“Experimental infection of rainbow trout with Piscine orthoreovirus 2 (PRV-2) (7119)”

Experimental infection of rainbow trout with Piscine orthoreovirus 2 (PRV-2)

Piscine orthoreoviruses (recently renamed Orthoreovirus piscis by ICTV) have emerged as a challenge for salmonid aquaculture worldwide.

Piscine orthoreovirus (PRV) belongs to the family Reoviridae, has a double stranded RNA genome consisting of 10 segments and double protein capsid with icosahedral symmetry.

Three subtypes of PRV have been reported: PRV-1, PRV-2, and PRV-3.

PRV-1 has caused heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar) in Norway since 1999. The major histopathological findings are in located in the heart and the red skeletal muscle. Affected fish show pancarditis, and severely affected fish also have red muscle inflammation.

PRV-3 was discovered in 2013 following a thorough investigation of unexplained mortalities in rainbow trout (Oncorhynchus mykiss) farmed in fresh water in Norway. Experimental trials showed that PRV-3 replicated in rainbow trout blood and efficiently transmitted to naïve host in a cohabitation trial. Pathology occurs predominantly in heart.

PRV-2 was shown to cause erythrocytic inclusion body syndrome (EIBS) in coho salmon (Oncorhynchus kisutch) in 2016 in Japan. The anaemic condition denoted as EIBS was first described in 1977 in rainbow trout and in 1987 in chinook salmon (Oncorhynchus tshawytscha) from the Pacific Northwest of North America. To date, PRV-2 like virus has been described in Alaska. Little is known about the capability of the virus to infect Rainbow trout.

Specific pathogen free rainbow trout were included in a cohabitation challenge trial. Briefly, PRV-2 infected tissue from Coho salmon was homogenized in EMEM supplemented with antibiotic. Shedder fish were tagged by adipose fin clipping and IP injected with supernatant from the aforementioned homogenate. Fish were cohabited in a tank with naïve non-tagged cohabitants. A group of mock- injected fish was included. The ratio of shedders to cohabitants  was set at 50:50.

At selected time points, 6 shedders and 6 cohabitants were euthanized and organ samples collected to assess viral load and heart histopathological changes. Blood was collected to measure heamatocrit (Hct). Fish were inspected daily over the 7 weeks trial to assess the presence of clinical signs and morbidity.

No morbidity nor reduced survival was observed in the experiment. Infected fish developed severe anemia, yielding Hct value of 10% in the lowest cases.

Efficient transfer of virus from shedders to cohabitants occurred starting from 3 weeks post challenge. Histopathological changes observed in heart were predominantly epicarditis.

Rainbow trout can be considered a susceptible host for PRV-2.

1. VENDRAMIN, NICCOLÓ, DTU AQUA, Presenter
2. Sørensen, Juliane, DTU AQUA, Author
3. Teena, Klinge, DTU AQUA, Author
4. Cuenca, Argelia, DTU AQUA, Author