Infectious Salmon Anemia Virus

Overview and Taxonomy of Infectious Salmon Anemia Virus

Taxonomic Classification and Virus Morphology

Infectious salmon anemia virus (ISAV) is the etiological agent of infectious salmon anemia (ISA), a devastating multisystemic disease of farmed Atlantic salmon (Salmo salar) that is listed as a notifiable pathogen by the World Organisation for Animal Health (WOAH) due to its substantial economic and welfare impacts [1, 3, 7]. ISAV is classified within the family Orthomyxoviridae, a family that also includes the influenza A, B, C, and D viruses of mammals and birds, as well as the recently described quaranjaviruses and thogotoviruses. Within this family, ISAV constitutes the sole member of the genus Isavirus, with the species designation Isavirus salaris [9, 19]. This taxonomic placement is supported by the virus's shared orthomyxoviral characteristics: an enveloped virion ranging from 90 to 140 nm in diameter, a segmented single-stranded negative-sense RNA genome, and a nuclear replication strategy [19, 21].

Electron microscopic examination of purified ISAV particles reveals spherical to pleomorphic virions adorned with surface projections approximately 10–12 nm in length [19]. These projections consist of a single glycoprotein, the hemagglutinin-esterase (HE) protein, which is unique among orthomyxoviruses in that it combines both receptor-binding and receptor-destroying esterase activities within the same molecule [6, 19]. The viral envelope is derived from the host cell membrane during budding, and beneath it lies a matrix layer composed of the M1 and M2 proteins, which provide structural integrity to the virion [14]. The genome is encapsidated by the nucleoprotein (NP) and associated with the RNA-dependent RNA polymerase complex comprising the PB1, PB2, and PA subunits, forming ribonucleoprotein complexes essential for transcription and replication [15, 19].

Genomic Organization and Protein Functions

The ISAV genome consists of eight negative-sense RNA segments, designated segments 1 through 8 in order of decreasing size, with a total genomic length of approximately 14.3 kilobases [10, 19]. Each segment encodes one or more proteins. Segment 1 encodes the PB2 polymerase subunit; segment 2 encodes PB1; segment 3 encodes the PA subunit; and segment 4 encodes the nucleoprotein (NP) [10, 19]. Segment 5 encodes the fusion (F) glycoprotein, which mediates pH-dependent fusion of the viral envelope with the endosomal membrane during entry [16]. Segment 6 encodes the aforementioned HE glycoprotein, which is critical for host-cell attachment to 4-O-acetylated sialic acids on the surface of endothelial cells, epithelial cells, and erythrocytes, as well as for receptor destruction via its esterase activity [6, 19]. Segment 7 encodes the non-structural protein 1 (NS1) and the nuclear export protein (NEP), both of which play roles in modulating host innate immunity and facilitating viral ribonucleoprotein export from the nucleus [14]. Segment 8 encodes the matrix proteins M1 and M2, with M2 functioning as an interferon antagonist [10, 14].

The structural organization of ISAV parallels that of influenza viruses, but important distinctions exist. For instance, the ISAV HE protein is unique in combining hemagglutinin and esterase functions into a single polypeptide, whereas influenza viruses possess separate hemagglutinin (HA) and neuraminidase (NA) proteins. The ISAV F protein, like influenza's HA, is a class I viral fusion protein whose post-fusion core structure has been resolved at 2.1 Å resolution, revealing a carboxyl-carboxylate pH sensor that stabilizes the fusion-active conformation at low pH [16]. The receptor-destroying activity of HE has been shown to prune sialic acids from erythrocyte surfaces during infection, modulating host cell interactions and potentially influencing viral pathogenesis [6].

Genetic Diversity and Variant Classification

One of the most significant features of ISAV biology is the existence of two phenotypically distinct variants that differ dramatically in virulence and clinical outcome [4, 13]. The virulent form, known as ISAV-HPRΔ (or HPR-deleted), is responsible for clinical ISA outbreaks characterized by severe anemia, hemorrhagic necrosis of internal organs, and high mortality rates that can approach 100% in naive populations [4, 10]. The avirulent form, ISAV-HPR0, carries a full-length, non-deleted hypervariable region (HPR) in segment 6 and is not known to cause disease or mortality [4, 17]. HPR0 infection is typically subclinical, transient, and restricted primarily to gill tissue, resembling a respiratory infection analogous to seasonal influenza in humans [17].

The genetic basis for this phenotypic dichotomy lies largely in segment 6 and, to a lesser extent, segment 5. The HPR region of segment 6, which encodes part of the HE stalk domain, contains a series of repeated amino acid motifs. Deletions in this region, ranging from small indels to large deletions of up to 85 nucleotides, are the hallmark of virulent HPRΔ variants [13, 18, 20]. These deletions are thought to alter the conformational stability or functional accessibility of the HE protein, potentially shifting the balance between receptor binding and destruction to favor systemic spread [13]. Importantly, HPR0 is considered the ancestral or progenitor form from which HPRΔ variants emerge through deletion events, and the coexistence of both variants in asymptomatic carriers has been documented, suggesting a dynamic evolutionary continuum [18].

In addition to segment 6, segment 5 encoding the F protein carries a virulence-associated marker: the presence or absence of a specific insertion of 11 amino acids in the F protein near the fusion peptide region. Highly virulent strains such as those from the 2007–2009 Chilean epidemic (e.g., ISAV-HPR7b) possessed this insertion, whereas attenuated strains lacked it [13, 20]. The interplay between segment 5 and 6 polymorphisms appears to determine the overall virulence phenotype in a combinatorial manner [10, 13]. Synthetic reassortment experiments using reverse genetics have confirmed that segments 5 and 6 are critical for infectivity and plaque formation, and that segment 2 can also modulate the immune response [10]. Furthermore, whole-genome amplicon sequencing of field isolates from a Norwegian local epidemic revealed that segment reassortment occurs naturally between co-circulating lineages, contributing to genetic diversity [5].

Phylogenetic analyses based on segment 6 sequences have identified two major clades: a North American clade and a European clade [8, 19]. The European clade is further subdivided into genotypes, and viruses from this clade have been introduced to Chile, where they have caused the most severe outbreaks in the industry [13, 20]. ISAV isolates from the Faroe Islands have also been classified as European, and distinct HPR0 subtypes have been identified that show close genetic association with prior pathogenic outbreak strains, supporting the hypothesis that HPR0 can serve as a reservoir for the emergence of new virulent variants [17].

Economic and Regulatory Significance

ISA is one of the most economically consequential viral diseases in aquaculture. The disease first emerged in Norway in 1984, and since then has been reported in all major Atlantic salmon-producing regions, including Scotland, Canada, the Faroe Islands, Chile, and the United States [2, 3, 9, 12]. The 2007–2009 Chilean outbreak, which was caused by a highly virulent ISAV-HPR7b variant, led to the collapse of the country's salmon farming industry at the time, with losses estimated at over US$2 billion [13]. Outbreaks continue to occur sporadically; in Newfoundland and Labrador, annual incidence of ISAV detection on marine sites ranged from 3% to 29% between 2012 and 2020, with European clade variants predominating [8]. The presence of ISAV triggers strict regulatory responses including quarantine, depopulation, and fallowing, all of which impose enormous financial burdens on producers and regional economies [3, 11].

Because of its high virulence and transmissibility, ISAV-HPRΔ is subject to international reporting obligations under WOAH guidelines. In contrast, HPR0 is not considered a cause of disease, but its detection still has regulatory implications because it indicates the potential for emergence of pathogenic variants through deletion or reassortment [4, 17]. The development of rapid multiplex RT-qPCR assays that can discriminate HPRΔ from HPR0 in a single reaction has greatly enhanced the speed of outbreak response and surveillance efforts, enabling earlier containment measures [4].

Evolutionary Dynamics and Quasispecies Nature

ISAV, like other orthomyxoviruses, exists as a quasispecies in infected hosts, with a high mutation rate driven by the error-prone RNA-dependent RNA polymerase [20]. Selection pressures in the farm environment, such as host immune responses, vaccination, and population density, promote the emergence and fixation of novel variants. The coexistence of HPR0 and HPRΔ variants in asymptomatic fish, as documented in Chilean samples, suggests that a shift from avirulence to virulence can occur within individual hosts without overt disease signs, representing a cryptic reservoir for future outbreaks [18]. Additionally, the role of anthropogenic factors such as ship movements and the sharing of equipment between farms has been implicated in the long-distance dispersal of ISAV-HPRΔ, with network analyses demonstrating that inadequate disinfection of vessels can connect distant production areas [3].

In summary, the taxonomy and genetic architecture of ISAV reveal a virus that is both typical of the Orthomyxoviridae in its segmented genome and replication strategy, yet distinct in its use of a bifunctional HE protein and the unique HPR-based modulation of virulence. The ongoing evolution and reassortment of ISAV populations, combined with the global connectivity of salmon aquaculture, ensure that comprehensive genetic surveillance and rapid discriminative diagnostics remain essential components of disease control.

Genomic Structure and Genetic Diversity of ISAV

The genomic architecture of Infectious Salmon Anemia Virus (ISAV) is a defining characteristic that underpins its evolutionary potential, virulence mechanisms, and the challenges it poses to global Atlantic salmon aquaculture. As a member of the genus Isavirus within the family Orthomyxoviridae, ISAV shares a fundamental structural and replicative strategy with influenza viruses, yet it possesses unique genomic features that dictate its host range and pathogenicity [19]. This section provides an exhaustive examination of the ISAV genome, from its segmented negative-sense RNA organization and the functional roles of its encoded proteins to the intricate molecular mechanisms driving its genetic diversity, including the critical role of segment reassortment and the evolutionary dynamics between the avirulent HPR0 and the virulent HPRΔ variants.

The Segmented Genome and Functional Architecture

The ISAV genome consists of eight segments of linear, single-stranded, negative-sense RNA, with a total estimated size of approximately 14.3 kilobases [19]. Each segment is encapsulated by the nucleoprotein (NP) to form viral ribonucleoprotein (vRNP) complexes, which are the functional templates for both transcription and replication. The six largest segments each contain a single open reading frame (ORF), encoding the three polymerase subunits (PB1, PB2, PA), the NP, the fusion protein (F), and the hemagglutinin-esterase protein (HE) [14, 19]. The two smallest segments, segments 7 and 8, are more complex, each encoding multiple proteins via alternative splicing or overlapping reading frames. Segment 7 encodes the non-structural protein 1 (NS1) and the nuclear export protein (NEP), while segment 8 encodes matrix protein 1 (M1) and matrix protein 2 (M2) [14, 19]. This functional division highlights a sophisticated replication cycle where the polymerase complex and NP manage the genetic material, the surface glycoproteins (HE and F) mediate entry and egress, and the internal proteins orchestrate assembly, nuclear export, and immune evasion.

The HE protein, encoded by segment 6, is a multifunctional surface glycoprotein of profound importance. It possesses both receptor-binding and receptor-destroying enzyme (RDE) activities, a combination that classically defines the attachment and release strategy of orthomyxoviruses [6, 19]. The receptor for ISAV is 4-O-acetylated sialic acid, and the HE’s esterase activity cleaves this receptor, a function critical for viral release from infected cells and for preventing viral self-aggregation [6]. Recent work has demonstrated that the ISAV esterase actively prunes the surface of circulating erythrocytes in infected Atlantic salmon, removing the viral receptor and exposing underlying glycans. This global loss of vascular 4-O-acetylated sialic acids is a direct consequence of HE hydrolytic activity, as confirmed by the observation that a recombinant esterase-silenced HE mutant failed to induce this modulation [6]. This phenomenon suggests that the viral RDE does not merely serve a passive role in viral release but actively reshapes the host cell surface, potentially altering interactions with endogenous lectins and the host immune system [6]. The F protein, encoded by segment 5, is a class I viral fusion protein responsible for the pH-dependent fusion of the viral and endosomal membranes after endocytosis [16]. High-resolution crystallographic analysis of the ISAV F fusion core at 2.1 Å resolution has confirmed its typical class I architecture and identified a central coil conserved across many post-fusion viral glycoproteins [16]. The fusion process is tightly regulated; the F protein exists in a prefusion state complexed with HE and is triggered upon receptor binding and low pH in the endosome [16]. The electrostatic stability of the F protein is inversely correlated with pH, a feature mediated by a unique carboxyl-carboxylate pH sensor that stabilizes the post-fusion conformation at low pH [16].

