Infectious Hematopoietic Necrosis Virus

Overview and Taxonomy of Infectious Hematopoietic Necrosis Virus

Infectious hematopoietic necrosis virus (IHNV) is the etiological agent of infectious hematopoietic necrosis (IHN), a devastating systemic disease of salmonid fish that is recognized as a notifiable pathogen by the World Organisation for Animal Health (WOAH) [5, 27]. The virus is a member of the family Rhabdoviridae, genus Novirhabdovirus, a taxonomic grouping that also includes other significant fish pathogens such as viral hemorrhagic septicemia virus (VHSV) and hirame rhabdovirus (HIRRV) [27, 33]. IHNV is an enveloped, bullet-shaped virion approximately 180 nm in length and 70 nm in diameter, a morphology that is archetypal of the rhabdovirus family and readily observable by electron microscopy in infected tissues, where virions are found in intercellular spaces or clustered around the nucleus of renal cells [4]. The viral genome consists of a single-stranded, negative-sense RNA molecule of approximately 11,100 to 11,130 nucleotides, organized into six genes in the canonical order 3′-N-P-M-G-NV-L-5′ [21, 27]. These genes encode, respectively, the nucleocapsid (N) protein, the phosphoprotein (P), the matrix (M) protein, the surface glycoprotein (G), the non-virion (NV) protein unique to novirhabdoviruses, and the large RNA-dependent RNA polymerase (L) [27, 33]. The glycoprotein G is the primary antigenic determinant and the target of neutralizing antibodies, making it the principal immunogen for vaccine development [1, 5, 14, 15, 32]. The N protein, which encapsidates the viral RNA, is highly conserved and serves as a reliable target for molecular and immunological diagnostics [3, 6, 16].

The taxonomic classification of IHNV has been refined extensively through phylogenetic analyses of the G gene, particularly the variable mid-G region, which has revealed five major genogroups: U, M, L, E, and J [4, 11, 16, 21, 29]. These genogroups exhibit distinct geographic distributions, host preferences, and virulence profiles. The U (Upper) and M (Middle) genogroups are endemic to North America, with overlapping ranges in the Columbia River Basin and along the Pacific coast [16, 31]. The L (Lower) genogroup is also found in North America, particularly in California, where it is associated with high mortality in Chinook salmon (Oncorhynchus tshawytscha) [17, 23]. The E (European) genogroup is prevalent across Europe, with isolates from Italy, France, and North Macedonia clustering within this clade [10, 22, 29]. The J (Japanese) genogroup, originally described in Japan, has since become the dominant lineage in East Asia, particularly in China and South Korea [4, 13, 20, 28]. Within the J genogroup, further subdivision into JRt-Shizuoka and JRt-Nagano lineages has been documented, with the former generally exhibiting higher virulence in rainbow trout (Oncorhynchus mykiss) [13, 20]. Importantly, a comprehensive phylogeographic analysis of Chinese IHNV isolates collected from 2012 to 2017 demonstrated that all strains belong to a monophyletic clade within the J genogroup, designated the JC subgroup, which shares a most recent common ancestor with the J Nagano (JN) subgroup [28]. This suggests that IHNV was introduced into China from Japan and has since undergone independent molecular evolution and diversification within Chinese salmonid farms [4, 28].

The evolutionary dynamics of IHNV are characterized by ongoing genetic drift and, in some regions, increasing virulence. Root-to-tip analyses of Italian isolates have revealed a strong temporal signal, indicating that the virus is evolving over time, with more recent strains (2015–2019) exhibiting moderate to high virulence in rainbow trout compared to historical isolates [22]. This increase in virulence is associated with specific genetic changes, though no single genetic determinant has been universally linked to the phenotype [22]. In China, the emergence of a genogroup U strain (BjLL) was reported for the first time in 2021, marking a significant expansion of the known geographic range of this genogroup beyond North America and Japan [21]. The BjLL strain was isolated from a local farm and, while less virulent than the co-circulating J genogroup strain GS-2014, it demonstrated the capacity to cause typical clinical signs and pathological damage in juvenile rainbow trout [21]. Comparative transcriptomic analyses of RTG-2 cells infected with U and J genogroup strains have revealed 2,238 differentially expressed genes, including those involved in immune responses, cellular signal transduction, and viral disease pathways, underscoring the distinct pathogenic mechanisms employed by different genogroups [11].

The host range of IHNV is primarily restricted to salmonid species, but susceptibility varies markedly among species and even among populations within a species. Rainbow trout and Chinook salmon are highly susceptible, while Atlantic salmon (Salmo salar) and coho salmon (Oncorhynchus kisutch) are more resistant [20, 29]. Steelhead trout (anadromous O. mykiss) exhibit intermediate susceptibility, and the L genogroup in California shows a pronounced host specificity for Chinook salmon, with steelhead trout experiencing very low mortality even under identical exposure conditions [17]. Notably, experimental infections have demonstrated that Japanese amberjack (Seriola quinqueradiata) and red sea bream (Pagrus major) show low or no susceptibility to IHNV, suggesting a narrow host tropism that is largely confined to salmonids [25]. Within the Columbia River Basin, the MD lineage of IHNV has specialized on O. mykiss (steelhead and rainbow trout), while the UC lineage displays a generalist phenotype capable of infecting both O. mykiss and Chinook salmon [31]. This specialist–generalist trade-off has important implications for viral transmission dynamics and disease management in multi-host hatchery systems [31]. The virus is known to persist in natural foci, such as the Kurile Lake in Kamchatka, where long-term virological studies (2004–2018) have demonstrated a closed system with unique genetic sequences, supporting the hypothesis that IHNV can maintain enzootic cycles in wild sockeye salmon (Oncorhynchus nerka) populations [18].

The molecular basis of IHNV virulence is multifactorial, involving both viral and host determinants. The N protein has been identified as a key virulence factor; alanine-scanning mutagenesis of the J genogroup N protein revealed that residues at positions 85 and 102 are critical for virulence in rainbow trout [24]. Recombinant viruses carrying substitutions at these sites exhibited significantly reduced replication in vivo and in vitro, induced higher expression of IFN1, IL-8, and IL-1β, and elicited earlier production of specific IgM antibodies, indicating that attenuation of virulence does not necessarily compromise immunogenicity [24]. The N protein also functions as an immune antagonist by targeting the host protein MITA (mediator of IRF3 activation), promoting its ubiquitination and degradation, thereby suppressing the production of type I interferon [33]. Similarly, the matrix (M) protein and glycoprotein (G) have been implicated in the upregulation of host miR-146a-3p, which in turn facilitates viral replication by targeting WNT3a and CCND1, two novel proteins that induce type I IFN responses [8]. The interplay between viral proteins and host microRNAs is a recurring theme in IHNV pathogenesis; for instance, miR-206 targets RIP2 to modulate IFN expression, while novel-m0065-3p targets IRF7 to regulate antiviral responses [7, 9]. These intricate host–virus interactions highlight the sophisticated strategies IHNV employs to subvert the innate immune system and establish productive infection.

The global distribution of IHNV has expanded considerably since its first description in the 1950s in North America, driven by the international trade of infected live fish and contaminated eggs [4, 27]. In China, IHNV was first detected in 2012 and has since become endemic in almost all rainbow trout farming regions, with isolates from Shandong, Chongqing, and Yunnan provinces forming a step-wise transmission model from north to south [4]. The virus has also been detected in autochthonous salmonid species in North Macedonia, including Macedonian trout (Salmo macedonicus) and Ohrid trout (Salmo letnica), indicating spillover from farmed rainbow trout into native populations [10]. The economic impact of IHNV is substantial, with mortality rates often exceeding 90% in naive juvenile populations, leading to significant losses in both aquaculture and conservation hatchery programs [3, 12, 26]. The virus is transmitted horizontally through waterborne exposure, with shedding kinetics characterized by a rapid onset, peak shedding at 2–3 days post-exposure, and a rapid decline to below detectable levels by 7 days [19]. Shedding magnitude varies by host species and viral strain; for example, spring-run Chinook salmon shed UC virus at 22-fold higher mean peak levels than fall-run fish, suggesting that spring-run populations may play a dominant role in the ecology and maintenance of IHNV in the Columbia River Basin [19]. Understanding these transmission dynamics is critical for designing effective control strategies, as vaccination primarily reduces mortality but has limited impact on viral shedding, particularly at high exposure dosages [2, 30].

Molecular Pathogenesis of Infectious Hematopoietic Necrosis Virus

The molecular pathogenesis of Infectious Hematopoietic Necrosis Virus (IHNV) represents a sophisticated interplay between a negative-sense single-stranded RNA virus of the Rhabdoviridae family and its salmonid hosts, primarily rainbow trout (Oncorhynchus mykiss), Chinook salmon, and sockeye salmon. As a pathogen notifiable to the World Organisation for Animal Health (WOAH), IHNV causes acute, systemic disease with mortality rates frequently exceeding 90% in naive juvenile populations, resulting in billion-dollar losses to the global salmonid aquaculture industry [3, 4, 27]. The pathogenic process is initiated at the molecular level through the viral glycoprotein (G), which facilitates attachment to host cell surface receptors and subsequent clathrin-mediated endocytosis. Once internalized, the viral ribonucleocapsid is released into the cytoplasm, where the RNA-dependent RNA polymerase drives rapid replication and transcription. However, the true complexity of IHNV’s molecular pathogenesis lies not merely in this replicative cycle, but in its sophisticated arsenal of immune evasion strategies, its manipulation of host cell death pathways, and its capacity to subvert the interferon (IFN) system, the cornerstone of the teleost antiviral defense.

Glycoprotein-Mediated Entry and Autophagy Induction

The G protein of IHNV serves as the primary determinant of viral tropism and pathogenicity, containing critical neutralizing epitopes that dictate host-cell recognition and fusion [1, 14, 27]. Structural studies have demonstrated that specific amino acid substitutions within the G protein, particularly at residues G230, G252, G270, and I277, correlate with variations in virulence among isolates, with the highly virulent J genogroup strains exhibiting distinct profiles compared to less pathogenic genogroups [12]. Beyond its canonical role in entry, the G protein has emerged as a key inducer of autophagy, a cellular degradation process that IHNV co-opts to enhance its replication. Using a pepscan approach spanning the complete G protein, researchers identified peptide p108 as a potent autophagy-inducing motif, with treated cells showing a 2.5-fold elevation in the LC3-II to LC3-I ratio, a hallmark of autophagosome formation [44]. Remarkably, UV-inactivated virus and purified G protein alone could trigger autophagosome formation, indicating that autophagy induction is independent of active viral replication [44]. This autophagy induction paradoxically suppresses IHNV replication, as cells pretreated with the autophagy-inducing peptide p108 exhibited significant inhibition of both intracellular viral mRNA levels and extracellular virion yields, suggesting that autophagy represents a host restriction mechanism that the virus must navigate [44]. The G protein also drives protective immunity; DNA vaccines encoding the G gene consistently elicit robust neutralizing antibody responses, with the J genogroup G protein providing superior cross-protection against heterologous genogroup U challenge compared to U genogroup-based vaccines [15].

