The Intestinal Helminths of Lesser Scaup: A Different Can of Worms

Lesser scaup (Aythya affinis) are the most abundant diving ducks in North America, but the continental population has declined drastically from peak numbers in the early 1970s. Today, the population is approximately 30 percent below the long-term goal of 6.3 million set by the North American Waterfowl Management Plan, and the lesser scaup has been identified as a species in greatest need of conservation under the Illinois Wildlife Action Plan. Researchers have been working to identify factors contributing to the decline, and most studies suggest that the bottleneck in population growth lies with lowered reproductive success.

Historically during migration, millions of lesser scaup and other diving ducks used stopover sites along the Mississippi and Illinois rivers. However, habitat loss and degradation, pollution, competition, and invasive species have contributed to reduced body condition of female scaup. Tens of thousands of migrating lesser scaup and other waterbirds have recently died in the Mississippi River pools in northern Illinois and southern Wisconsin—all from parasitic trematode infections.

The faucet snail (Bithynia tentaculata) is native to Eurasia and currently threatens waterfowl and food webs throughout the Great Lakes and a few other locations in the United States. First found in the Great Lakes in the 1870s, it is surmised that the snails spread with solid ballast used in timber transport ships or by contaminated vegetation used in packing crates. The faucet snail is the first and second intermediate host to two trematodes, (Cyathocotyle bushiensis and Sphaeridiotrema pseudoglobulus), both lethal to lesser scaup.

A single faucet snail may transmit a lethal dose of these introduced trematodes, and death typically occurs within 3 to 10 days post infection. The reoccurring die-offs in the upper Midwest led Illinois Natural History Survey (INHS) researchers to investigate the associations of intestinal helminth infections with various parameters of lesser scaup health.

Researchers collected 130 female scaup from four geographic locations in Illinois and Wisconsin during the springs of 2014 and 2015. Laboratory analyses were used to determine intestinal helminth loads, body nutrient composition, food use and selection, blood metabolite levels to index fat deposition or catabolism, hematological samples, and other measures to determine body condition and overall health of each bird.

The nonindigenous flukes responsible for the die-offs in the upper Midwest, C. bushiensis and S. pseudoglobulus, occurred in 10.8 percent and 30 percent of lesser scaup, respectively. Birds infected with these worms typically were in poorer condition than birds free from infection.

Results of the study are consistent with the feeding habits of these trematodes, which attach to the intestinal walls causing hemorrhage, often initiating an immune response from the host. Our results suggest that intestinal parasites may contribute to reduced body condition of lesser scaup, potentially affecting subsequent production.

Additional research is warranted to better understand the effects of these parasites in lesser scaup, and determine if management actions can limit mortality caused by these trematodes or reduce the density and distribution of the non-native host, the faucet snail.

This report was originally published in the June 2016 INHS Reports No. 414(6), see http://wwx.inhs.illinois.edu/files/8314/6498/4049/INHSReports_June2016-final.pdf.