Cat Ownership May Double Schizophrenia-Related Disorder Risk, Study Finds

Australian researchers have found that keeping a cat may be associated with roughly double the likelihood of schizophrenia-related disorders.

Their review examined 17 studies published over the past 44 years, spanning 11 countries, including the United States and the United Kingdom.

Cat ownership and schizophrenia-related disorders

“We found an association between broadly defined cat ownership and increased odds of developing schizophrenia-related disorders,” explained psychiatrist John McGrath and his colleagues at the Queensland Centre for Mental Health Research in their study, published last December.

The possibility that cat ownership may be connected with schizophrenia risk was first raised in a 1995 study. That research proposed exposure to the parasite Toxoplasma gondii as a possible explanation, although subsequent findings have been mixed.

Some studies suggest that contact with cats in childhood could increase the chance of developing schizophrenia, while others have identified no such association.

Research has also connected cat exposure with higher scores on measures of schizophrenia-related traits - a condition that can affect a person’s thoughts, feelings and behaviours - as well as psychotic-like experiences. Yet other studies have failed to find this relationship.

McGrath and his team said a comprehensive review and analysis of the available evidence was needed to clarify these questions.

Toxoplasma gondii and cat exposure

T. gondii is a parasite that is generally harmless and may be contracted through undercooked meat or contaminated water.

It can also be transmitted through the faeces of an infected cat. An estimated 40 million people in the United States may carry the parasite, typically without symptoms. Researchers continue to identify further unusual effects that infection might have.

After entering the body, T. gondii can reach the central nervous system and affect neurotransmitters. It has been associated with personality changes, the development of psychotic symptoms and certain neurological disorders, including schizophrenia.

However, an association does not establish that T. gondii causes these changes, nor does it demonstrate that a person acquired the parasite from a cat.

What the review found

The analysis of the 17 studies reported “a significant positive association between broadly defined cat ownership and an increased risk of schizophrenia-related disorders”.

“After adjusting for covariates, we found that individuals exposed to cats had approximately twice the odds of developing schizophrenia,” the researchers explained.

There are important limitations to consider. Fifteen of the 17 studies used a case-control design, which cannot establish cause and effect and often does not account for factors that may have influenced both the exposure and the outcome.

The authors also noted that several of the included studies were low quality.

Results varied between studies, although the higher-quality research indicated that associations in unadjusted models may have resulted from factors capable of affecting the findings.

One study found no significant link between owning a cat before the age of 13 and developing schizophrenia later in life. However, it identified a significant association when cat ownership was limited to a particular period, between ages 9 and 12. This inconsistency means the key window for cat exposure remains unclear.

A United States study involving 354 psychology students found no association between cat ownership and schizotypy scores. Those who had been bitten by a cat, however, recorded higher scores than participants who had not.

Another study, involving people both with and without mental disorders, found an association between cat bites and higher scores on tests assessing specific psychological experiences. Its authors suggested that other pathogens, such as Pasteurella multocida, might instead be responsible.

The researchers agree that stronger and more wide-ranging studies are required before firm conclusions can be drawn.

“In conclusion, our review provides support for an association between cat ownership and schizophrenia-related disorders,” the authors wrote.

“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 study was published in Schizophrenia Bulletin.

An earlier version of this article was published in December 2023.