IHN in salmonids: surveillance before an outbreak

Infectious haematopoietic necrosis (IHN) cannot be diagnosed from a dark fry or an increase in mortality alone. This WOAH-listed viral disease primarily concerns salmonids and requires a prepared response: retain animals that are useful for diagnosis, trace stock movements, restrict risky flows and contact the veterinarian and, where appropriate, the competent authority promptly. A farm should not wait for a textbook clinical picture. It should be able to detect, document and contain a credible suspicion.

Separate infection risk from clinical disease

IHN is caused by Salmonid novirhabdovirus, commonly known as infectious haematopoietic necrosis virus or IHNV. The WOAH Aquatic Manual describes early life stages as particularly susceptible. Resistance generally increases with age, while returning spawning adults may carry and shed substantial virus without obvious clinical disease.

This difference changes surveillance design. Acute mortality in fry and subclinical infection in broodstock are not investigated at the same time or with identical samples. Water temperature, species, stage, viral strain and rearing conditions all influence disease expression. A negative result from a low-risk group or an unfavourable sampling window is therefore not a blanket certificate for the whole farm.

Transmission between fish is mainly horizontal through contact with infected fish or contaminated water. WOAH considers the evidence insufficient to establish true vertical transmission within the egg. Egg movements still matter because surfaces, transport water, packaging and equipment may move the hazard when source health and disinfection are poorly controlled.

Define signals that trigger investigation

Lethargy, bursts of abnormal swimming, darkening, abdominal distension, exophthalmia and haemorrhage may occur during IHN outbreaks. None is pathognomonic. Water-quality deterioration, another infection or several simultaneous challenges can produce a similar presentation.

A useful trigger is often a departure from the normal pattern of the stock: mortality above baseline, weak fish accumulating near the outlet, reduced feeding or unusual distribution in the unit. Staff need to connect these observations with introduction dates, egg or fish origin, recent operations, water temperature and hydraulic links.

When an alert is raised, do not immediately discard every weak or freshly dead animal. The Manual prioritises moribund or clinically affected fish, ideally collected alive; older carcasses increase bacterial contamination and lose diagnostic value. Contact the veterinarian and laboratory before fixation, freezing or addition of a product that could compromise virus isolation or molecular detection.

Match samples to the diagnostic question

For clinically affected populations, WOAH identifies tissues including anterior kidney, spleen, heart or brain. In apparently healthy groups, anterior kidney and brain may be especially relevant. Very small fish may need to be pooled to provide enough material, but pooling should only be chosen when test performance and the sampling design support it.

Cell culture and real-time RT-PCR are among the methods recommended in the Manual for surveillance, presumptive diagnosis and confirmation, depending on the purpose and life stage. Amplification must be interpreted with the matrix, controls, contamination risk and confirmation criteria. Sequencing of an amplicon may contribute to confirmation of a suspect result.

Storage and transport are part of the test. Tissues intended for virus isolation should remain cool and be processed promptly. Histopathology uses a separate pathway with immediate fixation and an appropriate fixative-to-tissue ratio. Placing every tissue in one medium “to be safe” can remove diagnostic options. A response kit agreed with the laboratory prevents this mistake.

Contain the risk without destroying evidence

Stabilise movement patterns as soon as suspicion is credible. Pause non-essential transfers from the affected stock, identify shared equipment and dedicate tools to the tank or epidemiological unit. Work should proceed from units with the most protected status towards the suspect area, followed by the defined procedure for clothing, footwear, hands, surfaces and wastewater.

Disinfection only works when prior cleaning, concentration, contact time and material compatibility are controlled. Disinfectant residue on sampling tools may reduce viral titre or inhibit analysis. Biosecurity must therefore protect other stocks while preserving sample quality.

Hydraulic mapping is a priority: can the tank be isolated, does it share recirculation, is untreated surface water used, are effluents connected, and can equipment create a return path? Walls and doors do not create a health compartment on their own. Water, fish, eggs, equipment and staff flows determine whether separation is credible.

Place international guidance in the legal context

IHN is subject to aquatic animal health rules covering surveillance, movements and disease status. European Animal Health Law provides a general framework, while implementing acts and decisions by the competent authority determine the measures that apply to a territory, species, establishment type and epidemiological situation.

An internal response sheet should not convert a WOAH recommendation into an automatic national legal requirement. It should say who contacts the veterinarian, who can halt a movement, where certificates and source status are recorded, and how the competent authority is reached. This preparation speeds the response without pre-empting official decisions.

For businesses operating across jurisdictions, regulatory status and reporting routes must be checked locally. The biological principles travel more readily than the legal procedure. A corporate protocol should therefore preserve a shared evidence standard while leaving clear space for country-specific notification and movement rules.

Make routine surveillance decision-ready

Useful surveillance combines daily observations, mortality by unit, water temperature, stock origin and husbandry events. It defines escalation criteria suited to the production system before an incident. WOAH supports risk-based selection and inclusion of production units supplied by different water sources; weak, abnormally behaving or freshly dead fish are prioritised when present.

After a negative investigation, keep the original question visible. Did sampling cover the relevant stocks, life stages and time window? After a positive finding, distinguish confirmed facts from assumptions about origin. This prevents a laboratory result from being used to invent a transmission chain that the evidence has not demonstrated.

Conclusion

IHN management starts before the first suspect fish. Young salmonids may be highly susceptible, observable signs are non-specific and broodstock may be infected without an obvious clinical picture. A strong alert depends on traceable observations, appropriate animal selection, samples divided between the correct laboratory pathways and biosecurity that protects both other stocks and diagnostic evidence.

Vetofish can support farms with flow mapping, IHN response planning, laboratory sampling arrangements and interpretation of results within their epidemiological and regulatory context.