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Parasites Make Scaredy-Rats Foolhardy
26 July 2000 7:00 pm
In a life strategy straight out of science fiction, a parasitic protozoan benefits by warping its host's behavior. Infected rats appear to lose some of their fear of cats, potentially helping the parasite make its way to its ultimate destination, the cat.
The relatively innocuous protozoan Toxoplasma gondii is an unlikely candidate for a mind bender. Dwelling in a cat's bowels, it produces egglike oocysts that leave its host's body along with the feces. The oocysts can survive in soil for decades, waiting for a rat or some other warm-blooded mammal or bird to pick them up. Once inside an intermediate host, the parasite invades cells and replicates. Toxoplasma elicits a strong immune response, which prompts the parasite to form tough-coated cysts in which it finds refuge until its host happens to be eaten by a cat, when the cycle starts anew. The mildness of Toxoplasma's effects on its intermediate host make good evolutionary sense: It's not in the parasite's interest to be lethal, as cats won't eat dead animals.
To see if Toxoplasma might somehow increase its chances of getting into a cat, a team led by zoologist Manuel Berdoy and parasitologist Joanne Webster of Oxford University in the United Kingdom set up a maze with a nest box in each corner. On each nest they added a few drops of a particular odor: eau de rat's nest, fresh straw bedding, rabbit urine, or cat urine. Normal rats shied away from the cat odor and were unlikely to return to that part of the enclosure later in the night. The researchers then put Toxoplasma-carrying rats in the enclosure--and the cat scent had no effect. They would explore the nest treated with cat urine at least as often as anywhere else in the enclosure, the researchers report in the 7 August issue of the Proceedings of the Royal Society of London B.
In some cases, the rats even seemed to be attracted to the cat scent. In the wild, this could be fatal. It's isn't clear what causes the effect, but Berdoy speculates that Toxoplasma cysts may release a compound that interferes with a rat's own neurotransmitters, short-circuiting neurological pathways that would keep the rat out of danger.
"It's fascinating that this happens," says Hilary Hurd, a parasitologist at Keele University in the United Kingdom. But Hurd cautions that the study does not close the book on Toxoplasma. "One of the key elements that they haven't demonstrated is whether it actually works, whether the host really is predated more because of this behavior," she says. "This is interesting, but it's really only the beginning."