for National Geographic News
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.
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