Subversion of the Interferon System: The N Protein–MITA Axis

The most critical molecular determinant of IHNV pathogenesis is its ability to suppress the host type I IFN response, a mechanism mediated primarily by the nucleocapsid (N) protein. The N protein of IHNV directly targets the Mediator of IRF3 Activation (MITA), a central adaptor in the RIG-I-like receptor (RLR) signaling cascade. In rainbow trout, overexpression of IHNV N protein potently suppresses poly I:C-induced IFN1 promoter activity, and this suppression is achieved through targeted ubiquitination and proteasomal degradation of MITA [33]. By promoting MITA turnover, the N protein effectively severs the signaling axis that would otherwise activate IRF3 and drive IFN1 transcription, creating a permissive environment for viral replication [33]. This immune evasion strategy is remarkably effective: transcriptomic analyses of rainbow trout head kidney, spleen, liver, gill, and intestine all demonstrate that early IHNV infection (within 24–48 hours post-infection) triggers a massive transcriptomic reprogramming that paradoxically includes upregulation of key antiviral sensors such as TLR3, TLR7, TLR8, RIG-I (IFIH1), MDA5, LGP2, and TRIM25, yet the downstream IFN1 response is blunted [35-38]. The virus achieves this by simultaneously upregulating the sensors while degrading the central adaptor (MITA), creating a state of immunological paralysis. This is recapitulated in vivo: IHNV infection of rainbow trout liver results in marked upregulation of IFIH1, DHX58, MAVS, TRAF3, IRF3, and IRF7, yet IRF7 overexpression alone significantly inhibits IHNV replication, suggesting that the virus must actively suppress IRF7 function [7, 38]. The N protein virulence residues at positions 85 and 102 are critical for this suppressive activity; recombinant viruses bearing alanine substitutions at these positions show severely attenuated replication in vivo and in vitro, with survival rates in infected fish increasing from 10% (wild-type) to 52.5–55%, and these attenuated mutant viruses retain immunogenicity, inducing higher levels of IFN1, IL-8, and IL-1β and earlier IgM production [24].

miRNA-Mediated Immune Dysregulation

A major dimension of IHNV molecular pathogenesis involves the subversion of host microRNA (miRNA) regulatory networks. IHNV infection induces a dramatic shift in the splenic and hepatic miRNA landscape, with the virus selectively upregulating specific miRNAs that target and suppress key antiviral effectors. The most well-characterized of these is miR-146a-3p, which is robustly upregulated in rainbow trout spleen following IHNV infection due to the combined action of viral N, P, M, and G proteins [8]. This upregulation is dependent on the IFN signaling pathway itself, representing a sophisticated negative-feedback loop: the virus hijacks the host’s own IFN-induced regulatory machinery. miR-146a-3p directly targets Wingless-type MMTV integration site family 3a (WNT3a) and G1/S-specific cyclin-D1 (CCND1), both of which are novel inducers of the type I IFN response in RTG-2 cells [8]. By suppressing WNT3a and CCND1, this miRNA effectively abrogates the IFN-mediated antiviral state, promoting early viral replication. Knockdown of miR-146a-3p restores WNT3a and CCND1 expression and inhibits IHNV replication [8].

Conversely, some miRNAs act as part of the host antiviral defense and are targeted for suppression by the virus. Novel-m0065-3p directly targets interferon regulatory factor 7 (IRF7), and its overexpression in rainbow trout liver cells significantly reduces IRF7 expression while promoting cell proliferation and inhibiting apoptosis [7]. In vivo, injection of agomiR-m0065-3p suppresses IRF7 expression, facilitating viral replication [7]. Similarly, miR-206 targets receptor-interacting serine/threonine-protein kinase 2 (RIP2), a key adaptor in the NOD-like receptor signaling pathway. IHNV infection induces a negative correlation between miR-206 and RIP2 expression; silencing miR-206 in primary liver cells increases RIP2 and IFN expression while decreasing IHNV copy number, whereas overexpression suppresses the antiviral response [9]. The liver-specific miR-330-y targets TAP1, a transporter associated with antigen processing, and its overexpression reduces TAP1, IRF3, and IFN levels, while inhibition of this miRNA enhances the antiviral response [38]. Integrated miRNA-mRNA analyses across multiple tissues have identified a coordinated network of dysregulated miRNAs, including miR-141-y, miR-200-y, miR-144-y, miR-2188-y, miR-725-y, and miR-203-y, that collectively target TRIM25, LGP2, TLR3, TLR7, IRF3, and IRF7, creating a multi-layered suppression of the RIG-I and Toll-like receptor signaling cascades [36, 37].

Long Non-Coding RNA and DEAD-Box Helicase Dynamics

The molecular pathogenesis of IHNV extends to the regulation of long non-coding RNAs (lncRNAs). Comparative transcriptome analysis of IHNV-infected RTG-2 cells identified 3,693 differentially expressed lncRNAs and 3,503 differentially expressed mRNAs, with these lncRNAs predicted to regulate extracellular matrix metabolism, apoptosis, lipid synthesis, and autophagy [42]. Co-expression network analysis identified LTCONS_00146935 as a pivotal hub lncRNA, and its target genes were functionally enriched in immune response pathways, suggesting that IHNV manipulates the lncRNA landscape to create a cellular environment conducive to replication [42].

The DEAD-box RNA helicase DDX3, a conserved ATP-dependent RNA helicase involved in RNA metabolism and innate immunity, is also targeted by IHNV. Five transcript variants of rainbow trout DDX3 were identified, and their expression following IHNV infection initially increases but then markedly decreases as viral replication progresses in liver, spleen, and kidney [41]. siRNA-mediated knockdown of DDX3 significantly enhances IHNV replication in RTG-2 cells, indicating that DDX3 functions as a restriction factor that the virus must overcome through downregulation [41]. This pattern of initial upregulation followed by suppression, observed across multiple immune genes including tlr3, tlr7, traf3, ifih1, trim25, dhx58, irf3, irf7, and mx1, represents a common pathogenic strategy: the host mounts a transient antiviral response that is subsequently overwhelmed by the virus’s immune evasion capacity [35].

Metabolic Reprogramming and Apoptosis

Pathogenic IHNV infection drives a profound metabolic shift in host tissues. RNA-seq transcriptome profiling of the head kidney from rainbow trout infected with a highly virulent IHNV strain revealed that the earliest transcriptomic changes (day 1 post-infection) are centered on energy metabolism pathways, particularly gluconeogenesis, suggesting that the virus hijacks host metabolic machinery to fuel its replication [39]. By day 3, the predominant pathways shift to inflammatory response and immune system processes, and by day 5, cell proliferation pathways become dominant [39]. This temporal progression, from metabolic reprogramming, to immune dysregulation, to proliferative pathology, illustrates the phased nature of IHNV pathogenesis.

Apoptosis plays a dual role. On one hand, IHNV induces cellular apoptosis through mitochondrial dysfunction, as evidenced by decreased mitochondrial membrane potential (ΔΨm) in infected EPC cells [46]. The hydroxycoumarin derivative D5 protects against this by maintaining ΔΨm, enhancing antioxidant enzyme activities, and decreasing reactive oxygen species (ROS) [46]. Similarly, the imidazole coumarin derivative C4 inhibits IHNV-induced apoptosis and cellular morphological damage, with an IC50 of 2.53 μM [45]. However, IHNV also appears to suppress apoptosis in certain contexts to maintain the cellular machinery required for replication. The viral G protein’s ability to induce autophagy (as opposed to apoptosis) suggests that IHNV may favor autophagy as a replication platform while delaying apoptosis until later in the infection cycle [44].

Co-Infection Synergy and Host Susceptibility

The molecular pathogenesis of IHNV is profoundly altered in the context of co-infection, which is common in commercial aquaculture settings. Co-infection with Infectious Pancreatic Necrosis Virus (IPNV) produces complex, time-dependent interactions. In CHSE-214 cells, when IPNV is inoculated prior to or simultaneously with IHNV, IPNV inhibits IHNV replication through downregulation of clathrin and dynamin-2, critical components of IHNV entry [50]. However, when IHNV is established first (7 days prior to IPNV), this inhibition is lost and only minimal suppression occurs in the liver [40]. Conversely, in Chinook salmon embryo cells, IHNV enhances IPNV multiplication up to 40-fold, with the synergistic effect strengthening with increasing time intervals between infections [43]. In vivo, co-infection of rainbow trout with IHNV and IPNV results in higher mortality than either virus alone, likely due to the combined effects of hematopoietic necrosis and pancreatic necrosis [48]. Similarly, co-infection with sea lice (Lepeophtheirus salmonis) in sockeye salmon amplifies osmoregulatory dysfunction, leading to reduced survival through synergistic disruption of ion homeostasis and immune suppression [49].

Host genetic factors also modulate pathogenesis. Genome-wide association studies across multiple commercial rainbow trout breeding lines have confirmed an oligogenic architecture for IHNV resistance, with 17 quantitative trait loci (QTL) explaining 1.9–42% of additive genetic variance [26, 34, 47]. Genomic selection improves prediction accuracy by 15–33% over pedigree-based methods, indicating that the molecular determinants of resistance are polygenic and that the virus’s pathogenic success depends on host genotype [47]. Differences in susceptibility between Chinook salmon populations, with spring-run fish shedding 22-fold more UC genogroup virus than fall-run fish, further demonstrate how host population genetics shape transmission dynamics [19].

Epidemiology and Global Distribution of Infectious Hematopoietic Necrosis Virus

Infectious hematopoietic necrosis virus (IHNV) represents one of the most economically devastating viral pathogens affecting salmonid aquaculture and wild fisheries worldwide. Designated as a notifiable pathogen by the World Organisation for Animal Health (WOAH), IHNV induces a systemic, acute disease characterized by profound necrosis of hematopoietic tissues, with mortality rates frequently exceeding 90% in both farmed and naive wild populations [3]. The virus has demonstrated a remarkable capacity for transcontinental dissemination, evolving into distinct phylogenetic lineages that exhibit differential host specificity, virulence, and transmission dynamics across diverse geographic and ecological landscapes. Understanding the intricate tapestry of IHNV epidemiology requires a synthesis of virological, ecological, and anthropogenic factors that have shaped its global distribution over the past half-century.

Phylogeographic Framework and Global Genogroup Structure

The global population structure of IHNV has been resolved into five major genogroups, U, M, L, E, and J, through comprehensive phylogenetic analyses of the viral glycoprotein (G) gene and complete genome sequences [4, 11, 21, 28]. These genogroups reflect distinct evolutionary trajectories shaped by geographic isolation, host adaptation, and historical introduction events. The North American continent serves as the ancestral cradle of IHNV, harboring the greatest genetic diversity with the U, M, and L genogroups circulating predominantly in Pacific salmonid populations [16, 23, 31]. The U genogroup, considered the most ancestral lineage, is widely distributed along the Pacific coast from California to Alaska, infecting a broad range of salmonid hosts including sockeye salmon (Oncorhynchus nerka), Chinook salmon (O. tshawytscha), and steelhead trout (O. mykiss) [31]. The M genogroup emerged more recently within the Columbia River Basin and exhibits a pronounced host specialization for O. mykiss (rainbow trout and steelhead), a phenomenon supported by both field prevalence data and experimental challenge studies demonstrating higher infection probabilities in this host species compared to Chinook salmon [31, 51]. This specialization represents a classic example of pathogen adaptation in a multihost system, where viral fitness trade-offs result in lineage-specific host utilization strategies [31].

