Toxoplasma gondii is sometimes referred to as the “mind control” parasite: it can infect the brains of animals and disrupt their behavior in ways that can kill the host but help ensure the spread of the parasite. But now researchers have discovered that infected wolves can actually benefit from these mind-altering tricks. A Toxoplasma infection, they found, makes wolves bolder and more likely to become pack leaders or disperse to other habitats, making them more likely to reproduce.
“We really underestimated some of the impacts of this parasite,” said 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 parasite’s significance to the dynamics.” of wild ecosystems.”
T. gondii, a unicellular parasite, reproduces only in domestic cats and other felines. Infected cats excrete spore-packed oocysts in their feces, which can survive on plants or in soil or water. They can also persist in undercooked meat from livestock or game. When a host — including humans — consumes an oocyst, the spores are released and spread to the brain and muscles, forming new cysts. Worldwide, about one in four people is infected. Usually, the immune system keeps the parasite in check, but it can cause spontaneous abortion and other serious problems during pregnancy.
It has long been known that rodents are infected with Toxoplasma lose their fear of predators. Cysts in the brain somehow increase dopamine and testosterone, which encourage boldness and risk-taking and increase the likelihood of the host being eaten by cats. “These parasites use a kind of generic mind control or personality control that helps them fulfill their life cycle,” said Jaap de Roode, a biologist at Emory University who was not involved in the new study. “And that has all sorts of interesting implications that we may not have even thought of.”
The consequences are not limited to rodents. Researchers in Gabon discovered this in 2016 Toxoplasma-infected chimpanzees in captivity lost their aversion to leopard urine. And last year another team described how Toxoplasma-infected hyenas in Kenya venture closer to lions, making them more likely to be killed.
When researchers learned a few years ago that some wolves in Yellowstone National Park were infected with Toxoplasma, Connor Meyer, a 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 spent more than 26 years researching the park’s gray wolves, including Toxoplasma test results of blood samples collected in different park regions. They also examined data on cougars, in which Toxoplasma can reproduce. Wolves located in areas with many cougars were more likely to become infected Toxoplasma, they found. It’s likely, the authors say, that these wolves picked up their infections from the cougars, possibly by prowling around or eating the feces of the big cats.
Combining infection data and past field observations, they also found that infected wolves were much more likely to become pack leaders, the team reports today in Communication Biology. Infected wolves are also more likely to leave their pack at a younger age and start looking for new territory or other packs, just as infected rodents become more eager to explore. “There may be a few cases where wolves or even their pack become really successful because they push these boundaries and embrace more risk,” says Cassidy.
The study is one of the few that investigates Toxoplasma in the wild. “We know that infection can change animal behavior, but it’s very difficult to capture that in wildlife populations,” said Meggan Craft, a wildlife disease ecologist at the University of Minnesota. “The cool thing about this study is that it takes advantage of a fantastic long-term study to be able to tell these subtle effects of infection and behavior apart.”
As with rodents, daring in wolves also involves risk. Wolves that roam widely are more likely to get hit by a car or leave park boundaries and be shot by hunters. “Dispersal is one of the most dangerous things a wolf can do,” says Meyer. It is also possible for an infected pack leader to transmit the parasite during mating, as can happen in dogs, potentially endangering a pregnancy. On balance, Cassidy suspects the long-term risks of the infection likely outweigh the benefits. “Wolves live on the cutting edge to begin with,” says Cassidy.
Because wolves are one of the park’s cornerstone species, this parasite “could really have very important impacts on ecosystems,” says de Roode. “They can control food webs; they can control the energy flow within ecosystems.”
Infected pack leaders could even affect uninfected wolves, the researchers speculate in their paper. Members of the pack can imitate their leader’s boldness or curiosity for cougar scents, infecting more wolves. “This is a brilliant idea and I think it’s very likely,” says Gering.
In the end, the wolves turn out to be a dead-end host Toxoplasma, as they are unlikely to transmit the parasite to cougars, however. Still, Meyer wonders if the parasite’s effect on wolves means the animals may have 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 may have preyed on these infected — and emboldened — beasts.