypothesis that endogenous viral elements integrated into the shrimp genome may mediate an adaptive, non-pathological response to infection [5].

In the United States, IHHNV screening of P. vannamei from three commercial facilities in 2019 yielded positive detections, with nucleotide sequences of PCR amplicons showing 99%–100% similarity to isolates from Latin America and Asia [12]. Phylogenetic analysis of capsid gene and whole-genome sequences demonstrated clustering with an isolate from Ecuador, once again implicating transboundary movement of the virus via the international shrimp trade [12]. The detection of an WOAH-listed pathogen within US facilities highlights the pressing need for robust biosecurity protocols in hatcheries and grow-out ponds to mitigate economic losses. In Mexico, the epidemiological history of IHHNV is particularly instructive. The virus was responsible for major losses in both wild and farmed penaeid shrimp populations on the northwestern Pacific coast beginning in the early 1990s, with populations eventually recovering to pre-epizootic levels [27]. However, the genetic subdivision among viral populations in Mexican waters is highly significant, and the Bayesian analysis of capsid sequences indicates that this regional isolation has persisted, with the virus exhibiting an evolutionary rate that necessitates regular monitoring of both wild and farmed shrimp [27]. The Mexican shrimp breeding industry has also made strides in genetic management, with heritability estimates for log-transformed IHHNV viral load (VLₙ) in L. vannamei reported at 0.42 at the family mean level, indicating that viral load has a genetic component and can be selected against in breeding programs [32]. The study also found a favorable genetic correlation between lower viral load and increased survival (genetic correlation of -0.57), suggesting that selective breeding for reduced IHHNV viral burden could yield concurrent improvements in grow-out survival [32].

In Colombia, a report on two shrimp farms on the north coast in 2021 revealed an alarming IHHNV prevalence of 97% in one farm and 36% in the other, as determined by WOAH-recommended PCR [6]. Despite the high molecular prevalence, histopathological examination revealed no inclusion bodies, although in situ hybridization confirmed infection, and phylogenetic analysis identified the Colombian isolates as genotype II, similar to strains from Ecuador, Brazil, and the United States [6]. This discrepancy between molecular detection and histopathological change reinforces the assertion that IHHNV infection in P. vannamei often proceeds in a subclinical manner, especially when the host-virus relationship has reached an equilibrium state. In Brazil and the broader South American context, the detection of IHHNV in P. vannamei and P. monodon has been documented through phylogenetic analyses that place Brazilian isolates within the same Type II cluster as other Western Hemisphere strains [12, 18].

Asia: Persistent Infections, High Prevalence, and Emergent Threats

Asia represents the epicenter of global shrimp aquaculture and, consequently, a region of intense IHHNV epidemiological activity. In China, the virus has been detected across a wide array of crustacean species and geographical provinces. A survey of farmed Procambarus clarkii (red swamp crayfish) across six provinces in the middle and lower reaches of the Yangtze River, Anhui, Jiangsu, Zhejiang, Hunan, Hubei, and Sichuan, demonstrated an overall IHHNV-positive rate of 38.5%, with provincial prevalence ranging from 16.7% to 56.7% [9]. The isolated strains exhibited high homology (93.4%–99.4%) and split into two major distinct branches, both closely related to infectious IHHNV genotypes, suggesting a well-established and genetically diverse viral presence in Chinese crayfish aquaculture [9]. This is particularly concerning because P. clarkii is an economically significant species, and its high prevalence of IHHNV could serve as a viral reservoir that perpetuates infection cycles within pond ecosystems.

The susceptibility of Helice tientsinensis, a key benthic crab commonly found in shrimp ponds in northern China, was confirmed through experimental infection studies and long-term surveillance [3]. From 2021 to 2023, surveys revealed the persistent presence of IHHNV in H. tientsinensis populations across various shrimp farming periods along China's coastal regions, with notably high viral loads detected within pond environments [3]. The study confirmed cross-species transmission of IHHNV from infected H. tientsinensis to juvenile P. vannamei, meeting WOAH criteria for designating a susceptible host [3]. This finding underscores the overlooked risk of wild crab species serving as bridging vectors for pathogen transmission within aquaculture systems, with foraging behavior facilitating viral spread and posing a tangible threat to health management strategies. Furthermore, a detailed study on IHHNV tissue tropism in P. vannamei challenged by feeding revealed that the hepatopancreas, a tissue of endodermal origin previously considered refractory to IHHNV infection, exhibited the highest viral load (19.4 copies/mg, 100% positivity), followed by gills (10.6 copies/mg, 86.7% positivity), pleopods (10.5 copies/mg, 86.7% positivity), and muscle (4.7 copies/mg, 33.3% positivity) [8]. This demonstration of hepatopancreatic infection challenges the classical dogma that IHHNV is restricted to ectodermal and mesodermal tissues and has significant implications for understanding viral pathogenesis and transmission dynamics in Chinese shrimp farming operations.

In the Philippines, surveillance of market-sold L. vannamei from Central Luzon, a major shrimp-producing region, revealed a prevalence of approximately 20% (56 out of 276 samples) [15]. The sequences of the isolates acquired from different municipalities displayed a high degree of similarity, suggesting transboundary movement of the virus within the country, and phylogenetic analysis demonstrated a strong link between IHHNV strains in the western hemisphere and those in the Philippines, supporting the hypothesis that the virus was introduced to Asia from the Americas [15]. This finding indicates that despite the implementation of preventive measures such as broodstock testing, IHHNV persists in Philippine shrimp populations, necessitating tightened farm-monitoring processes to prevent further spread [15].

India presents a complex epidemiological picture, characterized by both clinical disease and covert infections. On the southeast coast of India, intensive grow-out ponds of L. vannamei have exhibited severe morphological deformities attributable to IHHNV, including reduced body size, bent rostrum, shrunken and twisted antennae, rough cuticles, and tumor-like hyperplasia at the lateral position of the carapace [24]. Scanning and transmission electron microscopy confirmed the presence of icosahedral viral particles within the nuclei and cytoplasm of gill cells, with the diagnosis confirmed by polymerase chain reaction [24]. Concurrently, a survey of wild crustaceans in the Andaman and Nicobar Archipelago detected co-infection of IHHNV and White Spot Syndrome Virus (WSSV) in 13.5% of shrimp and 4.5% of crab samples screened [13]. The highest rate of co-infection was observed in P. monodon and Scylla serrata, and histopathological analysis of infected gill sections confirmed the presence of both eosinophilic intranuclear Cowdry type A inclusion bodies (characteristic of IHHNV) and basophilic intranuclear inclusion bodies (characteristic of WSSV) [13]. This co-infection dynamic is epidemiologically significant because of the documented viral interference between IHHNV and WSSV, where prior or concurrent IHHNV infection can modulate the severity of WSSV disease, potentially through mechanisms involving the shrimp's immune system or direct viral competition [11].