The L genogroup occupies a distinct ecological niche in California, where it has been associated with recurrent disease outbreaks in Chinook salmon conservation hatcheries since at least the 1940s [17]. Remarkably, while Chinook salmon fry exhibit moderate to high susceptibility to L genogroup IHNV (cumulative percent mortality ranging from 47–87%), sympatric steelhead populations demonstrate striking resistance with mortality rates as low as 1.3–33% under identical exposure conditions [17]. This stark differential susceptibility underscores the coevolutionary relationships between IHNV lineages and their salmonid hosts and has profound implications for conservation hatchery management and reintroduction programs [17, 23].

European Epidemiology: Emergence, Expansion, and Virulence Evolution

The introduction of IHNV into Europe represents a classical example of pathogen translocation through anthropogenic activities, likely mediated by the international trade of infected fish and contaminated eggs. Molecular epidemiological investigations have revealed that European IHNV isolates belong exclusively to the E genogroup, which is further subdivided into clades E-1 through E-5 [10, 22]. The virus was first detected in France and Italy in the 1980s, and subsequent phylogeographic analyses suggest a complex pattern of dissemination across the continent, with evidence of independent introduction events and subsequent regional diversification [22]. Notably, recent studies from North Macedonia documented the first detection of IHNV in 2018, and subsequent surveillance from 2021–2023 revealed that 38.3% of sampled trout farms were positive, with the virus detected in both farmed rainbow trout and autochthonous salmonid species including Macedonian trout (Salmo macedonicus) and Ohrid trout (Salmo letnica) [10]. All Macedonian isolates clustered within the E-1 clade, sharing >99% nucleotide similarity with the initial 2018 isolate, indicating rapid viral spread throughout the country's salmonid aquaculture infrastructure following primary introduction [10].

Perhaps the most alarming epidemiological feature of IHNV in Europe is the documented increase in virulence observed among Italian isolates over the past decade. Systematic characterization of sixteen Italian IHNV strains collected between 2015 and 2019 revealed a temporal trend toward enhanced virulence in rainbow trout, with more recently isolated strains exhibiting significantly higher cumulative mortality, earlier mortality peaks, and greater intra-host viral loads compared to historical isolates [22]. Whole-genome phylogeny and root-to-tip regression analyses demonstrated a robust temporal signal, confirming that the IHNV genome is undergoing continuous evolution in European aquaculture environments [22]. This virulence escalation poses a critical challenge for disease management, as strains previously considered low-pathogenicity have transitioned to moderate or high virulence phenotypes, necessitating reevaluation of biosecurity protocols and vaccine strategies [22, 52].

Asian Epidemiology: The J Genogroup Dominance and Emergence of Novel Lineages

China has emerged as a major epicenter of IHNV activity in Asia, with the virus first detected in the 1990s and subsequently becoming endemic across virtually all rainbow trout farming regions [4, 12, 28]. Comprehensive surveillance conducted between 2012 and 2017 across nine provinces characterized 40 IHNV isolates, all of which clustered within a monophyletic clade designated JC (China subgroup) within the broader J genogroup [28]. The J genogroup itself originated from Japanese IHNV isolates, with phylogenetic analyses indicating that Chinese IHNV shares a most recent common ancestor with the J Nagano (JN) subgroup [28]. Within China, viral isolates obtained from individual farms over successive years formed strongly supported subclades, providing compelling evidence for viral maintenance and diversification within discrete aquaculture facilities [28]. Furthermore, phylogeographic reconstruction revealed both regional transmission within provinces and long-distance dissemination events, such as the transport of infected fish or eggs from Gansu to Yunnan provinces, highlighting the role of aquaculture trade networks in viral dispersal [4, 28].

A seminal epidemiological development in China was the first isolation of a genogroup U IHNV strain (BjLL) from diseased rainbow trout in a local farm, marking the first documented occurrence of this genogroup outside of North America and Japan since 1982 [21]. Comparative pathogenicity studies revealed that the U genogroup isolate BjLL exhibited significantly lower virulence than the co-circulating J genogroup strain GS-2014, suggesting that different selective pressures may govern the maintenance of these distinct lineages in Chinese aquaculture systems [21]. The co-circulation of J and U genogroups in China raises important questions regarding potential reassortment events, competitive exclusion, and the implications for vaccine cross-protection [15, 21].

In South Korea, IHNV epidemiology is characterized by the circulation of two distinct J genogroup subgroups: the JRt-Shizuoka and JRt-Nagano lineages [13, 20]. Experimental challenge studies demonstrated that both lineages are highly pathogenic to rainbow trout, with JRt-Shizuoka isolates inducing 100% cumulative mortality, while JRt-Nagano isolates resulted in 60% mortality over 14 days post-infection [20]. Importantly, the virulence of these Korean isolates is host-dependent, with rainbow trout exhibiting markedly higher susceptibility compared to Atlantic salmon and coho salmon, underscoring the species-specific nature of IHNV pathogenesis [20]. The Japanese archipelago represents another significant epidemiological zone, where IHNV has been documented in both freshwater rainbow trout farms and, notably, in marine fish species. Experimental infections demonstrated that Japanese amberjack (Seriola quinqueradiata) showed 70% cumulative mortality following intraperitoneal injection of an IHNV isolate from rainbow trout, while red sea bream (Pagrus major) exhibited complete resistance, with no mortality, no viral isolation from survivors, and no detectable viral replication in cell lines derived from this species [25]. These findings extend the known host range of IHNV beyond salmonids and indicate potential risks for marine aquaculture diversification [25].

North American Epidemiology: Complex Transmission Networks and Host Specialization

The Columbia River Basin (CRB) represents one of the most intensively studied ecosystems for IHNV epidemiology, owing to its critical importance for Pacific salmonid conservation and the presence of numerous hatchery facilities [19, 23, 31, 51]. Within the CRB, two major IHNV genogroups, UC and MD, co-circulate and exhibit distinct ecological niches [31, 51]. The UC genogroup functions as a generalist, infecting multiple host species including Chinook salmon, steelhead trout, and rainbow trout with moderate probabilities of infection across all host types [31]. In contrast, the MD genogroup displays a specialized phenotype, exhibiting high infection probability in steelhead and rainbow trout (O. mykiss) but significantly reduced infection in Chinook salmon [31, 51]. This specialist-generalist trade-off has profound implications for disease management, as the presence of specific viral lineages in a hatchery dictates the relative risk to different salmonid species [31].

Detailed modeling of IHNV transmission dynamics in the Snake River Basin (SRB), employing Bayesian susceptible-exposed-infected frameworks integrated with spatial contact networks, revealed that migrating adult salmonids are the primary source of exposure for juvenile fish in hatchery environments [51]. Crucially, the study demonstrated that the frequency of exposure by migrating adults was overestimated by 70 cohort-sites, and the infection probability was underestimated by approximately 0.09 when model coproduction, the integration of local expert knowledge regarding hatchery complexes, was not employed [51]. This finding highlights the critical importance of incorporating fine-scale operational knowledge into epidemiological models, as hatchery complexes consisting of main facilities and satellite sites exhibit distinct transmission patterns that are not captured by coarse-scale analyses [51].

Shedding kinetics represent a key epidemiological parameter governing transmission potential. In CRB Chinook salmon, IHNV shedding is characterized by a rapid onset, with peak viral shedding occurring within 2–3 days post-exposure, followed by a precipitous decline to undetectable levels by day 7 [19]. Strikingly, spring-run Chinook salmon shed UC genogroup virus at 22-fold higher mean peak magnitude, 33-fold higher mean total virus per fish, and 900-fold higher total virus per treatment group compared to fall-run Chinook salmon [19]. This intraspecies variation in shedding suggests that spring-run fish may serve as superspreaders, disproportionately contributing to viral maintenance and transmission within the CRB ecosystem [19]. Furthermore, all viral shedding occurred well before host mortality onset, indicating that clinically healthy fish actively contribute to viral dissemination, a phenomenon with significant implications for surveillance and containment strategies [19].

Host Range, Transmission Routes, and Ecological Determinants

The epidemiological landscape of IHNV is fundamentally shaped by its host range, which encompasses virtually all salmonid species, though with marked variation in susceptibility. Rainbow trout (Oncorhynchus mykiss) are considered the most susceptible species, with many isolates inducing 90–100% mortality under experimental conditions [4, 12, 20]. Atlantic salmon (Salmo salar) exhibit intermediate susceptibility, while certain Pacific salmon species such as Chinook salmon show differential susceptibility depending on the viral genogroup and host population [17, 20, 23, 29]. Comparative studies using isolates representing all five major genogroups demonstrated that virulence profiles are highly linked to precocious viral replication and the magnitude of innate and specific immune responses elicited in the host [29]. Notably, seroneutralization tests revealed efficient cross-neutralization among heterologous systems for all genogroups except the Asian representative (J genogroup), suggesting antigenic divergence that may complicate vaccine development and deployment [29].

Transmission of IHNV occurs through multiple routes, including horizontal transmission via waterborne exposure, direct fish-to-fish contact, and vertical transmission through infected broodstock and contaminated eggs [4, 51]. Evidence from China indicates that IHNV can be introduced into naïve populations through multiple pathways: SHandong and Chongqing isolates were potentially derived from purchased eyed eggs and parent fish carrying IHNV, while Yunnan isolates originated from external factors including cycling water, commercial fish, and vehicles [4]. A step-wise transmission model (SD→CQ→YN) was inferred, demonstrating the progressive dissemination of the virus through interconnected aquaculture networks [4].

Environmental factors exert potent modulating effects on IHNV epidemiology. Water temperature is a critical determinant of disease outcome, with studies demonstrating that rainbow trout infected at 12–13°C exhibited different gut microbiota and metabolite profiles compared to those infected at 16–17°C [53]. At lower temperatures, IHNV infection induced a significant increase in pathogenic bacteria including Aeromonas, Pseudomonas, and Yersiniaceae, while at higher temperatures, beneficial bacteria such as Streptococcaceae and Lactococcus lactis increased in abundance [53]. Additionally, temperature profoundly impacts viral replication kinetics and host immune responses, with higher temperatures generally reducing mortality in Chinook salmon challenged with L genogroup IHNV [17]. Stocking density, another husbandry parameter, has been evaluated as a potential stressor influencing disease susceptibility; however, controlled experiments demonstrated that high-density (20–40 kg/m³) and low-density (4–8 kg/m³) conditions did not significantly affect mortality following IHNV exposure in juvenile rainbow trout, despite measurable differences in serum cortisol and neutrophil:lymphocyte ratios [54].

