Owning an outdoor cat as a child is associated with an increased risk of psychotic experiences in adulthood – but only in males, new research suggests.
Investigators found
Courtesy McGill University
Dr. Vincent Paquin
The suspected culprit is not the cat itself but rather exposure to Toxoplasma gondii, a common parasite carried by rodents and sometimes found in cat feces. The study adds to a growing evidence showing exposure to T. gondii may be a risk factor for schizophrenia and other psychotic disorders.
“These are small pieces of evidence but it’s interesting to consider that there might be combinations of risk factors at play,” lead author Vincent Paquin, MD, psychiatry resident at McGill University, Montreal, said in an interview.
“And even if the magnitude of the risk is small at the individual level,” he added, “cats and Toxoplasma gondii are so present in our society that if we add up all these small potential effects then it becomes a potential public health question.”
The study was published online Jan. 30, 2022, in the Journal of Psychiatric Research.
Inconsistent evidence
T. gondii infects about 30% of the human population and is usually transmitted by cats. Most infections are asymptomatic, but T. gondii can cause toxoplasmosis in humans, which has been linked to increased risk of schizophrenia, suicide attempts, and more recently, mild cognitive impairment.
Although some studies show an association between cat ownership and increased risk of mental illness, the research findings have been inconsistent.
“The evidence has been mixed about the association between cat ownership and psychosis expression, so our approach was to consider whether specific factors or combinations of factors could explain this mixed evidence,” Dr. Paquin said.
For the study, 2206 individuals aged 18-40 years completed the Community Assessment of Psychic Experiences (CAPE-42) and a questionnaire to gather information about cat ownership at any time between birth and age 13 and if the cats lived exclusively indoors (nonhunting) or if they were allowed outside (rodent hunting).
Participants were also asked about the number of residential moves between birth and age 15, date and place of birth, lifetime history of head trauma, and tobacco smoking history.
Rodent-hunting cat ownership was associated with higher risk of psychosis in male participants, compared with owning no cat or a nonhunting cat. When the investigators added head trauma and residential moves to rodent-hunting cat ownership, psychosis risk was elevated in both men and women.
Independent of cat ownership, younger age, moving more than three times as a child, a history of head trauma, and being a smoker were all associated with higher psychosis risk.
Courtesy McGill University
Dr. Suzanne King
The study wasn’t designed to explore potential biological mechanisms to explain the sex differences in psychosis risk seen among rodent-hunting cat owners, but “one possible explanation based on the animal model literature is that the neurobiological effects of parasitic exposure may be greater with male sex,” senior author Suzanne King, PhD, professor of psychiatry at McGill, said in an interview.
The new study is part of a larger, long-term project called EnviroGen, led by Dr. King, examining the environmental and genetic risk factors for schizophrenia.
Need for replication
Commenting on the findings, E. Fuller Torrey, MD, who was among the first researchers to identify a link between cat ownership, T. gondii infection, and schizophrenia, said the study is “an interesting addition to the studies of cat ownership in childhood as a risk factor for psychosis.”
Of the approximately 10 published studies on the topic, about half suggest a link between cat ownership and psychosis later in life, said Dr. Torrey, associate director for research at the Stanley Medical Research Institute in Rockville, Md.
“The Canadian study is interesting in that it is the first study that separates exposure to permanently indoor cats from cats that are allowed to go outdoors, and the results were positive only for outdoor cats,” Dr. Torrey said.
The study has limitations, Dr. Torrey added, including its retrospective design and the use of a self-report questionnaire to assess psychotic experiences in adulthood.
Also commenting on findings, James Kirkbride, PhD, professor of psychiatric and social epidemiology, University College London, noted the same limitations.
Dr. Kirkbride is the lead author of a 2017 study that showed no link between cat ownership and serious mental illness that included nearly 5,000 people born in 1991 or 1992 and followed until age 18. In this study, there was no link between psychosis and cat ownership during pregnancy or at ages 4 or 10 years.
“Researchers have long been fascinated with the idea that cat ownership may affect mental health. This paper may have them chasing their own tail,” Dr. Kirkbride said.
“Evidence of any association is limited to certain subgroups without a strong theoretical basis for why this may be the case,” he added. “The retrospective and cross-sectional nature of the survey also raise the possibility that the results are impacted by differential recall bias, as well as the broader issues of chance and unobserved confounding.”
Dr. King noted that recall bias is a limitation the researchers highlighted in their study, but “considering the exposures are relatively objective and factual, we do not believe the potential for recall bias is substantial.”
“Nonetheless, we strongly believe that replication of our results in prospective, population-representative cohorts will be crucial to making firmer conclusions,” he added.
The study was funded by grants from the Quebec Health Research Fund. The study authors and Dr. Kirkbride disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.