Histopathological study of gills in experimentally amoebic gill disease (AGD) infected Atlantic salmon, Salmo salar, L.

Amoebic gill disease (AGD) is the most important parasitic disease of Atlantic salmon industry in Australia. Atlantic salmon (Salmo salar) experimentally infected with Neoparamoeba sp. apparently showed AGD gross signs on the gill and an amoebic-associated gill pathology. Physico-chemical factors of water during the experiment were monitored regularly and were approximately constant (temperature: 17°C, salinity: 35 g/1, total ammonia: 0.25 mg/l, pH = 7.9). In this study significant gill pathology was observed histologically, and in all of the sections a multifocal hyperplasia and fusion of adjacent secondary lamellae was seen. The severity of pathological changes observed in the sections did not always correspond with the number of amoebae and even occurred in the absence of amoebae. Some histopathological changes that were seen in the secondary lamellae are: thickening of the secondary lamellae due to hyperplasia, reduction in chloride cell density and an increase in mucous cell numbers of the epithelium. Some of neighboring secondary lamellae was seen attached to one another, but entire fusion of the primary lamellae was not observed. Amoebae were seen in all sections in significant densities mostly in the outer part of hyperplasic tissues.