Co-infection Dynamics and Epidemiological Interactions

Natural and experimental co-infections involving IHNV and other salmonid pathogens, particularly infectious pancreatic necrosis virus (IPNV), represent a significant and underappreciated dimension of IHNV epidemiology. In Chinese rainbow trout farms, co-infection with IHNV (genogroup J) and IPNV (genogroup 1) resulted in high mortality, with severe extensive necrosis observed across major organs [48]. Paradoxically, the dynamics of viral interference during co-infection are time-dependent and context-specific. In CHSE-214 cell lines, prior or simultaneous infection with IPNV significantly inhibited IHNV replication in all tissues examined, with the most pronounced inhibition occurring in the liver [40, 50]. This inhibition was associated with stronger antiviral responses, including upregulation of interferon-related genes, and resulted in reduced mortality in rainbow trout that received early IPNV injection prior to IHNV challenge [40]. Conversely, when IHNV was inoculated prior to IPNV, the inhibitory effect on IHNV replication diminished with increasing time intervals, suggesting that the timing of co-infection critically determines the outcome [50]. Mechanistically, IPNV appears to inhibit IHNV by downregulating entry-related genes such as clathrin and dynamin-2, thereby reducing viral invasion efficiency [50]. In contrast, IHNV enhances IPNV multiplication during co-infection, with the highest IPNV levels observed when IHNV was inoculated 12 hours after IPNV, resulting in a 40-fold increase in viral titer compared to single IPNV infection [43]. These complex viral interactions have profound implications for field epidemiology, as the prevalence and timing of co-infections will influence the transmission dynamics and disease severity of both pathogens [40, 43, 48].

Co-infection with non-viral pathogens also modulates IHNV epidemiology. Sockeye salmon smolts co-infected with salmon lice (Lepeophtheirus salmonis) and IHNV exhibited significantly lower survival compared to single-pathogen infections, driven by synergistic osmoregulatory dysfunction rather than immunomodulation [49]. Co-infected salmon demonstrated elevated osmoregulatory indicators and lowered hematocrit values, suggesting that the physiological stress imposed by ectoparasite infection amplifies the pathogenic effects of IHNV [49]. Furthermore, sublethal environmental contaminants such as glyphosate-based herbicides can modulate IHNV susceptibility across generations. Intergenerational exposure of rainbow trout to glyphosate and formulated glyphosate-based herbicides resulted in variable effects on offspring susceptibility: pure glyphosate induced comparable mortality to controls (35.8% vs. 37.0%), while Roundup Innovert® reduced mortality (−

Diagnostics and Detection Methods for Infectious Hematopoietic Necrosis Virus

The accurate and timely detection of infectious hematopoietic necrosis virus (IHNV) is paramount for effective disease surveillance, outbreak containment, and the implementation of control strategies within the global salmonid aquaculture industry. As a pathogen listed by the World Organisation for Animal Health (WOAH), IHNV necessitates robust diagnostic frameworks capable of identifying the virus across a spectrum of host species, genogroups, and clinical presentations. The diagnostic landscape for IHNV has evolved considerably, transitioning from classical virological and serological techniques to highly sensitive molecular and immunological platforms. This section provides a detailed, exhaustive analysis of the established and emerging methodologies employed for IHNV detection, critically evaluating their principles, performance characteristics, applicability, and limitations within the context of both research and field-based surveillance.

Virus Isolation and Cell Culture: The Historical Gold Standard

The traditional cornerstone of IHNV diagnosis has been virus isolation in susceptible cell lines, a technique that remains integral to confirmatory diagnostics and the generation of viral isolates for characterization. The most commonly employed cell lines include the Chinook salmon embryo (CHSE-214) cell line, the rainbow trout gonad (RTG-2) cell line, and the epithelioma papulosum cyprini (EPC) cell line [11, 12, 43, 50]. Upon inoculation with tissue homogenates from suspect fish, typically derived from kidney, spleen, heart, or encephalon tissue, IHNV induces a characteristic cytopathic effect (CPE), often observed within three to seven days post-infection depending on the viral load and isolate virulence [12, 21]. The CPE typically manifests as rounding and detachment of cells, progressing to cell lysis and plaque formation. The identity of the isolated virus is subsequently confirmed through immunological methods, such as immunofluorescence antibody tests (IFAT) using virus-specific monoclonal or polyclonal antibodies, or through molecular techniques like reverse-transcription polymerase chain reaction (RT-PCR) [3, 21, 48].

While virus isolation provides essential evidence of replicating, infectious virus and remains the reference standard for WOAH reporting, its utility is constrained by several factors. The process is time-consuming, requiring several days to weeks for definitive results [27]. It demands specialized laboratory infrastructure, sterile techniques, and the maintenance of continuous cell lines. Furthermore, the sensitivity of cell culture can be compromised by sample degradation due to improper storage or transport, the presence of cytotoxic substances in tissue homogenates, or interference from co-infecting pathogens [56]. Despite these limitations, cell culture remains indispensable for in-depth virological studies, such as virulence assessment, genogroup characterization via plaque purification, and the production of viral stocks for experimental vaccines or antiviral assays [4, 12, 20, 22].

Molecular Detection Methods: High Sensitivity and Specificity

The advent of molecular diagnostics, particularly nucleic acid amplification tests (NAATs), has revolutionized IHNV detection by offering unparalleled sensitivity, specificity, and speed compared to traditional culture methods. These techniques target conserved regions of the viral genome, most frequently the nucleocapsid (N) protein gene or the glycoprotein (G) gene, facilitating detection across diverse genogroups.

Reverse-Transcription Polymerase Chain Reaction (RT-PCR) and Real-Time Quantitative PCR (RT-qPCR)

Conventional RT-PCR and its quantitative counterpart, RT-qPCR (or real-time RT-PCR), have become the workhorse molecular tools for IHNV diagnosis. RT-qPCR offers the significant advantage of quantifying viral RNA copy numbers in clinical samples, providing data on viral load dynamics in tissues, blood, and shedding excreta [40, 47, 55]. This quantitative capacity is critical for understanding pathogenesis, evaluating vaccine efficacy by measuring reductions in viral replication [2, 52, 55], and monitoring the kinetics of infection and transmission [19, 30, 40]. Numerous studies have validated RT-qPCR protocols targeting the N gene or the G gene, demonstrating high analytical sensitivity, often down to 10-100 copies of viral RNA per reaction [6, 36].

A major advancement is the development of multiplex RT-qPCR assays capable of discriminating between different IHNV genogroups. Notably, Batts et al. (2022) developed a U/M multiplex RT-rPCR assay targeting the N gene, designed specifically to differentiate between the U and M genogroups that co-circulate in the Columbia River Basin of North America [16]. This assay demonstrated high diagnostic sensitivity (DSe > 94%) and specificity (DSp > 97%) when validated against cell culture in free-ranging adult Pacific salmon, offering a rapid and powerful tool for surveillance and management decisions that depend on knowing which genogroup is present [16]. Such genogroup-specific tests are crucial because the various IHNV genogroups (U, M, L, E, J) exhibit distinct host tropisms and virulence profiles, influencing risk assessment for different salmonid species [16, 23, 31, 51]. For example, the L genogroup is highly pathogenic for Chinook salmon, while the M and U groups show differential virulence in rainbow trout and Chinook salmon [16, 17, 23]. Molecular detection has also been instrumental in identifying the emergence of new IHNV genogroups and tracing their spread, as demonstrated by the discovery of a U genogroup strain (BjLL) in China, which was confirmed via RT-PCR and full genome sequencing [21, 28].

Reverse-Transcription Cross-Priming Amplification with Lateral Flow Assay (RT-CPA-LFA)

To address the need for rapid, point-of-care (POC) diagnostics suitable for on-farm use, isothermal amplification technologies have been adapted for IHNV detection. Choi et al. (2024) developed a reverse-transcription cross-priming amplification (RT-CPA) assay coupled with a lateral flow dipstick for visual readout [6]. This assay, targeting the N gene, circumvents the need for a thermocycler, performing amplification at a constant temperature and providing results within 5 minutes of completing the RT-CPA reaction via a simple lateral flow strip. Evaluated on 140 fish samples, the RT-CPA-LFA demonstrated a diagnostic sensitivity of 98.88% and specificity of 96.08% [6]. The detection limit was determined to be 3.28 × 10⁵ copies/μL, which, while less sensitive than some qPCR assays, is likely sufficient for detecting acute, high-viral-load infections. The 100% detection rate in dead, artificially infected fish underscores its potential for confirming presumptive IHN diagnoses during active outbreaks, particularly in fieldwork settings lacking sophisticated laboratory equipment [6].

Other Isothermal Amplification Methods

While not yet widely validated for IHNV specifically, isothermal amplification technologies like loop-mediated isothermal amplification (LAMP) and enzymatic recombinase amplification (ERA) have been successfully applied to related aquatic viruses such as the infectious hypodermal and hematopoietic necrosis virus (IHHNV) in shrimp, providing a proof-of-concept for their utility [7, 13, 57]. For example, a real-time triplex LAMP assay for IHHNV achieved a detection limit of 10 copies per reaction and 100% specificity and sensitivity, with results available in 45 minutes [56]. Similarly, a real-time ERA assay for IHHNV could detect as few as 1.4 × 10¹ copies in under 20 minutes at 42°C [57]. These platforms offer immense potential for developing rapid, field-deployable diagnostics for IHNV that could empower farmers and regulatory authorities to make swift biosecurity decisions, thus limiting pathogen spread.

Serological and Antigen Detection Methods

Immunological methods provide alternative or complementary diagnostic approaches, ranging from detecting host antibodies to directly identifying viral antigens in tissues.

Double-Antibody Sandwich Enzyme-Linked Immunosorbent Assay (DAS-ELISA)

A significant recent advancement in antigen detection is the development of a DAS-ELISA targeting the highly conserved IHNV N protein [3]. Zhao et al. (2025) generated specific monoclonal antibodies against two antigenic peptides of the N protein, creating a robust assay using one antibody (N-15) as a capture reagent and an HRP-conjugated second antibody (N-106) for detection [3]. This DAS-ELISA demonstrated excellent analytical specificity, showing no cross-reactivity with other common fish rhabdoviruses such as viral hemorrhagic septicemia virus (VHSV), spring viremia of carp virus (SVCV), or infectious pancreatic necrosis virus (IPNV) [3]. The sensitivity was determined to be 10³ TCID₅₀/mL, which is appropriate for detecting high viral loads typical of acute infection. Most critically, when applied to 293 clinical samples, the DAS-ELISA showed a concordance rate of 92.83% (κ = 0.856) compared to the WOAH reference method, confirming its reliability for high-throughput screening in clinical settings [3]. This method is particularly attractive for large-scale surveillance programs due to its speed, ease of use, and capacity to process numerous samples without specialized molecular equipment.

