Histopathological changes in the swimbladder wall of the European eel Anguilla anguilla due to infections with Anguillicola crassus.

The histopathological changes in swimbladders of European eels naturally and experimentally infected with Anguillicola crassus were studied using transmission and scanning electron microscopy. During the course of probably several infections swimbladders undergo characteristic changes. In addition to the thickening of the entire swimbladder wall, and to the folded internal surface of this organ, inflammation, migration of white blood cells, fibrosis and changes in the epithelial cells are frequently seen. Epithelial cells tend to proliferate heavily and form hyperplastic tissues; these processes are accompanied by changes in the internal structure of the cells. The normally cubic cells become spherical or columnar and form folds facing the lumen of the swimbladder. As a consequence, most of these cells lose contact with the blood vessels and show no strict polarity. In heavily affected swimbladders the basal labyrinth of the epithelial cells is reduced, i.e. becomes shorter and less densely packed. The lamina propria shows severe fibrosis with infiltration of white blood cells. Larvae of A. crassus, inhabiting the wall of the swimbladder, were found to be surrounded by cell debris, but this local necrosis does not affect the entire swimbladder in its overall structure. These histological findings can partly explain changes in the gas composition in eels infected with A. crassus.