Infectious salmon anaemia: confirm and contain

Infectious salmon anaemia is a regulated viral disease capable of causing severe losses in seawater Atlantic salmon production. Its presentation varies among populations: progressive mortality, lethargy, pale gills, haemorrhage or ascites may raise suspicion, but none confirms the cause. The operational challenge is therefore twofold. Farms must preserve traceability and restrict potentially hazardous contacts immediately, while building a diagnosis that distinguishes detection of viral RNA from systemic disease associated with a virulent variant.

Variants do not carry the same meaning

Infectious salmon anaemia virus, or ISAV, is an orthomyxovirus. Atlantic salmon stages from yolk-sac fry to adults are considered susceptible, although clinical outbreaks are predominantly reported in seawater cages. The World Organisation for Animal Health manual distinguishes HPR0 variants, primarily associated with localised gill infection without demonstrated clinical disease, from highly polymorphic region-deleted variants, or HPRΔ, associated with systemic disease.

That distinction prevents a positive molecular signal from becoming a diagnosis by itself. HPR0 detection in gills from apparently healthy fish does not have the same meaning as coherent HPRΔ detection across organs in fish with compatible lesions. Genotyping and anatomical distribution add essential information to the measured amount of RNA.

Clinical signs partly reflect the endothelial tropism of virulent variants. Vascular damage can produce gill pallor, anaemia, petechiae, exophthalmia, ascites and changes in the liver, kidney, spleen or intestine. Their absence does not rule out early infection, while their presence is not specific to ISAV.

Combine clinical evidence, lesions and laboratory methods

The WOAH manual recommends method combinations according to purpose: surveillance of apparently healthy animals, presumptive diagnosis in clinically affected fish, or confirmation. RT-qPCR offers sensitive detection of viral RNA, but its value depends on tissue, infection stage, sample quality and analytical controls. Gills are particularly relevant for HPR0. In suspected systemic disease, anterior kidney, heart, spleen and other protocol-defined tissues provide complementary evidence.

Virus isolation, molecular characterisation and histopathology answer different questions. Histopathology relates tissue damage to the clinical presentation; PCR identifies a genetic target; sequencing characterises the variant. A defensible conclusion compares these findings with mortality patterns, movements, population history and differential diagnoses.

Before sampling, teams should prepare clean instrument sets, unambiguous containers and a suitable cold chain. Recently moribund fish often have greater diagnostic value than decomposed carcasses. Allocation of tissues among molecular testing, virology and histology should be agreed with the laboratory so that material needed fresh is not inadvertently fixed.

What the 2026 challenge study adds

Johnston and colleagues compared forty North American Atlantic salmon families in an experimental ISAV challenge using injection or cohabitation. Fish were distributed across sixteen monitoring tanks, with one fish from each family in every tank. Mean cumulative mortality reached 34.3% among the twelve most resistant families and 78.5% among the thirteen most susceptible families.

The central finding is more nuanced than simple resistance to a virus. Families that resisted clinical disease more effectively still carried prolonged high viral transcript loads in the spleen. Their advantage was associated with earlier recognition and different immune expression profiles, particularly in the kidney. Families selected for growth and sea-lice resistance showed no clear incidental improvement in ISA resistance under this design.

The experiment does not establish a commercial selection test or outbreak procedure. It used a North American isolate, controlled exposure and high-containment conditions. It does reinforce a practical principle: clinical survival, viral RNA burden and potential transmission are separate dimensions. A population that becomes less ill cannot be assumed to be less infected without direct measurement.

Act while confirmatory results are pending

Biosecurity becomes a precautionary response as soon as ISA is suspected. Movements of fish, eggs, staff and equipment should be documented and restricted according to the health plan and competent-authority requirements. Mortality routes, vessels, nets, pumps and diving equipment all warrant review as possible pathways between production units.

Initial notification follows the rules of the relevant jurisdiction and should not wait for empirical treatment attempts. Decisions about containment, sanitary harvest or control zones belong to the competent authority; a general article cannot prescribe them. Site managers can immediately assemble population identity, origin, transfer dates, mortality curves, laboratory reports and contact maps.

At multi-unit sites, separating clean and dirty flows, assigning equipment and scheduling work from lower- to higher-risk units reduces indirect contact. Quarantine only works when it is physically and operationally separate and has explicit entry, monitoring and release criteria.

Limits and cautions

Clinical presentation overlaps with other infectious diseases, water-quality failures and toxic causes. A single quantification cycle value, detached from matrix, method and variant, does not describe severity. Conversely, one negative sample does not exclude infection when tissue selection, timing or preservation was unsuitable.

Genetic selection is a complementary strategy, not a replacement for prevention. The 2026 study indicates that resistance to disease is not necessarily accompanied by lower viral transcription. Performance must be confirmed in the relevant salmon populations, virus variants and production systems, while accounting for health, welfare and epidemiological risk.

Conclusion

Infectious salmon anaemia requires a combined interpretation of population history, lesions, RT-qPCR and virus characterisation. Separating HPR0 from HPRΔ avoids equating localised gill detection with systemic disease. Traceability and biosecurity begin at suspicion, while regulatory measures are coordinated with the competent authority.

Vetofish can support sampling design, contextual interpretation of results, biosecurity flow audits and preparation of the evidence required by aquatic veterinarians and authorities.