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Betanodaviruses are the causative agents of viral nervous necrosis (VNN) or viral encephalopathy and retinopathy (VER) in cultured marine fish. A total of 131 apparently healthy fish from 30 species were collected in two geographically remote aquaculture areas, Yashima Bay (Kagawa Prefecture) and Tamanoura Bay (Nagasaki Prefecture), in Japan. The brains of fish were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and nested PCR to detect the coat protein gene of betanodavirus. In Yashima Bay, two and 13 of 20 cultured fish were positive for nodavirus in RT-PCR and nested PCR, respectively, and four of five wild fish were positive only in nested PCR. In Tamanoura Bay, 28 and 99 of 106 wild fish were positive for the virus in RT-PCR and nested PCR, respectively. All the sequences of the nested PCR products (177 nt) from 27 fish species (10 cultured and 17 wild) were highly homologous to each other (99-100%) and were closely related to that of the known betanodavirus, redspotted grouper nervous necrosis virus (RGNNV). These results illustrate that large populations of cultured and wild marine fish in aquaculture areas are subclinically infected with genetically closely related betanodaviruses, suggesting an importance of such infected fish as a carrier or reservoir of betanodaviruses.

A rhabdoviral disease occurred in farmed populations of market sized Japanese flounder (hirame) Paralichthys olivaceus in the Seto Inland Sea of Japan in 1996. The causative agent was identified as viral hemorrhagic septicemia virus (VHSV) based on morphological, immunological, and genetic analyses. Diseased fish that were artificially injected with a representative virus isolate showed the same pathological signs and high mortality as observed in the natural outbreak. This is the first report of an outbreak of VHSV infection in cultured fish in Japan. Clinical signs of diseased fish included dark body coloration, an expanded abdomen due to ascites, congested liver, splenomegaly, and a swollen kidney. Myocardial necrosis was most prominent and accompanied by inflammatory reactions. Necrotic lesions also occurred in the liver, spleen and hematopoietic tissue, and were accompanied by circulatory disturbances due to cardiac failure. Hemorrhagic lesions did not always appear in the lateral musculature. Transmission electron microscopy revealed many rhabdovirus particles and associated inclusion bodies containing nucleocapsids in the necrotized myocardium. The histopathological findings indicated that the necrotizing myocarditis could be considered a pathognomonic sign of VHSV infection in Japanese flounder.KEY WORDS: Viral hemorrhagic septicemia · Viral hemorrhagic septicemia virus · Paralichthys olivaceus · Japanese flounder · Hirame · Necrotizing myocarditis Resale or republication not permitted without written consent of the publisherDis Aquat Org 47: [87][88][89][90][91][92][93][94][95][96][97][98][99] 2001 Japan during a survey on the distribution of fish viruses in wild marine fishes (Takano et al. 2000).Clinical signs and histopathological features of VHSV infection in some freshwater and marine fishes resemble those in rainbow trout. A common feature is the widespread hemorrhaging in the external and internal organs, although the severity of hemorrhaging varies among fish species. Rainbow trout displays most severe hemorrhagic lesions in the lateral musculature (Yasutake & Rasmussen 1968, Yasutake 1970, 1975, Horlyck et al. 1984, Smail & Munro 1989. Hemorrhagic lesions are also evident in pike Esox lucius (Meier & Jørgensen 1980), sea bass Dicentrarchus labrax (Castric & de Kinkelin 1984) and turbot (Ross et al. 1994), but are less obvious in Pacific herring (Kocan et al. 1997). Histopathologically, necrotic changes are observed in various visceral organs incluging the liver, spleen, hematopoietic tissue and pancreatic acini in many fish species except Pacific cod Gadus macrocephalus, which displays only dermal lesions (Meyers et al. 1992).VHSV is a member of the family Rhabdoviridae, genus Novirhabdovirus, to which infectious hematopoietic necrosis virus (IHNV) and hirame rhabdovirus (HIRRV) belong (Walker et al. 2000). HIRRV, the causative agent of the HIRRV disease (HIRRVD), was widely distributed in farmed hirame and other marine fishes in Japan , Nishizawa et al. 1991. HIRRV caused mass mor...

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