Environmentalist are concerned after several hindered American coot and smaller numbers of other waterfowl are found dead near Genoa.
Researchers say the birds consumed faucet snails which carry a small intestinal parasite that is deadly for birds.
Three to eight days after consuming the snails, birds die of blood loss, shock or a stomach infection.
Outbreaks of the snails have occurred since 2002 but experts say it's cause for concern because the snails are in a major Mississippi River refuge.
Officials say waterfowl are an important resource for the area.
"All the waterfowl species that we have through this area play a very important role in terms of maintaining ecological stability in these particular regions. They're feeding on all sorts of different organisms which in turn can keep populations of those organisms in check," said Greg Sandland, biology professor at UW-La Crosse.
Researchers are working to find ways to reduce the snail population in the area.