Neutralizing Antibody Assays (Seroneutralization Test, SNT)

Detecting a host's humoral immune response through serological assays is valuable for retrospective surveillance, epidemiological studies, and assessing vaccine-induced immunity. The seroneutralization test (SNT) remains the gold standard for detecting and quantifying specific neutralizing antibodies in fish sera. In this assay, serial dilutions of heat-inactivated serum are incubated with a standardized dose of infectious IHNV, and the mixture is then inoculated onto susceptible cell monolayers. The absence of CPE indicates the presence of neutralizing antibodies. SNT has been effectively used to confirm vaccine efficacy, showing that vaccinated fish produce neutralizing antibodies that correlate with protection [1, 14, 52]. Notably, Lin et al. (2022) demonstrated that a Montanide GEL 02-adjuvanted inactivated vaccine elicited neutralizing antibodies detectable at 30 and 60 days post-vaccination, which correlated with long-term protection [52]. However, the SNT is laborious, slow (taking several days), and requires cell culture facilities. Furthermore, cross-neutralization studies have shown that while SNT can detect antibodies across different genogroups, there can be reduced efficiency, particularly with more distantly related Asian isolates [29]. As an alternative to live virus assays, the development of DAS-ELISA for antigen detection in tissues offers a more rapid, high-throughput option for diagnosing active infection rather than past exposure [3].

Electron Microscopy and Histopathology

Direct visualization techniques, while not primary diagnostic tools, provide crucial supportive evidence for IHNV infection. Transmission electron microscopy (TEM) can identify the characteristic bullet-shaped rhabdovirus particles in tissue sections or cell culture supernatants. Reports have detailed the visualization of IHNV virions in the intercellular space and around the nucleus of renal cells from infected rainbow trout [4]. Histopathological examination of target organs, particularly kidney, spleen, and liver, reveals pathognomonic lesions such as extensive necrosis of hematopoietic tissue and hepatocytes [4, 48]. While specific IHNV inclusion bodies are not pathognomonic, histology can help differentiate IHNV from other causes of systemic necrosis. TUNEL staining has been employed to quantify apoptosis in the spleen of infected trout, providing insights into the pathological mechanisms of IHNV [55]. These techniques are primarily confined to research laboratories and diagnostic reference centers.

Integrated Genomic and Transcriptomic Approaches

Beyond routine diagnostics, advanced molecular tools are increasingly used for characterizing IHNV isolates and understanding host-pathogen interactions. Whole-genome sequencing (WGS) using next-generation sequencing (NGS) platforms allows for comprehensive phylogenetic analysis, identification of virulence markers, and tracking of viral evolution at the molecular level [11, 18, 22, 28, 39]. For instance, WGS of Italian IHNV isolates over time revealed a temporal signal, indicating an evolving nature of the virus with more recent strains exhibiting higher virulence [22]. Similarly, transcriptome profiling (RNA-seq) of infected tissues, such as kidney, spleen, and gills, provides a global view of the host's immune response. These studies have identified key differentially expressed genes (DEGs) and microRNAs (miRNAs) involved in antiviral signaling, revealing complex regulatory networks [11, 35-39]. For example, RNA-seq in rainbow trout head kidney revealed that IHNV infection drives pathogenesis by activating metabolic pathways and repressing type I interferon (IFN) production through the downregulation of critical signaling molecules [39]. While these methodologies are not used for routine primary diagnosis, they are indispensable for research into viral pathogenesis, vaccine design, and marker-assisted selection for disease-resistant fish populations [26, 34, 47]. The detection of viral RNA in eggs and fry via sensitive RT-qPCR has also been instrumental in tracing the sources of IHNV introduction into new geographic regions, such as infected eyed eggs imported into China [4].

Host Immune Response to Infectious Hematopoietic Necrosis Virus Infection

The host immune response to Infectious Hematopoietic Necrosis Virus (IHNV) is a complex, multi-layered process involving the coordinated activation of innate and adaptive arms of the teleost immune system. As a rhabdovirus of significant economic and regulatory concern, IHNV is listed by the World Organisation for Animal Health (WOAH) as a notifiable pathogen [5, 9], the virus has evolved sophisticated mechanisms to subvert host defenses, particularly the type I interferon (IFN) system. The immune response is profoundly influenced by viral genogroup, host species, environmental factors, and the intensity of exposure, making the host-pathogen interplay a dynamic and highly variable landscape. Understanding these intricate immunological cascades is paramount for the rational design of effective vaccines and immunotherapeutics.

Innate Immune Recognition and Signaling Cascades

The initial detection of IHNV relies on pattern recognition receptors (PRRs) that sense viral pathogen-associated molecular patterns (PAMPs), primarily the viral genomic RNA and replication intermediates. Transcriptomic analyses across multiple tissues, including the spleen [55], gill [35], head kidney [36, 39], liver [38], and intestine [37], have consistently demonstrated the profound upregulation of genes encoding Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), and NOD-like receptors (NLRs) following IHNV infection. Specifically, tlr3, tlr7, and tlr8 are markedly induced in the gill at 48 hours post-infection (hpi), alongside critical adaptor molecules such as traf3 [35]. In the head kidney, a primary hematopoietic and immune organ, the RLR pathway members lgp2 (also known as dhx58), mda5 (ifih1), and trim25 are significantly upregulated [36]. This activation triggers downstream signaling cascades that converge on the activation of interferon regulatory factors (IRFs), particularly irf3 and irf7, which are pivotal for the transcription of type I IFNs [35-38]. The integrated analysis of mRNA and miRNA profiles in the liver has further revealed the central role of the RIG-I-like receptor signaling pathway, with key genes such as MAVS and TRAF3 exhibiting robust induction [38]. This sentinel network ensures that the host can rapidly detect the presence of the virus across multiple mucosal and systemic compartments.

The Type I Interferon Response and Antiviral Effector Mechanisms

The induction of type I interferons (IFN-1) represents the cornerstone of the antiviral state against IHNV. The upregulation of ifn1 is a hallmark of infection, observed systemically and in mucosal tissues [32, 58, 59]. This response is tightly regulated by a complex interplay of transcription factors and non-coding RNAs. The IHNV N protein has been shown to suppress fish ifn1 production by targeting the mediator of IRF3 activation (MITA, also known as STING) for ubiquitination and degradation, revealing a direct immune evasion strategy [33]. Despite this, the host often mounts a robust IFN response that drives the expression of numerous interferon-stimulated genes (ISGs), which are the effectors of the antiviral state.

Among the most well-characterized ISGs in the context of IHNV are the Mx proteins (mx1, mx3) and the Vig proteins (vig1, vig2). The upregulation of mx1 is a sensitive and reliable marker of IFN pathway activation and is consistently induced by IHNV challenge in the kidney, spleen, liver, and gill [15, 20, 35, 38]. DNA vaccination studies have shown that mx1 expression in muscle can be significantly higher than in the spleen, indicating that local antiviral states are established at the site of antigen delivery [15]. The expression of vig1, ifit5, and isg-15 is also strongly induced, as demonstrated in studies using self-assembling ferritin nanovaccines and live vector vaccines [5, 62]. Furthermore, the upregulation of mx1 and other ISGs is not limited to the traditional immune organs; the gill, as a primary mucosal interface, mounts a significant ISG response following bath challenge [59]. The temporal dynamics of these effectors are critical, with peak expression often observed between 48 and 72 hpi, correlating with the peak of viral replication [35, 38]. This coordinated ISG response aims to limit viral spread and establish a refractory state in neighboring cells.

Immunomodulation by MicroRNAs and Long Non-Coding RNAs

The host immune response to IHNV is intricately regulated by non-coding RNAs, particularly microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), which fine-tune the expression of key immune genes. IHNV infection induces profound dysregulation of the host miRNA transcriptome across multiple tissues, including the skin [7], head kidney [36], gill [35], intestine [37], and liver [38]. These differentially expressed miRNAs (DEMs) often target critical nodes within the PRR and IFN signaling pathways. For instance, novel-m0065-3p has been identified as a negative regulator of the antiviral response by directly targeting irf7 in the skin, thereby facilitating viral replication [7]. Conversely, miR-206 plays a protective role; its silencing in vitro leads to increased expression of its target, receptor-interacting serine/threonine-protein kinase 2 (RIP2), and a subsequent increase in IFN expression, which significantly reduces IHNV copy numbers [9]. This suggests that while the host uses some miRNAs to restrain an over-exuberant inflammatory response, the virus can usurp this regulatory mechanism to its advantage.

A particularly elegant example of viral subversion is the role of miR-146a-3p. IHNV infection upregulates this miRNA, which in turn targets WNT3a and CCND1, both of which are novel inducers of the type I IFN response. By suppressing these host proteins, miR-146a-3p effectively dampens the IFN response, creating a permissive environment for early viral replication [8]. Similarly, in the liver, miR-330-y targets TAP1, a key component of the antigen processing pathway, and its overexpression reduces the expression of TAP1, IRF3, and IFN, thereby promoting infection [38]. The role of lncRNAs in this regulatory network is also emerging. A comparative transcriptome analysis of IHNV-infected RTG-2 cells identified thousands of differentially expressed lncRNAs, many of which are predicted to function as competing endogenous RNAs (ceRNAs) that modulate the expression of immune-related mRNAs, such as those involved in the RIG-I-like receptor and TLR signaling pathways [42]. The lncRNA LTCONS_00146935 was identified as a pivotal node in the co-expression network, suggesting a central role in orchestrating the immune response [42]. This complex regulatory architecture highlights the sophisticated battle for control that occurs at the post-transcriptional level.

Adaptive Humoral and Cellular Immune Responses

The adaptive immune response to IHNV is characterized by the production of specific neutralizing antibodies and the activation of T cell-like responses. The B cell response is marked by the upregulation of immunoglobulin (Ig) heavy chain genes, with igm and igt being the most prominent responders across various vaccine and infection models [1, 32, 55, 62]. IgT is considered the major immunoglobulin associated with mucosal immunity in teleosts, and its induction in the gill and intestine underscores the importance of the mucosal response [37, 59]. In the spleen, vaccination with an Astragalus polysaccharide (APS)-adjuvanted inactivated vaccine strongly induces igm, igt, and igd expression, correlating with reduced viral loads [55]. The production of neutralizing antibodies is a key correlate of protection, and DNA and inactivated vaccines have been shown to elicit significant neutralizing antibody titers in serum, typically by 30 days post-vaccination (dpv) [1, 52]. However, the relationship between neutralizing antibody titers and protection is not absolute. Studies on long-term protection with adjuvanted inactivated vaccines have demonstrated that protection can persist for over 200 days even in the absence of detectable neutralizing antibodies, suggesting that other mechanisms, such as cellular immunity or trained immunity, play a critical role [52].

The cellular arm of the adaptive response involves the upregulation of CD4+ and CD8+ T cell markers. Both cd4 and cd8 expression are induced in the spleen following vaccination with APS-adjuvanted vaccines and in the gill after IHNV challenge [55, 59]. The importance of the cytotoxic T lymphocyte (CTL) response, mediated by CD8+ cells, is inferred from the upregulation of genes involved in antigen processing and presentation, such as tap1 and cd83, in the liver [38]. The exact role of CTLs in clearing IHNV is an area of active research. Studies comparing DNA vaccines with single-cycle viral vaccines against VHSV and IHNV suggested that cross-protection may be independent of cytotoxic T cell epitopes, and instead point towards the potential involvement of trained immunity, an innate immune memory phenomenon [60]. The transcription factor irf7 is again crucial here, as it not only drives the IFN response but is also essential for the development of T cell-dependent antibody responses, as shown by the observation that irf7 overexpression significantly inhibits IHNV replication in vivo [7].