In Egypt, a study along the Mediterranean coast (Damietta and North Sinai) detected IHHNV in cultured Penaeus semisulcatus with a prevalence of 25%, with histopathological observation of eosinophilic intranuclear inclusion bodies and gross signs including opaque abdominal muscles, white milky to buff mottling on the shell, and runt-deformity syndrome [18]. Phylogenetic analysis revealed genetic similarity and cross-lineage relationships between Egyptian isolates and those from the USA, Brazil, Indonesia, China, Korea, Taiwan, and Ecuador, again illustrating the global interconnectedness of IHHNV strains through trade and movement of live animals [18].

Phylogenetic Diversity and Its Epidemiological Implications

The global phylogenetic structure of IHHNV has been elucidated through extensive sequencing of the capsid gene and, increasingly, whole-genome analyses. Three major genotypes are recognized: Type I, Type II, and Type III, though the epidemiological significance of these types is still under active investigation. Type II appears to be the most globally distributed genotype, encompassing isolates from Latin America (Ecuador, Peru, Brazil, Colombia, the United States), Asia (Philippines, China, South Korea), and Australia [6, 10, 12, 15, 19]. Type II genotypes have been consistently associated with both subclinical infections and the runt-deformity syndrome, particularly in P. vannamei and P. monodon [10, 16, 22]. In contrast, Type III genotypes, which are predominantly detected in Southeast Asia and Australia, have been linked to higher virulence in some studies, though experimental infections with Type II isolates in specific pathogen-free P. vannamei, P. monodon, and P. stylirostris have failed to induce mortality or clinical signs, even in the historically susceptible P. stylirostris [10]. This suggests that genotype alone does not determine virulence; rather, the host species, the presence of endogenous viral elements, and the prior immunological history of the host population are critical determinants of disease outcome.

The presence of endogenous viral elements (EVEs), non-infectious viral sequences integrated into the shrimp genome, has been documented in P. monodon and, to a lesser extent, in P. vannamei. These EVEs are thought to confer a degree of immunity or tolerance to exogenous IHHNV infection through mechanisms such as RNA interference or competitive inhibition of viral replication [5]. This has profound epidemiological implications: populations with a high prevalence of EVEs may be less susceptible to clinical disease,

Host Range and Susceptibility: Decapod Crustaceans

The host range of Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV), taxonomically classified as Decapod penstylhamaparvovirus 1 within the family Parvoviridae, represents a critical dimension of its epizootiology and economic impact. Initially recognized for its catastrophic effects on penaeid shrimp aquaculture, the virus has demonstrated a considerably broader ecological and taxonomic footprint than originally appreciated. This expansive host range, encompassing numerous species within the infraorder Caridea (true shrimp), the suborder Dendrobranchiata (prawns), and extending into the infraorder Brachyura (true crabs) and the superfamily Astacoidea (crayfish), underscores the complexity of its transmission dynamics and the challenges inherent in its control. Understanding the nuances of susceptibility, from permissive infection with high viral loads and clinical pathology to resistant or tolerant states, is paramount for developing biosecurity protocols and managing this notifiable pathogen, as recognized by the World Organisation for Animal Health (WOAH) [1, 11, 28].

Penaeidae: The Primary and Most Extensively Studied Hosts

The family Penaeidae constitutes the primary host reservoir and the principal source of economic loss attributed to IHHNV. The virus's initial documentation and subsequent characterization have been inextricably linked to the intensive cultivation of these species, particularly the Pacific white shrimp (Penaeus vannamei) and the blue shrimp (Penaeus stylirostris). The susceptibility of these species, however, is not uniform and is modulated by genotype, viral load, and life stage, leading to a spectrum of outcomes ranging from acute mortality to subclinical persistence.

1. Penaeus (Litopenaeus) vannamei (Pacific White Shrimp): As the most widely cultured shrimp species globally, P. vannamei serves as the archetypal host for IHHNV. Susceptibility in this species is characterized by a marked transition over time. Early outbreaks in the Americas were associated with severe epizootics in P. stylirostris, while P. vannamei was initially considered somewhat more resistant, presenting primarily with chronic, non-lethal deformities collectively known as Runt-Deformity Syndrome (RDS) [11, 24]. This syndrome, which includes severe morphological abnormalities such as bent rostrums, shrunken and twisted antennae, rough cuticles, and pronounced growth retardation, has been extensively documented in P. vannamei across its global distribution, from the intensive grow-out ponds of Southeast India to the farms of Ecuador and Peru [11, 16, 21, 24].

However, contemporary epidemiological data reveal a profound shift in this host-virus relationship. A large-scale surveillance study spanning major shrimp-producing regions in Ecuador and Peru reported that, despite an endemic IHHNV prevalence of 3.3–100% across farms, the currently cultured lines of P. vannamei exhibited a remarkable tolerance to the circulating viral genotypes [10]. Experimentally challenged specific-pathogen-free (SPF) P. vannamei with representative Ecuadorian and Peruvian isolates displayed no mortality, no clinical signs, and only low-grade histopathological lesions (Grade 1) characterized by mild nuclear alterations and Cowdry type A eosinophilic intranuclear inclusions in target tissues such as cuticular epithelium, connective tissue, and the epicardium [5, 10]. Strikingly, no associated inflammatory response was observed, aligning with the hypothesis that endogenous viral elements (EVEs) integrated into the shrimp genome may confer a form of adaptive tolerance [5]. This co-existence in a state of equilibrium suggests a co-evolutionary dynamic where the virus is no longer a catastrophic pathogen but a persistent, and potentially manageable, infection in commercial stocks. Indeed, survival rates in high-biosecurity facilities in the United States, where surveillance for WOAH-listed pathogens is stringent, have also been compromised by IHHNV detection, highlighting the constant threat of re-emergence [12]. The genetic architecture of this tolerance has been substantiated by heritability estimates for viral load (VLₗₙ) in P. vannamei, which demonstrated moderate heritability (0.42 ± 0.45 at family mean level), indicating a significant genetic component that can be leveraged in selective breeding programs [32].

2. Penaeus (Litopenaeus) stylirostris (Blue Shrimp): In stark contrast to P. vannamei, P. stylirostris is considered the most susceptible penaeid species to IHHNV. Historically, IHHNV caused catastrophic, large-scale mortality in cultured P. stylirostris throughout the Americas, a hallmark that defined the virus's pathogenic reputation [10, 11]. While more recent experimental bioassays using contemporary IHHNV isolates from Ecuador and Peru have shown a failure to induce mortality in P. stylirostris, these findings must be interpreted with caution. The experimental isolates were of the Type II genotype, which may represent a less virulent lineage than historical strains [10]. Critically, in these bioassays, P. stylirostris had the lowest viral load among the three species tested (P. vannamei, P. monodon, and P. stylirostris), and no Cowdry type A inclusion bodies were observed, suggesting a potential for resistance at the histopathological level to current circulating genotypes [10]. This genotype-specific susceptibility is a vital concept; it implies that pathogenicity is not an inherent property of the host but a complex interplay between host genetics, viral genotype, and environmental conditions.

