Wildlife pathologists from the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wis., have diagnosed intestinal parasites, known as trematodes or flukes, as the cause of a die-off of coots and lesser scaup currently occurring on Lake Onalaska (Pool 7) of the Upper Mississippi River. Similar die-offs have occurred on Lake Onalaska, located near La Crosse, Wis., each spring and fall since fall 2002. Last fall, almost 3,050 dead birds were found and total mortality was estimated at 6,300-7,000.
The first dead coot this fall was found September 25, near Broken Gun Island. To date, about 350 dead birds have been found, compared with nearly 500 by mid-October 2003 in comparable areas searched. Based on past experience, mortality is expected to increase as more coots and lesser scaup begin to migrate into the area and is expected to continue as long as the birds use the lake.
No human health threats have been documented from handling or consuming waterfowl infected with these parasites. Any bird suffering from these infections would not look or act normally, so the old warning for hunters not to harvest or consume any sick looking bird applies and should address this concern. Waterfowlers are also reminded to wear gloves when cleaning or handling birds and to cook them until well done.
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. A mollusk is often the first intermediate host infected with a stage of the parasite. At least two different species of trematodes have been found in the digestive tracts of birds involved in these die-offs. Both species are small, ranging in size from 1 millimeter to less than 2 millimeters.
Parasitologists from the National Wildlife Health Center and Minnesota State University at Mankato, Minn., sampled snails in selected areas of the lake each month through summer and early fall. 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 these trematodes.
Based on a review of the literature, this snail appears to be a newcomer to the Upper Mississippi River. Originally, faucet snails were found in Wisconsin in the Great Lakes basin in the early 1900s and then 1920s in Lake Winnebago. In 1996, faucet snails were documented in Shawano Lake. Die-offs of coots and lesser scaup from trematodes closely parallel locations within the state where faucet snails have been found.
Although small in size, hundreds of trematodes, more than sufficient to cause death, have been found in the lower intestines of most of the birds examined. From a recent necropsy, one lesser scaup had over 46,000 trematodes in the intestine. One of the species of trematodes feeds on blood, and death is due to severe blood loss, anemia, or other complications. The other species causes disruption in water and electrolyte balance, leading to death. A second effect of that trematode is damage to the intestinal wall, which could lead to bacterial infection in the abdomen of the birds.
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 pathologists at the National Wildlife Health Center, 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 coots and waterfowl in areas other than Lake Onalaska, contact the Service’s La Crosse District Office at 608-783-8405.
Established in 1924, the Upper Mississippi River National Wildlife and Fish Refuge is the most visited refuge in the United States. The 242,400 acre refuge extends 261 miles along the Upper Mississippi River from Wabasha, Minn. to Rock Island, Ill., and protects and preserves habitat for migratory birds, fish, and a variety of other wildlife.
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.
