The parasite Toxoplasma gondii uses a remarkable trick to spread from rodents to cats: It alters the brains of infected rats and mice so that they become attracted to—rather than repelled by—the scent of their predators.

A new study reveals that rodents infected with the parasitic protozoa are drawn to the smell of cat urine, apparently having lost their otherwise natural aversion to the scent.

The parasite can only sexually reproduce in the feline gut, so it’s advantageous for it to get from a rodent into a cat—if necessary, by helping the latter eat the former.

In rodents, “brain circuits for many behaviors overlap with the brain circuits responsible for fear,” said Ajai Vyas of Stanford University, who led the new study.

“One would thus assume that if something messes up fear of cat pee, it will also mess up a variety of related behaviors.”

But Vyas’s experiments showed that not to be the case.

In fact, his test demonstrated just how precise and efficient the mind-bending parasite is. While manipulating rodents’ innate fear of felines, T. gondii leaves other behaviors intact.

Toxoplasma-infected mice and rats retained most typical rodent phobias, including fears of dog odors, strange-smelling foods, and open spaces. Infected rodents also didn’t appear to be sick.

Only the animals’ response to cats was abnormal: Uninfected rodents avoided an area of a room that researchers had scented with cat urine. But infected rodents actually seemed drawn to the smell.

“Toxoplasma affects fear of cat odors with almost surgical precision,” Vyas concluded. “A large number of other behaviors remain intact.”

“There are a million examples of parasites manipulating host behavior,” said Robert Sapolsky, a Stanford University neuroscientist who collaborated with Vyas.

In most cases, he said, “they do something terribly unsubtle, like destroying the vision, so [infected animals] are much less capable of avoiding predators.”

T. gondii, by contrast, “is not just sledge-hammering a behavior out of existence,” Sapolsky said.

“It’s extinguishing a normal behavior”—avoidance of cats—”and replacing it with this incredibly maladaptive opposite.”

(Read related story: “Suicide Grasshoppers Brainwashed by Parasite Worms” [September 1, 2005].)

Vyas’s team found that Toxoplasma, which forms cysts in the brain, tends to concentrate in an area of the brain called the amygdala.

Because that region is linked to fear and anxiety, the finding provides a new clue to how the parasite manipulates behavior.

Sapolsky, Vyas, and their colleagues reported their findings Monday in the Proceedings of the National Academy of Sciences.

Manuel Berdoy, a zoologist at Oxford University in England, called the new finding “a delight.”

He and Joanne Webster, a researcher at Imperial College London, had previously found that Toxoplasma-infected rodents gravitated toward cat odors.

The new study advances scientists’ understanding of how the parasite pulls off the trick, Berdoy said.

He called it “astonishing that [T. gondii] may be able to target specifically the neural pathways responsible for processing cat odors.

“It’s incredible that the parasite would be able to alter a response—cat aversion—that is so ingrained in the rats’ psyche,” Berdoy said.