3. Penaeus monodon (Giant Tiger Prawn or Black Tiger Shrimp): P. monodon occupies a central role in the epizootiology of IHHNV, acting as both a highly susceptible host and a critical vector for global viral dissemination. The virus is known to cause RDS in this species, with affected animals exhibiting severe shell deformities, reduced growth, and substantial economic losses in aquaculture operations [22, 24]. The impact of IHHNV on P. monodon production has been quantified in commercial pond settings in Australia. Research demonstrated that postlarvae (PL) derived from broodstock with high IHHNV loads resulted in progeny with substantially higher and more rapidly increasing viral loads during grow-out [22]. By 155 days of culture, this high-infection burden led to a dramatic reduction in harvest yield (equivalent to 3.72 t/ha lower) and markedly reduced survival (79.5–84.5%) compared to low-infection ponds (95.9–99.8%), confirming a direct causal link between sustained high IHHNV loads and poor growth performance [22].

Beyond its commercial impact, P. monodon serves as a significant reservoir for the virus in both wild and farmed populations. The virus has been detected with high prevalence (up to 88.66% in some surveillance) in wild-caught populations, including the first reports from the Gulf of Mexico and Northwestern Atlantic Ocean, underscoring the role of wild stocks in viral maintenance and potential spread to naïve environments via invasive species or larval transport [7, 10]. Furthermore, P. monodon is frequently implicated in co-infections with other major pathogens, most notably White Spot Syndrome Virus (WSSV). A study in the Andaman and Nicobar Archipelago, India, reported that P. monodon had the highest rate of co-infection with IHHNV and WSSV among all shrimp and crab species screened, highlighting the complex disease ecology in natural and semi-natural systems [13].

4. Other Penaeid Hosts: The susceptibility of other penaeids is less comprehensively documented but indicates a broader range. Penaeus semisulcatus (green tiger prawn) has been confirmed as a host via PCR and histopathological identification of Cowdry type A inclusions in cultured specimens from Egypt, with a prevalence of 25% [18]. Fenneropenaeus merguiensis (banana prawn) has also been demonstrated susceptible, with IHHNV virus-like particle (VLP) administration showing a predilection for gill tissue and eliciting an immune response via upregulation of peroxiredoxin, confirming the host's ability to recognize and respond to the virus [30]. The giant tiger prawn (Penaeus monodon) from the Philippines also shows high prevalence, with ~20% of market-sold specimens testing positive, indicating ongoing circulation in aquaculture systems despite preventive measures [15].

Beyond Penaeidae: Expanding the Host Range to Crabs and Crayfish

A paradigm shift in understanding IHHNV epizootiology has been the formal recognition of susceptibility in non-penaeid decapods. This expansion carries profound implications for disease management, suggesting that IHHNV is not confined to the shrimp family but can exploit a diverse array of benthic and freshwater crustaceans as potential reservoirs, vectors, and intermediate hosts, thereby complicating biosecurity within integrated aquaculture systems.

1. Brachyura (True Crabs): The most compelling evidence for crab susceptibility comes from a study on the benthic crab Helice tientsinensis, a ubiquitous inhabitant of shrimp ponds in northern China [3]. This research conclusively met the WOAH criteria for confirming susceptibility (as outlined in Chapter 1.5 of the Aquatic Animal Health Code) through a combination of experimental infection, histopathology, in situ hybridization (ISH), and quantitative PCR (qPCR). Orally challenged H. tientsinensis developed IHHNV infections characterized by: (i) no mortality, indicating a tolerant carrier state; (ii) development of small black lesions on the carapace; (iii) presence of Cowdry type A eosinophilic intranuclear inclusion bodies in hemal sinuses and hepatopancreatic tubules; (iv) strong IHHNV-specific ISH signals in the hepatopancreas; and (v) detection of the highest viral loads in the hepatopancreas, gills, and stomach [3]. Crucially, this study confirmed cross-species transmission from infected H. tientsinensis to juvenile P. vannamei, providing definitive evidence that the crab serves as a competent host and vector [3]. Field surveys further revealed a persistent, high-level IHHNV presence in H. tientsinensis populations across Chinese coastal regions from 2021–2023, with the crabs' foraging behavior likely facilitating viral spread within the pond ecosystem [3]. This discovery challenges the traditional dogma that IHHNV targets only ectodermal and mesodermal tissues, as the hepatopancreas (an endodermal derivative) was a major site of replication [3, 8].

Further evidence for crab susceptibility is provided by the detection of co-infections with WSSV and IHHNV in the mud crab Scylla serrata from the Andaman and Nicobar Islands, India [13]. While the prevalence was lower than in P. monodon from the same study, it nonetheless confirms that natural IHHNV infection can occur in commercially important portunid crabs. The detection of IHHNV in wild crustaceans, including crabs, highlights the critical need for continuous monitoring of these reservoirs, which are often overlooked in farm-level biosecurity assessments [13].

2. Astacoidea (Freshwater Crayfish): Freshwater crayfish, previously considered relatively resistant to penaeid viruses, have emerged as significant hosts for IHHNV. Two species have been extensively studied:

• Procambarus clarkii (Red Swamp Crayfish): A large-scale survey of farmed P. clarkii in the middle and lower reaches of the Yangtze River in China revealed a surprisingly high overall IHHNV prevalence of 38.5%, with provincial rates ranging from 16.7% to 56.7% [9]. The IHHNV strains isolated from these crayfish exhibited high sequence homology (93.4–99.4%) to known infectious isolates and clustered into two distinct branches, indicating that the virus is not only present but actively circulating and evolving within crayfish populations [9]. This finding positions P. clarkii as a major reservoir host, capable of sustaining the virus in freshwater environments that are often adjacent to or integrated with shrimp ponds.
• Cherax quadricarinatus (Red Claw Crayfish): The vulnerability of this economically important ornamental and food species was demonstrated in a study on commodity crayfish imported into South Korea from Indonesia [19]. IHHNV was detected in 9 out of 10 batches, with a high prevalence in muscle tissue (66.7% of samples), followed by hepatopancreas (41.7%) and gills (25.0%). Phylogenetic analysis of the amplicons clustered the strains with infectious Type II sequences common in Southeast Asia, distinct from Type III strains previously found in P. vannamei in South Korea [19]. This highlights the role of international trade in moving infected crayfish and potentially introducing novel viral genotypes into new geographic regions.

