EPIZOOTIOLOGY, PATHOLOGY, AND ULTRASTRUCTURE OF THE MYXOSPOREAN ASSOCIATED WITH PARASITIC ENCEPHALITIS OF FARMED ATLANTIC SALMON SALMO-SALAR IN IRELAND

In 1995 at a sea-farm in Ireland, Atlantic salmon Salmo salar smelts w ere introduced in the spring and autumn into 2 marine rearing sites, o ne with and one without recent histories of neurologic disease and mas s mortality believed to be due to parasitic encephalitis. In order to monitor disease, determine the onset and anatomic distribution of para sites and encephalitis, and determine the ultrastructure and identity of the parasite, randomly selected smelts were necropsied from both si tes at intervals of 1 to 3 d for periods of 1 to 2 mo, and clinical si gns and cumulative mortalities were monitored. The prevalences of para sites and encephalitis were assessed by light microscopy, and sections of brain were examined by transmission electron microscopy. No clinic al signs of neurologic disease were observed, but parasites and enceph alitis were detected in smelts from both sites, with prevalences of ea ch that were significantly greater in smelts from the site with recent disease. Based on light microscopy, the parasite was first detected i n smelts sampled at 26 d post-introduction. Parasites were detected wi th significantly greater frequency in the optic tectum of the mesencep halon than in the diencephalon, metencephalon, and myelencephalon, and were not detected in the telencephalon. Foci of non-suppurative encep halitis were detected with significantly greater frequency in the myel encephalon than in each of the other 4 anatomic subdivisions. In each anatomic subdivision the prevalence of encephalitis was significantly greater than that of parasitic infection. The ultrastructure of the pa rasite was consistent with that of a histozoic presporogonic multicell ular developmental stage of a myxosporean, characterized by intercellu lar branching tubular structures containing generative cells arranged individually or in cell-in-cell doublets. Parasitic stages were locate d between bundles of axons, with compression of axons along segments o f their plasmalemma. No mature spores were detected.