In a stunning contribution to the ongoing quest to scientifically ascertain and describe known reality, researchers are now coming to grips with the undeniable link between exposure to cats and certain forms of insanity.

But the culprit is not the cats, or not exactly. It is the parasite Toxoplasma, long known to pose a serious risk to unborn fetuses, yet considered otherwise relatively harmless to infected humans, until recently.

A little protozoan backstory, courtesy of Harvard Stanford science professor Robert Sapolsky:

The normal life cycle for Toxo is one of these amazing bits of natural history. Toxo can only reproduce sexually in the gut of a cat. It comes out in the cat feces, feces get eaten by rodents. And Toxo’s evolutionary challenge at that point is to figure out how to get rodents inside cats’ stomachs.

Now for the cool part. Toxo makes its way back to the mother ship by undermining the rodent’s hard-wired aversion to the smell of cat urine. Better still, Toxo supplants that aversion with sexual attraction. As Professor Sapolsky explains of infected lab rats,

they’re no longer afraid of the smell of cats. In fact they become attracted to it. The most damn amazing thing you can ever see, Toxo knows how to make cat urine smell attractive to rats. And rats go and check it out and that rat is now much more likely to wind up in the cat’s stomach….

When you look at normal rats, and expose them to cat urine, cat pheromones, exactly as you would expect, they have a stress response: their stress hormone levels go up, and they activate this classical fear circuitry in the brain. Now you take Toxo-infected rats, right around the time when they start liking the smell of cat urine, you expose them to cat pheromones, and you don’t see the stress hormone release. What you see is that the fear circuit doesn’t activate normally, and instead the sexual arousal activates some. In other words, Toxo knows how to hijack the sexual reward pathway. And you get males infected with Toxo and expose them to a lot of the cat pheromones, and their testes get bigger. Somehow, this damn parasite knows how to make cat urine smell sexually arousing to rodents, and they go and check it out. Totally amazing.

In humans, on the other hand, Toxo infection is asymptomatic. Except, apparently, if you drive a car. As Professor Sapolsky conservatively puts it,

A small literature is coming out now reporting neuropsychological testing on men who are Toxo-infected, showing that they get a little bit impulsive.

Women less so, incidentally, which explains why the neighborhood cat lady hasn’t bought herself a motorcycle. Just as well, too, given that

two different groups independently have reported that people who are Toxo-infected have three to four times the likelihood of being killed in car accidents involving reckless speeding.


In other words, you take a Toxo-infected rat and it does some dumb-ass thing that it should be innately skittish about, like going right up to cat smells. Maybe you take a Toxo-infected human and they start having a proclivity towards doing dumb-ass things that we should be innately averse to, like having your body hurdle through space at high G-forces. Maybe this is the same neurobiology.

Yeah, and that cat urine is smelling better and better all the time, by the way.

Add to that the recent suggestion, according to infectious disease researcher Dr. Robert Yolkem, that Toxo infected humans stand an approximately two-fold higher risk for schizophrenia. Bummer. Anyone still want to talk TNR?

Now, it could be argued that if you’re hanging out with feral cats on a regular basis, you’ve likely lost a fair amount of innate inhibition already. Still, if you like cats, and like to do dumb-ass things, it wouldn’t hurt to get tested.

© Ruth Crisler and Spot Check, 2010.