Other Decapod Crustaceans

The known host range extends to other infraorders, though the clinical and epidemiological significance remains less well defined.

• Macrobrachium rosenbergii (Giant Freshwater Prawn): This ecologically and economically important palaemonid prawn has been identified as a host for IHHNV. A study in Malaysia confirmed infection in broodstock using OIE-recommended primers and further characterized the proteomic response in hemocytes during infection [26]. The identification of IHHNV in M. rosenbergii indicates that the virus can infect caridean shrimp, not just dendrobranchiates, and that hemocytes, the primary immune effector cells, are a key site of viral interaction and pathogenesis [26].
• Penaeus monodon and Farfantepenaeus spp.: The presence of IHHNV in wild populations of Penaeus species, including P. monodon and Farfantepenaeus spp. from the Gulf of California, has been linked to rapid viral expansion and significant genetic diversity [27]. The high substitution rate estimated for IHHNV (1.39×10⁻⁴ substitutions/site/year) suggests it is evolving more rapidly than initially believed, which may facilitate its adaptation to new hosts, including different penaeid species and potentially non-penaeid decapods [27]. This evolutionary dynamism underscores the need for continuous surveillance and genetic characterization to track the emergence of novel host-specific strains.

Refining the Concepts of Susceptibility and Tissue Tropism

The cumulative evidence from the past decade has necessitated a fundamental revision of the established dogma regarding IHHNV tissue tropism. It was classically held that IHHNV, like other parvoviruses, was restricted to tissues of ectodermal and mesodermal origin (e.g., cuticular epithelium, gills, connective tissue, hematopoietic organs) and did not infect endodermal tissues such as the hepatopancreas [8]. However, multiple lines of evidence now conclusively demonstrate that the hepatopancreas is a major target organ for IHHNV. Experimental feeding challenges in P. vannamei revealed that the hepatopancreas exhibited the strongest IHHNV positivity (100%, 19.4 copies/mg), surpassing gills, pleopods, and muscle [8]. This was confirmed histologically and corroborated by the finding of high viral loads in the hepatopancreas of H. tientsinensis [3] and its detection in the hepatopancreas of imported C. quadricarinatus [19]. The hepatopancreas, therefore, is not merely a site of viral clearance but is a primary site of viral replication and a likely major source for viral shedding into the environment via fecal material.

Furthermore, the concept of susceptibility must be decoupled from the presence or absence of clinical disease or mortality. High IHHNV prevalence (36–97%) has been reported in P. vannamei farms in Colombia where shrimp displayed no clinical signs and had no histopathological lesions, yet infection was confirmed by PCR and ISH [6]. Similarly, in the Peruvian surveillance study, 88.66% of shrimp were positive but with only grade 1 histopathological changes and no mortality [5]. These cases represent a state of asymptomatic or subclinical infection, where the virus replicates at a level detectable by molecular methods but does not trigger overt pathology. This carrier state is epidemiologically critical, as it allows for the unrecognized movement of infected stocks and the silent transmission of the virus within and between populations, undermining visual inspection-based biosecurity measures. The WOAH notification status of IHHNV underscores the global importance of detecting even these subclinical infections [1, 5, 28].

In summary, the host range of IHHNV is both broad and dynamic, encompassing a diverse array of commercially and ecologically vital decapod crustaceans. The susceptibility of each host species is not a fixed trait but is modulated by viral genotype, host genetic background (including the presence of EVEs), and environmental pressures. The discovery of high viral loads in hepatopancreatic tissue across multiple host taxa, the identification of true crab carriers that can transmit the virus to shrimp, and the high prevalence in freshwater crayfish populations collectively paint a picture of a highly adaptable virus with a complex, multi-host ecology. This demands a sophisticated, molecular-based surveillance approach that goes beyond clinical observation to include regular testing of not only the target crop species but also of cohabiting wild crustaceans and water sources.

Clinical Signs and Pathological Manifestations

Infection with the infectious hypodermal and hematopoietic necrosis virus (IHHNV), a non-enveloped, linear single-stranded DNA virus classified within the family Parvoviridae and designated as Decapod penstylhamaparvovirus 1 by the World Organisation for Animal Health (WOAH), presents a remarkably complex and variable clinical picture that is profoundly influenced by host species, viral genotype, developmental stage, and the presence of endogenous viral elements (EVEs) [1, 5, 11]. The clinical and pathological manifestations range from an acute, lethal epizootic in susceptible populations to a completely subclinical, chronic carrier state in tolerant lines, with a spectrum of developmental deformities and growth impairments in intermediate hosts. This section provides an exhaustive analysis of these manifestations, drawing upon histopathological, ultrastructural, and molecular evidence.

The Clinical Spectrum of IHHNV Infection: From Acute Lethality to Subclinical Carrier State

The most dramatic and historically significant clinical presentation of IHHNV is the acute epizootic form seen specifically in juvenile Penaeus stylirostris (blue shrimp). In this highly susceptible host, infection leads to mass mortality, often reaching 80-100% within a few weeks of an outbreak, with clinical signs being largely non-specific but severe, including lethargy, anorexia, and a characteristic whitish, opaque discoloration of the abdominal musculature [1, 11]. This acute phase is often accompanied by erratic swimming behavior and a tendency to settle at the bottom of ponds before death. In contrast, the clinical picture in Penaeus vannamei (Pacific white shrimp) and Penaeus monodon (giant tiger prawn) is fundamentally different. Here, the disease manifests as a chronic, non-lethal condition primarily characterized by the Runt-Deformity Syndrome (RDS) [1, 16, 18, 22]. Grossly, affected shrimp display a severe reduction in body size, becoming pronounced "runts" relative to their uninfected cohorts. This growth retardation is accompanied by a suite of characteristic morphological deformities: a bent or twisted rostrum, shrunken and twisted antennae, rough and wrinkled cuticles, soft, puffy, and dented antennal segments, and shrunken, deformed eyes [18, 24]. The clinical presentation of opaque, milky-white abdominal muscles can also be observed, often leading to confusion with other pathogens like infectious myonecrosis virus (IMNV) [18].

Crucially, a large and growing body of evidence from contemporary farming operations, particularly in Latin America and the United States, documents a predominantly subclinical infection in P. vannamei [5, 6, 10, 12]. In these scenarios, shrimp test positive for IHHNV via sensitive molecular assays (e.g., qPCR) but exhibit no overt clinical signs, no histopathological lesions, and no increase in mortality. For instance, surveillance programs along the Peruvian northern coast and farms in Colombia have reported IHHNV prevalences exceeding 88% with a complete absence of clinical disease or inflammatory response [5, 6]. This phenomenon is now largely attributed to the widespread presence of endogenous viral elements (EVEs) integrated into the shrimp genome, which confer a form of adaptive tolerance or resistance to exogenous IHHNV infection, as well as to the selection of genetically tolerant shrimp lines over decades of co-existence with the virus [5, 10]. Experimental bioassays using specific pathogen-free (SPF) P. vannamei challenged with contemporary Ecuadorian and Peruvian IHHNV isolates have confirmed this lack of clinical effect, reinforcing the notion of an evolving virus-host equilibrium [10]. Therefore, the clinical sign of "disease" is no longer a reliable indicator of IHHNV infection in many commercially farmed populations.

