VIRUSES Piscine (PRV)

I. Causative Agent and Disease species may be possible reservoirs. Piscine orthoreovirus (PRV), also known as reovirus, V. Diagnosis was identified in 2010 by next genera- HSMI disease is diagnosed by tion sequencing of tissues from farmed histological changes of mononuclear Atlantic salmon in Norway dying from inflammation and necrosis of the heart the disease “heart and skeletal muscle and red skeletal muscle with absence of inflammation” (HSMI). The has pancreatic lesions. PRV replicates in the double-stranded RNA with 10 segments cytoplasm of red blood cells producing and is 72 nm in diameter. There are inclusion bodies similar to EIBS with or three strains of the virus (PRV1, 2, 3) without anemia. This finding suggests a possibly influencing disease outcome relationship of PRV with EIBS as well in different host species under different as HSMI. PRV does not replicate well environmental conditions. in available fish cell lines, requiring molecular detection and sequencing for II. Host Species confirmation of the virus and strain. PRV is reported from Norway, Denmark, Ireland, Chile, Japan and the VI. Prognosis for Host Pacific Northwest (WA, AK, BC, Can- Rarely, 20% mortality from HSMI ada) infecting Atlantic salmon, Pacific has occurred in Atlantic salmon smolts salmon and trout (cutthroat, steelhead, 5-9 months after transfer to seawater. sea-run brown). In Alaska, PRV was However, high levels of PRV genetic ma- sequenced from three stocks of coho and terial are detected in asymptomatic wild one stock of Chinook and unconfirmed and cultured salmonids with no evidence in one stock of chum salmon. of HSMI disease. In one experiment, PRV was infectious for Chinook and III. Clinical Signs sockeye salmon and persisted but did not HSMI, described from Norway in cause fish mortality or HSMI, or other 1999, causes anorexia, lethargy, and apparent disease. Testing of archived tis- ascites with inflammatory lesions of the sues from BC indicated PRV was present heart and skeletal muscle. It is a disease in asymptomatic wild and farmed Pacific of farmed Atlantic salmon (PRV-1) and salmon since 1987, possibly as early (PRV-3) in both freshwater as 1977 before Atlantic salmon were and seawater. PRV-1 has been associated imported for . The ubiquity with jaundice, anemia and degenerative/ of PRV, apparent historic presence in necrotic lesions of the liver/kidney in wild Pacific salmon stocks in the PNW healthy farmed and and lack of clear association with disease PRV-2 is associated with EIBS and suggest the virus is of low risk to wild jaundice/anemia in farmed coho salmon species of Pacific salmon. in Japan. Stress may precipitate clinical disease. VII. Human Health Signifcance There are no human health concerns IV. Transmission regarding infection of fish with PRV. PRV can be transmitted by injection and horizontally. Marine forage fish

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PRV and EIBS viruses are likely related if not the same virus. EIBS/PRV virus in Chinook salmon: (A) Peripheral blood smear with single and multiple cytoplasmic erythrocytic inclusion bodies (arrowhead), scale bar = 5 um; (B) TEM of a single virus inclusion body (arrowhead) in erythrocyte cytoplasm, scale bar = 0.5 um; (C) Higher magnifcation of virus particles (60-70 nm) in the inclusion body of (B), scale bar = 100 nm; (D) Extracellular virus particles associated with cell debris in peripheral blood, scale bar = 100 nm. (From Meyers 2007).

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