How Cat Parasite Toxoplasma Hacks Mice Brains to Eliminate Fear

The terror of Toxoplasma gondii, the brain-controlling parasite that lives in cats, has by now been hyped enough to make everyone give their cats a sidelong glance. The parasite works something like this: It reproduces and grows in cats, but can be spread to any number of organisms. In rodents, toxoplasma makes mice fearless towards cats; in essence, the parasite seems to make rodents make themselves easier for a cat to eat, which is then of course infected by the parasite.

It's a brilliant example of the ruthless efficiency that makes many parasites work, and–considering people are regularly infected too–might explain why cats are so popular on the internet. Yes, I'm saying that /r/awww is populated by toxoplasma zombies who can't stop obsessing over their cats.

It all sounds like a rather fantastic tale, even if the parasite's effects have been well-observed. But with a quarter of the global population infected, you have to ask: Why don't our immune systems fight back against toxoplasma? Thanks to some research published today in PLoS Pathogens we've now got a better idea of how toxoplasma actually works.

The paper, published by a team led by Jonas Fuks of the Karolinska University Hospital Huddinge in Stockholm, suggests that toxoplasma can hijack our dendritic cells, which act as the "gatekeepers" of our immune system. According to the authors, toxoplasma–a single-celled, motile organism which would otherwise move around semi-randomly–seems to ride on the back of dendritic cells to take a quick ride straight to the brain. If that sounds familiar, it's because toxoplasma apparently works much like a Trojan horse.

The team showed that, after gaining access to dendritic cells, the parasite causes them to release a neurotransmitter called GABA, which counts among its effects the inhibition of fear and anxiety. (In fact, some anti-anxiety medications work by preventing the reuptake of GABA.) In essence, toxoplasma flips the neurochemical switch that helps you deal with fear.

For humans, the effects are minimal. But toxoplasma apparently makes mice feel like they're in God Mode, so when a cat comes running, they don't freak out. And just like that, they're eaten, and toxoplasma can complete its cycle. The team also demonstrated toxoplasma's GABA-pumping ability by infecting the brains of live mice directly with the parasite. In all, it's a pretty incredible feat of evolution, unless you're the mouse.

"For toxoplasma to make cells in the immune defense secrete GABA was as surprising as it was unexpected, and is very clever of the parasite," co-author Antonio Barragan said in a release. "It would now be worth studying the links that exist between toxoplasmosis, the GABA systems and major public health threats."

But most of you reading this are humans, so obviously you want to know if your cat is controlling your brain. Well, when I said toxoplasma is harmless, I was (only sort of!) lying. Toxoplasma is spread mostly through feces, which humans contract generally from eating poorly cooked food. Yes, your food often has poop on it.

For most adults, toxoplasmosis–the state of being infected by the parasite–might initially cause some flu-like symptoms, but eventually the parasite goes into a dormant state where it pretty much chills in your brain without doing anything. This is because, like most parasites, toxoplasma has to strike a balance between doing nothing and doing so much that it incites a full immune system response. But for fetuses and anyone with an already-weakened immune system, toxoplasmosis can be fatal. Don't touch your cat's litter box if you're pregnant, by the way.

Also, other studies have shown that even while dormant, the parasite can have neurological effects, and it's also been shown that toxoplasmosis is correlated with higher rates of anxiety, depression, and schizophrenia, as Greg Boustead noted in a definitive toxoplasma piece for Motherboard last year. Of course, there's also the counter argument that maybe anxious and depressed people have more cats than other folks, and thus have higher rates of toxoplasmosis.

In any case, it's fascinating that we now have a more solid idea of how toxoplasma actually takes over your brain. Parasites in general are absolutely fascinating, and parasites that can control the brains of other organisms are far from rare. Parasites in general are extremely common; hell, there are even parasites of parasites. But to think that something living in your cat's poop can crawl into your brain and cause your own cells to pump out neurochemicals? That's wild, no matter how you look at it. I'm just so glad Sir Abner goes outside.

@derektmead