Cellular and Histopathological Hallmarks: Cowdry Type A Inclusion Bodies and Tissue Tropism

The definitive histopathological hallmark of IHHNV infection is the presence of Cowdry type A (CA) eosinophilic intranuclear inclusion bodies within target cells [3, 5, 13, 18]. These structures are large, homogenous, acidophilic bodies that occupy the center of the nucleus, displacing the host cell chromatin to the nuclear periphery. Their identification in hematoxylin and eosin (H&E) stained tissue sections is considered pathognomonic for IHHNV in susceptible hosts and serves as a critical diagnostic tool, particularly in the absence of clinical signs. Classically, these inclusions are found in tissues of ectodermal and mesodermal origin, including the cuticular epithelium (particularly the subcuticular epidermis), connective tissue, hematopoietic tissue (the primary site of viral replication), gonadal tissue, and nerve cord [5]. However, a major revision to the understanding of IHHNV tissue tropism has emerged from recent experimental challenge studies. Contrary to the long-held belief that IHHNV spares tissues of endodermal origin, it is now conclusively demonstrated that the hepatopancreas is a major target organ. Challenge experiments using the feeding route have shown that the hepatopancreas exhibits the highest viral load (up to 19.4 copies/mg) and 100% positivity rate in P. vannamei, with CA inclusion bodies and strong in situ hybridization (ISH) signals confirming productive infection [3, 8]. This has been corroborated in other host species like the crab Helice tientsinensis, where CA inclusions were identified in hepatopancreatic tubules and hemal sinuses [3].

The severity of these histopathological changes is often graded on a semi-quantitative scale. In many contemporary, tolerant P. vannamei populations, the severity of lesions is uniformly low, typically classified as Grade 1, indicating only mild nuclear alterations, with the presence of eosinophilic intranuclear inclusions or hypertrophied nuclei limited to a few cells [5]. There is a notable absence of an inflammatory response (e.g., hemocytic infiltration, encapsulation, or melanization) surrounding these inclusion bodies, a finding that is consistent across both clinical and subclinical infections [5, 6]. This lack of inflammation is a key feature distinguishing IHHNV from many other shrimp viral pathogens and suggests a non-lytic or stealth-like mechanism of pathogenesis in tolerant hosts. It is critical to note that in some cases of infection, particularly in the presence of EVEs or in early-stage infections, histopathological lesions may be completely absent. The detection of IHHNV by ISH or PCR in shrimp with no detectable histopathology is a well-documented phenomenon, underscoring the necessity of integrating molecular diagnostics with histology for accurate diagnosis [6, 10]. For instance, in a Colombian outbreak, 97% of shrimp were PCR-positive, but H&E staining failed to reveal any inclusion bodies, while ISH confirmed the infection [6]. Similarly, experimental challenges in P. stylirostris with certain genotypes did not yield CA inclusions, despite successful infection [10].

Macroscopic Morphological Deformities: The Runt-Deformity Syndrome and Beyond

Beyond the classic RDS, severe and characteristic morphological alterations have been meticulously documented. Using scanning electron microscopy (SEM), affected P. vannamei have been shown to exhibit fused antennal segments that are also puffy, dented, bent, and rough. The rostrum is bent, and the gastrofrontal sulcus is distorted [24]. One of the most striking findings is the development of tumor-like growths or hyperplasia at the lateral position of the carapace. These growths are soft to the touch and represent a significant anatomical deformation [24]. Additionally, the muscle fibers are despoiled and rigid, and the connective tissue is in a degraded state [24]. These deformities are not merely cosmetic; they have direct and profound economic consequences. In P. monodon, high IHHNV infection loads have been causally linked to a significant reduction in growth performance after 120 days of culture (DOC), leading to an estimated harvest yield reduction of 3.72 t/ha compared to ponds stocked with low-load postlarvae. This growth penalty translates to a substantial economic loss of approximately $67,000 per hectare in farm gate value [22]. The deformities likely impair feeding efficiency, locomotion, and camouflage, contributing to the observed growth retardation and reduced survival rates [22, 24].

Histopathological Features of Co-Infections and In Vitro Cytopathic Effects

The pathological landscape becomes more complex in the context of co-infections, particularly with White Spot Syndrome Virus (WSSV). In such cases, histological examination of gill tissues can reveal a dual pathology: the eosinophilic intranuclear CA inclusion bodies characteristic of IHHNV co-existing with the basophilic intranuclear inclusion bodies pathognomonic for WSSV infection [13]. This co-occurrence is not merely additive; the phenomenon of viral interference has been observed, where IHHNV infection can reduce the severity of WSSV disease progression in certain contexts, although the precise mechanisms remain under investigation [11].

At the cellular level, in vitro studies using primary hemocyte cultures have provided detailed insights into the cytopathic effect (CPE) of IHHNV. Direct infection of cultured hemocytes leads to a characteristic sequence of morphological changes, including detachment of cells from the substrate, slendering (elongation and thinning), vacuolation of the cytoplasm, and eventually rounding and cell death [33]. Ultrastructural analysis of these infected cells via transmission electron microscopy (TEM) reveals the viral particles themselves. IHHNV virions are icosahedral, non-enveloped, and approximately 22-23 nm in diameter. They accumulate within hypertrophied nuclei, often forming paracrystalline arrays, and are also found scattered in the cytoplasm [24, 33]. The nucleus may show scattered chromatin and virogenic stroma, which are electron-dense, fibrillar areas where viral replication and assembly occur [33]. In the gills of infected P. vannamei, TEM has shown that the nucleus of infected cells is almost completely occupied by these viral particles [24]. This intracellular crowding is the physical basis for the nuclear hypertrophy and the displacement of chromatin observed in H&E-stained sections, ultimately leading to the formation of the Cowdry type A inclusion body.

