Trematode-caused waterfowl and coot mortality has been documented each spring and fall on Lake Onalaska since the 2002 spring migration. During the 2004 spring migration, about 1,060 sick/dead birds were found and total mortality was estimated at 2,400 to 2,700. Comparable losses occurred during the 2004 fall migration. Mortality this spring was first observed on March 28 and is expected to continue through the end of April.

Carcasses are being shipped to the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis. for examination.

Most trematodes have complex life cycles that require two intermediate hosts in which the parasites develop before they become infective for the definitive, final bird host. At least two different species of trematodes have been found in the digestive tracts of birds involved in past die-offs. Both species are small, ranging in size from 1 millimeter to less than 2 millimeters.

Last summer and early fall, parasitologists from the National Wildlife Health Center and Minnesota State University at Mankato, Minn. sampled snails in selected areas of Lake Onalaska. Among the findings, an exotic snail, known as the faucet snail (Bithynia tentaculata), is now present in the river and serves as the first and second intermediate host for both species of trematodes. A portion of the snails collected and examined from various sites on Lake Onalaska were infected with the trematodes. Based on a review of the literature, this snail appears to be a newcomer to the Upper Mississippi River. Native to Europe, faucet snails were first found in Wisconsin in the Great Lakes basin in the early 1900's. In 1998, these snails were documented in Shawano Lake. Die-offs of coots and lesser scaup from trematodes closely parallel locations within Wisconsin where faucet snails have been found.

Depending on how heavily snail populations are infected, some birds can receive a lethal dose during less than 24 hours of feeding. Susceptible waterfowl can die 3-8 days after ingesting a lethal dose of the trematodes.

Avian predators and scavengers, such as bald eagles, crows, and gulls, have been feeding on the sick/dead birds. Mammals, such as raccoons and coyotes, may also be feeding on the carcasses. According to Wildlife Disease Specialists, there appears to be no documented threat that raptors or scavengers feeding on infected carcasses are at risk.

For more information, or to report finding sick or dead waterfowl or coots in areas other than Lake Onalaska, contact the La Crosse District Office of the U.S. Fish and Wildlife Service at 608-783-8405.