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 an d experimentally infected with Anguillicola crassus were studied using tran smission and scanning electron microscopy. During the course of probably se veral infections swimbladders undergo characteristic changes. In addition t o the thickening of the entire swimbladder wall, and to the folded internal surface of this organ, inflammation, migration of white blood cells, fibro sis and changes in the epithelial cells are frequently seen. Epithelial cel ls tend to proliferate heavily and form hyperplastic tissues; these process es are accompanied by changes in the internal structure of the cells. The n ormally cubic cells become spherical or columnar and form folds facing the lumen of the swimbladder. As a consequence, most of these cells lose contac t with the blood vessels and show no strict polarity. In heavily affected s wimbladders the basal labyrinth of the epithelial cells is reduced, i.e. be comes shorter and less densely packed. The lamina propria shows severe fibr osis with infiltration of white blood cells. Larvae of A. crassus, inhabiti ng 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 over all structure. These histological findings can partly explain changes in th e gas composition in eels infected with A. crassus.