Diagnostic Methods for IHHNV Detection

The accurate and timely detection of Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV), a member of the family Parvoviridae and genus Penstyldensovirus, is a cornerstone of crustacean health management and international trade. As a notifiable pathogen to the World Organisation for Animal Health (WOAH), the deployment of validated diagnostic protocols is not merely a matter of research interest but a regulatory necessity for the global shrimp aquaculture industry [1, 11]. The diagnostic landscape for IHHNV has evolved considerably, progressing from classical histopathological observation to a sophisticated array of molecular and immunological techniques. Each method offers a distinct balance of sensitivity, specificity, throughput, and logistical feasibility, and their appropriate application is dictated by the specific diagnostic objective, whether it be surveillance, outbreak confirmation, broodstock screening, or point-of-care testing. A deep biological understanding of the virus, including its tissue tropism, genetic variability, and the existence of endogenous viral elements (EVE) integrated into host genomes, is critical for the correct interpretation of diagnostic results.

Histopathological and Cytological Examination: The Classical Gold Standard

Histopathology remains a foundational tool for the presumptive diagnosis of IHHNV, providing crucial morphological evidence of viral pathogenesis. The hallmark lesion of IHHNV infection is the formation of Cowdry type A inclusion bodies (CAIs). These are large, eosinophilic, intranuclear inclusions that often marginate the host chromatin, appearing as discrete, round bodies within hypertrophied nuclei [5, 13, 18]. These inclusions are predominantly observed in tissues of ectodermal and mesodermal origin, including the cuticular epithelium, gills, antennal gland, hematopoietic tissue, and connective tissue [1, 5]. The presence of CAIs in tissue sections stained with hematoxylin and eosin (H&E) is a strong indicator of active viral replication and a criterion used in WOAH diagnostic manuals.

However, the reliance on histopathology for definitive diagnosis is fraught with limitations that have been underscored by recent research. Crucially, the absence of CAIs does not exclude IHHNV infection. In a study of Penaeus vannamei from farms in Colombia, samples that were highly positive by PCR (97% prevalence in one farm) exhibited no histopathological lesions or inclusion bodies whatsoever, despite in situ hybridization (ISH) confirming the presence of viral nucleic acid [6]. Similarly, while experimentally challenged P. vannamei and P. monodon showed CAIs, P. stylirostris challenged with the same isolates did not develop these inclusions [10]. This variance in histopathological response may be linked to host genetic tolerance, strain virulence, or the phase of infection. In chronic or subclinical infections, where the viral load may be lower or the host is mounting an effective adaptive response, the classical cytopathic effects may be absent. The detection of CAIs is also a subjective, operator-dependent skill, and grading schemes (e.g., the "grade 1" severity reported in Peruvian farms) are only semi-quantitative [5]. Furthermore, the process is laborious, time-consuming (requiring fixation, embedding, sectioning, and staining), and requires significant expertise, making it unsuitable for rapid, high-throughput surveillance.

In situ hybridization (ISH) serves as a powerful adjunct to conventional histopathology, bridging the gap between morphological observation and molecular confirmation. ISH uses labeled DNA or RNA probes complementary to IHHNV-specific sequences to directly visualize the virus within tissue architecture. This technique has been instrumental in confirming infection in cases where inclusion bodies are absent [6] and in refining our understanding of tissue tropism. For instance, while early dogma suggested IHHNV did not infect endodermal tissues like the hepatopancreas, ISH, in conjunction with quantitative PCR (qPCR), has definitively localized IHHNV signals to hepatopancreatic tubule cells, challenging previous assumptions and demonstrating a broader tropism than originally appreciated [3, 8]. In Helice tientsinensis crabs, ISH confirmed strong IHHNV-specific signals in the hepatopancreas, corroborating qPCR data [3]. This technique, however, is more technically complex and costly than standard H&E staining and is typically reserved for confirmatory or research applications rather than primary screening.

Molecular Detection: The Reigning Standard of Sensitivity and Specificity

Molecular methods, particularly polymerase chain reaction (PCR) and its variants, have become the undisputed workhorses for IHHNV detection due to their unparalleled sensitivity, specificity, and capacity for high-throughput analysis. The WOAH-recommended assay for IHHNV is a conventional PCR targeting a 389-bp fragment of the virus, a method that has been widely employed for decades in surveillance and certification programs [9, 20]. This assay, however, has been progressively superseded by real-time quantitative PCR (qPCR), which offers several critical advantages. qPCR provides real-time quantification of viral DNA, enabling the determination of viral load (copies per mg of tissue or per reaction) rather than a mere positive/negative result. This quantification is essential for understanding infection dynamics, as exemplified by the finding that P. vannamei carries a significantly higher viral load than co-habiting P. monodon or P. stylirostris [10]. The ability to quantify viral burden also has profound epidemiological and economic implications, as higher IHHNV loads in broodstock have been directly correlated with reduced growth performance and survival in progeny, leading to substantial production losses in P. monodon [22]. Heritability studies in L. vannamei have even suggested that viral load itself could be a target for selective breeding programs to enhance tolerance [32].

The selection of target genes for molecular assays is a critical biological consideration. Most PCR assays target non-structural or structural protein genes, such as the capsid protein (CP) or non-structural protein 1 (NS1) regions [4, 29]. A major pitfall, however, is the presence of non-infectious endogenous viral elements (EVE) within the shrimp genome. These are fragments of IHHNV DNA that have been integrated into the host germline over evolutionary time and can be amplified by PCR, yielding a false-positive result for active infection. The WOAH-recommended PCR primers, for instance, can amplify these EVEs. To circumvent this, diagnostic assays must be specifically designed to differentiate between the infectious virus and its genomic relicts. The real-time ERA assay developed by Zhang et al. targets the ORF1 region, and careful primer design informed by genomic alignment is crucial to avoid this issue [29]. Moreover, the use of ISH or histopathology in parallel with PCR is frequently recommended to confirm that a positive PCR result corresponds to active viral replication rather than genomic integration [6].

Isothermal Amplification and Novel Point-of-Care Technologies

The logistical and infrastructural demands of conventional PCR (thermal cyclers, skilled personnel, centralized laboratories) have spurred the development of isothermal amplification platforms that are better suited for rapid, on-site field diagnosis. Loop-mediated isothermal amplification (LAMP) is a prominent example. The real-time triplex LAMP assay developed by Arunrut et al. achieves a remarkable 45-minute turnaround time with a detection limit of 10 copies per reaction, exhibiting 100% sensitivity and specificity against the qPCR gold standard [28]. This method uses a simple turbidimeter to monitor DNA amplification, eliminating the need for expensive thermocyclers and making it highly accessible for farm-level testing.

Similarly, enzymatic recombinase amplification (ERA) has emerged as a formidable alternative. The real-time ERA assay for IHHNV, targeting the ORF1 region, can amplify target DNA at a constant temperature of 42°C within 20 minutes, with a sensitivity of 1.4×10¹ copies/μL [29]. This assay outperformed the industry-standard qPCR in a field sample test (detection rate 18/20 vs. 17/20) and demonstrated no cross-reactivity with other major shrimp pathogens like WSSV and EHP [29]. The speed and simplicity of these isothermal methods represent a paradigm shift in disease surveillance, empowering farmers and field veterinarians to make real-time management decisions without the bottleneck of sample transport.

