Internal parasites, such as helminths, infect deer, caribou, bison, and other similar mammals. Even while many of these illnesses aren’t fatal, they can nonetheless have an impact on animal health and behavior.
Infected animals, for example, can consume less grass or other plants than they typically would. This has an interesting implication: a world with more sublethal parasite diseases is a greener planet.
A mathematical model and a worldwide meta-analysis were used in a new study led by Washington University in St. Louis to emphasize the cascading effects of common parasite illnesses in wild animals on terrestrial ecosystems.
“Parasites are well known for their negative impacts on the physiology and behavior of individual hosts and host populations, but these effects are rarely considered within the context of the broader ecosystems they inhabit,” said Amanda Koltz, senior scientist in biology in Arts & Sciences at Washington University, first author of the study published in the Proceedings of the National Academy of Sciences.
“In this study, we show that pervasive parasitic infections reduce herbivory rates and can therefore trigger trophic cascades that impact plant communities,” Koltz said. “This work helps fill a recognized knowledge gap regarding the ecological consequences of parasitic infections in natural ecosystems.”
“In bringing together experts in a transdisciplinary One Health approach, we bridged health, ecological and theoretical scientists to expand on traditional studies addressing impacts of parasites on individual hosts and host populations to better understand these impacts on the ecosystems where ruminant hosts and their parasites live,” said Sharon Deem, director of the Saint Louis Zoo Institute for Conservation Medicine, a co-author of the new study.
The potential for lethal infections that dramatically increase host mortality to have ecosystem-level effects is somewhat intuitive, especially when the hosts that are killed play a key role in the ecosystem. For example, rinderpest, a virus that killed millions of ruminants in sub-Saharan Africa prior to the 1960s, changed tree densities in the Serengeti ecosystem through its effect on wildebeest mortality.
Vanessa Ezenwa
A multidisciplinary and worldwide working group financed by the Living Earth Collaborative, a collaboration between Washington University, the Missouri Botanical Garden, and the Saint Louis Zoo, carried out the research. Biologists, wildlife veterinarians, epidemiologists, ecosystem ecologists, modelers, and infectious disease experts made up the team.
What’s eating you?
Ruminants, such as deer, bison, giraffes, gazelles, and antelopes, are vegetarians whose dietary patterns have demonstrable effects on local ecosystems. Consider how much vegetation a single grazing deer may consume in an unfenced garden.
However, the number of parasites that fill each wild ruminant’s gut and tissues as it goes about its grass-munching routine is an universe unto itself.
While other recent research suggests that parasites that finally kill their hosts can have ecosystem-wide cascade effects akin to predators, this study also looked at the effects of nonlethal parasitic infections.
The researchers utilized a mathematical model and a worldwide meta-analysis to see if helminth parasites, which are a group of common parasitic worms, might trigger trophic cascades by killing or injuring ruminant hosts.
“The potential for lethal infections that dramatically increase host mortality to have ecosystem-level effects is somewhat intuitive, especially when the hosts that are killed play a key role in the ecosystem. For example, rinderpest, a virus that killed millions of ruminants in sub-Saharan Africa prior to the 1960s, changed tree densities in the Serengeti ecosystem through its effect on wildebeest mortality,” said Vanessa Ezenwa, professor of ecology and evolutionary biology at Yale University, a senior author of the study.
“Our model suggests that sublethal infections, although more cryptic, can have equally important effects on ecosystems; and our meta-analysis shows how pervasive the sublethal effects of helminth parasites are in nature.”
Emory University’s Dave Civitello used well-documented examples of caribou and reindeer, as well as their parasites, in his modeling. Because of the ecological, economic, and cultural importance of caribou in arctic habitats, as well as the continuous disease concerns caused by climate change, caribou and their helminths are among the best-studied wild ruminant-parasite systems.
“Our models allowed us to explore the consequences of different ways that parasites harm their hosts,” said Rachel Penczykowski, assistant professor of biology in Arts & Sciences at Washington University, a senior author of the study. “We used the models to test effects of parasite infections on population densities of caribou hosts and on their plant and lichen food resources.”
“We discovered that any of three types of harm caused by parasitic infections that is, harm to host survival, feeding rates or reproduction can cause a cascading effect,” she said. “In short, diseases of herbivores matter to plants.”
The researchers then looked at data from more than 59 peer-reviewed articles to see if there were any correlations between helminth infections and the same important characteristics of free-living ruminant hosts in nature.
Their findings demonstrated that helminth infections affect host eating rates but are not connected with host survival or fecundity on average.
“Taken together, the broader relevance of these results is that there are likely widespread yet overlooked ecological consequences of sublethal parasitic infections,” Koltz said.
“Given that helminth parasites are ubiquitous within free-living populations of ruminants, our findings suggest that global herbivory rates by ruminants are lower than they otherwise would be due to pervasive helminth infections,” she said. “By reducing ruminant herbivory, these common infections may contribute to a greener world.”