The M1 protein, encoded by segment 8, and the NEP, encoded by segment 7, are critical for the nuclear export of newly synthesized vRNPs. Analogous to the mechanism in influenza A virus, the M1 and NEP proteins form a complex that bridges the vRNPs to the cellular nuclear export machinery. Molecular characterization has shown that ISAV M1 localizes to both the cytosol and nucleus, while NEP is predominantly cytosolic [14]. Crucially, co-expression experiments and pull-down assays have demonstrated that NEP interacts directly with both M1 and the host heat shock cognate protein Hsc70 [14]. This interaction is essential for the translocation of vRNP complexes from the nucleus to the cytoplasm, a critical step in the viral life cycle. The identification of Hsc70 as a host factor co-opted by the ISAV vRNP-M1-NEP complex underscores the virus’s reliance on cellular chaperone systems for efficient replication [14].

Segment 5 and 6: The Molecular Determinants of Virulence and Diversification

The preeminent drivers of ISAV genetic diversity and phenotypic variation are segments 5 and 6, which encode the F and HE proteins, respectively. Extensive global surveillance and reverse genetics studies have unequivocally demonstrated that the genetic makeup of these two segments is the primary determinant of viral virulence and infectivity [2, 10, 13]. The canonical distinction between the two major ISAV phenotypes, the avirulent ISAV-HPR0 and the virulent ISAV-HPRΔ, is defined by the sequence of segment 6. The HPR0 variant possesses a full-length, non-deleted hemagglutinin-esterase gene (the HPR region), while the HPRΔ variants are characterized by specific deletions in this region [4, 13, 17]. The deletion in the HPR region is considered a necessary, though not sufficient, step in the transition from a low-pathogenicity to a high-pathogenicity phenotype. However, it is not the sole determinant; the F protein on segment 5 also plays a critical and often synergistic role [10, 13, 18, 20].

Using synthetic reverse genetics to generate reassortant viruses, researchers have systematically dissected the contributions of each segment. Studies combining the highly virulent ISAV 752_09 (HPR7b) with the avirulent SK779/06 (HPR0) strain revealed that while multiple segments contribute to overall infectivity, segment 5 is of paramount importance [10]. For instance, the introduction of a virulent segment 5 into an HPR0 backbone conferred upon that virus the ability to produce plaques in CHSE-214 cells and replicate more efficiently, capabilities not observed with the parental HPR0 strain [10]. These experiments further demonstrated a high degree of genetic compatibility between the segments of HPR7b and HPR0, suggesting that reassortment events could readily generate novel viruses with unpredictable pathogenic potential [10]. The concept of a combinatorial effect between segments 5 and 6 is central to understanding ISAV virulence. Field observations in Chile following the devastating 2007-2009 outbreak have provided compelling real-world evidence. After the initial highly virulent HPR7b strain (which possessed both a segment 6 deletion and a specific insertion in segment 5) largely disappeared, a new HPR0 variant, lacking the segment 5 insertion, became prevalent. Later, when virulent outbreaks re-emerged in 2013 and 2014, the causative strains (HPR7a and HPR7b) exhibited variations in the presence of this segment 5 insertion, and differences in virulence were directly correlated with these genetic features [13, 20]. A theoretical integrative model has been proposed, where the specific pattern of deletions in segment 6 and the presence of an insertion in segment 5 form a molecular "code" that modulates the intensity of viral outbreaks [13].

The extensive genetic diversity within these two segments is not merely a laboratory curiosity but has profound implications for disease management and surveillance. The global genetic diversity of ISAV is largely defined by variations in segment 5 and 6 sequences, and epidemiological studies have shown that new variants are frequently reported in salmon-producing countries, with those emerging from segments 5 and 6 being the most common [2, 8]. This genetic variability necessitates robust and rapid diagnostic tools. The development of a multiplex RT-qPCR assay capable of simultaneously detecting all ISAV genotypes and discriminating between HPR0 and HPRΔ variants in a single reaction represents a significant advancement, allowing for faster response times in disease management [4].

Quasispecies Dynamics and Reassortment: Engines of Evolution

Beyond the point mutations and insertions/deletions in segments 5 and 6, ISAV evolution is driven by two other powerful forces: the quasispecies nature of its RNA genome and the capacity for genetic reassortment. As an RNA virus with an error-prone RNA-dependent RNA polymerase, ISAV exists within its host as a dynamic population of closely related but genetically distinct variants, known as a quasispecies [20]. This swarm of mutants provides a reservoir of genetic diversity that can be rapidly selected upon environmental or immunological pressure. The quasispecies theory explains how seemingly minor genetic variations can become dominant, leading to shifts in virulence and immune evasion. The variability in ISAV virulence markers observed in Chile, where different strains exhibited different combinations of deletions and insertions in segments 5 and 6 with no single stable "virulence signature," is a classic illustration of this dynamic evolutionary process [20].

Reassortment, the exchange of entire genomic segments between co-infecting viruses, is a hallmark of orthomyxoviruses and a major pathway for generating new and potentially pandemic strains. For ISAV, the potential for reassortment has been demonstrated both in the laboratory and in the field. Whole-genome amplicon sequencing of 12 different ISAV isolates from a local epidemic in northern Norway provided definitive evidence of natural reassortment events, allowing the identification of which specific segments had been exchanged between co-circulating strains [5]. The previously mentioned reverse genetics experiments have confirmed that there is a high degree of compatibility between the segments of different ISAV strains, including between virulent HPR7b and avirulent HPR0 viruses [10]. This compatibility is a latent risk factor, as reassortment could merge a segment 6 with a standard (HPR0) region from a non-virulent strain with, for example, a highly fusogenic segment 5 from a virulent strain, yielding a virus with a completely novel and unpredictable phenotype [10, 18]. Field evidence for this process has been observed in Chile, where asymptomatic fish were found to be co-infected with both an HPR0 variant and a highly pathogenic HPR7b variant, suggesting a natural and ongoing process of genetic mixing that could signal a shift toward pathogenicity [18].

The implications of this genetic dynamism are profound for fisheries management and regulatory frameworks. The high connectivity of farm networks, facilitated in part by ship movements, creates ample opportunities for co-infection and subsequent reassortment events to occur [3]. Understanding and tracking this genetic diversity is a cornerstone of modern disease control. Global efforts to catalog ISAV variants are critical for devising a universally accepted nomenclature for the virus, which is essential for comparing international surveillance data and customizing mitigation strategies [2]. The development of amplicon-based whole-genome sequencing protocols now enables high-resolution tracking of these evolutionary events, allowing for the early detection of reassortants and potentially dangerous new variants [5, 22]. This genomic surveillance is indispensable for informing international regulatory bodies, such as the World Organisation for Animal Health (WOAH), which lists ISAV as a notifiable pathogen, and for the design of effective, region-specific vaccination and biosecurity strategies.

Molecular Pathogenesis of Infectious Salmon Anemia Virus

The molecular pathogenesis of Infectious Salmon Anemia Virus (ISAV) is a remarkably intricate interplay between a segmented, negative-sense RNA virus and its host, the Atlantic salmon (Salmo salar). As a member of the Orthomyxoviridae family (genus Isavirus), ISAV shares fundamental virological features with influenza viruses, including a segmented genome, a reliance on sialic acid receptors, and the capacity for reassortment [10, 19]. However, its pathogenesis is distinguished by unique molecular adaptations, a nuanced dependence on environmental and nutritional cofactors, and a complex relationship with host cell death pathways that determines the trajectory from subclinical infection to fatal disease. The World Organisation for Animal Health (WOAH) classifies ISAV as a notifiable pathogen, underscoring its economic and epizootic significance. Understanding its molecular pathogenesis requires dissecting the virus’s entry strategy, its manipulation of host cellular machinery, the determinants of virulence encoded in its genome, and the molecular basis of host resistance and susceptibility.

Molecular Determinants of Viral Entry and Cellular Tropism

The initial steps of ISAV infection are governed by the coordinated action of two surface glycoproteins: the hemagglutinin-esterase (HE) protein, encoded by segment 6, and the fusion (F) protein, encoded by segment 5 [10, 16]. The HE protein serves a dual, seemingly paradoxical function: it binds to the host cell receptor, 4-O-acetylated sialic acid, and subsequently cleaves that same receptor via its receptor-destroying enzyme (RDE) activity, an esterase [6, 19]. This balance between attachment and detachment is critical for viral entry and spread. Recent work has demonstrated that the HE-mediated pruning of sialic acids from host cell surfaces is not merely a virological artifact but a profound modulator of the host cell surface. In infected Atlantic salmon, circulating erythrocytes progressively lose the ability to bind new ISAV particles due to HE esterase activity, which globally removes 4-O-acetylated sialic acids from vascular surfaces [6]. This process exposes underlying sugar moieties, such as those recognized by wheat germ agglutinin, potentially altering interactions with endogenous lectins and modulating vascular biology in ways that contribute to the anemia and circulatory collapse characteristic of ISA [6].

Following receptor binding, the virus is internalized via endocytosis. The F protein is a classical class I viral fusion protein, requiring a low-pH trigger within the endosome to undergo the conformational rearrangements that drive fusion of the viral and endosomal membranes [16]. The crystal structure of the ISAV F protein core at 2.1 Å resolution has revealed a carboxyl-carboxylate pH sensor that stabilizes the post-fusion conformation at acidic pH, a mechanism that is essential for productive infection [16]. This pH-dependent fusion strategy is a critical molecular switch, and its efficiency is modulated by the genetic compatibility between the F and HE proteins. Indeed, while segments 5 and 6 are recognized as major virulence determinants, their functional interaction, their compatibility at the molecular level, is a prerequisite for infectivity [10, 13]. Reassortment studies have demonstrated that a mismatch between the F and HE proteins from different strains can cripple viral entry, highlighting that pathogenesis is not simply a function of individual protein sequences but of the integrated function of the entry complex [10, 18].

The Genomic Architecture of Virulence: Segment 5, Segment 6, and the HPR0/HPRΔ Paradigm

The most significant axis of molecular pathogenesis in ISAV is defined by the distinction between the avirulent HPR0 variant and the virulent HPRΔ variants [4, 17]. This phenotypic dichotomy is primarily, though not exclusively, encoded in segment 6 (HE). HPR0 isolates possess a full-length, non-deleted highly polymorphic region (HPR) in the HE protein, whereas HPRΔ variants exhibit deletions of varying lengths in this region [13, 17]. The HPR0 variant is ubiquitous in farmed populations, causing a transient, non-clinical infection confined largely to the gill epithelium, a pattern reminiscent of seasonal influenza [17]. This suggests that HPR0 has a restricted tropism, likely unable to efficiently infect the endothelial cells that line the blood vessels, which are the primary target cells for HPRΔ-mediated systemic disease [19, 22]. The deletion in HPRΔ is thought to alter the receptor-binding specificity or avidity, enabling the virus to shift from a localized respiratory infection to a systemic, multi-organ infection targeting the kidney, liver, heart, and spleen [19, 22, 30].

However, segment 5 (encoding the F protein) plays an equally critical, and at times dominant, role in determining pathogenicity. Analysis of the devastating 2007-2009 Chilean ISA outbreak, caused by the HPR7b variant, revealed that a specific 22-amino acid insertion in the F protein was a key correlate of high virulence [13, 20]. Subsequent isolates from Chile, including later HPR7b strains that lacked this insertion, were associated with significantly lower mortality, demonstrating that the F protein insertion is a molecular switch for pathogenicity [20]. This finding aligns with the concept of a "combinatorial effect" between segments 5 and 6, where the full expression of virulence requires specific allelic combinations of both glycoproteins [13]. The presence of the segment 5 insertion appears to enhance fusogenicity, allowing the virus to spread more rapidly through the endothelial lining [20].

