Selenium Reduces Leucocytozoon Infection in Wild Birds: But at What Cost?
Courtney Werner is seen banding and collecting blood samples from one of the birds captured throughout this study. (Photo courtesy of Travis DeVault)
Parasitic infections in wildlife species are influenced by more than just their exposure to parasites and their vectors in the wild. The environments animals live in, including the quality of their soil, water, and habitat, can affect whether infections occur and how they spread through populations. Chemical elements and contaminants present in the environment may subtly alter an animal’s condition, changing interactions among hosts, parasites, and the insects that transmit them.
In an attempt to better understand these relationships, researchers working at the University of Georgia’s Savannah River Ecology Laboratory (SREL) studied parasite infections in wild birds at the Savannah River Site (SRS) in Aiken, South Carolina, where relatively undisturbed habitats and areas affected by past industrial activity occur side by side. This study was led by former master’s student Courtney Werner, who was co-advised by Olin E. Rhodes Jr., Director of SREL and UGA Athletic Association Professor of Applied Ecology in the Odum School of Ecology, and Travis DeVault, Associate Director for Research and Senior Research Scientist at SREL. The team focused on haemosporidian parasites, a group of avian blood parasites related to the organism that causes malaria, and examined whether environmental contamination influenced rates of infection.
In natural systems, disease depends on more than exposure alone: birds serve as hosts, mosquitoes transfer infections between individuals, and parasites rely on both to complete their life cycle. Because contaminants can affect nutrition and immune defenses, the researchers examined whether exposure to contaminants was associated with differences in parasitic infection patterns, rather than only with direct harm to birds.
Over the course of one breeding season, the team captured 329 birds representing 31 species across six wetland and streamside habitats on the SRS, and collected blood samples to measure contaminant exposure and test for parasite infections. Mosquitoes, who are vectors of parasite transmission, were also sampled to determine whether differences in bird infections were driven by changes in the parasite infection rates of the mosquito vectors or by changes in immunity to the parasites within the birds themselves.
The study compared relatively uncontaminated areas with locations influenced by legacy industrial activities, including coal-combustion waste and nuclear-related contamination, and researchers measured several trace elements and a radionuclide in the birds. These included zinc, copper, mercury, lead, arsenic, and selenium, and cesium-137. Among the contaminants examined, selenium showed the clearest relationship with infection patterns.
“One of the clearest effects that we observed within birds was the relationship between selenium, a trace element commonly found in coal combustion waste, and a parasite that is commonly found in the blood of birds,” says Rhodes. “Birds with concentrations of selenium above a certain level, just did not have the parasite, despite the fact that many other birds in those same areas were infected.”
Birds living in areas affected by coal-combustion waste had higher selenium concentrations in their blood, and those elevated levels were associated with fewer infections from one parasite group known as Leucocytozoon. Individuals with selenium levels above a certain threshold showed no infection by that parasite, although the same pattern was not observed for other parasites, such as avian malaria (Plasmodium) or Haemoproteus, suggesting that contaminants influenced specific host–parasite relationships rather than all infections equally.
When researchers examined mosquitoes, however, infection rates did not differ between contaminated and reference sites. Instead, they followed seasonal patterns, increasing during the breeding season when dormant infections in birds can re-emerge and spread. These results indicated the contaminant was affecting the birds’ ability to resist infection rather than altering parasite transmission.
“These results suggest that some environmental contaminants, such as selenium, may influence individual host immunity more than they disrupt broad parasite transmission cycles within vector populations,” explains Daniel Peach, an assistant professor from SREL and the Department of Infectious Diseases in the College of Veterinary Medicine.
Although fewer infections might appear positive, selenium exposure can also affect reproduction, making the overall effect more complicated. Birds can transfer contaminants into their eggs, and higher selenium concentrations may reduce hatching success even while certain parasite infections in adult birds decline, creating a tradeoff between disease resistance and reproductive health.
The findings suggest that environmental contamination can reshape disease dynamics by influencing interactions among hosts, parasites, and vectors, showing that wildlife health is closely linked to ecosystem conditions. Understanding these relationships helps scientists better predict how species respond to human-altered environments and reveals that pollution may affect wildlife not only through toxicity, but also through changes in infection dynamics.
The full study, Use of Contaminated Habitat and Associated Selenium Uptake Mediate Haemosporidian Parasite Infections in Wild Passerine Birds, was published in Ecology and Evolution. Authors include Courtney S. Werner, Mary Chapman, Daniel A. H. Peach, Travis L. DeVault, and Olin E. Rhodes Jr.