There’s a new paper out that extends the record of host manipulation by toxoplasma. We already knew that toxoplasma infections cause mice to lose fear of cat urine – turns out that toxoplasma infections also cause chimpanzees to develop a morbid attraction to leopard urine, a marker of their main predator. Uninfected chimps avoid it. Interestingly, infected chips don’t seem attracted to lion or tiger urine, which suggests a specific strain of toxo.
The background is that toxoplasmosis infects many warmblooded creatures as the intermediate host, but can only sexually reproduce in cats, their definitive host. These protozoans (apicomplexans, like malaria) need to have their intermediate host eaten by a cat, and they’ve apparently evolved methods of manipulating host behavior to help bring that about, probably through their colonization of the brain.
There is some evidence that toxoplasma in the brain has effects on human behavior, such as slowed reaction times, reduced long-term concentration, and, of course, liking the smell of cat urine.
The changes in mice sure look like host manipulation, and I have wondered if it might be happening in humans – in particular, cat ladies, but maybe this played a role in the whole human domestication-of-cats thing. Then again, perhaps it was toxoplasma domesticating humans. But if this manipulation happens in chimpanzees, you just know it has to work in humans. This suggests that if you eliminate the toxoplasma in the brains of cat ladies, say with Atovaquone and Clindamycin, you could perhaps cure their morbid attraction, just as antibiotics can cure parthenogenesis in parasitic wasps infected by Wolbachia. Cured, they might put all their flea-bitten parasites in a sack and throw them into the river. And get a dog.
About half the human race has toxo on the brain, as if we didn’t already have enough trouble.
The big question (other than helping explain human craziness) is whether this is an important part of how cats make a living. It may be that toxo is an essential ingredient in cat predation strategies: if so, it is probably very old, and may even go back before cats, perhaps switching from some creodont.
If toxo naturally can make people like cat piss, it’s already preadapted to become (with suitable genetic engineering) the model system for many kinds of infectious behavior modifiers.