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

Atlantic salmon were exposed to amoebic gill disease (AGD) immediately foll owing their acclimatization to sea water (group 1), or following a 2 week p eriod of maintenance in sea water (group 2). Three fish from each group wer e sampled on days 0, 1, 2, 4, 7, 14 and 28 post-infection. Characteristic g ill lesions began to occur between days 2 and 4, and dramatically increased by day 7. The number of gill lesions an fish from group 2 was significantl y higher than on fish from group 1 on days 7 and 14 (P < 0.001), but the tw o groups did not differ in any other parameter. Histologically, Paramoeba s p., the aetiological agent of AGD, could be seen on the gills of fish as so on as 1 day post-exposure, attached to healthy-appearing gills. Gill pathol ogy in the form of hyperplasia and lamellar fusion followed shortly. AGD in fection was accompanied by a significant increase in the number of gill muc ous cells (P = 0.002). Different methods for the diagnosis of AGD ate discu ssed.