VHS in salmonids: diagnose before taking action

A sudden mortality increase, darkened fish and haemorrhages can raise suspicion of viral haemorrhagic septicaemia (VHS), but none of these findings confirms infection. VHS is caused by viral haemorrhagic septicaemia virus (VHSV), a novirhabdovirus whose impact varies with viral strain, host species, life stage, temperature and production conditions. The useful farm response is not to treat a compatible appearance. It is to contain the event, preserve diagnostic evidence and engage veterinary and competent-authority pathways early.

Clinical appearance is variable and non-specific

In rainbow trout and other susceptible fish, acute disease may involve lethargy, abnormal swimming, darkening, exophthalmia, pale gills and haemorrhages in the skin, eyes or internal organs. Nervous, chronic and minimally apparent presentations also occur. Conversely, bacterial septicaemia, hypoxia, trauma, toxic exposure and other viral diseases can produce overlapping signs.

The lot investigation should record the mortality curve, distribution between units, fish size and origin, recent movements, temperature, dissolved oxygen, treatments and equipment failures. Temperature influences disease expression, but it is neither a diagnostic test nor a parameter to alter abruptly. An improvised change can increase stress and obscure interpretation.

Records should be assembled before staff memories diverge: deliveries, grading, transfers, feed intake, mortality disposal, visitors, shared equipment and water connections. A good timeline often reveals contact routes that a list of clinical signs cannot.

Sample while the evidence is still usable

Moribund, untreated fish from several representative units are generally more informative than old carcasses. Decomposition and secondary contamination rapidly reduce diagnostic value. Sampling should follow arrangements with the receiving laboratory, using dedicated equipment and identifiers that link every specimen to its lot, tank, date and time.

The World Organisation for Animal Health (WOAH) Aquatic Manual chapter describes virus isolation in cell culture and molecular methods for confirmation. A positive RT-PCR result must be interpreted against specimen type, validated method, controls and epidemiological context. A molecular signal alone does not automatically prove that VHSV accounts for the complete mortality event. Post-mortem examination, virology and pathology can help connect the agent to lesions and identify concurrent causes.

A 2024 experimental transcriptomic study in rainbow trout also showed that isolates classified as having low or high pathogenicity generated different host responses. Such work improves understanding of host–virus interaction. It does not allow a farm outcome to be predicted from genotype or a PCR result alone.

Protect unaffected units without spreading the hazard

Once VHS is suspected, unnecessary movement of fish, eggs, water and equipment should stop within the relevant containment area. Clean and dirty routes, footwear, nets, grading devices, vehicles and effluent pathways should be mapped. Biosecurity is most valuable when it prevents a local investigation becoming a multi-site incident.

Do not combine lots to balance stocking density or free tank space. Shared equipment should not move until it has been cleaned and disinfected through a procedure suitable for the suspected agent, materials and operator safety. The chosen disinfectant, concentration, contact time, temperature and organic-load limitations need to be recorded rather than assumed.

Quarantine is not simply a number of days. It combines hydraulic separation, dedicated staff or workflow, dedicated equipment, observation, sampling criteria and release rules. A short period without clinical signs does not establish freedom from infection.

A listed disease requires official coordination

WOAH lists infection with VHSV and maintains both an Aquatic Code chapter and a diagnostic manual chapter. National and regional legislation then determines notification, surveillance, movement controls and disease response. In the European Union, the Animal Health Law and its implementing acts provide the framework, but the precise obligations depend on territory, species, establishment type, intended movement and health status.

Elsewhere, authorities may use different categories and response procedures. An operator should therefore neither delay contact while seeking informal certainty nor announce a regulatory status independently. The attending aquatic veterinarian and competent authority can define official samples, restrictions and next steps. Measures written for one jurisdiction should not be transferred to another without checking the current rules.

Prepare the response before an outbreak

A practical contingency plan identifies whom to call, which laboratory will receive samples, how fish will be selected, what containers and forms are needed, how traffic will be separated, and which criteria trigger movement restrictions. Tabletop exercises can reveal missing phone numbers, unsuitable packaging or impossible clean–dirty routes before a real emergency.

The plan must also protect fish welfare while lots are held. Water quality, biomass, feeding, mortality removal and humane endpoints still require active management. Containment is not a reason to allow crowding or system deterioration.

Prevention combines documented sourcing, evaluated suppliers, cohort separation, mortality surveillance and the ability to sample quickly. It cannot guarantee that VHSV will never enter a site, but it can shorten the interval between the first signal and an evidence-based decision.

VHS demands speed without diagnostic haste. Representative fish, complete traceability and early coordination are more valuable than multiple poorly contextualised tests. Vetofish can support farms with contingency planning, sampling, result interpretation, biosecurity audits and coordination with the relevant veterinary and regulatory partners.