Furthermore, the virus exists as a quasispecies, and the coexistence of HPR0 and HPRΔ variants within the same asymptomatic fish has been documented, suggesting a dynamic and ongoing molecular evolution towards increased virulence [18]. This transition likely begins with the deletion in segment 6, followed by the acquisition of the segment 5 insertion, representing a stepwise molecular shift from an avirulent ancestor to a highly lethal epizootic strain [18]. The ability of the eight genomic segments to reassort, as evidenced by whole-genome sequencing of outbreak strains, adds another layer of complexity, allowing for the rapid generation of novel genotypes with unpredictable pathogenic properties [5, 10].

Host-Virus Interactions: Interferon Antagonism, Cell Death, and the Role of Oxidative Stress

Upon entry, ISAV must confront the host’s innate immune system, particularly the type I interferon (IFN) response. While Atlantic salmon IFNa1, IFNb, and IFNc can induce a classical antiviral state, ISAV has evolved sophisticated countermeasures [31]. The NS1 protein (from segment 7) and the M2 protein (from segment 8) are established antagonists of the type I IFN system [14]. This antagonism is partially effective in vitro, as ISAV replication is only transiently inhibited by IFN, with viral growth resuming 4-5 days post-infection, thereby explaining the limited protection conferred by IFN alone in standard cytopathic effect assays [31]. Interestingly, the expression of the HE protein itself can suppress plasmid-induced IFN-stimulated genes and chemokines, suggesting that the entry protein also plays a role in subverting the initial immune response at the site of infection [28].

The molecular pathogenesis of ISAV is also profoundly shaped by the mode of host cell death it triggers. This is strikingly cell-type dependent. In permissive cell lines like SHK-1 (derived from salmon head kidney), ISAV induces classical apoptosis, characterized by DNA fragmentation and caspase activation. This process is mediated, at least in part, by the direct binding of the viral PB2 protein and a protein from segment 7 to caspase-8 [21]. In contrast, infection of TO cells (a different salmonid cell line) leads to necrosis, characterized by the release of HMGB1 protein and a lack of apoptotic features [21]. This suggests that ISAV can manipulate the cell death machinery differently depending on the cell type, a feature that likely influences the nature of the ensuing inflammation and immune response in vivo.

A more recently elucidated and fascinating dimension of ISAV-induced cell death involves the interplay with host nutrition and oxidative stress. Infection triggers a strong respiratory burst and the production of reactive oxygen species (ROS), a process mediated by the viral nucleoprotein (NP) via the activation of the NADPH oxidase complex [15]. This oxidative stress, in turn, leads to the sumoylation of the NP itself, a post-translational modification that is essential for efficient viral progeny production [15]. Crucially, the composition of the host cell membrane, specifically the levels of polyunsaturated fatty acids (PUFAs) like eicosapentaenoic acid (EPA), can hijack this oxidative stress into a different cell death pathway: ferroptosis. High levels of membrane-incorporated EPA, combined with the ISAV-induced ROS formation, push cells into an iron-dependent, non-apoptotic cell death known as ferroptosis [1]. This mechanism appears to be a protective host response, as ferroptosis is a highly immunogenic and controlled form of cell death that can curtail viral replication [1]. This finding represents a paradigm shift, linking nutritional status (dietary EPA levels) to a specific molecular pathway of antiviral defense, and provides a molecular explanation for how functional feeds might modulate disease outcomes [1, 24, 25].

Molecular Correlates of Resistance and the Impact of Co-Infections

The molecular basis of host resistance to ISAV is not a classical antiviral response but rather a strategy of cellular resilience and controlled replication. Transcriptomic analyses comparing genetically resistant and susceptible Atlantic salmon families reveal that resistant fish are characterized by a coordinated downregulation of proteasome and translation initiation pathways, alongside upregulation of receptor internalization proteins like flotillins and NEDD4 E3 ubiquitin ligase [23]. This suggests resistant fish are not better at killing the virus but are better at tolerating its presence by limiting cellular damage and viral entry. A key genomic marker for resistance is a variant of the translation initiation factor EIF4G1 on chromosome 14, which is significantly downregulated in resistant fish, implying that a global reduction in protein synthesis may limit viral replication [23].

This resistance is profoundly temperature-dependent. At a lower temperature (10°C), susceptible fish show a dramatic activation of innate antiviral genes (e.g., mx1, isg15), but this response fails to protect them, leading to high mortality [23, 26]. At a higher temperature (20°C), mortality is paradoxically lower, and resistance is associated with a more tempered response and a faster shift towards adaptive immunity, including markers of Th1 and antigen-presenting cells [11, 26]. This indicates that the molecular pathogenesis of ISAV is thermodynamically gated; lower temperatures favor a futile, hyperinflammatory innate response, while higher temperatures permit a more effective, regulated immune clearance [23, 26].

Finally, the molecular landscape of ISAV pathogenesis is dramatically altered by co-infection with sea lice (Lepeophtheirus salmonis). The parasitic manipulation of the host immune system, characterized by the downregulation of key antiviral genes like Mx, MH class I β, and TRIM family members, creates a molecular window of vulnerability, making the host more susceptible to ISAV [27, 29]. Transcriptome analysis of co-infected fish reveals a massive upregulation of immune signaling pathways (Toll-like receptors, NOD-like receptors, type I interferon) that is not seen in single infections, coupled with suppression of complement activity [7, 25]. This dysregulated immune environment is the molecular underpinning of the significantly higher mortality rates observed in co-infected populations [27]. The molecular pathogenesis of ISAV, therefore, cannot be understood in isolation; it is a systems-level pathology shaped by the virus’s own genetic toolkit, the host’s genetic and nutritional background, the ambient temperature, and the wider microbial and parasitic community.

Host Innate Immune Responses and Modulation by ISAV

The host innate immune system constitutes the first line of defense against invading pathogens, and its interaction with Infectious Salmon Anemia Virus (ISAV) is a complex, multifaceted battleground that ultimately determines the trajectory of disease. As a member of the Orthomyxoviridae family, ISAV has evolved sophisticated strategies to subvert, evade, and even exploit the host's innate antiviral machinery, a dynamic that is central to its pathogenesis and the variable outcomes observed in infected Atlantic salmon (Salmo salar). Understanding these intricate host-pathogen interactions is not merely an academic exercise; it is fundamental to developing effective vaccines, selective breeding programs, and therapeutic interventions to mitigate the devastating economic and animal welfare impacts of this disease, which is notifiable to the World Organisation for Animal Health (WOAH).

The Interferon System: A Central Axis of Conflict

The type I interferon (IFN) system is the cornerstone of the antiviral innate immune response in vertebrates, and Atlantic salmon are no exception. Upon viral infection, pattern recognition receptors (PRRs) detect pathogen-associated molecular patterns (PAMPs), triggering a signaling cascade that culminates in the expression of IFN. This cytokine then acts in an autocrine and paracrine manner to induce hundreds of interferon-stimulated genes (ISGs) that establish an antiviral state. The relationship between ISAV and the IFN system is characterized by a transient and incomplete inhibition, a critical feature of its pathogenesis.

Early in vitro studies demonstrated that while ISAV replication is sensitive to the antiviral effects of recombinant Atlantic salmon type I IFN (IFNa1, IFNb, IFNc), this inhibition is not absolute. Using quantitative PCR and viral yield reduction assays, researchers showed that IFNa1 strongly suppresses ISAV replication in cell culture during the first 24 to 72 hours post-infection [31]. However, this inhibitory effect wanes significantly by four to five days post-infection, allowing viral replication to resume and ultimately leading to cytopathic effect (CPE) [31]. This transient nature of IFN-mediated control explains why earlier studies, which relied solely on CPE reduction assays, erroneously concluded that ISAV was resistant to IFN [31]. The virus does not completely block the IFN response; rather, it appears to delay or attenuate its effectiveness, creating a window for its own replication. This phenomenon is temperature-dependent, with higher temperatures (e.g., 20°C) slowing viral growth and allowing IFN to exert a more sustained protective effect compared to lower temperatures (e.g., 15°C) [31].

The virus’s ability to modulate the IFN response is encoded within its own genome. The NS1 protein, encoded by segment 7, is a well-characterized antagonist of the type I IFN system in ISAV, analogous to its role in influenza viruses [14]. Furthermore, the hemagglutinin-esterase (HE) protein itself may possess immunosuppressive properties. Transcriptomic analysis of a DNA vaccine against ISAV revealed that a plasmid expressing the HE protein (pHE) had a lower inductive effect on ISGs and chemokines compared to an empty plasmid control (pcDNA3.3) [28]. This suggests that the expression of HE actively inhibits the plasmid-induced innate immune response, a hypothesis supported by Mx-reporter assays [28]. This finding implies that the HE protein, beyond its role in receptor binding and destruction, may directly contribute to dampening the host's antiviral transcriptional program, a sophisticated strategy to facilitate viral establishment.

Transcriptional Landscapes of Resistance and Susceptibility

The outcome of ISAV infection is not uniform; it is heavily influenced by host genetics and environmental factors, particularly temperature. Large-scale transcriptomic studies have revealed starkly different molecular strategies between genetically resistant and susceptible families of Atlantic salmon, and these strategies are profoundly modulated by temperature.

A landmark study comparing resistant and susceptible families at 10°C and 20°C found that resistance mechanisms are dramatically temperature-dependent. At 10°C, resistant fish exhibited a coordinated downregulation of pathways related to proteasome activity, DNA replication, and translation initiation, alongside an upregulation of receptor internalization mechanisms involving flotillin proteins, β-arrestins, and NEDD4 E3 ubiquitin ligase [23]. This suggests that at lower temperatures, resistance is not about mounting a stronger classical antiviral response, but rather about employing a "cellular resilience" strategy, slowing down cellular machinery that the virus requires for replication and actively removing viral receptors from the cell surface. In stark contrast, at 20°C, the transcriptomic differences between resistant and susceptible families were minimal (only 64 DEGs vs. 3,690 at 10°C), indicating that high temperature itself may override or mask the genetic determinants of resistance [23]. This thermal modulation has profound implications for managing ISA under different environmental conditions, a concern echoed by the WOAH in its guidelines for aquatic animal health.

Further evidence for the importance of early, systemic recognition comes from a comprehensive family-wise challenge study. Fish that were phenotypically resistant to ISA (i.e., survived the disease) were not resistant to ISAV replication per se; they harbored viral loads as high as their susceptible counterparts in all organs screened [34]. The key difference was an early and robust transcriptional response in the kidney of resistant fish, characterized by the upregulation of antiviral, inflammatory, and endothelial growth factor genes [34]. This indicates that disease resistance is not about preventing infection or viral replication, but about mounting a rapid, controlled, and effective innate immune response that limits tissue damage and allows the host to endure the infection. The kidney, as a major hematopoietic and immune organ in fish, emerges as a critical site for this early recognition and response.

The Role of Co-Infections and Nutritional Modulation

In the natural environment, fish are rarely infected with a single pathogen. Co-infections, particularly with the ectoparasitic sea louse (Lepeophtheirus salmonis), are a major reality in salmon aquaculture and have a profound impact on the host's innate immune response to ISAV. Epidemiological studies have long suspected a link, and experimental challenges have confirmed that prior infestation with sea lice significantly increases the susceptibility of Atlantic salmon to ISAV, leading to higher mortality and faster death rates [27]. The immunological mechanism behind this phenomenon is a form of immune modulation. Sea lice infestation was shown to downregulate key antiviral genes, including Mx, MHC class I β, Galectin 9, and TRIM 16/25, in the period immediately before and after co-infection with ISAV [27]. This parasite-induced immunosuppression creates a permissive environment for the virus, effectively disarming the host's first line of defense.

Transcriptomic analyses of co-infected fish reveal a complex and dysregulated immune response. While a single infection with sea lice alone suppresses the innate immune system (e.g., the complement system), co-infection with ISAV triggers a strong but potentially pathological activation of immune pathways, including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and the interferon system [7]. This suggests that the host is trying to mount a response, but the prior manipulation by the parasite has skewed it in a way that is ineffective against the virus. The skin, as the primary site of sea lice attachment, also shows a distinct transcriptional profile during co-infection, with upregulation of glycolysis, interferon pathways, and complement, but downregulation of antigen presentation and T-cell activation pathways [25]. This localized immune dysregulation at the portal of entry may facilitate systemic viral spread.

