Do Cats Cause Schizophrenia? Believe the Science, Not the Hype
Cats, you might have heard, cause schizophrenia. Or—more recently—they do nothing of the sort. It’s a decades-long scientific investigation, infrequently punctuated by headline-grabbing stories that definitively claim one or the other, depending on whatever the newest sliver of research indicates.
The most recent study, published this Tuesday in the journal Psychological Medicine, inspired dozens of stories proclaiming felines do not, after all, cause schizophrenia. Which is wrong. Not because cats do cause schizophrenia. But because—like many health studies examining relationships between bacon and cancer, salt and obesity, or sugar and heart disease—the science here is not settled.
The link between schizophrenia and cats goes back to the 1970s, when psychiatrist E. Fuller Torrey learned that viruses from dogs might trigger multiple sclerosis—a neurological condition—in humans. “That got me thinking about which animals host which infectious agents,” he says. Soon, he learned that cats host the most successful infectious bacteria in the world: Toxoplasma gondii. Looking into previously published research, he found plenty of studies showing that schizophrenics often had higher levels of toxoplasma antibodies in their blood than people without the mental illness.
Then he started surveying schizophrenics about their life history, and found that many had indeed lived with cats. But what’s important isn’t just if, it’s when. See, Torrey’s theory isn’t merely that T. gondii causes mental illness, it’s that it somehow alters the development of a person’s brain during crucial periods of brain development—and probably only if that person is genetically predisposed to schizophrenia. It’s a complicated hypothesis, and even after four decades of study, Torrey says he’s still not totally convinced it’s fact. Hence, his continued research on the subject.
Still, every study he publishes—his most recent, dropped in July of 2015—attracts the media like nip. Same with refutations, like the one published this week. The authors analyzed a dataset of 5,000 UK children, looking for a correlation between cat ownership during critical ages of brain development and behavioral indicators of later psychosis (like dark thoughts) at the ages of 13 and 18. Their statistical analysis of the results showed no correlation. Most (but not all) news websites ran with some variation of “Relax, Cats Don’t Cause Schizophrenia.”
But that’s not what the study said. Granted, it was also very confusing; the title reads “No evidence of an association between cat ownership and psychotic symptoms at ages 13 and 18 years in a UK general population cohort.” That doesn’t say there is absolutely no link—just that there’s no link in the group of kids these researchers studied. Torrey also has problems with the study’s statistical analysis, which minimizes the effect that poverty, or crowded living conditions, can have on the cat hypothesis. “If you are living in a crowded house in lower socioeconomic conditions, you may be more exposed to cats,” he says. Besides all that, owning a cat is not the same as being exposed to a cat—sandboxes, gardens, and dirt playgrounds are littered with cat poop.
This stink of a problem goes well beyond the litterbox. Earlier this month, the Pew Internet Research Center reported that more than half of all US adults reported often reading contradictory health news. On the plus side, most adults also see this as a sign that scientists are constantly updating their knowledge. That’s heartening, but by no means an invitation for media outlets to double down on reporting in black and white.
Instead, let’s focus on what future studies this new data suggests. The kids in this study were too young to actually have schizophrenia—it typically manifests between 18 and 25—so the researchers used “disturbed thoughts” as a surrogate. “It’s a great cohort, and in 10 years they’ll have great data,” says Torrey. That’ll lead to another paper, surely, that leads to another rash of cat-crazies headlines.
But whether or not this analysis—or future work—is correct isn’t up to these researchers, Torrey, or the media. What scientists know with great confidence is that T. gondii infection is associated with risk of psychosis. Even the authors of the new paper acknowledge this: “There is good evidence to support an association between T. gondii infection and later risk of experiencing psychosis, and this research is consistent with possible inflammatory causes of schizophrenia and other psychotic disorders.”
However, establishing cat poop as the source for that schizophrenia is incredibly difficult. The disease is really rare, and scientists still don’t know a lot about its biological, or genetic, roots. Therefore, they still don’t know how T. gondii triggers schizophrenia to manifest, which brings the whole question right back around to whether cats are the source of that mental illness-inducing bacteria.
If you’re looking for a takeaway, that’s as good as you’re going to get. In the meantime, don’t drive yourself insane reading news that tells you otherwise.