Parasite Infects A Myriad of Mammals, Including Humans – Can Impact Behavior
FORT LAUDERDALE/DAVIE, Fla. – The parasite made me do it.
A new study involving Kenya’s wild hyenas illuminates how a parasite may control the behavior of mammals, including billions of humans worldwide.
Numerous studies suggest that the parasite Toxoplasma gondii (T. gondii), which is widely known as the “crazy cat parasite”, can alter the brains and behavior of mammals, including humans and lab rodents. Presumably, the parasite’s ability to manipulate these hosts evolved as a means of driving our ancestors into the bowls of feline (cat-like) predators, where the parasite sexually reproduces. Although it’s a compelling idea, interactions between felines and infected hosts have never been directly studied in the wild.
Credit: James Tyrrel
A new study of spotted hyenas in a remote region of Kenya, led by investigators at Nova Southeastern University (NSU), the University of Colorado-Boulder and Michigan State University, now offers exciting and long-awaited insight. The research team discovered that young hyenas who are infected with T. gondii act more boldly toward lions and have higher rates of lion-inflicted mortality. Importantly, this is the first study to show the parasite, which is believed to manipulate the brains and behavior of its hosts, impacts naturally occurring and often fatal interactions between infected wild hosts and felines.
The new study was led by Eben Gering, Ph.D., an assistant professor of biology at NSU’s Halmos College of Arts and Sciences, and Zachary Laubach, Ph.D. a research specialist at the University of Colorado-Boulder. Together with a diverse team of experts in animal behavior, veterinary medicine and parasitology, Drs. Laubach and Gering examined a 30 year dataset of interactions between wild lions and hyenas from Kenya’s Masai Mara ecosystem. By pairing these observations with T. gondii diagnostics, the research team obtained the first-ever confirmation that wild hosts infected with the parasite exhibit risky and costly behavior during naturally occurring interaction with felines.
The study was recently published in Nature Communications.
“This parasite is widely reported to reduce behavioral inhibition in wild hosts,” said Gering. “Our study confirms a key prediction of that hypothesis: T. gondii infected cubs show greater boldness during risky and often lethal interactions with lions, and they have higher rates of lion-inflicted mortality.”
The study supports a leading idea about how the parasite’s purported host manipulation evolved.
“More work is needed to test additional predictions of the host manipulation hypothesis and to rule out other possible reasons for our findings,” said Gering. “We also need to understand the consequences of infection for other wild species and ecosystems.”
T. gondii is abundant in a wide array of wild mammal species, including more than a billion humans worldwide, who acquired the parasite via contact with feces from housecats or consumption of undercooked meat. Within humans, hyenas and other mammal hosts, T. gondii infects diverse tissues including, most notably, the brain.
These infections are often benign, but studies of humans, chimpanzees and rodents have found they are also linked to alterations of host physiology (e.g. changes in dopamine and testosterone levels), and behavior (e.g. changes in boldness and motor coordination). Taken together, these findings spawned the widely discussed hypothesis that T. gondii has evolved a capacity to hijack its hosts and drive them toward the feline predators it requires for sexual reproduction. Among these feline hosts are the familiar domestic house cat, as well as its wild relatives such as lions, tigers and leopards.
The work illustrates how crucially a single-celled parasite may impact the health and survival of hyenas – and possibly other members of T. gondii’s myriad host species. Given the parasite’s global ubiquity, the new discoveries from Kenyan hyenas may be just the tip of the iceberg.
“One factor that sets this study apart from the vast majority of prior research is that we were able to test predictions about the parasite’s effects within the natural settings in which they purportedly evolved,” said Gering.
Drs. Gering and Laubach also emphasize that more work is needed to understand mechanisms linking infection to the behavior of wild animals, such as hyenas, and to assess the diversity of wild hosts in which these effects occur. It will also be important to ascertain how host behaviors influence the parasite’s transmission through natural ecosystems.
“Put simply, to conserve wild populations, we need to understand who eats whom, and when and why. T. gondii infections are known to directly cause of disease and death in several endangered species, but the consequences of behavioral alteration for these hosts (e.g. increased predation rates) are poorly known and potentially quite important,” said Gering.
After studying closely related malarial parasites for several years, Gering met the world’s leading experts on hyena biology and behavior during a postdoc at Michigan State University funded through the NSF-Beacon center for the study of evolution. The hyena group, led by Kay Holekamp, Ph.D., has painstakingly recorded behavioral data from animals in Kenya’s remote Masai Mara for three decades. The research group’s rich understanding of hyena behavior offered the perfect opportunity to break new ground in T. gondii research.
“Hyenas are a fantastic system for studying effects of early life events, such as infection by parasites, on the survival and behavior of host organisms,” said Laubach.
The study’s two lead authors are now working with mentored students to broaden their work to include longitudinal studies of focal hosts, studies of other host species, and impacts of infection by lesser-known parasites that may similarly influence the behaviors of their hosts.
This material is based in part upon work supported by the National Science Foundation under Cooperative Agreement No. DBI-0939454. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Sample collection and behavioral data collection were supported by NSF grants IOS1755089 and OISE1853934.
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