Parasite Makes Mice Fearless in Face of Cat Threat...Permanently

Ever hear of a fearless mouse? It turns out that a certain parasite can change the behavior of these rodents. When infected with the Toxoplamsma parasite, mice lose their fear of cats. Now, scientists have discovered a new twist on this phenomenon. It turns out that even when the mouse has recovered from the parasitic infection, it still remains fearless.

The protozoan parasite, Toxoplasma gondii, is known for causing spontaneous abortion in pregnant women and killing immune-compromised patients. In mice, though, it causes flu-like symptoms and makes them not fear cats. This is good for both the cat and the parasite; the cat gets an easy meal while the parasite can sexually reproduce inside the cat's intestinal tract.

Intrigued by this particular phenomenon, the researchers decided to examine how the parasite impacted a mouse's hardwired fear of cats. They checked to see whether infected mice would avoid bobcat urine (normal behavior) versus rabbit urine. While previous studies had shown that mice lose their fear of bobcat urine for only a few weeks after infection, the researchers revealed that the three most common strains of the parasite make mice less fearful of cats for at least four months.

"This would seem to refute--or at least make less likely--models in which the behavior effects are the result of direct physical action of parasites on specific parts of the brain," wrote Michael Eisen, one of the researchers, in a news release.

It's apparent that these parasites can impact behavior, though. In humans, these parasites can create dormant cysts in the brain. While they're often kept in check by the body's immune system, they can sometimes revive in immune-compromised people and cause death. Other studies have even shown that chronic infection may be linked to schizophrenia or suicidal behavior. The fact that these effects remain in mice also seem to indicate that there's more than meets the eye when it comes to this bacteria.

"The idea that this parasite knows more about our brains than we do, and has the ability to exert desired change in complicated rodent behavior, is absolutely fascinating," said Wendy Ingram, one of the researchers, in a news release. "Toxoplasma has done a phenomenal job of figuring out mammalian brains in order to enhance is transmission through a complicated life cycle."

The findings are published in the journal PLOS One.