Marine betanodavirus: diagnose before moving fish

Viral encephalopathy and retinopathy, also known as viral nervous necrosis, can cause severe losses in marine fish larvae and juveniles. Abnormal swimming draws attention, but it cannot identify a betanodavirus on its own. Sound control combines the epidemiology of the affected group, appropriate specimens, laboratory confirmation and immediate movement restrictions. A recent outbreak in wild groupers in the Azores also shows why adult fish and warm-water episodes should remain within the investigation.

Treat neurological signs as a suspicion

Spiral swimming, loss of balance, abnormal position in the water column, reduced feeding and darkening may be compatible with viral nervous necrosis. The same observations can occur with hypoxia, toxic exposure, trauma, other infections or abrupt water-quality failure. Staff should therefore document species, life stage, origin, onset, speed of spread, mortality, water temperature and recent husbandry events before assigning a cause.

The WOAH Aquatic Manual describes vacuolation and necrosis predominantly in the brain, spinal cord and retina. Compatible histological lesions strengthen the case, but decomposition, unsuitable freezing or late sampling can obscure them. Recently affected moribund fish are generally more informative than old carcasses. Tissues must be allocated separately for virology, molecular testing and histology according to laboratory instructions.

Match specimens to the diagnostic question

Brain and eye are priority tissues in fish with neurological signs. Whole small fish may be required at larval or early juvenile stages. Preservation differs between virus isolation, RNA detection and fixed tissue examination, so the diagnostic laboratory should specify containers, temperature, dispatch time and acceptable pooling before sampling begins.

RT-qPCR can detect and estimate the amount of a viral RNA target, but a positive result must be interpreted alongside controls, specimen type and pathology. It does not prove that the virus accounts for every death. A single negative result is likewise weak evidence if the wrong fish or tissue was sampled, the specimen deteriorated or the viral load was low. Cell culture, immunohistochemistry and sequencing may add evidence when they answer a defined question.

Genotyping can connect or separate epidemiological events, but genotype names are not automatic predictions of severity. Betanodaviruses include several genotypes and reassortants, while susceptibility varies with host species, age and temperature. Decisions should reflect the complete case record rather than one sequence label.

Stop movement while evidence is gathered

Once suspicion is credible, biosecurity is more urgent than an improvised treatment. Transfers of fish, eggs, water and shared equipment between units should pause. Nets, siphons, tanks and sampling equipment need clear identification and dedicated use. Work should progress from lower-risk groups toward suspect groups, with validated cleaning, equipment changes and disinfection between zones.

Quarantine works only when introductions have separate water and equipment, written surveillance criteria and a duration suited to the species, risk and temperature. Apparently healthy fish may carry viral material. Broodstock and their products require particular attention where vertical transmission is plausible. Screening, separation of egg batches and family-level traceability can reduce risk, although none provides absolute assurance.

Cleaning must remove organic matter before disinfection. The product, concentration, contact time and temperature should be validated for the surface or system. An improvised procedure may fail against the agent while harming fish or staff. Effluent, carcasses and biological waste need a defined route agreed with the attending veterinarian and competent authority when reporting or disposal rules apply.

Use temperature as context, not a verdict

The 2024 Azores event affected adult dusky groupers and coincided with a severe marine heatwave. The investigation confirmed betanodavirus as the primary cause of mortality and found highly homogeneous viral sequences, consistent with a recent introduction. The authors considered high temperature a plausible contributor to activation and spread, but did not establish it as the sole cause.

That distinction matters in aquaculture. Temperature influences viral replication, host immunity, metabolism and oxygen availability. Abruptly changing temperature without understanding the species’ tolerance may add another stressor. Farms should instead compare thermal records, mortality and viral results, check oxygen and other water variables, and agree any intervention with veterinary oversight.

Plan recovery around measurable criteria

Recovery is more than the disappearance of visible signs. A written plan identifies which groups may move, what tests are required, how units were cleaned and disinfected, and how replacement stock will be monitored. Repeated negative results are meaningful only when the sampling design defines the population, timing, target tissues and expected diagnostic sensitivity.

The outbreak record should retain mortality curves, raw laboratory reports, sequence data where available, temperatures, movements and interventions. These data help distinguish persistence from a new introduction, improve hatchery control points and train staff to recognise a genuine alert without treating every abnormal swim as VNN.

Conclusion

Visual signs cannot confirm marine betanodavirus, and preventing spread is the first operational priority. Carefully selected fish, well-preserved neural tissues and complementary tests establish the agent’s role. Prevention depends on separated groups, controlled introductions, traceability and cautious interpretation of temperature and husbandry conditions.

Vetofish can support sampling plans, clinical and virological interpretation, movement-risk mapping and practical recovery criteria for marine fish facilities.