Mucosal Immunity and Tissue-Specific Responses

The mucosal surfaces, gills, skin, and intestine, represent critical portals of IHNV entry and are therefore sites of intense immunological activity. The gill, in particular, mounts a robust and multifaceted response. Bath challenge with IHNV leads to severe gill pathology, including increased goblet cell counts in the primary and secondary lamellae, and a profound dysbiosis of the commensal microbiota, marked by a loss of beneficial taxa and an expansion of pathobionts [59]. This is accompanied by a strong innate and adaptive antiviral immune response, along with a paradoxical induction of antibacterial immune genes, suggesting that viral infection predisposes the host to secondary bacterial infections [59]. The skin, another mucosal barrier, shows a unique miRNA signature in response to IHNV. The novel miRNA novel-m0065-3p is a critical regulator in the skin, modulating irf7 expression and influencing liver cell proliferation and apoptosis [7].

The intestinal immune response is equally complex. IHNV infection disrupts intestinal integrity, causing oxidative damage and altering the composition of the gut microbiota. Dietary supplementation with crude lentinan (CLNT) has been shown to counteract these effects by enhancing the intestinal immune barrier, reducing permeability, boosting antioxidant and anti-inflammatory capacity, and promoting the growth of short-chain fatty acid (SCFA)-producing bacteria, such as Carnobacterium, which in turn increase the production of protective metabolites like acetic acid and butanoic acid [61]. Metabolomic analyses have further revealed that IHNV infection at different water temperatures leads to distinct shifts in gut metabolites, with significant changes in immune-related metabolites like 1-Octadecanoyl-glycero-3-phosphoethanolamine and L-Glutamate, which are correlated with shifts in specific microbial taxa [53]. This intricate tripartite interaction between the virus, the host mucosal immune system, and the resident microbiota is a crucial determinant of disease outcome.

The Impact of Viral Genogroup and Co-Infections on the Immune Response

The immune response elicited by IHNV is not uniform; it is heavily influenced by the specific viral genogroup. Comparative transcriptome analysis of RTG-2 cells infected with the U and J genogroups revealed significant differences in the host transcriptional landscape, with 2,238 differentially expressed genes between the two conditions. These host genes were associated with distinct cellular processes, including cellular signal transduction, viral disease pathways, and the immune response, with genes like trpm2, sting, ripk2, and irf1 showing differential regulation [11]. In vivo challenge studies have demonstrated that the J genogroup (e.g., IHNV-SD) is more virulent in rainbow trout than the U or CQ/YN isolates [4]. This differential virulence is reflected in the host response; the more virulent strains drive a more pronounced, but often less effective, immune activation, characterized by a rapid and overwhelming metabolic shift towards energy production for viral replication, coupled with an early shutdown of the type I IFN response [39]. The host species also dictates the outcome. While rainbow trout are highly susceptible to J and M genogroups, Chinook salmon are more susceptible to the L genogroup. This host-specific susceptibility is correlated with the ability of the virus to replicate and induce an immune response, with steelhead trout showing considerable resistance to many IHNV strains [17, 23].

Co-infections with other pathogens, such as Infectious Pancreatic Necrosis Virus (IPNV), further complicate the immune landscape. The interaction between IHNV and IPNV is time-dependent and can be antagonistic or synergistic. In CHSE-214 cells, IPNV infection prior to or simultaneous with IHNV significantly inhibits IHNV replication by downregulating clathrin and dynamin-2, key host factors for IHNV entry [50]. This inhibition is mirrored in vivo, where early IPNV infection reduces IHNV replication and mortality [40]. However, other studies have shown that in a co-infected clinical outbreak, IHNV loads were significantly higher than IPNV loads, suggesting that the dominant interaction can shift under different conditions or host states [48]. Furthermore, co-infection with sea lice (Lepeophtheirus salmonis) in sockeye salmon reduces survival and exacerbates osmoregulatory dysfunction, an effect linked to IHNV-induced downregulation of mhc I [49]. These findings underscore that the host immune response is rarely a response to a single pathogen, but rather a complex balancing act influenced by the entire microbial and parasitic community.

Vaccination Strategies and Immunoprophylaxis against Infectious Hematopoietic Necrosis Virus

The global salmonid aquaculture industry, a multi-billion dollar enterprise producing over three million tons annually, is perennially threatened by Infectious Hematopoietic Necrosis Virus (IHNV), a rhabdovirus listed by the World Organisation for Animal Health (WOAH) due to its devastating impact on both wild and farmed salmon and trout populations [3, 5, 27]. The pathogen is responsible for acute epizootics with mortality rates frequently exceeding 90% in juvenile fish, particularly in rainbow trout (Oncorhynchus mykiss) and various Pacific salmon species (Oncorhynchus spp.) [3, 4, 12]. Despite decades of research, the control of IHN remains a formidable challenge, as there is currently no globally approved, broadly commercialized vaccine, and therapeutic options are limited [5, 52, 63]. The sole licensed DNA vaccine (APEX-IHN®) is approved only in Canada, leaving vast aquaculture sectors in China, Europe, and the Americas reliant on stringent biosecurity, early detection via molecular assays like RT-qPCR or isothermal amplification, and depopulation protocols [6, 27, 63]. Consequently, the development and optimization of effective vaccination strategies, encompassing DNA, inactivated, live-attenuated, subunit, and viral-vectored platforms, represent the most critical frontier for sustainable immunoprophylaxis against IHNV.

DNA Vaccines: The Vanguard of IHNV Immunoprophylaxis

DNA vaccines encoding the viral glycoprotein (G) have emerged as the most potent and extensively studied platform for IHNV control, largely due to their ability to induce robust, long-lasting protective immunity in salmonids. The G protein is the primary antigenic determinant, responsible for viral attachment and fusion, and contains neutralizing epitopes that are the principal targets of the host protective immune response [15, 27, 32]. The foundational APEX-IHN® vaccine, based on a genogroup J strain, has demonstrated remarkable efficacy in reducing mortality, often achieving relative percent survival (RPS) values exceeding 90% under controlled laboratory conditions [27]. However, as with any antigen-specific platform, the degree of protection afforded by monovalent DNA vaccines is intricately linked to the genetic homology between the vaccine strain and the circulating field isolate. This specificity was underscored by studies examining cross-genogroup protection in China, where both the endemic J genogroup and an emerging U genogroup (typified by the BjLL isolate) are now prevalent. Huo et al. demonstrated that a DNA vaccine encoding the G gene of the GS2014 strain (genogroup J) provided superior homologous protection and, critically, exhibited cross-protective efficacy against the heterologous U genogroup challenge. In contrast, the U genogroup DNA vaccine and a divalent formulation (J + U) were comparatively less effective against the U genogroup virus, suggesting that the J genogroup G protein may present a broader array of conserved immunogenic epitopes [15]. This finding has significant implications for vaccine design in regions like China, where viral evolution and the introduction of new genogroups from North America via the movement of eyed eggs or broodstock are well-documented [4, 15, 28].

Beyond the canonical genogroup specificity, recent work has illuminated critical nuances in the mechanism of DNA vaccine action. While the induction of neutralizing antibodies is a hallmark of adaptive immunity, the kinetics and magnitude of this response are variable. Huo et al. observed that DNA vaccine immunization did not necessarily promote serum antibody titers at the early stage of immunization, despite activating the interferon-inducible Mx1 gene in muscle tissue [15]. This suggests that the early protective phase, often evident within days of vaccination, is driven predominantly by the innate antiviral state, mediated by type I interferons (IFNs) and downstream effectors like Mx proteins, rather than humoral immunity. This dual-phase protection is a defining feature of IHNV DNA vaccines: an immediate, non-specific antiviral response followed by a durable, specific B- and T-cell memory.

Cross-Protection and the Challenge of Diverse Genogroups

The global distribution of IHNV is characterized by five major genogroups, U, M, L, E, and J, each with distinct host tropisms and virulence profiles [21, 29]. In North America, the U and M genogroups co-circulate in the Columbia River Basin, where they present differential risks to Chinook salmon (O. tshawytscha) and steelhead trout (O. mykiss), respectively [23, 31]. In Europe, the E genogroup predominates, while the J genogroup is endemic to Japan and has become the dominant lineage in China, evolving into a distinct JC subgroup from imported Nagano strains [10, 28, 29]. This genetic diversity presents a formidable obstacle to universal vaccination. Kim et al. provided compelling evidence that cross-protection is not a given, even among closely related rhabdoviruses. In a comparative study, rainbow trout immunized with a Viral Hemorrhagic Septicemia Virus (VHSV) G protein-expressing DNA vaccine were fully protected against VHSV but not against IHNV, despite both being novirhabdoviruses [60]. In stark contrast, a single-cycle (G-gene deleted) VHSV genotype IVa vaccine (rVHSV-ΔG) conferred significant cross-protection against IHNV. The rVHSV-ΔG vaccine, which undergoes a single round of infection without producing progeny, likely stimulates a broader and more potent cellular immune response, including cytotoxic T lymphocytes (CTLs) and potentially innate immune memory (trained immunity), than a non-replicating DNA plasmid. The authors noted that the scarcity of shared CTL epitopes between VHSV and IHNV suggests that non-adaptive mechanisms, such as the epigenetic reprogramming of innate immune cells, may be central to this heterologous protection [60].

Inactivated and Adjuvanted Vaccines: Prolonging the Window of Protection

Historically, inactivated (killed) vaccines against IHNV were considered safe but poorly immunogenic, typically conferring robust but short-lived protection, often waning within 60 days post-vaccination (dpv) [52]. The primary challenge has been to overcome the limited duration of immunity and to enhance the activation of both the innate and adaptive arms of the piscine immune system. A landmark study by Lin et al. addressed this by systematically screening four commercial adjuvants for their ability to potentiate a formalin-inactivated IHNV vaccine. They identified the water-based adjuvant Montanide GEL 02 PR (Gel 02) as a superior immunostimulant. The Gel 02-adjuvanted inactivated vaccine not only provided strong early protection (89-100% RPS within 2 months) but, critically, extended protection to at least 285 dpv with an RPS of 60.79% at the higher dosage. This represents a paradigm shift, demonstrating that through careful adjuvant selection, a killed-virus platform can achieve the long-term immunological memory necessary for a commercially viable product in the multi-year production cycle of salmonids [52].

