Toxoplasma gondii is sometimes called a “mind control” parasite: It can infect the brains of animals and disrupt their behavior in ways that may kill the host but help ensure the parasite’s spread. But now scientists have discovered that infected wolves can actually benefit from these mind-altering tricks. AND Toxoplasma They found that the infection made the wolves bolder and more likely to become pack leaders or disperse to other habitats, giving them more opportunities to breed.
“We really underestimated some of the consequences that this parasite has,” says Eben Gering, a biologist at Nova Southeastern University who was not involved in the work. “The findings likely represent the tip of the iceberg regarding the importance of the parasite to the dynamics of wild ecosystems.”
T. gondii, a single-celled parasite, reproduces only in domesticated cats and other felines. Infected cats excrete oocysts full of spores in their feces, which can survive on plants or in soil or water. They can also persist in undercooked livestock or game meat. When a host—including a human—consumes the oocyst, the spores are released and spread to the brain and muscles, forming new cysts. Around one in four people worldwide is infected. Usually the immune system keeps the parasite at bay, but it can cause miscarriage and other serious problems during pregnancy.
Rodents have long been known to be infected Toxoplasma lose fear of predators. The cysts in the brain somehow increase dopamine and testosterone, increase courage and risk-taking, and increase the chance of the host being eaten by cats. “These parasites use some kind of generic mind control or personality control that helps them complete their life cycle,” says Jaap de Roode, a biologist at Emory University who was not involved in the new study. “And that has a lot of interesting implications that we might not have thought of before.”
The consequences are not limited to rodents. In 2016, scientists from Gabon found this out Toxoplasma-Infected captive chimpanzees lost their aversion to leopard urine. And last year another team described how Toxoplasma-Infected hyena cubs in Kenya move closer to lions, making them more likely to be killed.
When scientists learned a few years ago that some wolves in Yellowstone National Park were infected Toxoplasma, Connor Meyer, Ph.D. student at the University of Montana teamed up with park biologist Kira Cassidy to see if the parasite also changes wolf behavior.
Meyer and Cassidy reviewed more than 26 years of research on gray wolves in the park, incl Toxoplasma test results from blood samples taken in different areas of the park. They also examined data on cougars in which Toxoplasma can reproduce. Wolves that moved in areas with a lot of cougars were more likely to be infected Toxoplasma, they found. It’s likely, the authors say, that these wolves contracted the infection from cougars, perhaps by poking or eating the droppings of big cats.
By combining infection data and past observations in the field, they also found that infected wolves are much more likely to become pack leaders, the team reports today in Communication biology. Infected wolves were also more likely to leave their pack at a younger age to seek new territory or other packs, just as infected rodents are increasingly eager to explore. “There may be some cases where wolves or even a pack of wolves become really successful because they push those boundaries and take more risks,” Cassidy says.
The study is one of the few that research Toxoplasma in the wild. “We know that infection can change animal behavior, but it’s very hard to document that in wildlife populations,” says Meggan Craft, a wildlife disease ecologist at the University of Minnesota. “What’s great about this study is that it uses a wonderful long-term study to be able to dissect these subtle effects of infection and behavior.”
As with rodents, boldness in wolves also comes with risks. Wolves that roam at large are more likely to be hit by a car or leave park boundaries and be shot by hunters. “Distraction is one of the most dangerous things a wolf can do,” says Meyer. It is also possible that an infected pack leader can transmit the parasite during mating, which can happen in dogs, potentially putting the pregnancy at risk. Cassidy believes the long-term risks of infection likely outweigh the benefits. “Wolves live on a knife’s edge first,” Cassidy says.
Since wolves are one of the park’s keystone species, this parasite “can have really, really important impacts on ecosystems,” says de Roode. “They can control food webs; they can control the flow of energy in ecosystems.”
Infected pack leaders could even influence uninfected wolves, the researchers speculate in their paper. Pack members may imitate their leader’s boldness or curiosity about cougars, resulting in more wolves becoming infected. “It’s a great idea and I think it’s highly plausible,” says Gering.
Ultimately, wolves appear to be blind hosts Toxoplasma, however, they are unlikely to transmit the parasite back to cougars. Still, Meyer wonders if the parasite’s effect on wolves means the animals played a role in the infection cycle at some point in the distant past. During the last ice age, he notes, large lions roamed North America that could have hunted these infected—and energizing—beasts.