VNN/VER is a notifiable disease in Norway (Category F), and any suspicion or detection must be reported immediately to the Norwegian Food Safety Authority.
Pathogen and Transmission
Viral nervous necrosis (VNN) or viral encephalopathy and retinopathy (VER) is caused by infection with nodavirus.
There are different variants of the virus, and each fish species appears to have its own variant. However, these variants may also cause disease in other species, meaning cross-species transmission can occur. The virus is transmitted both vertically and horizontally. Infected fish are the main source of transmission, but the virus is highly resilient and can survive in water for long periods.
Clinical Signs
The virus damages nervous tissue in the brain, spinal cord, and retina, and maycause significant losses during the larval and juvenile stages. Typical clinical signs of VNN/VER include spiral swimming, rolling, swimming with belly-up, lack of appetite, and changes in pigmentation. Flatfish may lie on the bottom with elevated head and tail due to spasms. Upon necropsy, few changes are typically observed aside from an enlarged swim bladder.
Diagnosis
VNN/VER is diagnosed by identifying characteristic histopathological lesions in the central nervous system and detecting nodavirus via immunohistochemistry, virus isolation, or molecular techniques.
For routine testing related to monitoring, import, or export, notification of Mattilsynet is not required.
Occurrence
Nodavirus infection has been detected in marine farmed fish species and, in some cases, in wild fish. VNN/VER is globally distributed and is considered a major challenge in aquaculture due to the high losses it can cause in fry and juvenile fish.
In Norway, VNN/VER has been confirmed in Atlantic halibut and turbot since the mid-1990s. The disease was first detected in Atlantic cod in 2006.
The virus has also been detected in wild-caught cleaner fish species along the coast, but so far, these findings have not been linked to disease outbreaks in those species.
Limited farming of susceptible marine species in Norway has resulted in only sporadic cases in recent years.
Surveillance
The Norwegian Food Safety Authority and the Norwegian Veterinary Institute previously carried out a surveillance and control programme for VNN. This programme was concluded in 2003.
Control Measures
VNN/VER is notifiable in Norway (Category F), and any detection or suspicion must be reported immediately to the Norwegian Food Safety Authority. The disease was previously listed by the World Organisation for Animal Health (WOAH), but due to its global prevalence, it was removed from the list and is no longer subject to WOAH reporting requirements.
