Rats usually have an innate fear of cat urine. The fear extends to rodents that have never seen a feline and those generations removed from ever meeting a cat. After they get infected with the brain parasite Toxoplasma gondii, however, rats become attracted to cat pee, increasing the chance they'll become cat food.

This much researchers knew. But a new study shows the parasite, which also infects more half the world's human population, seems to target a rat's fear of cat urine with almost surgical precision, leaving other kinds of fear alone.

This discovery could shed light "on how fear is generated in the first place" and how people can potentially better manage phobias, researcher Ajai Vyas, a Stanford University neuroscientist, told LiveScience.

Hijacking the mind

T. gondii is a parasitic germ whose primary hosts are cats. However, it can be found in most warm-blooded animals, including an estimated 50 million people in the United States. One study suggests the parasite has altered human behavior enough to shape entire cultures.

In cats, the protozoan reproduces sexually, while it reproduces asexually in other animals.

The germ seems to especially like infesting the brain — "parasites hijacking the mind," Vyas said. Although the disease it causes in humans is rarely dangerous, it is the reason that pregnant women are sometimes told to avoid cat litter boxes (toxoplasmosis is risky for infants and others with compromised immune systems). Some scientists have suspected it might be linked to mental disorders such as schizophrenia and even neuroticism.

In 2000, scientists revealed T. gondii could modify the brains of rats to make them attracted to cat urine instead of afraid of it. Researchers suspect the germ does so to make it easier for it to jump into cats to begin the sexual part of its life cycle.

Vyas and his colleagues now show how specific this brain reprogramming is when it comes to rats, findings detailed online April 2 in the Proceedings of the National Academy of Sciences.

Just cat pee

Rats infected with the parasite became mildly attracted to bobcat pee. However, they remained as fearful of open spaces as normal rats. They reacted normally to sound cues that suggested mild electrical shocks were coming. Normally rats are somewhat reticent when it comes to eating food that smells unfamiliar. And the infected rats were, just like the normal rats, reticent when it came to food scented with the unfamiliar odor of coriander.

"One would thus assume that if something messes up with fear to cat pee, it will also mess up a variety of related behaviors," Vyas said. "We do not see that. Toxoplasma affects fear to cat odors with almost surgical precision."

In addition, "we show that parasites are a little more likely to be found in amygdala [a region of the brain] than in other brain areas," Vyas said. "This is important because the amygdala is involved in a variety of fear-related behaviors."

Future investigations can explore how exactly the parasite modifies the brain in such a precise manner. Potential targets in the brain for research include the stress hormone corticosterone and the brain chemical dopamine. Scientists might also want to see whether infected rats become less afraid of pictures of cats or scents of different predators of rats.