Even after infection with Toxoplasma gondii has been removed from rodents' brains, they continue to behave as if unafraid of the smell of cat urine, suggesting that the infection causes long-term changes in the brain. Wendy Ingram and Adrienne Greene / University of California Berkeley

A parasite that changes the brains of rats and mice so that they are attracted to cats and cat urine seems to work its magic almost right away, and continues to control the brain even after it’s gone, researchers reported on Wednesday.



The mind-controlling parasite, called Toxoplasma gondii, might make permanent changes in brain function as soon as it gets in there, the researchers report. They aren’t sure how yet.

“The parasite is able to create this behavior change as early as three weeks after infection,” says Wendy Ingram of the University of California, Berkeley, who worked on the study.

T. gondii has captured the imaginations of scientists and cat lovers ever since it was learned it can control the behavior of rodents. It changes their brains so they lose their innate fear of the smell of cat urine. In fact, it precisely alters their fear reaction so that they love the smell of cat pee.

This makes infected rodents much more likely to be caught by cats, which eat them and their mind-controlling parasites. T. gondii can only reproduce in the guts of cats, so its behavior directly affects its own survival.

It doesn’t just affect cats. People can be infected too -- pregnant women are told to stay away from cat feces for this very reason. It normally doesn’t bother people, but it can cause brain inflammation, called encephalitis, in some -- especially those with compromised immune systems like pregnant women.

“More than 60 million men, women, and children in the U.S. carry the Toxoplasma parasite, but very few have symptoms because the immune system usually keeps the parasite from causing illness,” the Centers for Disease Control and Prevention says on its website.

Chronic infection with the parasite Toxoplasma gondii can make mice lose their innate, hard-wired fear of cats. Wendy Ingram and Adrienne Greene / University of California Berkeley

Studies have linked toxoplasmosis with a range of human mental diseases, including schizophrenia, bipolar disease, obsessive compulsive disorder and even clumsiness. This study doesn’t answer questions about people, Ingram points out.

“It does not necessarily explain crazy cat ladies or why there are LOLCATS online,” she says.

But it does begin to hint at a potential mechanism for how and when the parasite changes the mouse brains.

“I want to know how the behavioral change is happening,” Ingram says.

Her team used a specially genetically engineered version of the parasite, made by a team at Stanford University.

Normal T. gondii parasites form a cyst in neurons. “It was assumed that the cysts … were doing something biologically that is actively changing the behavior,” Ingram told NBC News.

But the genetically engineered parasite wasn’t able to make cysts. And it was so weak that the rats’ immune systems were able to clear it from their brains. But even so, rats infected with this weakened form of the parasite just loved the smell of cat urine, Ingram and colleagues report in the Public Library of Science journal PLoS ONE.

“This suggests the parasite is flipping a switch rather than continually changing the behavior,” says Ingram.

She suspects it’s somehow activating the immune system in a way that then alters brain function. “That’s one of the very first things I am going to be checking,” Ingram says.