The study, which was published in the journal Psychological Medicine, is the latest in a field that’s yielded many alarmist headlines based on correlations, but not concrete conclusions, about cats making people crazy. And it amounts to a big “not so fast.”
“Many people own cats, which are an important part of the life of many families,” co-author James Kirkbride, a psychiatric epidemiologist at University College London, wrote in an email. “Our findings should reassure people that owning a cat in pregnancy or childhood is not related to later risk of psychotic symptoms.”
The cat-toxoplasmosis-psychosis nexus has gotten a lot of attention in recent years, but it’s hardly well understood.
It’s clear that the parasite that causes toxoplasmosis infections in people, T. gondii, depends entirely on cats, because it reproduces only in cat intestines and is spread via feline feces. And there’s “good evidence,” the study’s authors write, that T. gondii infections are associated with psychosis. So some researchers hypothesize that owning cats in childhood increases the risk of developing mental illnesses, particularly schizophrenia, and a handful of studies have supported this idea. But people can also become infected with T. gondii from undercooked meat or contaminated water.
The new study is much larger than previous ones, and it’s based on data collected from children born in early 1990s and tracked for decades as part of a longitudinal birth cohort study in the area of Bristol, England. That meant the authors could feel certain whether a child grew up with a cat, whereas past research depended on adults recalling whether they had cats as kids — a method that “can often lead to results that are biased,” said co-author Francesca Solmi, also an epidemiologist.
What the new research doesn’t answer is whether cat ownership during pregnancy and childhood is linked to later schizophrenia, because the participants haven’t yet reached the age of onset of that disorder, which typically develops between ages 18 and 25. But Solmi added that if there’s a cat-mental illness connection, the early signs would be detectable when the participants were screened for psychotic experiences at ages 13 and 18.
E. Fuller Torrey, a psychiatrist and prominent researcher on the links between T. gondii and mental illness, cited that as one weakness of the study.
“The measuring of unusual experiences and beliefs at age 13 is somewhat controversial. I’ve raised two children, and both had unusual thinking at age 13,” said Torrey, who was one of the paper’s reviewers. “I think this will become much more important when they’re measuring who developed schizophrenia out of this database.”
Torrey also noted that living with a kitty is not the only way cats might pass T. gondii to children.
“Even if we are suggesting that you get it at, say, age 4 when you’re playing in a sandbox, you don’t have to own a cat to have a cat go to the bathroom in it. The cat next door will go to the bathroom in it,” Torrey said, adding that he doesn’t think families with children should own cats. “I am not reassured by their findings that there’s no relationship.”
But Solmi said the findings indicate that owning a cat doesn’t amount to an additional risk for developing psychotic symptoms — in other words, if we assume T. gondii is a driver of mental health disorders, then having a cat around doesn’t seem to be more dangerous than other sources of infection, such as dirty vegetables. (Pregnant women, they emphasized, should continue to avoid the litter box, because T. gondii infections can cause serious birth defects and complications.)
At the end of the study, the authors appended an unusual conflict-of-interest statement: They all own or have owned cats, but that did not affect their work, it said.
“We were naturally curious about the poor quality of science until now on the link between cat ownership and psychotic outcomes,” Kirkbride said. “One day we were talking about this in our research group, and we said, ‘Wouldn’t it be great if we had better data on this?’ At that moment we realized we did — and we used the data to answer that question.”