A histopathological study of yellowtail ascites virus (YAV) infection of fingerling yellowtail, Seriola quinqueradiata.

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Abstract
A histopathological study was made on yellowtail fingerlings with ascites obtained from a hatchery in Kagawa Prefecture [Japan] in June, 1983 and those inoculated with YAV isolated form them. The inoculation experiment was done in June, 1985. Living inoculated fish were sampled at random daily after inoculation. The most characteristic pathological changes observed in naturally infected fish were extensive pancreatic acinar cell necrosis, hepatic parenchymal cell necrosis, and hepatic hemorrhage. Kidney tubule cell necrosis, gastric edema and desquamative catarrh of the pyloric caeca and intestine were observed in part of the fish studied. No pathological change was observed in any organ of all the inoculated fish examined 1 to 3 days after inoculation. Minor necrotic lesion was obseved in the pancreas of 1 of the 4 fish sampled 4 days after inoculation. Extensive pancreatic necrosis and hepatic necrosis and hemorrhage similar to those observed in naturally infected fish were observed in all the inoculated fish 5 days after inoculation. Kidney tubule cell necrosis was barely observed only in half of the fish. In 3 of the 11 fish sampled 6 to 10 days after inoculation necrosis was observed both in the pancreas and in the liver, but in 4 of the 11 fish extensive lesions were observed only in the pancreas. Moreover regeneration of pancreatic tissue was observed in extensive lesions in fish sampled 8 and 10 days after inoculation. No distinct pathological changes was observed in any organ in the remaining 4 fish. The observations of naturally and experimentally infected fish strongly suggested that acinous tissue of the pancreas and parenchymal tissue of the liver were primary tissues involved in fingerling yellowtail YAV infection. Moreover the observations of experimentally infected fish suggested that necrosis occurred earlier and progressed more rapidly in the pancreas than in the liver and that the fish in which the pancreas was extensively infected but the liver remained uninfected could survive. The kidney, stomach, pyloric caeca and intestine were involved in most severely infected fish.

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