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Cat Parasite Makes Mice Less Fearful, but Not Just of Cats, Study Finds

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The relationship between a notorious parasite spread by cats and the mice it infects might be more complex than we thought, according to new research out this week. It suggests that mice infected with Toxoplasma gondii aren’t just less afraid of cats—they’re more mellowed out in general.

Toxoplasma gondii, more plainly known as toxo, has earned plenty of attention for its creepy ways. The single-celled parasite’s typical life cycle begins in cats, who poop out undeveloped eggs that end up in the water or soil. If all goes well, a hapless rodent swallows those eggs, which then mature enough to bury into the cells of their new host, usually in muscle or brain tissue. They then mature further into a hardy cyst form. When a cat is infected by eating a rodent host, the parasites reach full adulthood, reproduce, and start the whole process over.

But toxo isn’t content to just play a passive bystander in the game of life. When it infects a rodent host, the parasite can subtly influence the animal’s behavior to make it less fearful of cat urine, research has shown. The rodent is then more likely to venture right into the jaws of Plumpers the tabby.


While there’s no shortage of parasites capable of mind-controlling their hosts, these examples have mostly involved smaller, less neurologically complex animals like insects. And there’s even some evidence that toxo could affect human minds, too. Humans aren’t natural hosts of toxo, but the parasite can infect us and lay hidden in our bodies for long periods of time (we’re more likely to get it from eating undercooked pork rather than through owning a cat, though). At least some studies have suggested that toxo-infected people are more likely to be impulsive or develop mental illnesses like schizophrenia.


The new study, published Tuesday in Cell Reports, doesn’t really wade into the contentious debate over toxo’s effects on human behavior, but it does throw into question the assumptions on how it affects mice.

Across a series of experiments with toxo-infected mice, the authors found evidence for many more behavioral changes than them simply loving cat pee. Infected mice, for instance, were more eager to explore new environments or spend more time out in the open compared to control mice. And the toxo-fied mice were also more willing to sniff the urine of several animals without recoiling in fear, both of predators like bobcats and non-predators like guinea pigs. In other words, the mice didn’t just become tolerant of cats but less anxious altogether.


“For 20 years, T. gondii has served as a textbook example for a parasitic adaptive manipulation, mainly because of the specificity of this manipulation,” said study author Ivan Rodriguez, a researcher in neurogenetics at the University of Geneva in Switzerland, in a statement. “We now show that the behavioral alteration does not only affect fear of feline predators but that major changes occur in the brain of infected mice, affecting various behaviors and neural function in general.”

Rodriguez and his team also found evidence that these changes are caused by inflammation in the brain, rather than any actions directly taken by the parasite. That’s potentially important, because it could mean an animal’s level of infection (i.e, the number of toxo cysts and where they are in the brain) will determine how affected they’ll be.


But the authors caution that their findings shouldn’t necessarily be applied to human toxo infections. And even if people can be mentally affected by these parasites, the effects are almost certain to be much less dramatic than what we see in mice.

“We hope that people understand that they will not get the ‘crazy cat lady syndrome’ if they are infected with T. gondii,” study author Dominique Soldati-Favre, also a researcher at the University of Geneva, said in a statement. “Although it seems that subtle behavioral changes may occur in humans, the inflammation in the human brain might never reach the same level as laboratory-infected mice.”