Schizophrenia can produce persistent delusions, hallucinations, and disorganized thinking. The precise cause is unknown but seems to involve a combination of genetics and environmental risk factors. One environmental factor may be an infectious agent, such as the common parasite Toxoplasma gondii, which causes toxoplasmosis.

Since cats can transmit Toxoplasma to humans, scientists have been investigating whether there is a link between cat ownership and schizophrenia. Many studies have tried to answer this question over the past 50 years; some studies show an association, but others do not. Researchers at the University of Queensland in Australia recently reanalyzed all these studies to determine the current consensus.

What Is Toxoplasma?

Toxoplasma is a single-celled parasite that infects all warm-blooded animals, including up to one-third of the human population. Cats are the only animals that support the sexual stage of the parasite’s life cycle, which culminates in the expulsion of infectious parasites in the feces. These fecal parasites are housed in sturdy containers called oocysts, which are stable in the environment for years and can spread the infection to a new individual if inhaled or ingested.

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In addition to litter boxes, people can pick up oocysts wherever a cat may have defecated, for example in the yard, sandbox, or garden (including unwashed fruits and vegetables). Oocysts have also made their way into streams and seawater, where they can infect people though shellfish.

Up to 40 million people in the U.S. are infected with Toxoplasma. While a healthy immune system can control the parasite’s growth, it cannot get rid of the infection entirely. Toxoplasma parasites remain in the brain and other tissues as latent cysts, which can resume growth if the immune system is weakened.

Most people with latent toxoplasmosis show no obvious signs of disease despite having parasite cysts in their brain. However, it has been noted that rodents with latent toxoplasmosis, which also show no signs of physical illness, do not behave normally. Most strikingly, infected rodents are more cavalier around threatening stimuli, including cat odors. Latent cysts have been shown to cause structural changes in the brain as well as alterations in hormones and neurochemistry. Some studies suggest that the behavioral changes in mice may be caused by the immune response to the parasite, which causes inflammation in the brain.

Humans with latent toxoplasmosis appear to display similar neurological anomalies. Correlations between Toxoplasma infection and increased risk-taking, aggression and impulsivity, and suicide and homicide have been documented. A caveat to these types of studies is that causation is not proven; in other words, they do not distinguish whether toxoplasmosis causes unusual behavior or whether unusual behavior increases the odds of acquiring toxoplasmosis.

What Does the New Study Show?

The correlation between latent toxoplasmosis and schizophrenia is well documented, prompting psychiatry professor John McGrath at the University of Queensland to perform a meta-analysis — a study of studies — that aimed to determine whether cat ownership also correlates with schizophrenia. His team scrutinized 17 distinct studies that have been published over the last 44 years in 11 countries to find the consensus.

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McGrath and his team found that people exposed to cats had approximately twice the odds of developing schizophrenia, although it was noted that several of the studies used in their analysis had low-quality scores. Consequently, the team calls for “more high-quality studies, based on large, representative samples to better understand cat ownership as a candidate risk-modifying factor for mental disorders.” Notably, their conclusion tracks with a study done last year that found people who tested positive for Toxoplasma were twice as likely to develop schizophrenia.

Evidence for a clear association between cat ownership and schizophrenia is not very strong, but sufficient to warrant further research. What is certain is that people can become infected with Toxoplasma through oocysts expelled by cats. It is also certain that toxoplasmosis poses a serious health risk to immune compromised persons and fetuses in the womb. Pregnant people should avoid tending to the litter box as congenital toxoplasmosis can cause miscarriage, vision loss, or birth defects.

How to Avoid Toxoplasmosis

There’s no need to panic and get rid of your beloved feline friend. Adhering to a few simple guidelines will protect you (and your cat) from contracting toxoplasmosis.

Like humans, cats can catch Toxoplasma from oocysts in the environment or through parasites contaminating the flesh of other animals. Keeping your cat indoors and on a diet free of raw or undercooked meat products will greatly reduce the risk of infection with Toxoplasma.

Toxoplasma oocysts do not become infectious until 1 to 5 days after they are shed in a cat’s feces. Changing the litter box daily ensures that the oocysts will not become a danger. Wear gloves and promptly wash your hands after cleaning the litter box or being outdoors where a cat may have defecated.

Finding direct evidence that cat ownership increases disease risk is tricky because it is difficult to rule out confounding variables. As we wait for more definitive data to better answer the question, heeding the warnings above will allow you to enjoy cats without worry.

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