Isolation of viruses from wild Japanese flounder Paralichthys olivaceus

For a survey on fish pathogenic viruses among wild Japanese flounder Parali chthys olivaceus, 274 fish (body weight: 29-2240 g) were collected in 9 coa stal areas of Japan in 1999 and 2000. Aquabirnaviruses were isolated, using RTG-2 and/or SSN-1 cells, from 111 fish (40.5%) caught in almost all the c oastal areas and the representative isolate was identified as YTAV (yellowt ail ascites virus) by cross-neutralization tests. Besides aquabirnaviruses, VHSV (viral hemorrhagic septicemia virus) was isolated, using EPC, SSN-1 a nd/or FHM cells, from 18 fish (6.6%) collected in 2 areas mainly in winter and spring. In a pathogenicity test, 4 isolates of aquabirnaviruses did not cause any mortality or morbidity in young Japanese flounder (average body weight 8.7 g) by intramuscular injection at 10(3.0)-10(7.6) TCID50/fish. In oculation of an isolate of VHSV at 10(2.0) and 10(6.0)TCID(50)/fish produce d mortalities of 60% and 100%, respectively, in flounder (24.8 g). Common e xternal signs of flounder experimentally infected with VHSV were dark color ation of the body with hemorrhagic fins and abdominal swelling, and the int ernal signs included ascites and extensive hemorrhage in the muscle and/or viscera.