Nutrition is another powerful modulator of innate immunity. The shift from marine-based to plant-based feeds in aquaculture has altered the fatty acid profile of salmon diets, with potential consequences for immune function. A study using Atlantic salmon kidney (ASK) cells demonstrated that high concentrations of the omega-3 fatty acid eicosapentaenoic acid (EPA) have a significant and independent effect on the gene expression of ISAV-infected cells [1]. While EPA alone had a limited impact on the innate immune system, it profoundly altered the response to viral infection. High EPA levels, in the context of ISAV infection, induced gene expression patterns associated with increased formation of oxygen radicals and activation of ferroptosis, a form of regulated cell death dependent on iron and lipid peroxidation [1]. This suggests that a diet rich in EPA may prime cells to undergo ferroptosis upon viral infection, a mechanism that could limit viral replication by eliminating infected cells in a controlled manner. Conversely, functional feeds enriched with specific fatty acid profiles have been shown to modulate the expression of antiviral genes like irf7b and mxb during co-infection with sea lice and ISAV, although the feeds most successful at reducing lice were often the least successful in promoting survival during subsequent viral co-infection [24]. This highlights the delicate balance required in formulating feeds to support overall health in a multi-pathogen environment.

Cellular Stress, Apoptosis, and the Role of microRNAs

The host's response to ISAV extends beyond classical immune signaling to encompass cellular stress pathways. The viral nucleoprotein (NP) itself has been shown to trigger a strong respiratory burst, activating the NADPH oxidase complex and leading to the production of reactive oxygen species (ROS) [15]. This oxidative stress, in turn, induces a shift in the host's post-translational modification profile, specifically an increase in SUMOylation (SUMO-2/3 conjugation) of cellular proteins [15]. This NP-mediated oxidative stress and subsequent SUMOylation are not merely bystander effects; they appear to be necessary for efficient viral progeny production, as blocking the NADPH oxidase complex or the p38MAPK signaling pathway significantly reduces viral yield [15]. This reveals a cunning viral strategy: the virus actively induces a state of cellular stress that it then hijacks to facilitate its own replication.

The mechanism of cell death induced by ISAV is also cell-type specific and has implications for pathogenesis. In SHK-1 and CHSE-214 cell lines, ISAV infection leads to apoptosis, a programmed and immunologically "silent" form of cell death characterized by DNA fragmentation and caspase activation [21]. In contrast, infection of TO cells results in necrosis, a more inflammatory form of cell death involving the leakage of cellular contents like HMGB1 protein [21]. The ISAV PB2 protein and proteins encoded by segment 7 were found to bind caspase-8 in vitro, suggesting a direct viral role in triggering the apoptotic cascade in certain cell types [21]. This differential induction of cell death pathways may influence the extent of tissue inflammation and the overall pathogenesis of the disease.

Finally, the host's innate response is also regulated at the post-transcriptional level by microRNAs (miRNAs). ISAV infection leads to the differential expression of a suite of host miRNAs in ASK cells [32, 33]. Among these, miR-148a/b and miR-152 have been identified as putative anti-viral miRNAs, as they are predicted to directly target viral genes such as HA, P3, and NP [32]. Conversely, miR-462a-5p is upregulated, while miR-125b-5p is downregulated during infection [33]. The functional consequences of these miRNA changes are still being elucidated, but they clearly represent an additional layer of complexity in the host-virus arms race, where the host attempts to use small RNAs to silence viral gene expression, and the virus may in turn evolve mechanisms to subvert this pathway. The use of RNA interference (RNAi) as a therapeutic strategy has been explored, with inactivated E. coli producing double-stranded RNA (dsRNA) targeting the HE gene showing antiviral activity when added to infected ASK cells [35], highlighting the potential for harnessing this innate mechanism for disease control.

Epidemiology and Global Distribution of ISAV Outbreaks

Infectious salmon anemia virus (ISAV), the etiological agent of infectious salmon anemia (ISA), represents one of the most economically consequential pathogens in global salmonid aquaculture. Designated as a notifiable pathogen by the World Organisation for Animal Health (WOAH), ISAV has been responsible for devastating epizootics across multiple continents, with mortality rates reaching 100% in naïve populations. Understanding the epidemiology and global distribution of ISAV requires a multi‑faceted analysis encompassing phylogenetic divergence, transmission networks, environmental drivers, host–pathogen interactions, and the complex interplay between virulent and avirulent variants. This section provides an exhaustive examination of the spatiotemporal patterns, risk factors, and molecular epidemiological features that define ISAV’s global footprint.

Historical Emergence and Global Dissemination

The first recorded ISA outbreaks occurred in Norway in the mid‑1980s, although retrospective analyses suggest the virus may have circulated subclinically for years prior [19]. From Scandinavia, ISAV rapidly disseminated to other major salmon‑producing regions, including Scotland (1998), the Faroe Islands (early 2000s), Canada (New Brunswick in 1996 and Newfoundland in 2012), the United States (Maine), and Chile (2007). The Chilean outbreak of 2007–2010 was particularly catastrophic, nearly collapsing the country’s booming salmon industry and resulting in estimated losses exceeding USD 2 billion [13, 20]. This event underscored the vulnerability of intensive aquaculture systems to emerging viral pathogens and highlighted the critical need for robust epidemiological surveillance.

Phylogenetic analyses consistently classify ISAV isolates into two major clades: the North American clade and the European clade. The European clade includes isolates from Norway, Scotland, the Faroe Islands, and Chile, while the North American clade encompasses isolates from Canada (New Brunswick and Newfoundland) and the northeastern United States [8, 19]. Notably, Chilean isolates belong to European genotype I, strongly suggesting that the virus was introduced to South America via infected smolts or contaminated biological materials from Europe, most likely in the mid‑1990s [20]. This introduction event, followed by local adaptation and subsequent emergence of highly virulent variants, exemplifies the risks associated with global movement of live fish and germplasm.

Genetic Diversity and the HPR0–HPRΔ Paradigm

A defining feature of ISAV epidemiology is the existence of two phenotypically distinct variants: the avirulent HPR0 and the virulent HPRΔ (also referred to as HPR‑deleted). The HPR0 variant possesses a full‑length hemagglutinin‑esterase (HE) gene (segment 6) and is not associated with clinical disease or mortality. In contrast, HPRΔ variants carry characteristic deletions in the highly polymorphic region (HPR) of segment 6, and these deletions are strongly correlated with virulence [4, 10, 13]. However, deletions in segment 6 alone are not sufficient for full pathogenicity; molecular analyses have demonstrated that an insertion in segment 5 (encoding the fusion protein) is also required for the high‑virulence phenotype observed in devastating outbreaks such as the 2007 Chilean epizootic [13, 20]. This combinatorial effect, a deletion in segment 6 combined with a specific insertion in segment 5, has been proposed as an integrative model that explains the wide spectrum of virulence observed in the field [13].

The HPR0 variant is now recognized as the ancestral, avirulent precursor from which pathogenic HPRΔ strains evolve. Field surveillance in the Faroe Islands revealed that HPR0 is highly prevalent among apparently healthy fish, with a mean time to first detection in marine sites of 7.7 months after seawater transfer, suggesting an unknown marine reservoir [17]. Importantly, HPR0 infections are transient and seasonal, causing no detectable mortality or pathology, and display strong gill tropism, analogous to a subclinical respiratory infection in mammals [17]. The detection of both HPR0 and HPRΔ variants coexisting in asymptomatic fish has been reported in Chile, providing direct evidence of ongoing evolutionary transitions that may precede clinical outbreaks [18]. This genetic plasticity, combined with the segmented nature of the ISAV genome, enables frequent reassortment events. Whole‑genome sequencing of isolates from a small local epidemic in northern Norway confirmed segment reassortment among co‑circulating strains, further complicating epidemiological tracking and virulence prediction [5]. Synthetic reassortant studies have shown that segment 5 is particularly critical for infectivity and plaque‑forming ability, while segments 2 and 6 also contribute to viral fitness and immune modulation [10]. These findings underscore the necessity of continuous molecular surveillance to detect emerging reassortants with pandemic potential.

Transmission Dynamics: Networks, Dispersal, and Shedding

ISAV transmission occurs primarily via horizontal routes, direct contact between fish, waterborne spread, and fomites including ships, equipment, and personnel. A network analysis of ship movements among Norwegian fish farms revealed exceptionally high connectivity: the largest strongly connected component encompassed at least 72% of farms, and the ship‑contact network was significantly associated with the spatiotemporal distribution of ISA cases. Inadequate disinfection of vessels could facilitate long‑distance transmission of ISAV‑HPRΔ across distant production areas, emphasizing the role of anthropogenic movement in viral dissemination [3].

Waterborne dispersal has been rigorously modelled using ocean circulation and particle‑tracking frameworks. In the Quoddy region (New Brunswick, Canada, and Maine, USA), simulations incorporating virus inactivation by ultraviolet radiation and microbial communities demonstrated that ISAV concentration varies spatiotemporally with outbreak progression, current speed, tidal amplitude, and environmental decay. Connectivity among farm sites was asymmetrical and strongly influenced by seaway distance, yet under certain conditions, long‑distance transmission remained plausible [37]. Coupling these dispersal models with wild Atlantic salmon post‑smolt migration models indicated that wild fish could encounter relatively high viral concentrations, although infection probability under a strict minimum infectious dose threshold was low [38]. Nevertheless, reliance on laboratory‑derived thresholds may underestimate real‑world infection risk, particularly when host susceptibility is heightened by co‑infections or environmental stressors [38].

Quantification of viral shedding using droplet digital PCR in seawater revealed that ISAV RNA is first detectable two days post‑challenge, peaking one day before mortality onset in high‑dose challenges. This large shedding event prior to death represents a critical window for transmission and underscores the value of water‑based surveillance for early outbreak detection [39]. The rapid kinetics of shedding, combined with the potential for fomite‑mediated spread, necessitate stringent biosecurity protocols, including the use of accelerated hydrogen peroxide disinfectants diluted in seawater, which have demonstrated efficacy against a surrogate virus at low temperatures [40].

Risk Factors: Co‑Infection, Temperature, and Host Genetics

Epidemiological studies consistently identify sea lice (Lepeophtheirus salmonis) infestation as a major risk factor for ISA outbreaks. Controlled experimental infections demonstrated that Atlantic salmon infested with sea lice prior to ISAV exposure experienced significantly higher cumulative mortality and faster death rates compared to fish infected with ISAV alone. This effect was consistent across two different host strains (Penobscot and Saint John River) and was associated with downregulation of antiviral genes (Mx, MHC class I β, galectin‑9, TRIM16/25) in lice‑infested fish [27, 29]. Transcriptomic analyses of co‑infected head kidney revealed 994 differentially expressed genes, with co‑infection triggering strong activation of Toll‑like and NOD‑like receptor pathways, interferon signaling, and MHC class I antigen presentation, yet this immune activation was insufficient to offset the immunosuppressive effects of lice [7]. Skin transcriptomic profiling further showed that co‑infection with sea lice and ISAV upregulated glycolysis, interferon pathways, and complement cascade, while downregulating antigen presentation and T‑cell activation [25]. These data establish sea lice as true epidemiological synergists that enhance viral susceptibility and severity.

Water temperature profoundly modulates ISAV infection dynamics. Experimental challenges at 10 °C versus 20 °C revealed that despite higher acute mortality at 20 °C, cumulative mortality was significantly greater at 10 °C, and viral clearance occurred more rapidly at the higher temperature [26]. Transcriptomic analyses of resistant and susceptible Atlantic salmon families at these two temperatures identified 3,690 differentially expressed genes between resistance groups at 10 °C but only 64 at 20 °C, indicating that thermal conditions fundamentally alter the molecular basis of host resistance. At 10 °C, resistance was associated with coordinated downregulation of proteasome, DNA replication, and translation pathways, coupled with upregulation of receptor internalization machinery (flotillins, β‑arrestins, NEDD4). This suggests that resistant fish employ a broad cellular resilience strategy rather than classical antiviral responses, and that this strategy is temperature‑dependent [23]. Genome‑wide association analysis identified the translation initiation factor EIF4G1 on chromosome 14 as a candidate resistance marker, significantly downregulated in resistant fish [23].

