A Light and Electron Microscopic Study on Yellowtail Fingerlings with Ascites

Abstract

A light and electron microscopic study was made on yellowtail fingerlings with ascites obtained from a fish farm in Shizuoka Prefecture in June, 1984. The most characteristic symptom of the disease was abdominal ventral distention caused by accumulation of low viscous and light yellow transparent filuid. General gross findings were redness of the liver and pallor of the gills. Histopathological examination revealed hepatic partial necrosis with congestion as well as severe pancreatic necrosis, which occurred earlier than the hepatic lesions, and partial edematous changes in both submucosa of the stomach and ellipsoids of the spleen. The kidney showed vacuolar degeneration of renal tubules. In addition, slight catarrh was seen in the pyloric caeca and intestine. On examination under the electron microscope, degenerated hepatocytes showed vesiculation of rough endoplasmic reticulum or vacuolation of mitochondria. Hexagonal or round virus particles measuring 62.2-68.9 nm in diameter without envelope were found in the cytoplsm of degenerated hepatocytes. These particles were gathered in vaculoles or scattered in the whole cytoplasm but never seen in the nucleus. Almost all pancreatic acinar cells showed decrease of zymogen granules and vesiculation of rough endoplasmic reticulum. A few virus-like particles measuring 58.8-64.7 nm in diameter were found in a small cytoplasmic vacuole of degenerated pancreatic acinar cells.