The very good WNYC program Radiolab describes the journey of the parasite Toxoplasma gondii. Said parasite wants to breed inside the guts of cats. But after breeding, toxo—along with the other contents of cats’ guts—tends to end up in the litter box or beneath your neighbor’s hedge. What happens to expelled cat poop? Well, in a stunningly lovely visual, much of it’s eaten by rats. So now, the excreted toxo is living inside rats … and it wants to get back into the cat so that it can reproduce.
How does it do this?
Toxoplasma gondii migrates from the rat’s gut to its brain, where it invades the amygdala and snips the rat’s fear response to the smell of cat pee. In fact, not only does it turn off the fear of cat smell, but it connects this snipped wire with the area that controls sexual arousal: Now the rat is hot for cats.
This is not good for the rat. Soon it ends up in the cat’s belly, where T. gondii can continue the never-ending circle of life.
Here’s the thing: Toxoplasma gondii can live in humans too. And when we eat cat poop (unintentionally, one would hope) or scratch ourselves with something crusted with cat feces (same), we give it the chance to enter our bodies. How does Toxoplasma gondii know it’s in a human and not in a rat? Well, it doesn’t. And so it migrates to our brains, where it bounces around looking for trouble.
Does toxo make us like cats? Maybe. There’s clinical conjecture, if not outright support, for toxo’s contribution to the stereotypical crazy cat lady. And there’s definitely clinical confirmation of the crazy part: Toxoplasma gondii is a proven risk factor for schizophrenia (and cases of schizophrenia bloomed in the 1800s, along with the practice of keeping cats as pets—coincidence?).
There’s also evidence that, like in rats, Toxoplasma gondii decreases our experience of fear: it’s been shown to be a risk factor in traffic accidents, implying that toxo-infected drivers are reckless.