A fascinating and counterintuitive observation from the Lin et al. study was the late antiviral response in vaccinated fish that lacked detectable neutralizing antibodies. After 60 dpv, despite undetectable serum neutralizing titers, the Gel 02-vaccinated fish remained resistant to challenge. This contradicts the mammalian immunological dogma where long-term protection is typically antibody-dependent, and instead suggests a unique feature of the fish adaptive immune system. The authors posited that protection at later time points may rely on cellular immunity, including memory CTLs and long-lived plasma cells residing in lymphoid tissues like the spleen and kidney, which are not reflected in circulating antibody levels [52]. This finding underscores the necessity of using functional challenge models rather than relying solely on serological correlates to evaluate vaccine efficacy in teleosts.

Synergistic Strategies: Plant Polysaccharides and Traditional Chinese Medicine

Adjuvants are not restricted to synthetic formulations; immunostimulatory compounds from natural sources have shown remarkable potential to augment the efficacy of inactivated vaccines. Pan et al. demonstrated that combining the IHNV inactivated vaccine with Astragalus polysaccharide (APS) significantly enhanced protection in a mechanistic fashion. Transcriptomic analysis of the spleen revealed that the APS+vaccine group exhibited a profound upregulation of hub genes central to antiviral immunity, including IgM, IgT, CD4, CD8, IRF7, and TLR7. Histopathological and TUNEL assays confirmed that this combination dramatically inhibited splenic apoptosis and tissue damage caused by IHNV, while simultaneously bolstering the activity of antioxidant enzymes (SOD, CAT) and acid/alkaline phosphatases [55]. This work illustrates a potent synergy: the inactivated vaccine provides the antigenic template for specific immunity, while APS acts as a molecular adjuvant, fine-tuning the innate and adaptive response to both neutralize the virus and mitigate the immunopathology that contributes to mortality.

Live Attenuated and Vector-Based Vaccines: Mimicking Natural Infection

Live attenuated vaccines (LAVs) offer the theoretical advantage of inducing a robust, multi-faceted immune response that closely mimics natural infection, often with a single dose and without the need for adjuvants. However, the primary concern with LAVs is the risk of reversion to virulence. A rational approach to attenuation was taken by Li et al., who used reverse genetics to introduce specific alanine substitutions in the nucleoprotein (N) of a virulent IHNV strain (HLJ-09). By targeting residues 85 and 102, which are critical for virulence, they engineered recombinant viruses (e.g., rIHNV-N85, rIHNV-N102) that were significantly attenuated (survival rates of 52.5-55% vs. 10% for wild-type) while retaining full immunogenicity. These mutant viruses elicited higher and earlier expression of IFN1, IL-1β, and anti-IHNV IgM antibodies compared to the wild-type, suggesting that they are powerful vaccine candidates that strike a favorable balance between safety and efficacy [24].

Viral Vectors: Adenovirus and Salmonella as Delivery Platforms

The use of replication-defective viral vectors provides a safety-catch mechanism, delivering the antigenic payload without the risk of dissemination. Adenovirus vectors have proven highly efficacious in fish. Li et al. demonstrated that a single immersion vaccination with a recombinant adenovirus expressing the IHNV G protein (rAd-G) conferred an RPS of 92% against a lethal challenge. This is particularly significant because immersion is a non-stressful, scalable route suitable for mass vaccination of fry, in contrast to labor-intensive intraperitoneal injection. The vector drove expression of TLR3, TLR7, TLR8, IFN1, and Mx genes, followed by a sustained adaptive response marked by CD4, CD8, IgM, and IgT upregulation [62]. Extending this concept, Li et al. developed a bivalent adenovirus vaccine co-expressing the IHNV G and the infectious pancreatic necrosis virus (IPNV) VP2 proteins. Given the high prevalence of IHNV-IPNV co-infections in aquaculture, this bivalent approach is highly pragmatic. The bivalent vaccine induced high levels of neutralizing antibodies against both viruses and provided an RPS of 81.25% against IHNV and 78.95% against IPNV, demonstrating that the adenovirus platform can accommodate and effectively deliver multiple large antigens simultaneously without immunological interference [14].

For oral vaccination, the holy grail of piscine immunoprophylaxis, bacterial vectors offer a compelling solution. Jalali et al. utilized attenuated Salmonella enterica serovar Typhimurium as a live carrier to deliver a eukaryotic expression plasmid (pcDNA3.1-IHNG) encoding the IHNV glycoprotein. Upon oral administration, the Salmonella vector invaded the gut-associated lymphoid tissue (GALT), delivering the plasmid directly to host cells. The vaccine was detected in various tissues up to 45 dpv and induced a strong transcriptomic response, upregulating ifn-1, mx-1, igM, and igT, alongside elevated serum neutralizing antibodies. Although the RPS of 58.2% was lower than that of injected DNA vaccines, it represents a significant step toward a non-invasive, oral delivery strategy that can be administered with feed, drastically reducing handling stress and labor costs [1].

Subunit and Nanovaccines: Engineering for Stability and Immunogenicity

The development of affordable, stable, and safe subunit vaccines has been a long-standing goal. The IHNV truncated G protein expressed in E. coli has been shown to be immunogenic; when emulsified with Freund's incomplete adjuvant, it induced specific IgM antibodies and upregulated IFN1 and IL-8 in the head kidney and spleen, culminating in a 60% cumulative survival rate post-challenge [32]. While this demonstrates proof-of-concept, the reliance on harsh adjuvants limits its commercial applicability.

A revolutionary advance in this space is the use of self-assembling protein nanoparticles. Ahmadivand et al. designed a nanovaccine (FerritVac) by genetically fusing the IHNV glycoprotein to Helicobacter pylori ferritin, which spontaneously assembles into 24-mer nanocages. This platform is a masterpiece of biomimetic engineering: the ferritin scaffold confers extraordinary thermal and pH stability, resisting degradation in the harsh gastrointestinal environment, which is critical for oral delivery. The FerritVac nanoparticles were non-cytotoxic at high doses and potently upregulated innate antiviral genes (mx, vig1, ifit5, isg-15) in host macrophages. This nanoplatform addresses two major bottlenecks of subunit vaccines, instability and poor immunogenicity, by presenting the antigen in a dense, repetitive array that mimics a viral particle, thereby efficiently cross-linking B cell receptors and enhancing uptake by antigen-presenting cells [5].

The Critical Gap: Vaccination versus Transmission

Perhaps the most sobering finding from recent vaccine efficacy studies is the profound disconnect between protection against disease (mortality) and protection against transmission. The fundamental goal of herd immunity is to reduce the basic reproductive number (R0) of a pathogen to below 1, which requires vaccines to block not just clinical signs but also shedding and onward transmission. Doumayrou et al. and Jones et al. independently demonstrated that while DNA, inactivated, and attenuated vaccines all significantly reduced mortality in rainbow trout, they had a minimal impact on viral shedding kinetics. Vaccinated fish still shed substantial quantities of infectious IHNV, particularly when exposed to high viral dosages that mimic real-world outbreak conditions [2, 30].

In direct cohabitation experiments, the DNA vaccine reduced transmission from 100% in naive fish to only 50%, while inactivated and attenuated vaccines had little to no impact. This means that a vaccinated population, while appearing healthy, can still act as a silent reservoir, contaminating water sources and infecting vulnerable cohorts, including unvaccinated juveniles or wild fish [2]. The implication is profound: current IHNV vaccines are excellent therapeutics for individual fish but are inadequate as stand-alone tools for eradication or long-term field control. This work contributes to a growing body of evidence that future vaccine development must pivot from an exclusive focus on survival to a dual mandate of reducing both morbidity and virus shedding [2, 30].

Genetic Resistance as an Adjunct to

Antiviral Therapeutics and Control Measures for Infectious Hematopoietic Necrosis Virus

Infectious hematopoietic necrosis virus (IHNV), a member of the family Rhabdoviridae and genus Novirhabdovirus, represents one of the most significant viral pathogens confronting global salmonid aquaculture. The World Organisation for Animal Health (WOAH) lists IHNV as a notifiable pathogen, reflecting its capacity to cause epizootics with mortality rates frequently exceeding 90% in naive juvenile populations [3, 27]. The economic burden is substantial, with outbreaks in rainbow trout (Oncorhynchus mykiss), Chinook salmon (O. tshawytscha), and Atlantic salmon (Salmo salar) operations causing direct losses through mortality, as well as indirect costs associated with quarantine, depopulation, and trade restrictions. Despite decades of research, no globally licensed, universally effective therapeutic agent exists for IHNV [5, 63]. Consequently, control measures have historically relied upon a combination of biosecurity protocols, stamping out of infected cohorts, and increasingly, vaccination strategies. However, a paradigm shift is underway, driven by advances in vaccinology, the discovery of novel small-molecule antivirals, the exploration of immunomodulatory natural products, and a deeper understanding of host genetics and viral pathogenesis. Present-day approaches are moving toward integrated control frameworks that combine prophylactic immunization with strategic antiviral therapies and selective breeding for resistance.

Vaccination Strategies: From DNA to Nanotechnology

The development of effective vaccines against IHNV has been a central focus of fish health research for decades, given the impracticality and economic infeasibility of treating viral infections in large aquaculture settings. The only commercialized vaccine for IHNV is a DNA vaccine based on the viral glycoprotein (G) gene, approved in Canada for use in Atlantic salmon [27, 63]. This platform leverages the ability of plasmid DNA encoding the G protein to induce robust innate and adaptive immune responses. The G protein is the primary target for neutralizing antibodies, and its expression within host cells following intramuscular injection triggers a potent type I interferon (IFN) response, upregulation of Mx proteins, and the generation of specific IgM antibodies [1, 15]. Studies have consistently shown that DNA vaccines provide high relative percent survival (RPS) rates. For instance, a DNA vaccine encoding the G gene of the J genogroup (GS2014) provided superior protection compared to a U genogroup vaccine in cross-genogroup challenges in China, achieving RPS values greater than 80% in some trials [15]. Similarly, an oral DNA vaccine delivered via attenuated Salmonella enterica serovar Typhimurium achieved an RPS of 58.2% in rainbow trout, demonstrating the feasibility of non-invasive delivery routes [1]. This bacterial vector approach is particularly promising because it stimulates both mucosal and systemic immunity, and the antigen gene (pcDNA3.1-IHNG) was detectable in multiple tissues up to 45 days post-vaccination [1].

Despite these successes, significant challenges remain. A critical and often overlooked aspect of vaccine efficacy is the impact on viral transmission, as distinct from protection against clinical disease and mortality. Seminal work by Doumayrou et al. [2] directly compared the efficacy of three vaccine regimens, DNA, inactivated, and attenuated, against IHNV transmission potential in rainbow trout. While all three vaccines significantly reduced mortality, their effect on viral shedding was surprisingly minimal. Cumulative shedding of infectious virus was only marginally reduced compared to unvaccinated controls. The DNA vaccine reduced direct fish-to-fish transmission from 100% to 50% in cohabitation experiments, whereas the inactivated and attenuated vaccines had negligible impact on transmission [2]. This finding has profound implications for field efficacy, suggesting that vaccinated fish, while surviving infection, may still act as shedders and sustain viral circulation within the population. The study further indicated that exposure dosage modulates shedding; at higher viral challenge doses, even DNA-vaccinated fish shed substantial quantities of virus [30]. These data underscore the necessity of developing next-generation vaccines that not only prevent disease but also block transmission, a concept analogous to "sterilizing immunity" in mammalian vaccinology.

