Intestinal parasites, known as trematodes or flukes, are believed to be the cause of a large scale die-off of lesser scaup, coots, and ring-necked ducks on Lake Onalaska and along the main channel of the Upper Mississippi River immediately below Lock and Dam 7 near Dresbach, Minn. Refuge staff at the La Crosse District of the Upper Mississippi River National Wildlife and Fish Refuge have observed large numbers of the sick and dead water birds and say higher river flows are moving some of them through the dam and depositing them along the main channel in the upper part of Pool 8.
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.
The U.S. Fish and Wildlife Service is the principal federal agency responsible for conserving, protecting and enhancing fish, wildlife and plants and their habitats for the continuing benefit of the American people. The Service manages the 95-million-acre National Wildlife Refuge System, which encompasses 545 national wildlife refuges, thousands of small wetlands and other special management areas. It also operates 69 national fish hatcheries, 63 Fish and Wildlife Management offices and 81 ecological services field stations. The agency enforces federal wildlife laws, administers the Endangered Species Act, manages migratory bird populations, restores nationally significant fisheries, conserves and restores wildlife habitat such as wetlands, and helps foreign governments with their conservation efforts. It also oversees the Federal Assistance program, which distributes hundreds of millions of dollars in excise taxes on fishing and hunting equipment to state fish and wildlife agencies.
