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Host-specificity, modes of transmission and pathogenicity of the parasite Heterosporis sp. (Microsporida: Pleistophoridae) in yellow perch (Perca flavescens) from the Bay of Quinte in Lake Ontario and inland lakes of Minnesota and Wisconsin
Daniel Sutherland and Peggy Stelzig
River Studies Center and Department of Biology
1725 State Street
University of Wisconsin-La Crosse
La Crosse, WI 5460
Heterosporis sp. is a newly reported intracellular parasite in skeletal muscle cells of yellow perch (Percaflavescens) from the Eagle River Chain of Lakes in Vilas Co., Wisconsin, Leech Lake in Cass Co., Minnesota and Bay of Quinte in northern Lake Ontario. The parasite has also been confirmed in perch from eleven other lakes in Minnesota, walleye (Stizostedionvitreum) from Minnesota and Wisconsin, northern pike (Esoxlucius) from Minnesota, burbot (Lotalota), mottled sculpin (Cottusbairdi), trout-perch (Percopsisomiscomaycus), rockbass (Ambloplitesrupestris) and pumpkinseed (Lepomisgibbosus) from Wisconsin and rockbass and pumpkinseed from Lake Ontario. Heterosporis has previously been reported from cultured eels (Anguillajaponica) in Japan and Taiwan and from several ornamental fish species in France (Pterophyllumscalare), Germany (Pseudocrenilabrusmulticolor, Ancistruscirrhosus) and Thailand (Bettasplendens). The North American source of the parasite is unknown. Prevalence is usually less than 15-30% in inland lakes and approximately 5-7% in the Bay of Quinte. The presence of this parasite in Great Lakes perch requires commercial fishers to fillet the fish thereby ensuring that infected fillets are not sent to market; such additional handling is significant for a commercial fishery that is already depressed by declining perch stocks. Infected fillets are opaque and milky white in color, and 90% of the fillet may contain parasites. Examination with LM and TEM shows infected muscle cells filled with sporophorocysts containing numerous intact sporophorous vesicles, each containing 8 or 16 mature spores. A polymerase chain reaction diagnostic assay has been developed and made available to any interested diagnostic facilities. Laboratory exposures indicate that eurasian perch (P. fluviatilis), carp (Cyprinuscarpio), goldfish (Carassiusauratus), rainbow trout (Oncorhynchusmykiss), brook trout (Salvelinusfontinalis), lake trout (S. namaycush), brown trout (Salmotrutta), channel catfish (Ictaluruspunctatus) and fathead minnows (Pimephalespromelas) are extremely susceptible to infection, while bluegill (Lepomismacrochirus), largemouth bass (Micropterussalmoides), coho salmon (Oncorhynchuskisutch) and white suckers (Catostomuscommersoni) are much less susceptible to infection. Golden shiners (Notemigonuscrysoleucas) and smallmouth bass (M. dolomieui) appear to be refractive to infection. Therefore, Heterosporis exhibits an extremely wide host specificity. The single confirmed report of Heterosporis in northern pike is considered to be an accidental infection since we were unable to infect northern pike during our experimental exposures. Laboratory studies showed that Heterosporis-infected fathead minnows were significantly more likely to be consumed by largemouth bass than were uninfected minnows. Initial studies conducted to show that infected fathead minnows exhibit reduced swimming stamina than uninfected minnows were inconclusive. Laboratory studies investigating the potential of viable Heterosporis spores passing through the alimentary canal of cormorants are ongoing. The appearance of Heterosporis in North America has serious implications for culture operations that harvest wild fish as bait fish or as forage for production fish.