Epizootiology, pathology, and ultrastructure of the myxosporean associated with parasitic encephalitis of farmed Atlantic salmon Salmo salar in Ireland.

Abstract

In 1995 at a sea-farm in Ireland, Atlantic salmon Salmo salar smolts were introduced in the spring and autumn into 2 marine rearing sites, one with and one without recent histories of neurologic disease and mass mortality believed to be due to parasitic encephalitis. In order to monitor disease, determine the onset and anatomic distribution of parasites and encephalitis, and determine the ultrastructure and identity of the parasite, randomly selected smolts were necropsied from both sites at intervals of 1 to 3 d for periods of 1 to 2 mo, and clinical signs and cumulative mortalities were monitored. The prevalences of parasites and encephalitis were assessed by light microscopy, and sections of brain were examined by transmission electron microscopy. No clinical signs of neurologic disease were observed, but parasites and encephalitis were detected in smolts from both sites, with prevalences of each that were significantly greater in smolts from the site with recent disease. Based on light microscopy, the parasite was first detected in smolts sampled at 26 d post-introduction. Parasites were detected with significantly greater frequency in the optic tectum of the mesencephalon than in the diencephalon, metencephalon, and myelencephalon, and were not detected in the telencephalon. Foci of non-suppurative encephalitis were detected with significantly greater frequency in the myelencephalon 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 parasite was consistent with that of a histozoic presporogonic multicellular developmental stage of a myxosporean, characterized by intercellular branching tubular structures containing generative cells arranged individually or in cell-in-cell doublets. Parasitic stages were located between bundles of axons, with compression of axons along segments of their plasmalemma. No mature spores were detected.