Toxoplasmosis gondii may pose a threat to boys. Image: Nina Buday/Shutterstock.com

Growing up in the same household as a pet cat may enhance the risk of experiencing a psychotic episode later in life, but only for men. According to a new study, this increased likelihood of mental illness could be caused by a common parasite called Toxoplasmosis gondii, which can be transmitted to humans who come into contact with cat poop.

The link between T. gondii and psychosis has been hotly debated for decades, with some studies suggesting a higher prevalence of schizophrenia among those infected with the parasite, while others have failed to identify any such link. Sometimes present in uncooked meat or contaminated drinking water, the nasty little protozoan can infect all warm-blooded animals and is very prevalent among humans.

While the vast majority of infected individuals experience no symptoms and don’t even know they carry the parasite, some people can develop moderate to severe complications such as fever or respiratory issues. Previous studies have also shown that kids who grow up in cat-owning households are more likely to experience psychiatric disorders as adults.

To investigate, the authors of the new study published in the Journal of Psychiatric Research interviewed 2,206 adults from downtown Montreal about their cat ownership in childhood, as well as their history of psychotic episodes. Participants were also asked about other risk factors for psychosis, such as head trauma, smoking, and the number of times they moved house as a child.

“Cat ownership in childhood was associated with greater psychosis expression in adulthood, but only in presence of certain factors,” write the authors. Specifically, they observed an increased risk for psychosis in men who owned outdoor cats as kids, yet found no such link for women or for any adults who owned housecats in their youth.

Importantly, the researchers didn’t analyze participants’ blood for T. gondii antibodies, which means they can’t be certain that this elevated risk is linked to the parasite. However, their results hint that this may be the case, as cats can only pick up the parasite by hunting infected rodents, which is obviously something that outdoor cats do more frequently that housecats.

The researchers also note that cat ownership alone may not enhance the risk of psychosis, as the greatest increase in likelihood of psychotic episodes was seen in those who had a history of head trauma, moved house multiple times in childhood, and owned a rodent-hunting cat. Furthermore, the authors are unable to explain why only men appear to be affected and do not provide evidence for any neurobiological mechanism linking T. gondii to psychiatric disorders.

Having said that, previous studies have indicated that the parasite may interfere with a type of brain cell called microglia and disrupt the formation of neurological connections, all of which might explain how T. gondii affects the mind.

Overall, the link between cat ownership and psychosis remains a point of contention, although anyone concerned about the risk posed by T. gondii can minimize their chance of infection by washing their hands thoroughly after cleaning out their cat’s litter tray.

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