Pathology of experimental amoebic gill disease in Atlantic salmon, Salmo salar L., and the effect of pre‐maintenance of fish in sea water on the infection

Abstract

Atlantic salmon were exposed to amoebic gill disease (AGD) immediately following their acclimatization to sea water (group 1), or following a 2 week period of maintenance in sea water (group 2). Three fish from each group were sampled on days 0, 1, 2, 4, 7, 14 and 28 post‐infection. Characteristic gill lesions began to occur between days 2 and 4, and dramatically increased by day 7. The number of gill lesions on fish from group 2 was significantly higher than on fish from group 1 on days 7 and 14 (P<0.001), but the two groups did not differ in any other parameter. Histologically, Paramoeba sp., the aetiological agent of AGD, could be seen on the gills of fish as soon as 1 day post‐exposure, attached to healthy‐appearing gills. Gill pathology in the form of hyperplasia and lamellar fusion followed shortly. AGD infection was accompanied by a significant increase in the number of gill mucous cells (P=0.002). Different methods for the diagnosis of AGD are discussed.