A new review suggests that having a cat as a pet could potentially double a person's risk of schizophrenia-related disorders.
Australian researchers conducted an analysis of 17 studies published during the last 44 years, from 11 countries including the US and the UK.
"We found an association between broadly defined cat ownership and increased odds of developing schizophrenia-related disorders," writes psychiatrist John McGrath and fellow researchers, all from the Queensland Centre for Mental Health Research.
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 studies have found an association.
Some 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 feces of an infected cat can also transmit T. gondii. It's estimated that around 40 million people in the US may be infected, usually 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.
The new analysis of 17 studies found "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 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 cannot prove cause and effect, and it often does not look at things that might have affected both the exposure and the outcome.
A number of the studies that were looked at were of low quality, which is something that the authors also highlight.
Findings 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 find a significant link when narrowing down cat ownership to a specific period (ages 9 to 12). This inconsistency suggests that the crucial time frame for cat exposure is not clearly 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, that included people with and without mental disorders discovered a connection between cat bites and higher scores on tests measuring particular psychological experiences. But they suggest other pathogens such as Pasteurella multocida may be responsible instead.
The researchers agree that better and broader research is needed before we can make any firm interpretations.
"In conclusion, our review provides support for an association between cat ownership and schizophrenia-related disorders," the authors write.
"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 has been published in Schizophrenia Bulletin.