In response to the limitations of current injectable platforms, researchers have pursued alternative delivery systems and antigen formulations to enhance mucosal immunity and stability. Oral vaccines represent an attractive avenue for mass vaccination of fry, eliminating handling stress. Shao et al. [64] evaluated four delivery vectors, poly(lactic-co-glycolic acid) (PLGA), chitosan, sodium alginate, and MONTANIDE™ GR01, for an oral DNA vaccine. MONTANIDE™ GR01 emerged as the most effective, significantly inhibiting IHNV proliferation and improving survival, likely due to its ability to protect the DNA from gastric degradation and enhance uptake by gut-associated lymphoid tissue [64]. Inactivated vaccines, while safer than live-attenuated versions, have historically suffered from short duration of immunity. However, Lin et al. [52] demonstrated that a formaldehyde-inactivated IHNV vaccine adjuvanted with Montanide GEL 02 PR (Gel 02) provided long-term protection, with an RPS of 89-100% within the first two months and extending to 60.79% at 285 days post-vaccination. Remarkably, this protection persisted even after neutralizing antibody titers had waned, indicating that cellular immune mechanisms or innate immune memory (trained immunity) may contribute to long-term defense [52]. This represents a pivotal advance over earlier inactivated vaccines that failed beyond 60 days.

The most innovative frontier in IHNV vaccinology is the application of self-assembling protein nanoparticles. Ahmadivand et al. [5] engineered a ferritin-based nanovaccine (FerritVac) by genetically fusing the IHNV glycoprotein to Helicobacter pylori ferritin. The resulting 24-mer nanocages demonstrated exceptional stability under harsh gastrointestinal conditions (mimicking the trout gut), no cytotoxicity in zebrafish cell lines up to 100 µg/mL, and potent upregulation of antiviral gene markers (mx, vig1, ifit5, isg-15) in host macrophages [5]. This platform has significant commercial potential due to its biocompatibility, stability, and suitability for oral administration, addressing a key bottleneck in fish vaccine development, the need for thermostable, deliverable formulations. Additionally, recombinant vector vaccines offer another strategy. Li et al. [14] developed a bivalent recombinant adenovirus vaccine co-expressing IHNV G and infectious pancreatic necrosis virus (IPNV) VP2 proteins. Immersion vaccination of juvenile trout induced neutralizing antibodies against both pathogens and provided an RPS of 81.25% against IHNV [14]. Such multivalent approaches are critical in aquaculture settings where co-infections (e.g., IHNV and IPNV) are common and cause compounded economic losses [40, 48].

Small-Molecule Antivirals and Immunomodulatory Agents

The development of direct-acting antivirals (DAAs) for IHNV is an area of intense investigation, driven by the need for therapeutic options to treat outbreaks when vaccination is not feasible or has failed. Screening of compound libraries has identified several promising candidates, often derived from natural products or repositioned drugs, that target specific stages of the viral life cycle. Bufalin, a traditional Chinese medicine derived from toad venom, emerged from a screen of 1,483 compounds as a potent IHNV inhibitor [63]. Bufalin exhibited a 50% inhibitory concentration (IC50) of 0.1223 µM against IHNV, with a selectivity index >163. Mechanistic studies revealed that bufalin blocks viral attachment and RNA replication stages, but not internalization, by targeting host cell Na+/K+-ATPase. The antiviral activity was dependent on extracellular Na+ and K+ concentrations, confirming the Na+/K+-ATPase as the molecular target. Crucially, bufalin significantly increased survival of rainbow trout in vivo and reduced viral load, making it a strong candidate for further development [63]. The targeting of a host factor, rather than a viral protein, may reduce the likelihood of resistance emergence, a principle increasingly embraced in antiviral therapy.

Coumarin derivatives have also shown considerable anti-IHNV activity. Hu et al. [45] synthesized 24 coumarin derivatives and identified imidazole coumarin C4, with an IC50 of 2.53 µM against IHNV glycoprotein. C4 significantly inhibited apoptosis and cellular morphological damage. A hydroxycoumarin derivative, D5 (7-[6-(2-methylimidazole) hexyloxy] coumarin), further demonstrated therapeutic potential [46]. D5 at 10 mg/L inhibited IHNV replication by >90% in epithelioma papulosum cyprini (EPC) cells, maintained mitochondrial membrane potential, and reduced reactive oxygen species. In vivo, intraperitoneal injection of D5 enhanced survival rates and upregulated interferon-related gene expression (ifn1, mx1, vig1), suggesting that its mechanism involves both direct antiviral effects and immunomodulation [46]. Ribavirin, a broad-spectrum nucleoside analog, was evaluated against IHNV and showed 99.88% inhibition of replication in EPC cells with an IC50 of 0.40 mg/L on glycoprotein expression [66]. Ribavirin was found to damage viral particles directly and could be used prophylactically, although its in vivo efficacy and safety in trout require further validation. Another synthetic approach involved arctigenin derivatives. A hybrid arctigenin-imidazole compound (derivative 15) with an eight-carbon linker reduced IHNV replication with an IC50 of 1.3 µM and acted primarily at the early replication stage without affecting adsorption [65].

Beyond synthetic and natural compounds, immunomodulatory interventions have been explored to bolster host antiviral defenses. Astragalus polysaccharide (APS), a bioactive component of the traditional Chinese herb Astragalus membranaceus, has been extensively studied. Pan et al. [55] demonstrated that APS combined with an IHNV inactivated vaccine significantly inhibited spleen damage and apoptosis in rainbow trout. The combination enhanced serum activities of superoxide dismutase (SOD), catalase (CAT), total antioxidant capacity (T-AOC), alkaline phosphatase (AKP), and acid phosphatase (ACP). Transcriptome analysis revealed enrichment of immune-related pathways, and protein-protein interaction networks identified hub genes including IgM, IRF7, IgT, IgD, TLR7, CD4, CD8, and IL-1β [55]. The APS-vaccine combination synergistically induced these genes and profoundly suppressed IHNV replication in the spleen. This work highlights the potential of vaccine-adjuvant combinations to achieve stronger and more durable immune responses. Similarly, a Chinese herbal medicine mixture (CHMM) containing multiple herbs enhanced T-SOD, CAT, ACP, and AKP activities, upregulated NF-κB, TNF-α, IFN-β, IL-1β, and heat shock proteins (HSP70, HSP90), while downregulating SOCS2, leading to reduced IHNV G protein expression at a dosage of 20 g/kg feed [58]. Crude lentinan (CLNT), a β-glucan from Lentinula edodes, also demonstrated antiviral effects by strengthening the intestinal immune barrier, promoting the growth of short-chain fatty acid-producing bacteria such as Carnobacterium, and increasing production of acetic, butanoic, and hexanoic acids in the gut [61]. These prebiotic-like effects highlight the importance of the gut-liver axis in antiviral immunity, a relatively underexplored area in fish health.

Autophagy Modulation and microRNA-Based Therapeutics

A deeper understanding of host-virus interactions has revealed novel targets for therapeutic intervention. Autophagy, a conserved cellular degradation process, plays a dual role in viral infections. Zhao et al. [44] demonstrated that IHNV glycoprotein can induce autophagy independently of viral infectivity. Using pepscan mapping, they identified peptide p108, derived from the glycoprotein, as a potent autophagy inducer. Pretreatment of EPC cells with p108 significantly inhibited intracellular viral mRNA replication and extracellular viral yields, indicating that autophagy induction can be harnessed as an antiviral strategy [44]. This finding opens the door for peptide-based therapeutics that modulate this pathway.

microRNAs (miRNAs) are emerging as critical regulators of the host response to IHNV. Several studies have identified miRNAs that either restrict or facilitate viral replication. Wu et al. [9] found that miR-206 targets receptor-interacting serine/threonine-protein kinase 2 (RIP2), a key adapter in the NOD-like receptor pathway. Silencing miR-206 in rainbow trout primary liver cells increased RIP2 and IFN expression and significantly decreased IHNV copy numbers, whereas overexpression of miR-206 had the opposite effect [9]. This suggests that inhibiting miR-206 could boost antiviral immunity. Conversely, Huang et al. [8] revealed that IHNV exploits host miR-146a-3p to promote its own replication. IHNV infection upregulated miR-146a-3p, which in turn targeted WNT3a and CCND1, both of which are novel inducers of type I IFN responses. By suppressing WNT3a and CCND1, miR-146a-3p dampens the interferon response, facilitating early viral replication [8]. Therefore, antagonizing miR-146a-3p represents a logical therapeutic approach. Similarly, novel-m0065-3p was shown to target interferon regulatory factor 7 (IRF7); overexpression of this miRNA reduced IRF7 expression and promoted liver cell proliferation while inhibiting apoptosis, and injection of agomiR-m0065-3p in vivo suppressed IRF7 expression [7]. Zhao et al. [38] further identified miR-330-y as a negative regulator of TAP1, a key antigen processing gene; overexpression of miR-330-y reduced TAP1, IRF3, and IFN levels, while inhibition had the opposite effect. These findings collectively indicate that anti-miRNA oligonucleotides (antagomiRs) or miRNA mimics could be developed as novel therapeutics to recalibrate the host immune response.

Host Genetics and Selective Breeding as a Control Measure

A complementary and increasingly important control strategy is the genetic improvement of host resistance through selective breeding. Classical genetic studies have established that resistance to IHNV is moderately heritable (h² = 0.15–0.25) and controlled by an oligogenic architecture with several moderate-effect quantitative trait loci (QTL) [26, 34]. Vallejo et al. [26] used genome-wide association studies (GWAS) in a commercial rainbow trout population from Clear Springs Foods, Inc. and identified 10 moderate-effect QTL that jointly explained up to 42% of the additive genetic variance. Genomic selection (GS) models, using the single-step genomic best linear unbiased prediction (ssGBLUP) method, substantially outperformed traditional pedigree-based predictions, with accuracy improving from 0.13–0.24 (pedigree) to 0.33–0.39 (genomic) [26]. Subsequent progeny testing confirmed that GS accuracy reached 0.38 with ssGBLUP, a 15% improvement over pedigree-based methods [47]. These results demonstrate that genomic selection is a cost-effective, sustainable approach for enhancing IHNV resistance over generations.

Further GWAS in two rainbow trout lines naive to IHNV confirmed the absence of major-effect loci and reiterated the oligogenic nature of resistance, with five QTL shared across lines, suggesting common genetic mechanisms [34]. Notably, some of the identified QTL regions contain candidate genes involved in antiviral immunity, such as those encoding interferon regulatory factors and toll-like receptors [34]. The application of marker-assisted selection or genomic selection, combined with vaccination, could create a synergistic effect, reducing both mortality and viral shedding. However, it is important to note that resistant lines may still become infected and shed virus, albeit at lower levels [30]. Therefore, genetic improvement should be integrated with other control measures rather than viewed as a standalone solution.

Biosecurity, Co-Infection Management

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