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Viral Encephalopathy and Retinopathy

What is it?

Viral encephalopathy and retinopathy, also known as viral nervous necrosis, is a disease affecting a wide range of fish, caused by a Betanodavirus, which is in the family Nodaviridae.

The disease has been seen to have the biggest impact in populations of sea bass.

Where and When Might it Occur?

The disease is thought to mainly affect fish in the larval or fry stage, but has been seen in older fish.

The disease has been reported in all inhabited continents apart from Africa.

Transmission is believed to occur horizontally, through the water column and vertically (parent to offspring).

The rate of transmission may be influenced by stressors, including handling, repeated spawning, high stocking densities, high ambient temperature and virulence of the particular Betanodavirus strain.

Sand worms of the family Nereidae, genus Nereis, collected in proximity to an infected farm have had positive detection of Betanodavirus.

Virus can survive for one year in the right environmental conditions (pH 2–9 and 15°C) and can persist subclinically in infected live fish. Therefore, fish products and byproducts may facilitate the spread of virus to unaffected areas.

Diagnosis

The disease often results in 50–100 per cent cumulative mortality over a period of 48 hours to several weeks.

Fish infected with the disease may show anorexia, abnormal swimming behaviours, including erratic, uncoordinated darting, spiral and/or looping swim pattern; corkscrew swimming.

Hyperactivity, sporadic protrusion of the head from the water and fish resting belly-up (loss of equilibrium)can also be seen.

In some fish, a change in colouration is an important indicator of disease. Species differ in how they are affected (e.g. barramundi show lighter colouration when affected).

Gross pathological signs are:

  • colour change—larval barramundi become lighter, but groupers become darker
  • blindness
  • abrasions
  • emaciation
  • overinflated swim bladder (the only significant internal gross pathological sign).

Microscopic pathological signs are:

  • vacuolation of central nervous tissues, including retina
  • intracytoplasmic inclusions in brain tissues as crystalline arrays or aggregates.

SOURCE: Australian Government, Department of Agriculture, Fisheries and Forestry

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