Can Cats Infect You With a Psychosis-Inducing Brain Parasite? The Answer Is Complicated

Hello!

If you spend any time online, you have probably come across jokes about Toxoplasma gondii, the parasite that humans can contract from cat feces. The topic keeps resurfacing because some studies—though still debated—suggest the organism may influence human psychology or even increase affection for cats. Whether or not those claims hold up, the parasite has become a popular metaphor for the sometimes irrational devotion cat lovers feel toward their pets.

New Research from McGill University

Now fresh data are adding nuance to the discussion. Researchers at McGill University in Montreal have published findings suggesting that exposure to cats during childhood may be linked to a higher likelihood of psychotic experiences later in life. The study, which appears in the Journal of Psychiatric Research, does not prove causation but identifies a statistical association worth further investigation.

Can Cats Infect You With a Psychosis-Inducing Brain Parasite? The Answer Is Complicated

Interview with Lead Researcher Vincent Paquin

Quasa spoke with Vincent Paquin, a psychiatry resident at McGill, who conducted the analysis together with his mentor, psychologist and lecturer Dr. Suzanne King. The conversation below has been edited for length and clarity.

What the study actually shows

Quasa: What should the headline of this research be?

Vincent Paquin: The easy answer is what the headline should not be. It should not claim that cat exposure causes psychosis—our study cannot demonstrate that. What we can say is that cat ownership in childhood appears to be associated with a greater number of psychotic experiences in adulthood, especially among men. That could serve as an accurate headline.

Origins of the project

Q: How did you become involved in this line of research?

VP: When I began my residency at McGill, I looked for a new research mentor and started working with Dr. Suzanne King. She has long studied prenatal stress as a risk factor for psychosis. Several years earlier she had recruited roughly 2,000 people in downtown Montreal and asked them about various environmental exposures, including cat ownership, head injuries, and smoking. That dataset formed part of a larger effort to examine how genetic and environmental factors interact in the development of psychosis. One of the questions concerned whether participants had owned a cat that hunted rodents—an item included because of the known life cycle of Toxoplasma gondii. When Dr. King mentioned the unpublished data, I offered to analyze it.

Common misconceptions

Q: What do people most often misunderstand about the study?

VP: Many readers leap from the statistical association we found to the conclusion that owning a cat in childhood causes psychosis. That is not what the data show, and it is not the claim we are making. We view the finding as a hypothesis that merits further testing. Because randomized controlled trials are impossible in this area, researchers must rely on observational methods that have inherent limitations. Our own study is no exception. Still, the association was stronger when the cat was reported to hunt rodents and was therefore more likely to carry the parasite, which lends some support to the Toxoplasma gondii hypothesis. We hope the work encourages additional research rather than discouraging cat ownership; our study did not measure the many documented benefits of living with a companion animal.

Broader implications of toxoplasmosis research

Q: How do you view some of the more unusual theories linking toxoplasmosis to behaviors such as an interest in sadomasochistic sex?

VP: The broader field examining possible health effects of Toxoplasma gondii exposure is important because the parasite is widespread. Its potential impacts on human health remain incompletely understood—much as the health consequences of tobacco smoking were once unrecognized. Some studies have reported associations between antibodies to the parasite and outcomes such as traffic accidents, though confounding factors cannot yet be ruled out. We must remain cautious about drawing definitive conclusions, yet questions about psychosis, cognitive function, and other clinically relevant outcomes deserve rigorous investigation.

Other routes of infection

Q: Are there sources of the parasite besides cat ownership?

VP: The main sources for humans are undercooked or raw meat, cat feces, and mother-to-child transmission during pregnancy.

Future research directions

Q: Where would you like to see this field head next?

VP: The first study linking cat ownership to psychosis appeared in 1995, yet relatively few follow-up studies have been conducted. A logical next step is to measure antibodies repeatedly across the lifespan rather than at a single time point. Such data would allow researchers to determine whether infection during childhood confers a higher risk of psychosis than exposure before birth or in adulthood. Our study did not include biological confirmation of infection—an acknowledged limitation—so future work incorporating serial antibody testing would strengthen the evidence base.