That’s been one common takeaway from a study, published last week in the Journal of Clinical Psychiatry, that is the latest to link the common parasitic infection toxoplasmosis to behavior changes or psychiatric disorders in humans. This one found that people with intermittent explosive disorder — who have recurrent outbursts of extreme anger, like road rage — are more than twice as likely as healthy people with no psychiatric disorders to have been exposed to t. gondii, as the parasite is known.
T. gondii‘s survival depends on cats, because it reproduces only in cat intestines and is spread via feline feces. So if t. gondii can control your mind or behavior, the thinking goes, then cats are the culprit. But don’t disavow your cat just yet.
First, some background. About one-third of people worldwide have been infected with t. gondii, though the infection rate varies widely from country to country. The figure is as high as about 75 percent in Costa Rica; in the United States, it’s believed to be 10 to 20 percent. After being deposited into the world by cat poop, the parasite spreads, and people can get it from that excrement, but also from undercooked meat or contaminated water.
It can cause brain damage or even death to fetuses, which is why pregnant women are advised not to clean litter boxes, and are treated with antibiotics if they test positive for toxoplasmosis. But most healthy people who are infected feel a bit flu-like or show no symptoms, even though the parasite can remain latent in brain tissue. So for a long time, science didn’t worry much about it.
It is, however, known to mess with the minds of rats and mice. Scientists have established that t. gondii causes them to lose their fear of the odor of cat urine — and actually be attracted to it. Scientists think the parasite has evolved to make that happen, because it increases the chance that the rodents will get chomped by cats. That’s a good deal for t. gondii, because it takes it to the cat’s intestines, where it can happily reproduce. This strange effect has been dubbed the “fatal feline attraction.”
In recent decades, scientists have been probing the connections between t. gondii and psychiatric disorders and behavior changes in humans, and they’ve found several. Studies have linked infection to schizophrenia, suicide attempts and traffic accidents. One 2015 study found that children with latent infections had poorer reading skills and reduced memory.
The cat connection sailed into public view in 2012, when the Atlantic published an article headlined “How your cat is making you crazy.” It focused on a Czech evolutionary biologist named Jaroslav Flegr who blamed t. gondii for causing untold car crashes and suicides and for screwing with his own brain, making him act strangely and self-destructively.
“Toxoplasma might even kill as many people as malaria, or at least a million people a year,” he told the magazine. It’s “wild, bizarre neurobiology,” he said.
But in humans, the links between toxoplasmosis and psychiatric diseases or behavior changes so far are just that — correlations, some stronger than others. Last month, in fact, cat owners breathed a sigh of relief with the release of another study, published in PLoSOne, that found “little evidence” of links between t. gondii and schizophrenia, depression, suicidal behavior, criminality or effects on personality or memory. The authors said maybe previous links have been “exaggerated” and noted that the idea has “captured the imagination of researchers, but also the lay public.”
The researchers behind the study on intermittent explosive disorder, or IED, didn’t set out to figure out anything about cats, said co-author Royce Lee, an associate professor of psychiatry and behavioral neuroscience at the University of Chicago. They study aggression and wanted to know why it’s often related to inflammation in the brain and the body; toxoplasmosis could be one reason.
They tested 358 U.S. adults, about one-third of whom had IED, which afflicts about 4 percent of Americans — and can lead not only to road rage, but lost jobs and broken marriages, Lee said. Another third had another kind of psychiatric disorder, and the remaining third had no psychiatric history nor first-degree relatives with one.
Overall, 14 percent of the participants tested positive for toxoplasmosis exposure. But 22 percent of those with IED did, compared to 9 percent of those with no psychiatric history.
The results “were surprising to us,” Lee said. “We had no really strong reasons to link IED to toxoplasmosis.”
But there’s no reason to shun cats, Lee said. This study, like many others, didn’t come close to determining that toxoplasmosis causes the psychiatric disorder. It could be, for example, that people who already have these kinds of disorders seek out the company of cats or eat more raw meat, and then get toxoplasmosis, he said. There’s much more research to be done, he said.
“We know that having pets is very good for mental health. It seems to reduce stress and help with social problems,” Lee said. “So I wouldn’t advise people to do anything different. Because you probably would lose more than you would gain.”