First report of erythrocytic inclusion body syndrome (EIBS) in chinook salmon Oncorhynchus tshawytscha in Alaska, USA

Abstract

Juvenile seawater-reared chinook salmon Oncorhynchus tshawytscha at a hatchery on Baranoff Island, Alaska, USA, sustained chronic mortality associated with anemia and mixed infections by various obligate and opportunistic pathogens including a cytoplasmic erythrocytic virus resembling the agent of erythrocytic inclusion body syndrome (EIBS). This is the first case report of EIBS in Alaska and is a range extension of the disease. KEY WORDS: Erythrocytic inclusion body syndrome · EIBS · Anemia · Chinook salmon Resale or republication not permitted without written consent of the publisherDis Aquat Org 76: [169][170][171][172] 2007 erythrocytes (Leek 1987, Michak et al. 1992. Isolation and replication of the virus in available fish cell lines has been unsuccessful, thus preventing further characterization. A more recent study of an epizootic in Atlantic salmon (Graham et al. 2002) reported EIBSlike inclusions and virus particles in erythrocytes as well as similar virus particles associated with necrosis in hematopoietic tissues of the spleen and kidney and hyperplasia of gill tissues. The histopathology and virus were morphologically similar to the previously described phagocytolytic syndrome (PCLS) in Atlantic salmon (Palmer et al. 1992). Graham et al. (2002) suggested that EIBS and PCLS may represent alternate or concurrent manifestations of infection by the same virus. The EIBS virus has also been associated with heart and skeletal muscle inflammation in Atlantic salmon from western Norway where typical virus particles were present in vacuoles within erythrocytes and unidentified head kidney cells (Watanabe et al. 2006). This note presents a case study of the first report of EIBS in Alaska, which represents a range extension of the disease. MATERIALS AND METHODSCase history. A salmon hatchery on Baranof Island near Sitka, Alaska, sustained ongoing chronic mortality in brood year 2002 juvenile chinook salmon in late October after transfer to seawater net pens in September 2003. The immediate cause of the fish mortality was diagnosed as vibriosis and was successfully treated by dietary administration of oxytetracycline (OTC). By late December 2003 through early March 2004, additional disease outbreaks in these fish included furunculosis and low level bacterial kidney disease (BKD). Fish mortality was 120 to 400 fish d -1 within a population of 1.25 million fish in 6 net pens. Isolates of Aeromonas salmonicida were resistant to OTC and marginally sensitive to Romet B. Therefore, Romet was not used for treatment due to concerns over less than optimum sensitivity to this antibiotic. Instead, the freshwater lens in each pen was removed, and larger net pens were deployed to optimize the rearing environment. Mortality continued in the smaller fish and was higher than could be expected from osmoincompetency.In early ). Smolt size ranged from 10 to 50 g total weight, and 50% of the mortality occurred in larger fish over 20 g. The seawater temperature was 5°C, fish were lethargic, and the feeding response was ...

Full-Genome Sequencing and Confirmation of the Causative Agent of Erythrocytic Inclusion Body Syndrome in Coho Salmon Identifies a New Type of Piscine Orthoreovirus

“…Similarly, in a previous study of PRV, the blood, spleen, and kidney had the highest viral RNA loads of the 11 tissues tested in HSMI-affected Atlantic salmon [29]. It is generally understood that erythrocytes are the target cells of both PRV species (causative agents of EIBS and/or HSMI), because they form inclusion bodies in these cells [2, 7, 8, 12, 14, 30, 31]. An analysis of the incidence of immature erythrocytes and inclusion bodies led to the hypothesis that immature erythrocytes are initially infected with the causative agent of EIBS [32, 33].…”

“…The virions are also occasionally free in the cytoplasm of erythrocytes or combined within large cytoplasmic vacuoles [10]. The preliminary characterization of this virus with electron microscopy revealed similar characteristics to those of a member of the family Togaviridae [11], but the isolation of the virus in a fish cell line has been unsuccessful, preventing its further characterization [12]. …”

Molecular testing of adult Pacific salmon and trout (Oncorhynchus spp.) for several RNA viruses demonstrates widespread distribution of piscine orthoreovirus in Alaska and Washington

“…PRV replicates preferentially in red blood cells, forming intraerythrocytic inclusion bodies (IBs; Finstad et al, 2014;Haatveit et al, 2016;Wessel, Olsen, Rimstad, & Dahle, 2015) that are morphologically reminiscent of the inclusion bodies associated with erythrocytic inclusion body syndrome (EIBS; Leek, 1987;Piacentini, Rohovec, & Fryer, 1989). However, EIBS is associated with anaemia and is typically found in association with other diseases (Meyers, 2007;Piacentini et al, 1989). No changes in haematocrit values were observed in challenge studies of Atlantic and sockeye salmon conducted using PRV from western North American (Garver, Johnson et al, 2016).…”

Piscine orthoreovirus (PRV) infects Atlantic salmon erythrocytes

“…There have been a number of reports of viral inclusions in salmonid erythrocytes that have been collectively termed EIBS [ 26 ]. No specific viruses have been recognized as causative agents and erythrocytic inclusions assumed to be of viral origin have been observed in salmonid species in both the Atlantic and Pacific, including Atlantic salmon, rainbow trout ( O. mykiss ), Chinook salmon ( O. tshawythscha ) and Coho salmon ( O. kisutch ) [ 21 , 23 , 55 ]. The PRV particles and inclusions described in our study have a striking resemblance to several of the previously reported viral inclusions described as EIBS [ 21 - 25 , 55 ].…”

“…No specific viruses have been recognized as causative agents and erythrocytic inclusions assumed to be of viral origin have been observed in salmonid species in both the Atlantic and Pacific, including Atlantic salmon, rainbow trout ( O. mykiss ), Chinook salmon ( O. tshawythscha ) and Coho salmon ( O. kisutch ) [ 21 , 23 , 55 ]. The PRV particles and inclusions described in our study have a striking resemblance to several of the previously reported viral inclusions described as EIBS [ 21 - 25 , 55 ]. Pinacyanol chloride staining has previously been used in identification of EIBS [ 26 , 56 ], and in the present study we observed stained inclusions using this techniques.…”