Viral Nervous Necrosis (VNN / VER): A Guide for Marine Aquaculture
Viral nervous necrosis (betanodavirus) is a neurotropic disease causing very high mortality in marine fish larvae. There is no treatment; control relies on biosecurity.
Overview
Viral nervous necrosis (VNN), also known as viral encephalopathy and retinopathy (VER), is one of the most devastating diseases of marine aquaculture. It is caused by betanodavirus, a non-enveloped, icosahedral, bipartite positive-sense single-stranded RNA virus of the family Nodaviridae, with a genome divided into segments RNA1 and RNA2. The virus is neurotropic, targeting the central nervous system and retina. It is a notifiable disease of the World Organisation for Animal Health (WOAH).
Genotypes and affected species
Four recognized genotypes are red-spotted grouper nervous necrosis virus (RGNNV), striped jack nervous necrosis virus (SJNNV), barfin flounder nervous necrosis virus (BFNNV), and tiger puffer nervous necrosis virus (TPNNV). The disease affects a very wide host range of marine fish, with more than 40 to 50 reported susceptible species. Important cultured hosts include groupers (Epinephelus spp.), European sea bass (Dicentrarchus labrax), Asian sea bass or barramundi (Lates calcarifer), striped jack (Pseudocaranx dentex), and, through reassortant strains, gilthead sea bream (Sparus aurata). Larvae and juveniles are most severely affected, though adults can also be infected.
Clinical signs
- Abnormal swimming behavior, including erratic, spiral, or whirling movement
- Anorexia and lethargy
- Darkening or other changes in skin pigmentation
- Swim bladder hyperinflation and loss of balance
- Blindness and other ocular signs
- Vacuolation and neuronal degeneration in the brain and retina (on histology)
Transmission
Betanodavirus spreads both horizontally and vertically. Horizontal transmission occurs through water and direct contact within hatchery and grow-out systems. Vertical transmission passes the virus from infected broodstock to eggs and larvae, which is a major route for outbreaks in hatcheries because it seeds infection before larvae are stocked.
Diagnosis
Diagnosis is based on reverse transcription PCR (including real-time RT-PCR), supported by histopathology showing characteristic vacuolation of the brain and retina, and immunohistochemistry to localize viral antigen. Because nervous signs overlap with other conditions, laboratory confirmation is required.
Treatment, control and prevention
There is no treatment for VNN. Management depends on preventing introduction and breaking vertical transmission.
- Screen broodstock by real-time RT-PCR and exclude positive animals from breeding
- Use specific-pathogen-free (SPF) broodstock and eggs where available
- Disinfect eggs and treat incoming water, including UV sterilization
- Apply strict hatchery biosecurity and equipment disinfection
- Fallow and disinfect facilities between production cycles
- Follow WOAH surveillance and reporting protocols