Three thousand scaup and a few hundred coot are dead on Minnesota's Lake Winnibigoshish, the apparent victims of a deadly intestinal parasite, reports the Wildlife Management Institute. The affected lake is a known stopover for scaup and other migrating waterfowl, and state wildlife officials fear that, as more birds arrive, the number of deaths will increase.

The parasite indicated in the Lake Winnibigoshish deaths is a trematode that is most likely passed to the ducks by way of a favored food source-snail. The small intestinal parasites use the snails as an intermediate host during their life cycle and, when ingested by scaup and other waterfowl, attack the birds' internal organs and blood, causing death in just a few days.

The scaup, commonly known as "bluebills," feed heavily there on banded mystery snails, a snail native to the eastern United States and the likely carrier of the parasites. The snail has been documented in the lake for the last eight years, but no one is sure of the full extent of its distribution. According to Steve Cordts, waterfowl specialist for the Minnesota Department of Natural Resources (DNR), no one is sure how the snails got into the lake, but because they are popular for aquariums, it is possible an aquarium owner released them.

Scaup die-offs from trematodes also have occurred on the upper Mississippi River in Minnesota. The likely carrier there has been the exotic faucet snail.

The Lake Winnibigoshish deaths are particularly troublesome considering the dramatic decline of scaup populations during just the last decade, from 6 million to 4 million breeding birds. Habitat loss and reduced nest success appear to be the leading causes of the decline, but virulent infections, such as the one in Minnesota this month, can contribute significantly to the mortality rate and can hamper efforts to rebuild populations.

Trematodes are not a known health risk to humans, but the Minnesota DNR is recommending that hunters avoid eating ducks that appeared sick or acted abnormally. Hunters are advised to wear rubber gloves while cleaning waterfowl.