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Virus associated disease in rainbow trout (virus Y)

The Norwegian Veterinary Institute diagnosed in 2013 a new disease in rainbow trout in freshwater. Typical findings were circulatory failure, anaemia, inflammation of the heart and skeletal muscle and liver necrosis. There is strong evidence of a viral etiology. The virus is closely related to Piscine orthoreovirus associated with heart- and skeletal muscle inflammation in Atlantic salmon. No new cases of the disease were observed in 2015.

Infectious agent and transmission routes
During disease investigation in hatcheries of rainbow trout in 2013, the Norwegian Veterinary Institute (NVI) isolated a gene sequence from a new virus. This virus is closely related to Piscine orthoreovirus (PRV) which is associated with heart- and skeletal muscle inflammation in Atlantic salmon (HSMI). NVI uses the working name "virus Y" pending taxonomic data that can provide the basis for naming. The gene sequence was found in tissue and blood from diseased fingerlings of rainbow trout and in contact farms for these hatcheries. This included both broodstock groups and on-growing farms. In addition, small amounts of virus Y were detected in randomly selected historical material from the counties Hordaland and Møre and Romsdal back to 2011. Virus Y was detected until 15 months after seawater transfer and there is reason to believe that subclinical disease may occur.

Infection trials conducted in 2014 and 2015 showed that the virus can be transmitted to both rainbow trout and Atlantic salmon by the injection of red blood cells from diseased rainbow trout. It is shown that the virus is transmitted horizontally through the water between individuals of the same species. The virus replicates more slowly in salmon than in rainbow trout. No tests are carried out to show whether the disease is transmitted between species.

Occurrence
NVI received in the period August 2013 to January 2014 samples from four rainbow trout hatcheries. The fish was from 25 to 100 g. Moderate mortality rates were reported except in one of the farms where high mortality was observed in some tanks. All farms had received eggs or juveniles from the same group of brood fish.

Diseased fish were exposed to freshwater or water with low salinity (< 1‰). Fish from hatcheries affected by the disease were transferred to seawater. In two cases high mortalities after seawater transfer was reported, but the mortality ceased after a few days. Two farms experienced disease two to three months after sea transfer. In the other cases no specific problems in seawater were recorded.

From a preventive perspective, the fish farmers carried out measures to mitigate the risk of spreading the disease. This virus associated disease was not diagnosed at the NVI during 2015. However, the virus was detected in a risk based surveillance program. Six out of 29 farms investigated per November 2015 were positive for virus Y. These were located in the southwest and mid of Norway. In three of these farms the virus had not been detected earlier. The sites were, however, located in areas where the virus previously had been detected. Neither the disease nor the virus has been detected in Norwegian farmed Atlantic salmon.

Disease signs and necropsy findings
Diseased rainbow trout had reduced appetite and signs of circulatory failure with bulging eyes and bloody fluid in the abdominal cavity. Gills and viscera were often pale due to anaemia. The histopathological findings showed varying degrees of inflammation of the heart and red muscle and liver necrosis. There were no significant clinical signs in fish that were experimentally infected with the infectious agent, but heart inflammation in experimentally infected rainbow trout was detected.

Prevention and control
The diagnosis is confirmed by histopathology and detection of virus by PCR developed by the Norwegian Veterinary Institute. There is no treatment or vaccine against the disease. A general advice is that diseased fish should be handled as little as possible.

Future work at the Norwegian Veterinary Institute
The Norwegian Veterinary Institutehas conducted infection trials in collaboration with the Technical University of Denmark and is still working on further characterization of the virus and the disease. In 2016 the NVI in cooperation with the Norwegian Food Safety Authority, will conduct a surveillance program to investigate the prevalence of virus Y in rainbow trout and salmon in areas where the virus previously has been detected.

 

Contact person, Norwegian Veterinary Institute
Brit Hjeltnes

More information can be found on the web-page of NFSA.

 

Updated December 2015/AGG