If you're a mouse, you're afraid of cats. That's just biology.

But, if you're a mouse infected with the parasite Toxoplasmosis gondii it's a different story. These mice will march right up to a cat, the very picture of foolish bravado. Of course, this usually doesn't go well for the mouse.

But that's the point. T. gondii needs to enter a cat's intestine to reproduce. The easiest way to do that is by riding a carrier straight to the source, and the parasite has a sneaky way of doing it. In a chilling display of mind-control, T. gondii is able to insinuate itself into the brain and turn off a mouse's reflexive aversion to cats, scientists say.

But new research in the journal Cell Reports says that's not quite the case. Rather than only losing their innate fear of felines, infected mice are instead markedly less anxious overall, making the parasite something like a courage booster for the small rodents.

I think the story of having a parasite hijacking the behavior of a mammal is fascinating, says study co-author Ivan Rodriguez, a neurogeneticist at the University of Geneva. It is rare for parasites to influence mammal behavior let alone for the effects to be this strong, Rodriguez says.

From the parasites perspective, making a mouse less afraid of cats makes sense: That increases the likelihood that a feline will catch, digest and breed the organism. T. gondii relies on cat intestines to reproduce, so eating infected prey is key to its survival. But researchers werent sure what T. gondii does in mouse brains to alter their behavior so radically. Rodriguez partnered with a medical researcher at his university, Dominique Soldati-Favre, to investigate.

Behavior tests showed that infected mice were not only less afraid, they were actually more willing to interact with all kinds of things cats and otherwise. They explored the perimeter of an open field for longer than uninfected mice and prodded a human hand in their cage, something their healthy and more naturally wary relatives wouldn't do. Infected mice were also just as willing to sniff guinea pig (a non-predator) odors as they were odors from a fox (a predator, and obvious source of fear.)

So while the parasite makes mice more comfortable with cats, it might be that the disease isnt rerouting mice brains as specifically as people thought. They could be attracted to crocodiles, Rodriguez says. T. gondii doesnt care.

Rodriguez thinks researchers might need to rethink their search for the exact brain structures the parasite impacts. The network of neurons that only controls a mouse's fear of cats is smaller than the set of neurons controlling overall anxiety, he says. Now that we know its something more general, were not looking for such [a] specific and minute change of circuitry.

The study also has more direct implications for human healthcare. Humans can contract toxoplasmosis as well, and while the effects are slightly different (there's no sudden love of cats, of course), some studies suggest the infection could lead to mental health issues.

In the study, blood tests and assessments of messenger DNA, called RNA, in infected mice brains showed that those with the most severe behavioral changes also showed higher levels of inflammation-related molecules.This relationship indicates that degree of inflammation in mice could serve as a stand-in for how bad their T. gondii infection is.

Human symptoms of the disease are rarely as obvious and severe as the changes mice undergo, and theres no way to tap into patient brains while theyre alive and possibly sick. And of course, this research was in mice not people. But the finding indicates that blood tests could help indicate the severity of a human T. gondii infection.

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The Parasite That Infects Mouse Brains and Makes Them More Curious - Discover Magazine