Having a cat as a pet could potentially double a person's risk of schizophrenia-related conditions, according to an analysis of 17 studies.
Psychiatrist John McGrath and colleagues at the Queensland Centre for Mental Health Research in Australia looked at papers published over the last 44 years in 11 countries, including the US and the UK.
Their 2023 study found "a significant positive association between broadly defined cat ownership and an increased risk of schizophrenia-related disorders."
"There is a need for more high-quality studies in this field," the authors emphasize in their published paper.
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This idea that cat ownership could be linked to schizophrenia risk was proposed in a 1995 study, with exposure to a parasite called Toxoplasma gondii suggested as a cause. But the research so far has put forth mixed conclusions.
Studies have found that being around cats during childhood might make a person more likely to develop schizophrenia; however, not all research has found an association.
Some papers also link cat exposure to higher scores on scales that measure traits related to schizophrenia – which affects a person's thoughts, feelings, and behaviors – and psychotic-like experiences, but again, other studies don't show this connection.
To get a clearer picture, McGrath and his team say there's a need for a thorough review and analysis of all the research on these topics.
T. gondii is a mostly harmless parasite that can be transmitted through undercooked meat or contaminated water.
A bite from an infected cat, or the cat's feces, can also transmit T. gondii can also be transmitted through a bite from an infected cat or through the cat's feces.
Estimates suggest that T. gondii infects about 40 million people in the US, typically without any symptoms. Meanwhile, researchers keep finding more strange effects that infections may have.
Once inside our bodies, T. gondii can infiltrate the central nervous system and influence neurotransmitters. The parasite has been linked to personality changes, the emergence of psychotic symptoms, and some neurological disorders, including schizophrenia.
However, a link doesn't prove T. gondii causes these changes or that the parasite was passed on to a human from a cat.
"After adjusting for covariates, we found that individuals exposed to cats had approximately twice the odds of developing schizophrenia," the Australian team writes.
There are some important things to keep in mind here, like the fact that 15 of the 17 studies were case-control studies.
This kind of research can't show cause and effect, and it often doesn't account for factors that may have affected both the exposure and the outcome.
The researchers also highlight the low quality of a number of the examined studies.
Results were inconsistent across studies, but those of higher quality suggested that associations in unadjusted models might have been due to factors that could have influenced the results.
One study found no significant association between owning a cat before age 13 and later developing schizophrenia, but it did identify a significant link when narrowing down cat ownership to a specific period (ages 9 to 12). This inconsistency suggests that the critical window for cat exposure is not well defined.
A study in the US, which involved 354 psychology students, didn't find a connection between owning a cat and schizotypy scores. However, those who had received a cat bite had higher scores when compared to those who had not.
Another study, which included people with and without mental disorders, discovered a connection between cat bites and higher scores on tests measuring particular psychological experiences. But they suggested other pathogens, such as Pasteurella multocida, may be responsible instead.
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Before we can make any firm interpretations, the researchers reiterate that we need better and broader research.
"Our review provides support for an association between cat ownership and schizophrenia-related disorders," the authors conclude.
"There is a need for more high-quality studies, based on large, representative samples to better understand cat ownership as a candidate risk-modifying factor for mental disorders."
The research was published in Schizophrenia Bulletin.
An earlier version of this article was published in December 2023.