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Nodavirus

 

Nodavirus

Fish nodavirus
Viral encephalopathy and retinopathy (VER)
Persistent nodavirus infection
Diagnosis

List of papers

Fish nodavirus

Nodaviruses are small (25-34 nm) icosahedral, single stranded viruses with positive sense RNA genome.		  

Viral encephalopathy and retinopathy (VER)

Nodavirus infects the central nervous system (CNS) of the fish and the disease is called viral encephalopathy and retinopathy (VER) or viral nervous necrosis (VNN). Nodavirus are found in bouth cold and warm seawater and VER is documented in more then 30 marine fish species worldwide. Grotmol et al first described VER in Atlantic halibut in 1995. Since then VER has been diagnosed in several halibut farms in Norway and is one of the major problems in the halibut farming industry. The clinical signs of acute VER in juvenile fish are lack of appetite, change of pigmentation, neurological symptoms like abnormal swimming behavior and high mortality rates up to 100%. Histopathology shows degeneration and vacuolation of brain and retina.

 

Persistent nodavirus infection

Subclinical persistent nodavirus infections in juvenile Atlantic halibut are under investigation at to fish farms in Norway. The pathological findings in these fish differ from acute VER. The degeneration and vacuolization are moderate and the virus infected cells form clusters. The cell clusters are immunopositive for nodavirus and are seen in all parts of the brain and all cell layers of the retina. So fare we have looked at juvenile Atlantic halibut up to 8 months age.

Electron microscopic studies of the cell clusters show large macrophage like cells with multiple large membrane bound inclusions filed with virus particles. The virus particles have sometimes form membrane-bound necklace-like arrangements. In the retina some of the virus-infected cells contain melanin-like granula mostly inside the virus inclusions. The source and role of the melanin-like granulas are unknown.

Diagnosis

VER is diagnosed by histopatology and immunohistochemistry. (Photo 1 and 2) Histopathology shows degeneration and vacuolation of brain and retina.

Spawners are believed to be the major source of infection. In Japan they lowered the nodavirus problems in the striped jack larvae by using nodavirus free spawners. The spawners where tested for nodavirus with enzyme linked immunosorbent assay (ELISA) and polymerize chain reaction (PCR). We now have an ELISA and PCR test for nodavirus in Atlantic halibut and the test will be examined on field material in 2001.