You Probably Don't Have Toxoplasmosis

Disclaimer: I'm a cat enthusiast, not a doctor or veterinarian. This post is for informational purposes and should not replace professional medical advice.

I've had cats for 25 years. I've been scooping their litter boxes for most of that time. Naturally, sometimes I wonder whether I have toxoplasmosis, a parasite that can infect humans who interact with infected cat poop.¹ Surely, after all these years, I have it, right?

Probably not. And, assuming you live in the U.S., you probably don't either, even if you're a longtime cat owner like me.

Let's start with base rates. Globally, the lifetime toxoplasmosis infection rate for a human is about 1 in 3, and in the U.S. it's about 1 in 9.² Among cat owners, a group we expect to be at higher risk, the prevalence rates are a little higher than among non-owners, but not by as much as you might expect. Some studies find an odds ratio of around 1.4, and other studies don't consider cat ownership a statistically significant risk factor for toxoplasmosis at all. If we apply that 1.4 odds ratio to the American base rate (1 in 9), it suggests that roughly 15% of American cat owners carry the parasite.

How can that be? Cats carry toxoplasmosis! You'd think being a cat owner would be the strongest risk factor; after all, we're scooping their dusty poop all the time. However, other factors actually matter more.

Toxoplasmosis isn't only transmitted by cat poop. It's also transmitted from eating meat that hasn't been cooked enough, eating unwashed fruits and vegetables from contaminated soil, or gardening and then eating without washing your hands.³ People are MOSTLY getting infections from that stuff and SOMETIMES from their pet cats - not the other way around.

Even if you're around cats a lot (and scooping their poop), assuming you live in the U.S. and are therefore subject to the pretty low base rate here, you probably don't have toxoplasmosis (and neither do I).

Why don't more cat owners have it?

There are a few factors that help to explain that.

Factor 1: Cat poop isn't infectious right away. It takes 1 to 5 days after leaving the cat for the eggs (known as oocysts) to activate (sporulate) and become infectious. Many people scoop their litter boxes every day, which means they're cleaning them before the eggs are infectious.

Factor 2: Most cats only shed the parasite's eggs once, shortly after they're infected, and then never again. Rarely, cats shed again, especially when re-infected with a new strain, but most don't. When cats shed the eggs, it tends to be for only 1 to 3 weeks. So, for most cats, assuming they get infected with the parasite, there's a relatively short window of opportunity for a human to become infected from that cat.

Factor 3: Many pet cats are indoor-only. Cats usually contract the parasite by eating infected meat while hunting outdoors.⁴ If your cat is strictly indoors and eating cooked commercial cat food (no raw cat food allowed), the chance of them becoming infected is very low.

Factor 4: Many cats contract toxoplasmosis, but most don't; it isn't inevitable. The European Advisory Board on Cat Diseases states that only around one third of cats get it at some point during their lives, and other sources put the rate at closer to 50% in some cat populations.

To summarize: For you to get toxoplasmosis from your cat, (A) they have to get it at all, and many cats don't, especially the ones who never eat infected prey or undercooked meat; (B) you have to get it during the short window when your cat is shedding the parasite eggs; and (C) you have to leave the poop in the litter box for at least 24 hours without scooping it out.⁵

What about the other vectors that can infect humans with toxoplasmosis?

Like I mentioned earlier, raw meat is a big one, so cook your meat. Wearing gloves while gardening and washing your hands after is a good idea too, since cats can poop in your garden, and the parasite's eggs can live for months there. Also, wash fruits and veggies before eating them, because if any infected cats have pooped in the soil where the produce is growing, you can get infected from eating that. Toxoplasmosis has been found on fruits and vegetables before, and the CDC recommends washing or cooking produce before consumption to reduce your risk of getting the parasite.

Finally, although tangential to my main argument, let's explore what actually happens if you get toxoplasmosis.

Most people get no symptoms at all.⁶ People who do get sick from the infection usually get mild flu-like symptoms that eventually go away. However, people with compromised immune systems (e.g., from HIV or cancer) can get nasty, dangerous complications like brain inflammation. And, unfortunately, the fetuses of people who get infected for the first time during pregnancy or just before getting pregnant can have a variety of devastating health effects, like blindness or intellectual disability.

Summarizing all this: Most Americans don't have toxoplasmosis. More than half of cats don't. Cat owners CAN get it from their cats, and that does happen SOMETIMES, but most infections are due to eating infected meat or accidentally ingesting the parasites' eggs from soil or unwashed produce.

If you're not immunocompromised, pregnant, or trying to become pregnant, then go forth and clean thy litter box!⁷ You probably won't get toxoplasmosis from doing that (even after years of scooping).