Cat exposure in early life could double risk of schizophrenia, meta-analysis suggests

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Recent research published in Schizophrenia Bulletin suggests that exposure to cats in early life may increase the risk of developing schizophrenia and related disorders in young adulthood. The systematic review and meta-analysis found that cat ownership was associated with more than twice the odds of experiencing schizophrenia-related disorders. However, the relationship between cat ownership and less severe psychotic experiences is unclear.

Schizophrenia, a serious mental disorder characterized by distortions in thinking, perception, emotions, language, sense of self, and behavior, has long been the focus of scientific research to identify possible factors of risk Schizophrenia symptoms typically fall into three categories: positive, negative, and cognitive.

Positive symptoms include hallucinations (such as hearing voices) and delusions (fixed, false beliefs). Negative symptoms involve emotional flatness or lack of function, such as the inability to initiate and follow through with activities, and cognitive symptoms cover problems with attention, memory, and executive functions such as planning and organization.

Psychotic-type experiences are subclinical symptoms of psychosis that can occur in the general population without a diagnosis of psychotic disorder. These include experiences similar to hallucinations and delusions, but are usually less intense and less frequent than those seen in full-blown psychotic disorders such as schizophrenia. For example, a person may report hearing a voice calling their name when they are alone, but unlike in schizophrenia, these experiences may not significantly affect functioning or require clinical intervention.

Previous research into environmental factors contributing to schizophrenia has sometimes pointed to cat ownership as a potential risk factor. This interest is largely due to the role of the Toxoplasma gondii parasite, a microscopic organism that can infect most animals and birds, but reproduces sexually only in cats, making them the primary hosts.

Humans can become infected through several routes, including ingestion of undercooked meat, consumption of food or water contaminated with cat feces, or, less commonly, transmission from mother to fetus during pregnancy. In most healthy people, T. gondii It causes no obvious symptoms, but can pose serious risks to those with weakened immune systems and pregnant women.

Studies have also suggested that infection with T. gondii may increase the risk of developing schizophrenia, possibly due to the parasites’ ability to persist in the human brain and affect neurotransmitter functions and immune responses. However, results have been mixed, with some studies showing a significant association between childhood cat ownership and increased risk of schizophrenia, while others found no such link.

To provide a clearer picture of the relationship between cat ownership and schizophrenia, the researchers conducted a meta-analysis, a statistical technique that combines the results of multiple scientific studies. With larger data sets from combined studies, meta-analyses provide more precise estimates of effects. In addition, meta-analyses often include diverse populations and varied settings, which may improve the generalizability of findings.

To capture all relevant studies without restrictions of language or geographic location, the researchers searched several electronic databases, including PubMed-Medline, Embase, CINAHL, and Web of Science, covering publications from January 1980 to May 2023. Analysis did not limit types. of included studies provided they provided risk estimates such as relative risks, odds ratios or hazard ratios, with 95% confidence intervals or similar measures of uncertainty.

The researchers ended up with a sample of 17 studies from 11 countries: Canada, Ethiopia, Egypt, Finland, France, Lebanon, Saudi Arabia, Tunisia, Turkey, the United Kingdom and the United States of America. All of these studies explored the potential link between cat ownership and the risk of developing schizophrenia-related disorders and psychotic experiences.

The population focus was the general population, with a specific emphasis on cat ownership from prenatal to age 25. This age range was chosen because of the typical onset of schizophrenia in the early twenties. Data on cat ownership were collected using self-reports or reports from others, such as parents.

The researchers identified a clear link: people who had been exposed to cats during the critical period from prenatal development to age 25 showed a greater likelihood of developing schizophrenia-related disorders. Specifically, the unadjusted pooled odds ratio was 2.35, indicating that the risk of developing these disorders was more than double for those exposed to cats compared to those who were not. This association persisted even after adjusting for various confounders.

However, the analysis on psychotic-type experiences produced conflicting results. Although some studies within the meta-analysis reported a correlation between cat ownership and increased scores on psychotic experience scales, others did not find a significant association. This inconsistency highlights the complexity of psychotic experiences as a diagnostic category and suggests that the relationship between cat exposure and subclinical psychotic symptoms may be influenced by factors that were not uniformly controlled for across studies.

Despite the strong findings, the study faced several limitations. In particular, heterogeneity among studies, in terms of how cat exposure was defined and how outcomes were measured, posed challenges. This variability may have influenced the results and the ability to draw broader generalizations from the findings.

Most of the included studies were unable to control for all potential confounders, which could affect the observed associations. The role of T. gondiifor example, it remains an area ripe for further research to understand its exact mechanism for potentially increasing the risk of schizophrenia.

Future research should focus on more uniformly designed studies that can better address these inconsistencies and explore the biological underpinnings of the observed associations. Understanding the critical periods of exposure and the specific conditions under which cat ownership may increase the risk of schizophrenia will be crucial. Furthermore, refining the measurement of psychotic experiences and their relationship to cat exposure will help clarify this complex link.

“In conclusion, our review provides support for an association between cat ownership and schizophrenia-related disorders. Our field needs to generate new candidate environmental risk factors, especially those that are potentially modifiable. In this context , more high-quality studies based on large, representative samples are needed to better understand cat ownership as a candidate risk-modifying factor for mental disorders,” the researchers wrote.

The study, “Cat Ownership and Schizophrenia-Related Disorders and Psychotic Experiences: A Systematic Review and Meta-Analysis,” was authored by John J. McGrath, Carmen CW Lim, and Sukanta Saha.

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