Toxoplasma gondii is a single-cell parasite usually associated with cat feces, although undercooked meat is the more common form of infection. For years, everyone knew that T. gondii was totally harmless unless a pregnant woman caught it at a very particular stage in the pregnancy, at which point it caused miscarriage or devastating birth defects. I probably learned about this younger than most because this was my parents official reason for not letting me have a cat while they were trying to conceive. But eventually I got my cat and never thought about it again*, because I was not a pregnant woman. While the concept was gross, 20% of the US and 30-60% of the world has it, so clearly it’s harmless.
Then science began to poke around a bit more. Toxoplasmosis causes pretty drastic behavior changes in rat, as demonstrated by this adorable video of rats attempting to cuddle a cat…
…which is actually a video of a paramecium attempting to get this cat to eat the rats so it can sexually reproduce in the stomach. Enjoy that mental image. If it can have such a strong effect in rats, might it have some measurable effect in humans as well?
First, T. gondii was always considered dangerous in immunocompromised individuals (e.g. AIDS patients). But it gets worse. Research revealed associations between T. gondii and lower IQ in children (which may reverse with treatment), suicide attempts, decreased novelty seeking, car accidents, lower IQ in men, greater friendliness and sexuality in women , and perhaps 20% of all schizophrenia.**
Here is what I think is going on. The human body is incredibly robust. It can take a number of hits and show only a very minor decrease in function. But if you already have enough hits against you (HIV, age, genetic predisposition to schizophrenia), it can have a big effect. Or maybe it will do nothing, but it uses up one of your hits, so when the next blow comes, you don’t have the energy to fight it. This is why the phrase “only dangerous in immunocompromised individuals” bugs me so much. First, everyone who doesn’t die of trauma lives at the mercy of their immune system. Second, immune function is not bimodal. There’s lots of people that don’t have AIDS, but do have, I don’t know, multiple chronic complex infections in their jaw requiring extensive surgery to remove. Or they’re poor and have substandard housing and nutrition. Or they pick up a second parasite while camping.
Telling these people- who don’t have AIDS or leukemia, but aren’t functioning at optimal either- that T. gondii, or any other aggravator, can’t affect them is like telling a working-poor person that ATM fees can’t hurt her because she’s not homeless. It’s great that the fees are a rounding error to you, but don’t discount the cost they impose on others
*Which turned out to be totally justified. Owning a cat is not a risk factor for toxoplasmosis, and I happen to have been tested as part of a larger parasite screen last year and am certifiably toxoplasmosis free.
**A lot of these studies are associational, which I usually frown upon. I find it more valid in this case because causational studies in animals show similar effects.