Immunological and Nanotechnology-Based Detection: From Nanobodies to Colloidal Gold

While molecular methods dominate, immunological tools are gaining traction, particularly for their potential for rapid, low-cost screening. The development of specific antibodies, and more recently nanobodies (single-domain antibodies derived from sharks or camelids), has opened new avenues for antigen detection. Yang et al. successfully generated high-affinity nanobodies (vNARs) against the IHHNV capsid protein (CP) and non-structural proteins (NS1, NS2) using an immune phage-display library from white-spotted bamboo sharks [4]. These nanobodies were then utilized to create colloidal gold immunochromatographic test strips. Such lateral flow devices offer a "dipstick" format with a short detection time, low cost, and simplicity, ideal for on-site, preliminary screening [4]. While their sensitivity may not match qPCR, they provide a valuable first-line tool for rapid triage in farms or quarantine stations.

A further logistical innovation that complements these detection methods is the use of Flinders Technology Associates (FTA) cards for sample stabilization and transport. Lian et al. demonstrated that IHHNV DNA stored on FTA cards remained stable for at least 60 days at temperatures ranging from -20°C to 37°C, with no significant degradation [34]. Nucleic acid extracted directly from shrimp tissue applied to FTA cards yielded results comparable in magnitude to conventional reagent-based extraction kits when analyzed by real-time PCR [34]. This technology circumvents the need for cold-chain logistics, drastically simplifying the process of sample collection in remote areas and transportation to diagnostic laboratories, thereby enhancing the reach and effectiveness of any molecular surveillance program.

Disease Management and Control Strategies

The management and control of Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) represents a formidable challenge in global crustacean aquaculture, demanding a multi-faceted, evidence-based approach that integrates rigorous biosecurity, advanced diagnostics, selective breeding, and emerging therapeutic technologies. As a notifiable crustacean pathogen listed by the World Organisation for Animal Health (WOAH), IHHNV imposes significant economic burdens, particularly through the induction of runt-deformity syndrome (RDS) in Penaeus vannamei and Penaeus monodon, and historical mass mortalities in Penaeus stylirostris [1, 11, 18]. The strategic framework for disease management must therefore be predicated on a deep understanding of viral epidemiology, host-pathogen interactions, and the ecological dynamics of transmission within and beyond aquaculture systems.

### Biosecurity and Quarantine Protocols: The First Line of Defense

The cornerstone of IHHNV prevention lies in the implementation of stringent biosecurity measures, particularly at the hatchery and broodstock level. The vertical transmission of IHHNV from infected broodstock to progeny is a well-documented and critical pathway for introducing the virus into production cycles. Empirical evidence from commercial P. monodon ponds in Australia demonstrated that postlarvae (PL) derived from broodstock with high IHHNV loads exhibited substantially higher viral burdens during grow-out, culminating in a 3.72 t/ha reduction in harvest yield and markedly lower survival (79.5–84.5%) compared to ponds stocked with low-load PL (95.9–99.8%) [22]. This stark production differential underscores the immense economic value of screening-based selection of specific pathogen-free (SPF) or low-load broodstock. The use of quantitative PCR (qPCR) for pre-spawning screening is not merely a recommendation but a mandatory economic imperative, with extrapolations indicating an increase in farm gate value of approximately $67,000 per hectare when low-load PL are used [22].

Beyond broodstock management, the movement of live crustaceans across international borders represents a major risk for the transboundary spread of IHHNV. Surveillance studies have repeatedly detected IHHNV in imported commodity crustaceans, including red claw crayfish (Cherax quadricarinatus) imported into South Korea from Indonesia, where 9 out of 10 batches tested positive [19]. Similarly, whiteleg shrimp imported from Vietnam to South Korea harbored IHHNV in 23 of 58 samples [20]. These findings highlight a critical gap in quarantine enforcement and the need for harmonized, WOAH-recommended diagnostic protocols at all points of entry. The detection of IHHNV in wild-caught Penaeus monodon from the Gulf of Mexico and Northwestern Atlantic Ocean further illustrates the virus's capacity for long-distance translocation, likely via ballast water or the international trade of live bait, necessitating enhanced surveillance of wild populations as sentinels for viral incursion [7].

### Advanced Diagnostic Surveillance: From Lab to Farm

Effective disease management is inseparable from the capacity for rapid, sensitive, and field-deployable diagnostics. While conventional PCR and qPCR remain the gold standard for confirmatory diagnosis, their reliance on expensive thermal cyclers and specialized personnel limits their utility for on-site, real-time decision-making in shrimp farming [28, 29]. To address this, significant advances have been made in isothermal amplification technologies. A real-time triplex loop-mediated isothermal amplification (LAMP) assay, combined with a turbidimeter, has demonstrated 100% sensitivity and specificity against standard qPCR, achieving a detection limit of 10 copies per reaction within 45 minutes [28]. This platform offers a low-cost, high-technology-readiness-level solution for farm-level screening.

Further expanding the diagnostic toolkit, a real-time enzymatic recombinase amplification (ERA) assay has been developed, targeting the ORF1 region of IHHNV. This method operates at a constant 42°C, eliminating the need for thermal cycling, and can detect as few as 1.4×10¹ copies/μL within 15 minutes [29]. The ERA assay exhibited a superior detection rate (18/20) compared to the industrial standard qPCR (17/20) in practical sample testing, and showed no cross-reactivity with other major shrimp pathogens such as White Spot Syndrome Virus (WSSV) or Vibrio parahaemolyticus causing acute hepatopancreatic necrosis disease (AHPND) [29]. For ultra-rapid, point-of-care testing, nanobody-based colloidal gold immunochromatographic test strips have been engineered. By screening variable new antigen receptors (vNARs) from bamboo sharks against IHHNV capsid and non-structural proteins, researchers have produced test strips that offer a short detection time, convenience, and low cost, making them ideal for on-site deployment in remote shrimp farms [4].

A practical innovation for sample logistics is the use of Flinders Technology Associates (FTA) cards. These cards allow for the collection, storage, and transportation of IHHNV DNA at ambient temperatures without degradation, as nucleic acids remained stable for at least 60 days under temperatures ranging from -20°C to 37°C [34]. This method circumvents the need for cold chain logistics and simplifies cross-border transportation of diagnostic samples, facilitating more widespread and frequent surveillance [34]. The integration of these diverse diagnostic modalities, from high-throughput qPCR in centralized labs to rapid LAMP and lateral flow strips at the pond-side, is essential for a comprehensive surveillance architecture that can detect IHHNV before clinical signs emerge, particularly given that infections can be subclinical or present with low-grade histopathological lesions [5, 6].

