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Role of Ribeiroia ondatrae (Platyhelminthes: Trematoda) Metacercariae in the Development of Malformed Frogs in Minnesota and Wisconsin

Daniel R. Sutherland and Joshua M. Kapfer
University of Wisconsin-La Crosse,
Department of Biology and River Studies Center,
1725 State St., La Crosse, Wisconsin 54601

Michael J. Lannoo
Muncie Center for Medical Education, MT 201
Indiana University School of Medicine
Ball State University, Muncie, Indiana 47306


Melinda G. Knutson
U.S. Geological Survey
Upper Midwest Environmental Sciences center
2630 Fanta Reed Road
La Crosse, Wisconsin 54603

Metacercariae of Ribeiroia ondatrae have been shown in laboratory and field studies to elicit significant limb malformations in metamorphs of several amphibian species. During a five-year study of trematode metacercariae from Minnesota and Wisconsin anurans, Ribeiroia was distributed throughout eastern Minnesota and western Wisconsin. Ribeiroia was most abundant (100% prevalence, mean intensity >110 worms per host) at two Minnesota malformation hotspots during the same years that extremely high (>50%) malformation rates occurred at the sites. Subsequent declines in malformation rates at these two sites have been correlated with reduced Ribeiroia abundance in metamorphs. Ribeiroia has not been found at malformation hotspots in western Minnesota, indicating that causes other than Ribeiroia are responsible. Preliminary studies on malformed anurans from eastern U.S. wildlife refuges indicated that, though infrequent, Ribeiroia is present at some sites. We suggest that in order for high rates of malformations to occur at a site, Ribeiroia infection rates must exceed a species-specific threshold. Environmental conditions that support increased intermediate snail host populations will, in turn, provide more Ribeiroia cercariae to penetrate tadpoles at critical times during limb formation. High Ribeiroia infection rates may elicit more frequent and severe amphibian malformations.

Keywords: Anura, malformations, parasites, Ribeiroia, Trematoda, Minnesota, Wisconsin, amphibian, deformity.

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