Toxo is the craziest parasite you’ll hear about all day
Long time readers of stuckk are probably aware by now that I’m fascinated by neuroscience, evolutionary biology and philosophy of mind. So it’s no surprise that the recent story on Edge.org about Toxoplasma caught my eye.
Toxo is a parasite that can only reproduce in the guts of cats. The parasite exits its host cat in its feces, which (who knew) gets eaten by rodents. Rodents get eaten by cats, and so the cycle continues. So that’s kinda cool because it’s gross, but what makes it interesting is what toxo does to the rodents it inhabits.
Rodents have a natural revulsion to the smell of cat urine. This is an “oh no I might get eaten” revulsion, not a “dude clean up your side of the room” revulsion. Anyway, toxo flips a switch (lots of switches?) in the brains of rodents that makes them ridiculously attracted to cat pee. Turns out at least in males it’s the sexual channel. Yes, that’s right — rats become sexually aroused by the smell of cat pee and go check it out.
This of course leads to the rodent getting eaten a lot more often and toxo getting a free ride back into the cat guts it calls home. The circle of life.
This isn’t the first parasite to change the brains of its host. Grasshoppers get turned suicidal by some types of worms. It’s really crazy stuff. This one is interesting because it affects humans as well. In tropical locales, often more than 50% of people are infected. Pregnant women are evidently told to be wary of cats because if toxo gets into the system of an infant it can wreak havoc. We think that it doesn’t have an effect in adults, or at least we used to.
Turns out people with traces of toxo are way more likely to be in car crashes involving reckless speeding. Three to four times more likely. It seems that whatever sort of fear process that gets turned off in rodents wrt cats also affects us — makes us a bit less fearful in cars.
But it gets even weirder. Being infected by toxo increases your risk of schizophrenia. Additionally, giving toxo-infected rats schizophrenia drugs makes them less attracted to cat urine. There’s a real suggestion here that the “crazy cat lady” trope might be a real thing — just a severe toxo infection.
Just one more thought. Humans have tons of bacteria in our stomachs that help us digest stuff. Assume they implement the mechanism of hunger — that is, they let us know when we want to eat. Are our desires for, say, a PB&J at 1AM less ours for that fact? I’d say certainly not. Yet our desire to be surrounded by cat urine seems alien and imposed. I’ve long argued against a notion of medicine that references some normal state of affairs but when it comes to changes in one’s desires I think it’s really difficult to say what sorts of things we should regard as normal changes in behavior wrt changing circumstance (e.g. changing religion to marry someone) and what changes we should regard as coercion.
