Epizootiology, ultrastructure, and molecular characterization 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) smolts from a freshwater hatchery were introduced in spring and autumn into two marine rearing sites, one with and one without a history of epizootic mortality believed due to parasitic encephalitis. No neurologic signs were observed, but parasites and encephalitis were detected in brains of smolts from both sites, with prevalences of each that were significantly greater in smolts from the site with a previous history of epizootic mortality. Parasites and encephalitis were first detected in smolts sampled at 26 and 4 days post-introduction, respectively. Histopathologic examinations of brains showed that parasites were detected with significantly greater frequency in the optic tectum of the mesencephalon than in the diencephalon, metencephalon, or myelencephalon, and were not detected in the telencephalon. Foci of encephalitis were non-suppurative, multifocal, and were detected in each neuroanatomical subdivision, with significantly greater frequency in the myelencephalon. The ultrastructure of the parasite was that of an intercellular, presporogonic, multicellular, developmental stage of a histozoic myxosporean. Parasite stages were branching tubular structures containing generative cells arranged individually or in cell-in-cell doublets, and were located between bundles of axons with compression of axons along segments of their plasmalemma. No mature spores were detected. Polymerase chain reaction, Southern hybridization, dideoxynucleotide chain-termination DNA sequencing, and in situ hybridization were used in concert to characterize segments of the small subunit ribosomal RNA gene. Assessments of homology were made between sequences from the SSU rRNA gene of the parasite and those of seven myxosporeans from three separate genera, e.g., Myxobolus, Henneguya, and Ceratomyxa. The partial SSU rRNA gene sequence from the parasite was most homologous to SSU rRNA gene sequences from species of Myxobolus, and was 99% homologous to sequences from the SSU rRNA gene of Myxobolus cerebralis. These sequence data represent the first molecular characterization of a myxosporean based entirely on genetic material from the vegetative stage, and indicate that the myxosporean associated with parasitic encephalitis of farmed Atlantic salmon is most likely a neurotropic species of the genus Myxobolus.