Host genetics play a substantial role in ISA outcomes. In a transmission trial comparing families with high, mid, and low genomic estimated breeding values (GEBV), neither vaccination nor selective breeding completely prevented transmission, but both reduced the probability of infection in contact fish. Genetic resistance had a larger effect (odds ratio 8.35) than vaccination (odds ratio 4.52), though differences were not statistically significant. Importantly, low‑GEBV fish died 14 days earlier than high‑GEBV fish, indicating that endurance, the ability to survive prolonged viral replication, is a key component of genetic resistance [11]. A separate study of 40 North American families challenged with ISAV revealed that the most resistant families had mean cumulative mortality of 34.3% compared to 78.5% in the most susceptible families. Phenotypic resistance was not associated with reduced viral load; rather, resistant fish showed early upregulation of antiviral, inflammatory, and endothelial growth factor genes in the kidney, highlighting the importance of early systemic recognition for disease mitigation [34].

Global Distribution Patterns and Surveillance

Systematic surveillance data from Newfoundland and Labrador (2012–2020) provide insights into the regional epidemiology of ISAV. Site‑level annual incidence risk ranged from 3% to 29%, with European clade variants more common than North American clade variants. Time to first detection and time to depopulation were not associated with clade type, suggesting that control measures are equally effective regardless of the genetic lineage [8]. In Norway, despite stringent control programs, outbreaks continue to occur, and the ship‑movement network remains a persistent risk [3]. In Chile, after the devastating 2007–2010 epizootic, the highly virulent HPRΔ variants seemingly disappeared, but HPR0 variants persisted and new HPRΔ strains with varying virulence, such as HPR7a and HPR7b, have re‑emerged in subsequent years, indicating that HPR0 acts as a genetic reservoir from which pathogenic strains can evolve [13, 17, 18, 20].

Rapid, accurate detection is paramount for effective control. A novel multiplex RT‑qPCR assay capable of simultaneously detecting all ISAV strains, discriminating HPRΔ from HPR0, and validating RNA extraction in a single reaction has been developed and validated against 31 international strains, offering a significant improvement over sequencing‑dependent methods [4]. Whole‑genome amplicon sequencing protocols now enable routine genomic surveillance, as demonstrated in the Norwegian local epidemic where reassortment was identified [5]. These molecular tools, combined with enhanced water‑based sampling using droplet digital PCR [39] and environmental dispersal models [37, 38], provide a comprehensive toolkit for real‑time epidemiological monitoring.

The interplay between wild and farmed salmon populations remains a critical unknown. Mathematical models of a two‑patch system (wild and farmed) coupled by linear viral diffusion show that if the basic reproductive ratio (R₀) exceeds one, the virus persists in both patches. Interestingly, positive diffusivity can allow viral persistence even when only one patch contains susceptible hosts, highlighting the potential for wild reservoirs to sustain ISAV in the absence of aquaculture [41]. Although HPR0 has not been detected in eggs or progeny of infected broodstock, suggesting negligible vertical transmission [36], the marine reservoir hypothesis remains open, as evidenced by the consistent 7.7‑month lag before infection in Faroese marine sites [17].

In summary, the epidemiology of ISAV is shaped by a tripartite interplay of viral genetic diversity (HPR0→HPRΔ transitions and reassortment), anthropogenic transmission networks (ship movements, poor biosecurity), and environmental–host factors (temperature, co‑infection, host genetics). The global distribution of ISAV is dynamic, with persistent endemicity in Norway, Chile, Canada, and the Faroe Islands, and the ever‑present risk of emergence in new regions via international fish movements. Continued investment in genomic surveillance, transmission modelling, and risk factor analysis is essential to mitigate the impact of this devastating pathogen on global salmon production.

Diagnostic Approaches for ISAV Detection and Surveillance

The accurate and timely detection of infectious salmon anemia virus (ISAV) is a cornerstone of global efforts to control and prevent outbreaks of infectious salmon anemia (ISA), a notifiable disease of major economic consequence for Atlantic salmon aquaculture [3, 9]. The diagnostic landscape for ISAV has evolved considerably from its initial reliance on clinical observation, histopathology, and virus isolation, now incorporating a suite of molecular, serological, and environmental surveillance tools that enable rapid genotype discrimination, quantitative viral load assessment, and even prediction of transmission risks. Given the existence of two phenotypically distinct variants, the virulent ISAV-HPRΔ (responsible for clinical ISA) and the avirulent ISAV-HPR0 (an asymptomatic, gill-tropic form), diagnostic approaches must not only detect the virus but also differentiate these variants, as their regulatory and management implications diverge dramatically [4, 13, 17]. The World Organisation for Animal Health (WOAH) has long recognized ISAV as a reportable pathogen, and standardized international diagnostic protocols are critical for trade and biosecurity.

Reverse Transcription Quantitative PCR (RT‑qPCR) and Multiplex Differentiation

RT‑qPCR targeting the ISAV genome, most often segment 8 (encoding matrix protein) or segment 7, remains the global gold standard for routine surveillance and outbreak confirmation. This method offers high sensitivity, rapid turnaround, and the capacity to process large numbers of samples from gill, kidney, heart, liver, and spleen [22, 42]. Critically, the choice of genomic target influences detection sensitivity: segment 3 (polymerase basic 1) has been shown to yield higher analytical sensitivity than segment 8, making it a preferred target for early detection in low‑prevalence scenarios [42]. Traditional RT‑qPCR, however, cannot distinguish HPRΔ from HPR0, necessitating secondary genetic sequencing of the hemagglutinin‑esterase (HE) gene (segment 6) to confirm the deletion pattern, a process that can delay management responses.

To overcome this bottleneck, Rounsville et al. [4] developed and validated a multiplex RT‑qPCR assay that simultaneously (i) detects all ISAV genotypes, (ii) discriminates HPRΔ from HPR0 using probes targeting the hypervariable region (HPR) of segment 6, and (iii) includes an internal RNA extraction control, all in a single reaction. The assay was tested against 31 ISAV strains from North America and Europe (28 HPRΔ, 3 HPR0) and demonstrated 100% concordance with conventional sequencing. Inter‑laboratory comparison with commercial ISAV testing further confirmed equivalence [4]. This multiplex approach dramatically accelerates phenotype determination, enabling fish health authorities to implement variant‑specific control measures (e.g., depopulation for HPRΔ versus surveillance for HPR0) without sequencing delays. The design of the assay exploits the fact that HPRΔ variants possess characteristic deletions in the HPR region; probes spanning the deleted area fail to bind to HPR0, while conserved probes detect all strains.

Droplet Digital PCR (ddPCR) and Environmental Surveillance

A paradigm shift in ISAV surveillance has been the application of droplet digital PCR (ddPCR) to detect viral RNA directly in seawater, eliminating the need for fish sampling. Traditional surveillance relies on lethal sampling of sentinel or moribund fish, which is labor‑intensive, welfare‑compromising, and may miss early viral shedding before clinical signs appear. Weli et al. [39] demonstrated that an electronegative membrane filter coupled with lysis buffer concentration achieved significantly higher recovery of ISAV RNA from seawater than alternative methods, with a detection limit of approximately 5.6 × 10⁴ copies/L. In a bath challenge model, viral shedding was detectable by ddPCR as early as two days post‑challenge, peaked at 11 days (one day before the onset of mortality), and correlated with the high‑dose infection group [39]. This pre‑mortality shedding window is critical for models of between‑farm transmission; the data indicate that substantial viral release occurs before fish become moribund, providing a wide window for environmental detection.

DdPCR offers the advantage of absolute quantification without the need for standard curves, making it particularly robust for the variable matrix of seawater. When integrated with oceanographic and particle‑tracking models, such quantification becomes the input for spatially explicit risk mapping. For instance, Ding et al. [37, 38] combined a virus dispersal model (incorporating UV inactivation and microbial decay) with a particle‑tracking framework to simulate ISAV plumes from infected farms in the Quoddy region (New Brunswick and Maine). Their simulations revealed asymmetrical connectivity between sites, depending on currents and tidal phase, and demonstrated that even under worst‑case scenarios, a single outbreak may expose wild post‑smolts to viral concentrations below the laboratory‑derived minimum infectious dose, though this threshold may be too stringent given environmental co‑factors that increase host susceptibility [38]. These modeling efforts underscore the need for empirical validation of ddPCR‑based environmental detection to calibrate dispersal and infection risk.

Whole‑Genome Sequencing and Reassortment Detection

While targeted RT‑qPCR and ddPCR meet most diagnostic needs, understanding ISAV evolution and the emergence of new virulent strains requires whole‑genome sequencing (WGS). Spilsberg et al. [5] developed an amplicon‑based sequencing protocol using 80 ISAV‑specific primers covering 92% of the genome, designed for the Illumina MiSeq platform. The method produced sequences nearly identical to Sanger sequencing and was applied to 12 isolates from a small local epidemic in northern Norway. Analysis of the contiguous full‑genome data revealed segment reassortment, a mechanism by which a low‑pathogenicity HPR0 virus can acquire genomic segments from a co‑circulating HPRΔ strain, potentially generating a novel virulent reassortant. This risk is supported by reverse genetics experiments showing high genetic compatibility between HPR7b and HPR0 segments, particularly for segments 5 and 6 [10, 18]. The amplicon‑based WGS approach is cost‑effective and scalable for routine surveillance, generating the depth of data needed to track the spatiotemporal dynamics of segment reassortment.

A complementary proof‑of‑concept study by Eckstrand et al. [22] compared histopathology, virus isolation, WGS via shotgun metagenomics, electron microscopy, in situ hybridization (ISH), and RT‑rtPCR for ISAV detection. Interestingly, microscopic differences between infected and uninfected fish were absent, highlighting the limitations of histopathology for early diagnosis. However, ISH with a probe targeting ISAV genome reliably detected viral RNA in renal hematopoietic tissue, while WGS from tissue homogenates proved challenging owing to low viral abundance relative to host nucleic acids [22]. Virus isolation in cell culture (ASK or SHK‑1 cells) remains a valuable confirmatory method, particularly for live virus characterization, but is time‑consuming and requires biosafety level 2+ facilities.

Serological and Immunological Approaches

Detection of anti‑ISAV antibodies provides a retrospective measure of exposure and is useful for population‑level surveillance, particularly in broodstock or surveillance zones where active infection may have been cleared. Enzyme‑linked immunosorbent assays (ELISAs) using recombinant HE or F proteins as antigens have been developed but are not yet standardized across international reference laboratories. Tobar et al. [43] demonstrated that serum IgM specific for ISAV correlates with protective status in vaccinated fish; when antibody levels dropped below 2000 pg/mL, farms entered a “window of susceptibility” to ISA. This serological monitoring, combined with oral booster vaccinations, can maintain protective immunity throughout the production cycle. However, serology cannot distinguish between vaccinated and naturally infected animals, limiting its utility for regulatory confirmation.

The type I interferon (IFN) response has been exploited as an indirect marker of infection. Svingerud et al. [31] showed that pretreatment of ASK cells with recombinant IFNa1 transiently suppressed ISAV replication (measured by qPCR of segment 6), but the inhibitory effect waned after 4–5 days, explaining why CPE‑based assays underestimate IFN activity. Transcriptomic profiling of infected fish has further identified early upregulation of interferon‑stimulated genes (e.g., mx1, isg15) as a hallmark of resistance [23, 34], and these biomarkers could be incorporated into diagnostic panels to assess individual or family‑level susceptibility.