### Genetic Selection for Tolerance and Resistance

The observation that IHHNV has become endemic in many shrimp farming regions without causing catastrophic mortality, particularly in P. vannamei, suggests a co-evolutionary shift towards tolerance. In Ecuador and Peru, for example, currently circulating IHHNV genotypes (Type II) are infectious but do not induce clinical lesions or mortality in commercially farmed P. vannamei, P. monodon, or P. stylirostris [10]. This tolerance is hypothesized to be linked to the presence of endogenous viral elements (EVE) integrated into the shrimp genome, which may confer an adaptive, RNAi-mediated resistance [5]. This natural phenomenon provides a powerful foundation for selective breeding programs.

Quantitative genetic analyses have demonstrated that IHHNV viral load (VL) is a heritable trait. In a Mexican breeding population of L. vannamei, the heritability for log-transformed IHHNV load was estimated at 0.42 ± 0.45 at the family mean level, indicating substantial genetic variation [32]. Crucially, the genetic correlation between VL and survival was favorable (-0.57), meaning that selecting for lower viral loads is genetically associated with higher survival rates. Furthermore, the genetic correlation between VL and body weight at harvest was negligible (-0.04), suggesting that selection for reduced viral burden does not negatively impact growth [32]. These findings provide a clear roadmap for breeding programs: by incorporating IHHNV VL as a selection criterion, it is possible to simultaneously improve survival and maintain growth performance, effectively breeding shrimp populations that are more tolerant to endemic IHHNV. This strategy is particularly valuable in regions like the Philippines and China, where high prevalence (20-38.5%) persists despite existing management efforts [9, 15].

### Ecological Management and Cross-Species Transmission Risks

A critical and often overlooked dimension of IHHNV management is the role of non-penaeid crustaceans as reservoirs and vectors. Recent research has identified the benthic crab Helice tientsinensis as a susceptible host that meets WOAH criteria for susceptibility [3]. This crab, commonly found in shrimp ponds in northern China, can become infected via oral exposure, develop high viral loads (particularly in the hepatopancreas), and subsequently transmit IHHNV to juvenile P. vannamei [3]. Field surveys from 2021-2023 confirmed the persistent presence of IHHNV in H. tientsinensis populations across various shrimp farming periods, with notably high viral loads detected within pond environments [3]. Similarly, the red swamp crayfish (Procambarus clarkii) in China's Yangtze River basin shows an overall IHHNV-positive rate of 38.5%, with strains closely related to infectious genotypes [9]. These findings necessitate a paradigm shift in biosecurity planning: management strategies must extend beyond the target species to include the active control or exclusion of wild crustacean vectors. This may involve the installation of physical barriers (e.g., fine-mesh netting), regular removal of benthic crabs from pond systems, and the implementation of integrated pest management strategies that reduce the foraging opportunities for these reservoir hosts.

The phenomenon of viral interference, particularly between IHHNV and WSSV, adds another layer of complexity. Co-infection is common in wild and farmed crustaceans, with studies from the Andaman and Nicobar Archipelago reporting co-infection rates of 13.5% in shrimp and 4.5% in crabs [13]. While the precise mechanisms remain under investigation, the potential for one virus to modulate the pathogenesis of another must be considered in disease risk assessments [11]. A management strategy that inadvertently reduces IHHNV prevalence could, in theory, alter the ecological balance and potentially increase susceptibility to WSSV, a far more lethal pathogen. Therefore, any intervention must be evaluated within the context of the entire pathogen community.

### Therapeutic and Prophylactic Innovations: Virus-Like Particles and RNA Interference

The development of virus-like particles (VLPs) derived from IHHNV capsid proteins represents a groundbreaking avenue for both prophylactic immunization and therapeutic delivery. IHHNV-VLPs are non-infectious nanoparticles that self-assemble from recombinant capsid proteins, retaining the native tropism for shrimp tissues such as gills, muscle, and connective tissue [17, 25]. Their physical stability is remarkable; they remain intact at 4°C for up to 30 days, are stable across a broad pH range (4-9), and resist digestion by trypsin and chymotrypsin, making them viable candidates for oral administration [25].

The dual functionality of IHHNV-VLPs has been demonstrated in several key studies. First, as an immunostimulant, VLP administration in Fenneropenaeus merguiensis induced a significant upregulation of the antioxidant gene peroxiredoxin (FmPrx) and a corresponding two-fold increase in peroxidase activity in gill tissue, which correlated with a reduction in WSSV copy number upon subsequent challenge [30]. Second, VLPs serve as highly effective nano-containers for double-stranded RNA (dsRNA) therapeutics. Encapsulation of VP28 dsRNA (targeting WSSV) within IHHNV-VLPs resulted in superior gene silencing and lower cumulative mortality in WSSV-challenged shrimp compared to naked dsRNA, an effect attributed to both the targeted delivery and the inherent immunostimulatory properties of the VLP [31]. Furthermore, IHHNV-VLPs have been successfully used to encapsulate and deliver plasmid DNA (e.g., pcDNA3.1(+)-EGFP) into primary shrimp hemocytes, demonstrating their potential as gene therapy vectors to express antiviral proteins directly within host cells [23]. The ability to control VLP disassembly and reassembly using reducing agents (DTT) and calcium chelators (EGTA) provides a tunable platform for loading a variety of therapeutic cargoes [25]. While these technologies are still in the experimental phase, their translation to commercial application could revolutionize the management of not only IHHNV but also other viral co-infections in shrimp aquaculture.

### Regulatory and Policy Frameworks

Given the global distribution of IHHNV and its status as a WOAH-notifiable pathogen, national and international regulatory frameworks are essential for coordinated control. The detection of IHHNV in commercial shrimp facilities in the United States, a country with robust biosecurity protocols, highlights the constant threat of incursion and the need for vigilant import testing and quarantine [12]. In developing nations, where the majority of shrimp aquaculture occurs, the challenge is compounded by limited diagnostic capacity and enforcement of health certificates. Studies from Egypt and Colombia emphasize the need for strict regulations during live shrimp transportation and the implementation of health control certificates over all imports and exports [6, 18]. The use of standardized, WOAH-recommended molecular diagnostics (e.g., PCR targeting the 389 bp fragment) must be mandated for all broodstock and postlarvae movements [26]. Furthermore, the establishment of national surveillance programs, such as the Pathogens Surveillance Plan in Peru, should be adopted globally to provide baseline prevalence data and detect emerging virulent strains early [5]. The rapid evolutionary rate of IHHNV, estimated at 1.39×10⁻⁴ substitutions/site/year, comparable to RNA viruses, underscores the necessity for continuous genomic monitoring to track the emergence of new genotypes that may evade current management strategies [27].

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