Histopathological Studies on Gill-I

A Bacterial Gill Disease of Cultured Eel

Abstract

A bacterial gill disease named “Eragusare-byo” occurred among pond-cultured eels in a eel farm in Fukui Prefecture during March 1974. The pond-cultured eels with the disease included two species, Japanese eels (Anguilla japonica) and European eels (A. anguilla), and the water temperature of the ponds was above 20°C throughout the year. Histopathological studies on these diseased eels were done by the author and the following results were obtained.
The eariest histopathological changes were observed on the gill lamellae of the two species of eels as inflammatory edema, swelling of the epithelial cells and excessive secretion of mucin. Column-like protrusions formed by a number of slender, rod-shaped Gram-negative bacteria were often found on the surfaces of the gill lamellae in these eels.
In the subsequent inflammatory processes, there were marked differences regarding histological changes between Japanese and European eels. In the case of Japanese eels, there were three types of processes in the formation of “clubbing” of gill filaments. The type I was caused by hyperplasia of the lamellar epithelium at the distal tips of the lamellae. The type II was caused by hyperplasia of the epithelium in all the regions of the lamellae. The type III was caused by hyperplasia of the lamellar epithelium at the distal tips and basal parts of the lamellae. In many cases these three types of processes of the clubbing formation were recognized in the same individual. In a more aggravated case the clubbed filaments had gangrenous inflammation owing to combination of circulatory failure and putrifying bacterial infection, and finally they fell off from the gill arches. In European eels, the clubbing of the filaments was mainly caused by hyperplasia of mucous cells, but necrosis or defect of the clubbed filaments was never recognized.