Surveillance Strategies and Network‑Based Prediction

Diagnostic approaches extend beyond individual animal testing to system‑level surveillance. The spatiotemporal analysis of ISAV detections in Newfoundland and Labrador revealed annual incidence risks ranging from 3% to 29%, with European clade variants predominating over North American clades, and no association between clade type and time to depopulation [8]. Such epidemiological surveillance is enhanced by network models that incorporate ship movements, which Duault et al. [3] identified as a potential vector for long‑distance ISAV transmission. Their analysis of Norwegian fish farms showed that ship contact networks, reconstructed with delays of 1, 8, and 15 days, connected ≥72% of farms, and that these networks were significantly associated with the spatiotemporal distribution of ISA case farms. Inadequate disinfection of ships could permit viral survival over long distances, particularly at low temperatures [40]. Incorporating genetic data from WGS into these network models would strengthen the attribution of transmission events.

The identification of ISAV‑HPR0 in broodstock and the subsequent screening of their eggs and progeny represents another critical surveillance application. Polinski et al. [36] used qPCR to screen eggs from 16 family crosses derived from HPR0‑infected broodstock; none of the 781 eggs nor the 870 gill clips from offspring reared for two years in biocontainment tested positive, providing strong evidence against vertical transmission of HPR0. This diagnostic finding has profound implications for broodstock management: it suggests that HPR0‑positive brood need not be culled, and that vertical transmission does not contribute to the maintenance of avirulent ISAV in the population.

Future Directions: Integration of Multi‑Omics and Machine Learning

The diagnostic frontier for ISAV is moving toward predictive and pre‑symptomatic detection. Transcriptomic signatures from early infection, such as those identified by Johnston et al. [34] in kidney (early upregulation of antiviral, inflammatory, and endothelial growth factors in resistant families), could be converted into rapid, multiplex gene expression assays. Similarly, microRNA profiling has identified miR‑462a‑5p (upregulated) and miR‑125b‑5p (downregulated) as consistent markers of ISAV infection in both in vitro and in vivo models [32, 33], and these small RNAs are stable in plasma, offering a non‑lethal diagnostic target. Machine learning applied to qPCR or ddPCR datasets from environmental samples could farm‑scale risk predictions, while amplicon‑based WGS, when coupled with phylogenetic analysis, will continue to detect emerging reassortants before they spread. The continued validation of these tools against field outbreaks, as emphasized by the modeling studies of Ding et al. [37, 38], will be essential to translate diagnostic advances into actionable surveillance and control measures.

Nutritional Factors and Environmental Influences on ISAV Pathogenesis

The pathogenesis of infectious salmon anemia virus (ISAV) is not determined solely by viral genetics or host susceptibility; rather, it emerges from a complex interplay between nutritional status, environmental conditions, and the physiological state of the host. As a notifiable pathogen under the World Organisation for Animal Health (WOAH) standards, understanding these modulatory factors is critical for developing effective disease management strategies in Atlantic salmon aquaculture. The following analysis synthesizes current research on how dietary composition, temperature regimes, co-infecting parasites, and environmental stressors collectively shape ISAV pathogenesis.

Dietary Lipid Composition and Polyunsaturated Fatty Acid Modulation of Viral Pathogenesis

The shift from marine-based to plant-based feed formulations in salmon aquaculture has fundamentally altered the fatty acid profiles available to farmed fish, with profound implications for ISAV pathogenesis. Holmlund et al. [1] demonstrated that eicosapentaenoic acid (EPA) concentration exerts a dose-dependent effect on the transcriptional landscape of ISAV-infected Atlantic salmon kidney (ASK) cells. Their RNA sequencing analysis revealed that while ISAV infection alone dysregulated over 3,000 genes, the addition of increasing EPA concentrations triggered an additional 2,500 differentially expressed genes specifically in virus-infected cells, a phenomenon not observed in uninfected cells [1]. This indicates that EPA is not merely a passive membrane component but actively modulates the host-virus interface.

The mechanistic underpinnings of this interaction are particularly illuminating. Cells exposed to both high EPA concentrations and ISAV exhibited gene expression signatures consistent with ferroptosis activation, including upregulation of oxygen radical formation pathways [1]. Ferroptosis, an iron-dependent form of regulated cell death driven by phospholipid peroxidation, represents a potentially beneficial host response when sufficient polyunsaturated fatty acids (PUFAs) are present in cellular membranes. The authors propose that ferroptosis may serve as a controlled cell death mechanism that limits viral replication by eliminating infected cells before progeny virions can be assembled [1]. This finding carries significant practical implications: dietary formulations that optimize EPA content could theoretically enhance the host's ability to restrict ISAV dissemination through ferroptotic elimination of infected cells. However, the relationship is not straightforward, as the same study found that EPA had limited independent effects on innate immune gene expression in the absence of infection, suggesting that its antiviral potential is context-dependent and requires active viral replication to manifest [1].

The broader lipid environment also influences disease outcomes through interactions with other pathogens. Carvalho et al. [24] investigated functional feeds enriched with different fatty acid profiles during sea lice (Lepeophtheirus salmonis) and ISAV co-infection. Diets with higher ω6 content relative to ω3 (designated FA+I) were most effective at reducing sea lice abundance but paradoxically resulted in the poorest survival outcomes during subsequent ISAV co-infection [24]. This trade-off highlights a critical consideration for feed formulation: nutritional strategies optimized for one pathogen may inadvertently increase susceptibility to another. The transcriptomic analysis revealed that antiviral gene expression, including interferon regulatory factor 7b (irf7b) and Mx protein (mxb), was induced in all dietary groups except those receiving the immunostimulating diet, suggesting that certain functional feed additives may suppress the very antiviral pathways required for ISAV control [24].

Temperature as a Master Regulator of Host Resistance and Viral Pathogenesis

Environmental temperature represents perhaps the most potent environmental determinant of ISAV pathogenesis, acting through both direct effects on viral replication kinetics and profound modulation of host immune responses. Groves et al. [26] conducted experimental infections at 10°C and 20°C, revealing that cumulative mortality was significantly higher at the lower temperature despite more acute mortality at 20°C. This seemingly paradoxical finding is explained by differential immune kinetics: at 10°C, fish mounted a stronger innate antiviral response characterized by elevated expression of mx1, isg15, and vprn, along with a Th2-polarized response (il10, il4/13a). Conversely, at 20°C, the Th1 and antigen-presenting cell responses (il12rb2, nkl) were initially more robust, potentially facilitating more rapid viral clearance [26]. Viral prevalence also disappeared more quickly at 20°C, suggesting that higher temperatures may accelerate the transition from innate to adaptive immunity.

The molecular mechanisms underlying this thermal modulation have been elucidated through transcriptomic analyses. Misk et al. [23] compared resistant and susceptible Atlantic salmon families at 10°C and 20°C, discovering that resistance mechanisms were dramatically temperature-dependent. At 10°C, resistant families exhibited 3,690 differentially expressed genes (DEGs) compared to susceptible families, characterized by coordinated downregulation of proteasome, DNA replication, and translation initiation pathways, alongside upregulation of receptor internalization mechanisms including flotillin proteins, β-arrestins, and NEDD4 E3 ubiquitin ligase [23]. At 20°C, only 64 DEGs distinguished resistant from susceptible families, indicating that thermal conditions fundamentally alter the molecular strategies employed for viral resistance. Genome-wide association analysis identified the translation initiation factor EIF4G1 on chromosome 14, which was significantly downregulated in resistant fish (log₂FC = -0.51, FDR = 0.03), suggesting that translational suppression may be a conserved resistance mechanism at lower temperatures [23].

The temperature-dependent nature of ISAV resistance has direct implications for aquaculture management. Selective breeding programs that identify resistant families at one temperature may not predict performance at another, as the genetic architecture of resistance shifts with environmental conditions. Furthermore, the finding that resistant families at 10°C prioritize broad cellular resilience strategies rather than classical antiviral responses challenges conventional approaches to vaccine development and immunostimulant design, which typically target canonical interferon pathways [23].

Sea Lice Co-Infection: Parasitic Modulation of Viral Susceptibility

The epidemiological association between sea lice infestations and ISAV outbreaks has been recognized since the 1998 Scottish ISA epidemic, but recent experimental evidence has established a causal mechanistic basis for this relationship. Barker et al. [27] demonstrated that prior infestation with Lepeophtheirus salmonis significantly increased mortality rates and accelerated death in Atlantic salmon subsequently exposed to ISAV, an effect consistent across two genetically distinct salmon strains (Penobscot and Saint John River). The immunological mechanism involves parasite-mediated immunosuppression: lice infestation downregulated antiviral genes including Mx, MHC class I β, galectin 9, and TRIM 16/25 prior to and shortly after ISAV co-infection [27]. This suppression of the type I interferon system creates a permissive environment for viral replication, effectively lowering the threshold for ISAV pathogenesis.

Transcriptomic analyses have further characterized the molecular consequences of co-infection. Zhong et al. [7] identified 994 DEGs in the head kidney of co-infected fish compared to only 240 DEGs in lice-only infections, with co-infection inducing a strong activation of Toll-like receptor and NOD-like receptor signaling pathways, along with significant upregulation of interferon and MHC class I protein complex genes. The cumulative mortality rate of 47.8% in co-infected groups, contrasted with zero mortality in lice-only infections, underscores the synergistic pathogenic potential of these two pathogens [7]. Importantly, the immune response during co-infection was qualitatively different from simple additive effects: single infection with lice significantly suppressed the complement system, while co-infection triggered a robust but ultimately ineffective immune activation [7].

The skin transcriptome provides additional insights into the co-infection dynamics. Cai et al. [25] found that co-infected fish exhibited upregulation of glycolysis, interferon pathway components, complement cascade activity, and heat shock protein family members, while downregulating antigen presentation and processing, T-cell activation, collagen formation, and extracellular matrix genes. This pattern suggests that co-infection redirects host resources away from adaptive immunity and tissue repair toward metabolically costly innate antiviral responses, potentially explaining the increased mortality observed [25]. The identification of unique pathway enrichments in co-infected groups, including "autophagosome" and "cytosolic DNA-sensing pathway," points to specific molecular vulnerabilities that could be targeted therapeutically.

Environmental Transmission Dynamics and Viral Persistence

The physical environment mediates ISAV transmission through complex hydrodynamic and biological interactions that influence both viral persistence and host exposure. Ding et al. [37] developed a sophisticated modeling framework integrating ocean circulation, particle tracking, dynamic infection kinetics, and viral inactivation to simulate ISAV dispersal from infected aquaculture sites in the Quoddy Region (New Brunswick, Canada and Maine, USA). Their simulations revealed that ISAV concentration varies spatiotemporally with outbreak progression, current speed and direction, tidal elevation amplitude, and environmental decay mediated by ultraviolet radiation and natural microbial communities [37]. Connectivity among aquaculture sites was asymmetrical, meaning that the risk of transmission from Farm A to Farm B was not equivalent to the reverse direction, a finding with significant implications for coordinated disease management strategies.

The importance of environmental viral persistence is underscored by studies of viral shedding kinetics. Weli et al. [39] used droplet digital PCR to quantify ISAV in seawater during experimental bath challenges, detecting viral shedding as early as two days post-challenge with peak shedding occurring at 11 days post-challenge, one day before mortality onset in high-dose groups. This temporal pattern indicates that substantial viral release precedes clinical disease, creating opportunities for undetected transmission between farms [39]. The development of water-based surveillance tools, as advocated by these authors, could provide earlier warning of outbreaks than traditional fish sampling approaches.

Ship movements represent another critical environmental transmission pathway. Duault et al. [3] constructed network models of vessel movements among Norwegian fish farms, demonstrating that ship contact networks exhibited high connectivity, with the largest strongly connected component encompassing ≥72% of farms. Both ship contact networks and company affiliation networks were significantly associated with the spatiotemporal distribution of ISA cases [3]. Importantly, increasing the time window between ship visits (from 1 to 15 days) enabled connection of distant regions, suggesting that inadequate disinfection protocols could facilitate long-distance ISAV-HPRΔ transmission. This finding aligns with WOAH guidelines emphasizing biosecurity measures for equipment and vessels moving between aquaculture sites.

