(Washington, DC, June 3, 2015) Two new peer-reviewed studies have just been published evaluating the link between serious mental disorders and a common cat-carried parasite, Toxoplasma gondii, which can infect any warm-blooded species (i.e., birds, mammals) but relies on felids to sexually reproduce.

In a study led by A.L. Sutterland from the Department of Psychiatry, Academic Medical Centre (AMC) in Amsterdam, Netherlands, a team of researchers reported on the prevalence of T. gondii infection in patients with psychiatric disorders as compared to healthy controls. The study, called “Beyond the Association: Toxoplasma gondii in Schizophrenia, Bipolar Disorder, and Addiction: Systematic Review and Meta-analysis,” was published in the journal Acta Psychiatrica Scandinavica.

Using a systematic review of the published scientific literature, the authors evaluated nearly 3,000 publications, of which 50 were finally included in the study. The analyses confirmed that a T. gondii infection is associated with schizophrenia and also suggested an association with bipolar disorder and possibly addiction and obsessive-compulsive disorder (OCD). The odds ratios – which measure the likelihood of a particular outcome as compared to control groups –  for an individual with a T. gondii infection also having the following mental disorders was 3.4 for OCD, 1.81 for schizophrenia, 1.52 for bipolar disorder,  and 1.91 for addiction. Therefore, an individual with T. gondii was nearly twice as likely to have schizophrenia and 3.4 times more likely to develop OCD as someone without a T. gondii infection. The authors did note, however, that the findings on addiction and OCD should be interpreted with caution.

“In schizophrenia, the evidence of an association with T. gondii is overwhelming,” the study says. The authors went on to state that the findings “give an indication that T. gondii infection…may be a causal factor in the etiology of developing schizophrenia.”  Furthermore, “These findings may give further clues about how T. gondii infection can possibly [alter] the risk of specific psychiatric disorders.”

The second recently published study was led by E. Fuller Torrey and Wendy Simmons of the Stanley Medical Research Institute and Robert H. Yolken of Stanley Laboratory of Developmental Neurovirology, Johns Hopkins University, School of Medicine and was titled: “Is childhood cat ownership a risk factor for schizophrenia later in life?” It was published in the journal Schizophrenia Research.

Based on two previously published studies of members of the National Alliance for the Mentally Ill (NAMI) from 1992 and 1997 that indicated that cat ownership during childhood might be a risk factor for schizophrenia later in life, the authors analyzed data from a previously unpublished survey of NAMI members conducted in 1982. Notably, this information was collected 10 years before any data on cat ownership and mental illness had been published.

Evaluating over 2,000 questionnaires from families across the United States, the authors identified a “remarkably similar” result to the 1992 and 1997 surveys. According to the authors, “Cat ownership in childhood has now been reported in three studies to be significantly more common in families in which the child is later diagnosed with schizophrenia or another serious mental illness” and that cat exposure in childhood may be a risk factor for developing the disease.

Infection with T. gondii has long been associated with a variety of negative health consequences in people and wildlife. In people, this parasite can cause miscarriages, fetal developmental disorders, blindness, memory loss, and even death. Dozens of studies have also linked T. gondii to schizophrenia and suggested links with other neurodegenerative diseases. An ever-increasing list of terrestrial, freshwater, and marine birds and other wildlife also suffer from T. gondii infections, which have resulted in the deaths of threatened and endangered species.

With around 100 million outdoor cats now roaming the United States, the likelihood of infection with T. gondii is concerning. Cats may excrete hundreds of millions of tiny infectious eggs, called oocysts, in their feces, and these oocysts are incredibly hardy. Oocysts can remain infectious for years in soil or water, where they may be accidentally ingested or inhaled by people or wildlife. According to a study published in 2013, “There is evidence that accumulating T. gondii oocysts in the environment pose a significant public health hazard, especially in sandboxes of children, gardens, and other places favored by cats for defecation” and the authors of the Torrey et al 2015 study acknowledged that “children could theoretically become infected by playing in any infected public play area even if their family did not own a cat.” Toxoplasma gondii can infect any warm-blooded species (i.e., birds, mammals) but relies on felids to sexually reproduce.

In a related study published in 2014 in the scientific journal Brain, Behavior, and Immunity, scientists documented what they described as “remarkable” working memory performance reductions in seniors 65 and older that test positive for infection by the Toxoplasma gondii parasite.