Mortalities in Atlantic salmon, Salmo salar L., associated with pathology of the melano-macrophage and haemopoietic systems

In 1987 and 1988, a previously undescribed syndrome associated with mortalities in farmed Atlantic salmon, Salmo salar L., occurred in Ireland. Initially, mortalities with a distinct pathology occurred in S1 smolt stocks on six sea-farms from June to July 1987, when seawater (SW) temperatures ranged between 13.5 and 16.5 degree C. All losses were in fish transferred approximately 6 weeks previously from a common freshwater (FW) unit. Adjacent stocks of other origins remained apparently unaffected. Mortality levels in affected stocks ranged from approximately 0.5 to 20%, with a mean loss per farm of 6%. From June to July 1988, mortalities occurred in six sea-farms in S1 smolts from the same FW unit, also approximately 6 weeks after SW transfer. Two sea-farms receiving smolts from the unit were apparently unaffected. Mortalities in affected stocks were 0.4-9%. Adjacent stocks of other origins were unaffected. SW temperatures ranged between 13.0 and 16.0 degree C. In 1987, one farm reported a reduction in mortalities with oxytetracycline administration. However, no reductions were found on four other farms with administration of oxytetracycline, oxolinic acid or erythromycin. In 1988, a few mortalities were found with no gross abnormalities and with food in the gut. Tissues fixed in 10% buffered formalin were wax sectioned and stained with haematoxylin and eosin (H&E), Lillie's allochrome (modified periodic acid-Schiff, PAS), Ziehl-Nielsen (ZN) or modified Gram stains. Histopathological changes were seen most commonly in the spleen, and in many cases, were confined to this organ. In typical cases, lesions were present in spleen, kidney and to varying extents in other organs.