Nutritional Interventions and Antiviral Feed Additives

Beyond fatty acid composition, specific dietary components have demonstrated potential to modulate ISAV pathogenesis. Lozano et al. [44] evaluated Chilean red macroalgae (Pyropia columbina and Gracilaria chilensis) as functional feed additives, finding that sera from fish fed diets containing 1.0% and 10% G. chilensis exhibited significantly increased antiviral activity against ISAV in plaque reduction neutralization assays. Fish receiving 10% G. chilensis also showed improved specific growth rates without compromising feed conversion ratios [44]. The antiviral mechanism likely involves bioactive metabolites present in these macroalgae, though the specific compounds responsible remain to be identified.

The concept of oral immunization to maintain protective antibody levels has been explored by Tobar et al. [43], who demonstrated that successive oral immunizations are required to sustain high concentrations of anti-ISAV IgM antibodies (above 2750 ng/mL) for extended periods. When antibody levels fell below 2000 pg/mL, a window of susceptibility to infection was observed, establishing a quantitative threshold for protective immunity [43]. This finding has practical implications for vaccination programs, suggesting that booster oral immunizations at 600-800 degree-day intervals are necessary to maintain protection throughout the production cycle.

Genetic Resistance and Environmental Interactions

The interaction between host genetics and environmental factors represents a frontier in understanding ISAV pathogenesis. Johnston et al. [34] challenged 40 families of North American Atlantic salmon with ISAV, finding that the most resistant families had mean cumulative mortality of 34.3% compared to 78.5% in susceptible families. Critically, selective breeding for growth and sea lice resistance did not incidentally impact ISA resistance, indicating that these traits are genetically independent [34]. However, ISA resistance was not synonymous with ISAV resistance: resistant and susceptible fish harbored equivalent viral loads across all organs screened. Instead, resistance was associated with early viral recognition in the kidney, characterized by upregulation of antiviral, inflammatory, and endothelial growth factors [34]. This suggests that genetic resistance operates through enhanced early detection and containment rather than suppression of viral replication per se.

Chase-Topping et al. [11] further demonstrated that both vaccination and selective breeding reduce the probability of ISAV transmission, though neither prevents transmission entirely. The effect of genetic resistance (odds ratio: 8.35) was larger than vaccination (odds ratio: 4.52), though neither reached statistical significance in their transmission trial [11]. Importantly, fish with low genomic breeding values died 14 days earlier than those with high breeding values, indicating that genetic resistance primarily enhances endurance to infection rather than susceptibility to initial infection. The authors suggest that incorporating mucus viral load as an additional phenotype in breeding programs could more effectively reduce ISA transmission than mortality-based selection alone [11].

The molecular basis of genetic resistance involves complex regulatory networks. Jørgensen et al. [30] compared gene expression in fish dying early versus late following ISAV infection, finding that early mortality was associated with high viral replication and dramatic activation of innate immune responses that failed to provide protection. Late mortality fish, by contrast, showed lower inflammatory responses and activation of adaptive immunity markers, including immunoglobulin-like genes [30]. Linear discriminant analysis identified four hepatic genes (5-lipoxygenase activating protein, cytochrome P450 2K4-1, galectin-9, and annexin A1) that could correctly classify individuals into early mortality, late mortality, and uninfected groups, with three of these markers involved in metabolism of inflammatory regulators [30]. This suggests that the capacity to modulate inflammation, rather than the magnitude of antiviral response, may be the key determinant of survival.

Viral Genetic Diversity and Environmental Adaptation

The genetic plasticity of ISAV, particularly in segments 5 and 6 encoding the fusion protein and hemagglutinin-esterase, respectively, enables adaptation to environmental conditions and host populations. Cárdenas et al. [13] proposed a theoretical integrative model wherein the combinatorial effect of segment 5 and 6 features modulates virulence intensity in the field. The non-pathogenic HPR0 variant, characterized by a complete, non-deleted segment 6, appears to represent the ancestral strain from which pathogenic HPRΔ variants emerge through deletion events [13]. This evolutionary dynamic is influenced by environmental pressures, as demonstrated by the seasonal and transient nature of HPR0 infections in Faroe Islands farmed salmon, which exhibit gill tropism and cause subclinical respiratory infections reminiscent of seasonal influenza [17].

The potential for segment reassortment to generate novel pathogenic variants has been demonstrated experimentally. Cárdenas et al. [10] used reverse genetics to create synthetic reassortant viruses combining segments from highly virulent ISAV 752_09 (HPR7b) and avirulent SK779/06 (HPR0), finding that most segments contribute to infectivity characteristics, with segment 5 being particularly important. Segment 5 enabled HPR0 viruses to produce plaques and replicate in CHSE-214 cells, while segments 5 and 6 together participated in different stages of the viral cycle, with their compatibility being critical for infection [10]. The high degree of genetic compatibility between HPR7b and HPR0 segments represents a latent risk for the emergence of reassortant viruses with unpredictable phenotypes, particularly under environmental conditions that favor co-infection with multiple ISAV variants.

Implications for Disease Management and Future Research

The integration of nutritional, environmental, and genetic factors into ISAV pathogenesis models has direct implications for disease control. The WOAH-listed status of ISAV necessitates rigorous biosecurity protocols, but the current understanding suggests that management strategies must be tailored to local environmental conditions. Temperature-dependent resistance mechanisms imply that selective breeding programs should evaluate families under relevant thermal regimes, while feed formulations should consider the trade-offs between controlling sea lice and maintaining antiviral capacity. The development of water-based surveillance tools, coupled with network analysis of vessel movements, could enable earlier detection of outbreaks and more targeted interventions.

Future research should prioritize the validation of environmental transmission models with field data, as emphasized by Ding et al. [38], who noted that reliance on laboratory-derived minimum infectious doses may underestimate infection probability in wild populations where host susceptibility and environmental variability permit infection at lower doses. Additionally, the identification of specific bioactive compounds in macroalgae and other functional feed ingredients could lead to the development of targeted nutritional interventions that enhance antiviral resistance without compromising growth performance or susceptibility to other pathogens.

Current and Emerging Control Strategies for Infectious Salmon Anemia

The control of Infectious Salmon Anemia (ISA) represents one of the most formidable challenges in contemporary marine aquaculture, demanding a multifaceted approach that integrates rapid molecular diagnostics, rigorous biosecurity protocols, selective breeding programs, and the continued development of effective immunoprophylaxis. Given that the World Organisation for Animal Health (WOAH) classifies ISAV as a notifiable pathogen with significant economic consequences for global salmon production, the strategic deployment of control measures must be informed by a deep understanding of viral transmission dynamics, host genetics, and the molecular underpinnings of pathogenesis. The following sections critically examine both established and emergent strategies that collectively form the current armamentarium against this devastating orthomyxovirus.

Enhanced Molecular Surveillance and Rapid Diagnostics as a Cornerstone of Control

The foundation of any effective ISA control program rests upon the capacity for rapid, accurate detection and differentiation of viral variants. A critical advancement in this domain is the development of multiplex reverse-transcription quantitative PCR (RT-qPCR) assays that can simultaneously detect the presence of ISAV and discriminate between the virulent HPRΔ (highly polymorphic region deleted) and the avirulent HPR0 phenotypes in a single reaction [4]. This capability is of paramount importance, as HPR0 is ubiquitous in many salmon-producing regions and causes a non-clinical, transient infection of the gill epithelium, whereas HPRΔ is responsible for the systemic, lethal disease that triggers regulatory action and depopulation [17]. Traditional methods requiring genetic sequencing to differentiate these phenotypes introduce critical delays in disease management; the multiplex RT-qPCR approach, validated across 31 strains from North America and Europe, dramatically shortens the time from sample collection to actionable intelligence [4]. This technological leap allows producers and regulatory authorities to implement immediate containment measures for HPRΔ detections without the ambiguity that previously hampered response efforts.

Complementing rapid phenotyping, whole-genome amplicon sequencing (WGS) has emerged as an indispensable tool for molecular epidemiology. A recently developed protocol employing 80 ISAV-specific primers covering 92% of the genome enables high-resolution tracking of viral evolution during outbreaks [5]. Application of this method during a localized epidemic in northern Norway revealed that segment reassortment, a process whereby genomic segments are exchanged between co-infecting viral strains, occurred between isolates, providing definitive evidence for a mechanism that can generate novel genotypes with altered virulence [5]. This finding aligns with earlier work demonstrating that segments 5 and 6, encoding the fusion (F) and hemagglutinin-esterase (HE) proteins respectively, are critical determinants of infectivity, and that their compatibility is essential for productive viral infection [10]. Furthermore, the coexistence of both HPR0 and HPR7b variants in asymptomatic fish, as documented in Chilean populations, suggests that the transition from an avirulent to a virulent phenotype may involve a stepwise process initiated by deletion events in segment 6 followed by insertions in segment 5 [18]. This underscores the necessity for ongoing genomic surveillance to detect the emergence of virulent reassortants before they trigger widespread epizootics.

The integration of environmental sampling into surveillance frameworks represents an emerging frontier. Quantification of ISAV RNA in seawater using droplet digital PCR (ddPCR) has demonstrated that massive viral shedding occurs in infected populations prior to the onset of mortality, with peak concentrations reaching (5.6 \times 10^4) RNA copies per liter [39]. This pre-mortem shedding event creates a window of opportunity for early detection through passive water sampling, which could complement traditional sentinel fish programs. Simultaneously, sophisticated particle-tracking models coupled with ocean circulation data have been developed to simulate the dispersal of ISAV from infected aquaculture sites, incorporating viral inactivation rates due to ultraviolet radiation and microbial communities [37]. These models reveal that connectivity among farms is not merely a function of seaway distance but is profoundly influenced by tidal amplitude, current vectors, and the temporal dynamics of the outbreak itself. Such frameworks are essential for predicting the spatial extent of risk and for designing coordinated fallowing and zone-based management strategies.

Biosecurity, Farm Management, and the Disruption of Transmission Networks

Despite advances in diagnostics, the primary defense against ISA remains the rigorous implementation of biosecurity protocols that sever transmission pathways. Network analysis of ship movements among Norwegian fish farms has provided compelling evidence that vessel traffic constitutes a significant risk factor for the long-distance dissemination of ISAV-HPRΔ [3]. With the largest strongly connected component of the ship contact network encompassing over 72% of farms, the potential for viral spread via contaminated ballast water, equipment, or personnel is substantial. Critically, while production areas were found to structure the network, increasing the time window between consecutive farm visits (from 1 to 15 days) enabled the connection of distant regions, highlighting the role of viral environmental persistence in mediating long-range transmission [3]. This finding mandates the implementation of rigorous disinfection protocols for all vessels moving between management zones, with particular attention to the efficacy of disinfectants under marine conditions.

The evaluation of accelerated hydrogen peroxide (AHP) as a disinfectant, using avian influenza virus as a surrogate for ISAV, has established that natural seawater can be used as a diluent provided that contact times and concentrations are appropriately adjusted [40]. At 4°C, a concentration of 0.44% hydrogen peroxide in natural seawater was required to achieve a 6-log reduction in viral titer within five minutes, compared to lower concentrations needed at 21°C. This temperature-dependent efficacy has practical implications for farms operating in colder waters where disinfection of nets, well-boats, and processing equipment is most critical. Furthermore, the demonstrated ability of ISAV to survive on fomites and in water for extended periods reinforces the need for comprehensive biosecurity plans that include dedicated equipment for each farm, footbaths, and restricted access protocols for personnel.

Mathematical modeling has further informed control strategies by quantifying the effects of vaccination and selective breeding on transmission dynamics. A transmission trial using 420 Atlantic salmon parr demonstrated that neither vaccination nor genetic resistance prevented transmission entirely, but both reduced the probability of infection in naïve contact fish [11]. Interestingly, the effect of genetic resistance (comparing high versus low genomic breeding value families) was larger than the effect of vaccination, with odds ratios of 8.35 and 4.52 respectively, although neither reached statistical significance due to the inherent variability in challenge models. Importantly, fish with high genetic resistance demonstrated significantly greater endurance once infected, surviving 14 days longer than susceptible cohorts, which has profound implications for reducing the total viral load shed into the environment and thus the force of infection on neighboring populations [11]. The temporal dynamics of shedding are critical: peak viral concentrations in water occur at 11 days post-challenge, one day before mortalities commence, meaning that fish that survive longer will contribute to a protracted shedding period [39]. Therefore, breeding programs that enhance survival without eliminating viral replication may inadvertently prolong the infectious period, a paradox that necessitates careful evaluation of mucus viral load as a potential phenotype for selection to more effectively curb transmission [11].

