- This article has been reviewed by a pet health professional. It is not a substitute for professional veterinary advice.
My dad used to hate cats. When I was a kid, we weren’t allowed to have indoor cats, though a rotation of semi-wild outdoor kitties roamed in and out of the yard. As an adult, I’ve had three indoor cats for over 15 years, and my father never bothered to learn their names. It’s safe to say, I never imagined he’d become a cat guy.
But a few months ago, my dad found a tiny, jet-black kitten huddling under the diesel tank on his farm. Now, that kitten (named Diesel, of course) runs the house, galloping up and down the hall with my parents’ yellow lab, eating premium kibble out of ceramic dishes, lounging on a brand-new cat tree, and even—gasp!—sleeping on the bed at night.
There can be only one explanation for this total personality and lifestyle change: the cat parasite is controlling my father’s brain.
What Is “The Cat Parasite?”
Toxoplasma gondii (T. gondii) is a protozoan (single-celled) parasite that infects most species of warm-blooded animals, and causes the disease toxoplasmosis. A 2014 New York Times article called it “one of the most successful parasites on Earth.”
If any warm-blooded animal can get it, how did T. gondii get the nickname “the cat parasite?” It turns out, cats are uniquely essential to T. gondii’s life cycle: it can only sexually reproduce inside feline intestines. As the New York Times explains, “the parasites produce cysts that get passed out of the cat with its feces; once in the soil, the cysts infect new hosts.” From there, Toxoplasma gondii returns to cats through their prey.
Here’s where things get really interesting (and kind of gross, imo): studies show that T. gondii changes the behavior of animals it infects. For instance, rodents have evolved to avoid cat smells as much as possible in order to not get eaten, but once infected with Toxoplasma gondii, many lose their natural fear of cat smells. At times, they even seem to be attracted to it. The “cat parasite” directly influences their behavior and puts them in harm’s way—and conveniently, for rodent-loving cats.
How Toxoplasma Affects Animals (Including Your Cat)
Toxoplasma gondii doesn’t just help outdoor cats catch their dinner. In recent years, studies have shown that toxoplasmosis can cause behavioral changes in hyena cubs, leading them to be more at risk of lion attack.
And in Hawaii, toxoplasmosis is a leading cause of mortality in the endangered monk seal.
As Rover’s resident cat expert Dr. Mikel Delgado explains, “toxoplasmosis can be a problem for several species including marine mammals such as seals and otters.” One recent study shows that places with higher human populations see higher cases of T. gondii in wild animals. Why? Because more humans means more free-roaming cats who poop outside. That cat poop, full of T. gondii eggs, gets carried through soil and rainwater to vulnerable marine animals.
Unfortunately, if your cat is a carrier, you’ll probably never know. Although domestic cats are the primary carriers of T. gondii, according to the Cornell College of Veterinary Medicine, “the parasite rarely causes clinical disease in them.” But you can protect nearby wildlife by keeping your cat indoors (which has the added bonus of extending their lives!) and carefully disposing of their waste. “Cat waste should be disposed of in the trash, not flushed in the toilet,” Dr. Delgado says. “Flushing infected feces can lead to water contamination that spreads the parasite to other species.”
It’s also always a good idea to have your cat spayed or neutered, and support trap-neuter-release programs in your area. Although spaying and neutering doesn’t prevent cats from carrying Toxoplasma, it does prevent unwanted litters that are more likely to roam free and deposit the parasite in the wild.
Can Toxoplasmosis Alter the Human Brain?
In the now-famous article “How Your Cat Is Making You Crazy,” published by The Atlantic in 2012, science writer Kathleen McAuliffe profiled Czech scientist Jaroslav Flegr, who hypothesized that the Toxoplasma parasite might change human behavior. Another scientist quoted in the story said, “the organism rewires circuits in parts of the brain that deal with such primal emotions as fear, anxiety, and sexual arousal.”
While this is true for rodents, the evidence is less-conclusive when it comes to humans. According to the Atlantic article, there is some evidence that men carrying Toxoplasma gondii are “more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules.” Women carrying the parasite, on the other hand, tend to present as “more outgoing, trusting, image-conscious, and rule-abiding than uninfected women.” However, even Dr. Flegr admits, “the parasite’s effects on personality are very subtle.”
Although changes in human behavior make for interesting headlines, over the past 5-10 years, there hasn’t been much new evidence to conclusively prove that Toxoplasma gondii alters human brains the way it impacts wild animals.
And there’s absolutely no data about whether the cat parasite can turn previously cat-hating fathers into a kitten-snuggling softies, no matter how much certain writers think it might be true.
How to Know if You Have Toxoplasmosis (and What to Do About It)
The Center for Disease Control (CDC) estimates that in America, “11% of the population 6 years and older have been infected with Toxoplasma.” In developing countries, infection rates go up to 60%, especially in places with hot, humid climates. In other words, millions of people around the world are carrying T. gondii—but most of them never feel the effects.
As Dr. Delgado explains, the majority of people carrying Toxoplasma gondii “have asymptomatic infections.” If you did develop symptoms, they would resemble a mild flu—muscle aches and tender lymph nodes—and would likely clear up within a few weeks.
Pregnant or immunocompromised people are most at risk of developing symptoms and complications, but that doesn’t mean pregnant or immunocompromised people shouldn’t have cats. Like anybody else, they should simply practice good litter box hygiene. Dr. Delgado explains, “because it takes at least 24 hours for the Toxoplasma parasites in feces to become infectious, cat owners will not be at risk if they scoop the box twice daily, which I recommend anyway. If someone is in an at-risk group, they can wear gloves, or ask someone else in the household to clean the litter box.”
Do Cats Control our Brains, or Not?
The truth is, cats aren’t even the primary cause of toxoplasmosis in human beings. It’s much more common to contract toxoplasmosis through contaminated water, or by eating undercooked, contaminated meat or shellfish. As Dr. Delgado says, “People are still at more risk of getting toxoplasmosis from handling raw meat than from their cats.”
So, maybe my dad isn’t under the influence of a tiny one-celled parasite that compels him to let the kitten lick his mustache. Maybe there’s a much more obvious explanation for his seemingly-sudden change in behavior: Kittens are cute, and it’s impossible not to love them.
I suppose, in that way, cats do control our brains—but it’s not the parasite’s fault.