Parasite likely cause of scaup, coot deaths at Lake Winnibigoshish (November 6, 2007)
Trematodes, a small intestinal parasite, are believed to have killed about 3,000 waterfowl on Lake Winnibigoshish beginning last week, according to the Minnesota Department of Natural Resources.
These parasites have a complex life history and require two intermediate hosts, such as snails, for the parasites to develop. Waterfowl then consume the infected snails, and the adult trematodes attack the internal organs or blood of the birds. Infected birds appear lethargic and have difficulty diving and flying before eventually dying.
Dead and sick birds were first observed on Oct. 28 on the west shore of the lake. Specimens were shipped to the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wis., for examination and confirmation of the parasite. The parasite was confirmed in two birds, but further lab results are pending.
Staff from the Minnesota DNR and U.S. Department of Agriculture’s Wildlife Services removed about 1,000 dead scaup from Lake Winnibigoshish on Nov. 3. Wildlife officials estimate that about 3,000 scaup and a few hundred coots have died so far.
“We did some reconnaissance of the entire lake and it appears most of the mortality was concentrated on the west side of the lake, particularly around Rabbit flowage and Raven’s point,” said Steve Cordts, DNR waterfowl specialist. “We cleaned up a small stretch of shoreline with the highest concentration of dead birds and plan to continue to monitor this area to better estimate the magnitude of the die off or whether other species may be susceptible.”
Waterfowl and coot deaths caused by trematodes have been documented on the Upper Mississippi river in southern Minnesota during the past five years. The exotic faucet snail serves as the host for the trematode on that stretch of the river.
“We suspect a different snail (the banded mystery snail) may be serving as a host for the parasite on Lake Winnibigoshish, but further investigation is needed,” Cordts said.
Fisheries biologists have documented this snail along the western side of Lake Winnibigoshish for at least eight years, but the full extent of their distribution in the lake, or other lakes, still is unknown. The species is native to eastern North America and has been documented in other lakes in Minnesota.
In previous cases of waterfowl dies-offs caused by trematodes, ducks usually died three to eight days after ingesting a lethal dose of the trematodes. Because these birds appear to be dying within one or two days, the snails may be either extremely abundant or be carrying very high levels of the trematode.
“Since this is the first suspected occurrence related to trematode mortality on the lake, we still have a number of questions to answer about the disease and the snails,” Cordts said.
Avian predators and mammalian scavengers, particularly bald eagles and raccoons, have been feeding on the sick and dead birds. There appears to be no documented threat that they are at risk from feeding on carcasses, however.
Avian trematodes are not known to be a health risk to humans, but the DNR continues to recommend that hunters not consume sick waterfowl and use standard precautions, such as wearing rubber gloves and thoroughly washing hands when cleaning waterfowl.
