Abstract

The morbidities and mortalities caused by parasitic infections comprise a considerable portion of the estimated 3 billion dollar losses to fish diseases that the worldwide aquaculture industry incurs yearly. In North America and Europe, spore-forming parasites, such as the myxosporean, microsporidians, Dermocystidium spp, and Dermocystidium-like protozoa, are agents of salmonid diseases that significantly, and often catastrophically, impact production. In 1993, an unclassified parasite was identified in association with mass mortality of net-pen reared Atlantic salmon (Salmo salar) smolts on a sea-farm in Ireland.^4,5 Affected smolts displayed abnormal neurologic behaviors characterized by gyrating motions, inappropriate postures in the water column, and stacking on the pen floors. Bacteriologic, virologic and toxicologic tests yielded no significant findings⁵; however, foci of parasites, presumptively diagnosed myxosporeans,⁴ with and without attendant encephalitis were identified in affected smolts based on histologic interpretations of brains. ^4,5

To determine the onset of infection in the brain, the distribution of foci of parasites and encephalitis, and the ultrastructure of the parasite, a sampling protocol was implemented at this sea-farm in cooperation with an Irish aquaculture company in 1995. Atlantic salmon smolts from a single freshwater hatchery were introduced in July and October into two marine sites, one without a history of neurologic disease and the other with a history of neurologic mass mortality. Smolts were necropsied from both sites daily for the first five days and every third day thereafter during July through August, and October through December. Brains were immersion-fixed in a solution of 1.5% glutaraldehyde and 1.5% paraformaldehyde and 3mM MgCl₂ in 0.1 M cacodylate buffer, trimmed sagittally or transversely and serially, and routinely paraffin-embedded and sectioned. Sections of brain complementary to those wherein foci of parasites and encephalitis had been identified were routinely processed for transmission electron microscopy.

Aggregates of the parasite were identified at seventeen days post-introduction, and were morphologically to those identified in prior studies of smolts from 1993 based on fight microscopy. Foci of non-suppurative encephalitis were identified in each of the five major subdivisions of the brain and were identified with greater frequency in the optic tectum, cerebellum, and medulla. Based on transmission electron microscopy, the ultrastructure of the parasite was consistent with that of an extrasporogonic stage of a myxosporean, characterized by branching tubular plasmodia containing primary and secondary cells arranged in cell-in-cell doublets.

Identification of this parasite as a developmental stage of a myxosporean indicates that its life cycle may involve obligate intermediate hosts (e.g. tubificid worms), and provides knowledge essential to the development of management practices designed to minimize infection. Identification of the parasite also focuses future immunohistochemical or molecular studies designed to develop probes for its detection and stricter taxonomic characterization.

Acknowledgements

This work is partially funded by a grant from the Storrs Agricultural Experiment Station, College of Agriculture and Natural Resources, University of Connecticut and from a private aquaculture company in Ireland.

References

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