Characteristics and Pathogenicity of a Virus Isolated from Yellowtail Fingerlings Showing Ascites

Abstract

In early summer, 1983, an acute disease characterized by ascites occurred among yellowtail fingerlings, Seriola quinqueradiata, cultured in a farm in the Seto Inland Sea, Japan, and accordingly virological investigations were carried out in July. All three inoculants prepared from the viscera of the fish with ascites produced cytopathic effects (CPE) of pyknosis on RTG-2, CHSE-214 and EK-1 cells within 1.5–5 days after inoculation at 20°C. The virus replicated in CHSE-214 and EK-1 cells at temperature between 5–30°C. Electron micrographs revealed pentagonal or hexagonal particles of 62–69 nm. The virus was resistant to pH 3, pH 11, ether and chloroform and was stable at 56�C for 30 minutes. Replication of the virus was not inhibited by 5-Iodo-2′-deoxyuridine. Neutralization tests with antisera proved that this virus resembled to IPNV. Pathogenicity of the virus was studied using yellowtail fingerlings (average body weight 1.1 g). Fish were exposed to the virus by immersion for 1 hour in seawater containing 106.6 TCID50/ml of the virus. The fish infected with the virus showed mortality of 62% at water temperature of 20°C and 14% at 25°C, while controls showed mortality of only 4% at 20°C and 10% at 25°C, respectively. Moribund fish infected with the virus were often accompanied by severe ascites and hemorrhage in the liver. The virus reisolated from all the infected fish died. The results strongly suggest that this virus was the causative agent of the ascites of yellowtail fingerlings. The agent was tentatively named yellowtail ascites virus (YAV). © 1985, The Japanese Society of Fish Pathology. All rights Reserved.