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Further Information Released on New Trout Disease

03 March 2015

NORWAY - The Norwegian Veterinary Institute has released a fact sheet around the new disease which was detected in freshwater rainbow trout in 2013. A new virus - Virus Y- was discovered but it is still not characterized and it is not yet confirmed if virus Y causes the disease or not.

The National Veterinary Institute received in August 2013 the first samples of juveniles rainbow trout with an unusual illness. Samples from the second case were received in October and a third case was detected in early November.

The last known eruption in hatcheries was in January/February 2014.

The fish which became ill were around 25-100 g. It was reported that mortality was moderate except on one of the units that had high mortality. All plants had eggs or fry from the same brood group.

Sick fish had gone to clean freshwater or freshwater with added seawater (<1 ‰).

Disease Signs and autopsy findings

Sick fish show signs of circulatory failure. The fish may have protruding eyes and skin bleeding.

Gills and organs are often bleached because of anemia and bloody fluid can be found in the abdominal cavity.

The histopathological findings show varying degrees of inflammation in the heart and red muscle and liver necrosis.

Disease Study

Molecular biological studies of known pathogenic fish viruses only resulted in the detection of small amounts of IPN virus in one of the units. Cultivation for the detection of viruses in cell lines were negative. Pathogenic bacteria was also not detected and a therapeutic trial with antibiotics had no effect.

A new virus (currently called virus Y) was found in samples from diseased fish. The virus is found in blood and tissues and may be associated with the disease. The Veterinary Institute has used sequences from the virus to establish a PCR method for detection of virus Y.

It is not yet confirmed that virus Y causes the disease characterised by heart inflammation and anemia that was detected in the four hatcheries.

In 2014 the Veterinary Institute conducted minor infection experiments with rainbow trout and salmon.

Amounts of virus Y were increased in the blood of rainbow trout injected with infectious material and fresh rainbow trout that were infected via the water. No rainbow was clinically sick, but findings indicated that the disease may have been transmitted.

For salmon, it was also shown that the amount of virus in the blood increased in fish injected with infectious material, but less and slower than in rainbow trout. The salmon showed no signs of illness. There are no results confirming that the virus infects salmon through water.

Although virus Y survives in the blood of fish and is transmitted horizontally to rainbow trout, the relationship between the disease and virus Y is not clarified.

It is possible that virus Y alone can be the cause of disease in rainbow trout or that the virus contributes together with another agent to the disease.

Virus Y can also be a chance finding that can not be linked to the disease. Furthermore it is possible that the virus Y exists in several varieties, where not everyone is pathogenic.

The disease is not transmitted to humans.

Nextgeneration sequencing (NGS) is now being used for further characterization of the virus, as well as looking for other possible pathogenic agents. 

In 2015, two major infection experiments were initiated. One, conducted in cooperation with Technical University of Denmark, aims to clarify the relationship between the disease and virus Y in rainbow trout and salmon if susceptible to the disease.

In 2015, the National Veterinary Institute in cooperation with the Food Safety Authority will implement a monitoring programme to investigate the prevalence of virus Y.

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