Helminths and Health of Spring-Migrating Lesser Scaup in the Upper Midwest

The continental lesser scaup (Aythya affinis) population has declined for more than 40 years, and the breeding population has remained well below the goal of 6.3 million set by the North American Waterfowl Management Plan. Although many mechanisms have been proposed as factors in the decline of scaup (e.g., habitat degradation and loss, pollution, climate change), research indicates that females reach the breeding grounds in poor body condition due to the lack of adequate forage along spring migration corridors, particularly in the upper Midwest. Additionally, infection with exotic trematodes mediated by the invasive faucet snail (Bithynia tentaculata) may lead to further declines in body condition and the consequential deaths of thousands of lesser scaup annually. Remarkably, few researchers have examined the associations of intestinal helminths and health parameters in migrating lesser scaup. To date, most parasite assessment has been performed on birds that were found dead. For this study, the collection of random individuals across a continuum of body condition and parasite loads allowed us to examine sub-lethal effects of helminthiases. Our objectives were to 1) quantify and compare intestinal helminth infections; 2) examine body composition (i.e., protein and lipid content of carcasses) and blood parameters (i.e., plasma metabolites, packed cell volume, white blood cell differentials); and 3) compare these parameters with other metrics of waterfowl health and wetland condition for spring-migrating female lesser scaup. During the spring migrations of 2014 and 2015, we experimentally collected 130 foraging female lesser scaup, obtained blood and tissue samples, removed and preserved intestines, determined total lipids and protein from carcasses, and enumerated and identified all helminths in a 10% aliquot of intestinal ingesta. Preliminary analyses indicated that % carcass lipids (F2,56=4.87, P=0.01) and packed cell volume (F2,57=10.78, P<0.001) generally decreased with collection latitude. Plasma metabolites differed across regions in both years of the study, with the northernmost region having the lowest quality scores (2014 - F14,88=3.70, P<0.001; 2015 - F21,161.35=1.71, P=0.035). Further analyses suggest that intestinal helminth loads are indeed negatively impacting the health of lesser scaup in the upper pools of the Mississippi River where faucet snails are present. Most scaup collected from northerly pools of the Mississippi River were infected with the invasive trematode species Cyathocotyle bushiensis and Sphaeridiotrema pseudoglobulus and were typically in poorer condition than scaup without infections. Due to the complexity of host-parasite interactions, parasitism is often misunderstood and therefore overlooked as an influential regulator of wildlife populations. However, when parasitic infections result in the reduction of host survival and/or fecundity in a density dependent manner, the idea becomes plausible. To understand the effect of a parasite on a host population, one needs to understand the effect of a parasite on the individual host, the prevalence and intensity in the host population, and the context within which the interaction is occurring. These findings emphasize the need for continued investigations of the effects of parasites on waterfowl populations during critical periods of their annual cycle.