A histopathological study on the etiology of intense congestion of the gills of Japanese eel, Anguilla japonica

Histopathology of the disease, which was reported to be characterized by intense congestion in central venus sinuses of gill filaments found in cultured eel was studied with spontaneously diseased fish and exprimentally infected fish by light and electron microscopy. Experimental infection produced similar histopathological changes to those observed in the spontaneously diseased fish. In gill filaments of diseased fish, dilatation of the central venous sinuses was observed. In the liver, hemorrhage in the parenchyma and destruction of blood vessels were observed. Furthermore, hemorrhage in the haematopoietic tissue and degeneration of blood vessels and glomeruli were observed in the kidney. These pathological changes were always accompanied by degeneration of endothelial cell nuclei of blood vessels. The degeneration of the nuclei were characterized by swelling, intense staining of the nuclear rim with hematoxylin, and a homogenous appearance of the nucleoplasm. Electron microscopy revealed hexagonal viral particles in the nuclei of the degenerated endothelial cells. Each virion measured about 80 nm in diameter. The virions were observed only in the degenerated endothelial cells. These results suggest that the disease of cultured eel is a systemic viral infection, which is characterized by the necrosis of endothelial cells of blood vessels.