Fatal Attraction

Natural History, April, 2001 by Richard Milner

Rats have evolved a strong, innate aversion to the smells of their predators. Healthy rats--even those bred for hundreds of generations in the laboratory--show distinct anxiety around feline odors. When the amoebic parasite Toxoplasma gondii gets into their brains, however, many of the rodents seem to lose their fear.

Zoologist Manuel Berdoy, epidemiologist Joanne Webster, and colleagues at the University of Oxford have studied the Life cycle of T. gondii to test the hypothesis that the parasite manipulates the behavior of its intermediate host, the rat, to reach its ultimate target, the cat. The researchers infected captive rats with oocysts of the parasite and then placed them in large outdoor pens containing both cat and rabbit urine. Uninfected rats tended to shun the cat-scented areas, while the parasitized rats became substantially less afraid. With their fear of felines diminished, parasitized rats may become uninhibited about approaching their arch predators, with the predictable result of ending up as dinner.

Although all mammals are susceptible to infection by T. gondii, the amoebas can reproduce only within members of the cat family. Once T. gondii has bred in the brain of a cat that has ingested an infected rat, the parasite's oocysts are expelled with the cat's feces. After being washed away by rain, these oocysts can remain infectious in moist soil for more than a year. They can be ingested by beetles and worms, which are readily eaten by rats.

While the parasite affects rats' fear of cats, it appears to leave other aspects of rat behavior intact and to leave cat behavior completely unchanged. The Oxford researchers believe that T. gondii has evolved to alter rat behavior in this specific way and that the reduction of fear is not simply a side effect of cerebral malfunction. ("Fatal Attraction in Rats Infected With Toxoplasma gondii," Proceedings of the Royal Society of London B 267, 2000)