Selective Breeding and the Molecular Architecture of Host Resistance

The substantial heritable variation in resistance to ISAV has made selective breeding a cornerstone of long-term disease control. Family-wise challenge experiments consistently demonstrate that cumulative mortality can range from approximately 34% in resistant families to 79% in susceptible families [34]. Crucially, resistance to ISA disease is not synonymous with resistance to ISAV replication; resistant and susceptible fish were found to be equally burdened with virus in all organs screened, indicating that the phenotypic difference lies in the host's ability to tolerate infection rather than to prevent it [34]. Transcriptomic analyses have revealed that early recognition of the virus in the kidney, characterized by the upregulation of antiviral, inflammatory, and endothelial growth factor genes, is a hallmark of the resistant phenotype [34]. This early systemic response appears to mitigate the pathological consequences of infection without reducing viral transcription, a strategy akin to disease tolerance observed in other host-pathogen systems.

The interaction between genetic resistance and environmental temperature adds another layer of complexity. Experimental infections conducted at 10°C and 20°C revealed that resistance mechanisms are dramatically temperature-dependent, with 3,690 differentially expressed genes (DEGs) distinguishing resistant from susceptible families at the lower temperature compared to only 64 DEGs at the higher temperature [23]. At 10°C, resistance was characterized by coordinated downregulation of proteasome, DNA replication, and translation initiation pathways, coupled with upregulation of receptor internalization mechanisms mediated by flotillin proteins, β-arrestins, and NEDD4 E3 ubiquitin ligase [23]. This suggests that resistant fish restrict viral entry or promote degradation of viral components. Furthermore, genome-wide association analysis identified the translation initiation factor EIF4G1 on chromosome 14 as a candidate resistance locus, with significant downregulation in resistant fish [23]. These findings indicate that ISAV resistance relies on broad cellular resilience strategies rather than classical antiviral interferon responses, and that selection programs must account for the thermal regime of the production environment to maximize efficacy. Conversely, selective breeding for growth and sea lice resistance does not appear to inadvertently impact ISA resistance, as families selected for these traits showed survival rates comparable to randomly mated controls [34].

Vaccination Strategies: From Current Limitations to Next-Generation Platforms

Vaccination remains the most economically viable long-term strategy for ISA control, yet current commercial vaccines provide incomplete protection. The protective immune response against ISAV is complex and involves both humoral and cellular arms. An immunoinformatic approach has predicted 32 linear B-cell epitopes and six antigenic cytotoxic T lymphocyte (CTL) epitopes from the fusion (F), HE, and matrix (M) proteins, with demonstrated high-affinity binding to Atlantic salmon MHC class I molecules [12]. These predicted epitopes provide a rational basis for the design of peptide-based vaccines that could elicit robust CD8+ T-cell responses, which are likely critical for clearing infected cells given the virus's ability to persist in the face of antibody responses.

DNA vaccines encoding the HE protein (pHE) have shown promise in experimental settings, particularly when co-administered with a plasmid expressing type I interferon (pIFNa). Transcriptomic analysis of the injection site revealed that pIFNa mediates upregulation of interferon-stimulated genes (ISGs), chemokines (CCL5, CCL8, CCL19, CXCL10), and markers of B-cells, T-cells, and antigen-presenting cells [28]. Remarkably, expression of the HE antigen alone had an inhibitory effect on ISG and chemokine induction, suggesting that the virus has evolved mechanisms to suppress the host's innate antiviral response, which the interferon adjuvant effectively overcomes [28]. This antagonism is also evident at the cellular level, where ISAV infection in vitro inhibits the induction of type I interferon and ISG15 expression, although recombinant interferon administered prior to infection can transiently suppress viral replication [31]. The transient nature of this suppression, waning 4–5 days post-infection despite initial strong inhibition, explains why interferon-based therapies alone are insufficient for long-term control.

Oral vaccination represents an emerging strategy to maintain protective immunity throughout the production cycle. Field studies in Chile involving over 600 farms demonstrated that successive oral immunizations are required to uphold high levels of specific IgM antibodies against both ISAV and Piscirickettsia salmonis [43]. After injectable priming with oil-adjuvanted vaccines, oral boosters maintained antibody concentrations above the protective threshold (approximately 2750 ng/mL for ISAV) for periods of 800 degree-days. Critically, when antibody levels fell below 2000 pg/mL, a window of susceptibility to infection was observed, demonstrating a direct association between circulating antibody titers and farm-level protective status [43]. This finding underscores the need for regular booster vaccinations, particularly during periods of high environmental stress or elevated pathogen pressure.

The potential for antibody-dependent enhancement (ADE) of ISAV infection remains a concern that complicates vaccine development. Early work demonstrated that antibodies can mediate uptake and replication of ISAV in macrophage-like cell lines via Fc receptors, suggesting a mechanism by which suboptimal vaccine responses could paradoxically exacerbate disease [19]. This phenomenon, well-characterized in other orthomyxoviruses, necessitates careful vaccine design to ensure that elicited antibodies are neutralizing rather than enhancing. The structural characterization of the ISAV F protein, which exhibits a canonical class I fusion architecture with a pH-sensitive carboxyl-carboxylate sensor, provides a molecular target for the design of broadly neutralizing antibodies that block membrane fusion [16].

Functional Feeds and Nutritional Modulation of Innate Immunity

The shift from marine-based to plant-based diets in Atlantic salmon aquaculture has reduced the levels of key polyunsaturated fatty acids (PUFAs), particularly eicosapentaenoic acid (EPA), with potential consequences for antiviral immunity. In vitro studies using Atlantic salmon kidney (ASK) cells revealed that high concentrations of EPA (200 µM) have an independent effect on gene expression in ISAV-infected cells, affecting over 2,500 genes beyond those altered by the virus alone [1]. Pathway analysis indicated that EPA promotes peroxisome proliferator-activated receptor (PPAR) signaling and induces a gene expression pattern consistent with ferroptosis, a regulated form of cell death driven by iron-dependent lipid peroxidation. This is a critical observation, as ferroptosis may serve as a mechanism for controlled elimination of infected cells, thereby limiting viral replication when sufficient PUFAs are present in cellular membranes [1]. The practical implication is that dietary formulations optimized for EPA content could prime host cells for a more effective antiviral response, although the balance must be carefully managed to avoid excessive inflammation.

Functional feeds incorporating other bioactive compounds have also been investigated. Red macroalgae, including Gracilaria chilensis and Pyropia columbina, when included in the diet at levels up to 10%, significantly increased the antiviral activity of serum against ISAV as measured by plaque reduction neutralization assays [44]. Fish fed G. chilensis showed improved growth rates and a substantial increase in serum-mediated viral inhibition, suggesting that algal-derived polysaccharides or other metabolites may stimulate innate immune effectors. The transcriptomic response to functional feeds during co-infection with sea lice and ISAV revealed that diets enriched with omega-3 fatty acids and immunostimulants modulated the expression of interferon pathway genes, complement components, and glycolysis enzymes in the skin [25]. However, a critical finding was that feeds most successful at reducing sea lice burden were often the least successful at improving survival during subsequent ISAV co-infection, highlighting the trade-offs inherent in nutritional immunomodulation and the need for diet formulations tailored to specific pathogen risks [24].

RNA Interference and Antiviral microRNAs

The application of RNA interference (RNAi) technology represents a promising avenue for therapeutic intervention. Inactivated Escherichia coli transformed with plasmids producing double-stranded RNA (dsRNA) targeting conserved regions of the ISAV genome have demonstrated antiviral activity when added to infected ASK cells [35]. Of the four target genes evaluated, nucleoprotein (NP), fusion (F), HE, and matrix (M), only dsRNA targeting the HE gene produced significant inhibition of cytopathic effect and reduction in viral load [35]. This specificity likely reflects the critical role of HE in receptor binding and membrane fusion, making it an ideal target for RNAi-mediated silencing. The use of bacterial vectors for dsRNA delivery offers a cost-effective and scalable approach for feed-based delivery, although challenges remain in ensuring stability in the gastrointestinal tract and efficient uptake by target cells.

The role of host-encoded microRNAs (miRNAs) in the antiviral response has also been characterized. Deep sequencing of the small RNA transcriptome in ISAV-infected ASK cells identified a panel of differentially expressed miRNAs, with miR-148a/b and miR-152 emerging as putative antiviral factors that directly target viral genes including hemagglutinin, polymerase subunit P3, and nucleoprotein [32]. Similarly, miR-462a-5p was found to be upregulated during infection, while miR-125b-5p was downregulated, and these changes were observed both in vitro and in vivo [33]. The manipulation of these endogenous miRNA pathways, either through dietary supplementation with miRNA mimics or through antisense oligonucleotides that block proviral miRNAs, represents an unexploited therapeutic opportunity. However, the pleiotropic effects of miRNAs on host gene expression necessitate careful validation to avoid unintended immunosuppression or oncogenesis.

The Challenge of Co-Infections and Immune Modulation by Sea Lice

Any control strategy that fails to account for the immunological context of the host is likely to be suboptimal. Epidemiological and experimental evidence has firmly established that infestation with the sea louse Lepeophtheirus salmonis profoundly increases susceptibility to ISAV. Cohabitation trials demonstrated that lice-infested Atlantic salmon experienced significantly higher mortality rates and faster death rates when subsequently exposed to ISAV compared to naïve fish, and this effect was consistent across two distinct host strains [27]. The mechanistic basis for this synergism lies in the parasite's manipulation of the host immune system: lice infection prior to ISAV exposure downregulated key antiviral genes, including Mx, MHC class I β, galectin 9, and TRIM 16/25, while simultaneously inducing pro-inflammatory genes [27]. This suppression of the type I interferon axis creates a permissive environment for viral replication.

Transcriptomic analysis of co-infected fish has confirmed that lice alone suppress the innate immune system, particularly the complement cascade, whereas co-infection induces a strong but ultimately dysregulated immune response characterized by activation of Toll-like receptor and NOD-like receptor pathways, alongside significant upregulation of interferon and MHC class I genes [7]. Interestingly, the co-infection groups displayed upregulation of glycolysis and heat shock proteins, while genes related to antigen presentation, T-cell activation, and collagen formation were downregulated [25]. This metabolic reprogramming towards aerobic glycolysis (the Warburg effect) may support rapid viral replication but impair long-term adaptive immunity. Consequently, effective ISA control must be integrated with sea lice management programs. The use of functional feeds that simultaneously deter lice attachment while preserving or enhancing antiviral capacity remains an active area of investigation, although the trade-offs documented to date [24] suggest that a single "magic bullet" feed is unlikely to emerge.

Emerging Therapeutic Targets: Ferroptosis, SUMOylation, and the Interferon Paradox

The identification of specific host pathways that are hijacked by ISAV has opened new therapeutic avenues. The observation that the viral nucleoprotein (NP) triggers a robust respiratory burst via NADPH oxidase complex activation, leading to increased reactive oxygen species (ROS) and altered SUMOylation profiles, has identified novel drug targets [15]. Treatment with the NADPH oxidase inhibitor diphenyleneiodonium (DPI) or blockade of the p38 MAPK signaling pathway significantly reduced viral progeny production, suggesting that pharmacological inhibition of ROS generation could limit replication. Similarly, the use of extracts from Aristotelia chilensis, a plant with known antioxidant properties, decreased viral yields, pointing to the potential of natural products as adjunctive therapies [15].

The discovery that ferroptosis is activated in ISAV-infected cells exposed to high EPA levels [1] suggests that controlled induction of this cell death pathway could be exploited therapeutically. Unlike apoptosis, which can be subverted by viral anti-apoptotic proteins, ferroptosis involves the catastrophic perox

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