Massive study links mind-altering cat parasite to schizophrenia

7 purr-fect cat facts

More than 40 million people in the United States may be infected with the single-celled cat-borne parasite Toxoplasma gondii, according to the Atlanta-based Centers for Disease Control and Prevention.

The infection typically occurs by eating or handling undercooked, contaminated meat or shellfish; drinking contaminated water or accidentally swallowing the parasite through contact with cat feces.

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Most infected individuals don’t have symptoms, but pregnant women and those with compromised immune systems are at heightened risk of a flu-like illness or serious eye and brain damage. Prenatal infection may also cause abortion or a congenital syndrome involving seizures and intellectual disability.

While very few infected individuals are known to experience the physiological symptoms associated with a Toxoplasma infection, some studies have found the parasite can alter the psychological behavior of mice and possibly, alter human cognition.

In 2012, a study of more than 45,000 Danish women found those infected with T. gondii were 1.5 times more likely to attempt suicide than uninfected women. Other studies have argued against any "overblown" psychological fears.

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Now new research led by the Copenhagen University Hospital in Denmark suggests the parasite may in fact be a "contributing causal factor for schizophrenia." The study—the largest of its kind—was published this week in the journal Brain, Behavior and Immunity.

For the large-scale serological study, scientists examined blood samples of 81,912 individuals through the Danish Blood Donor Study and identified 2,591 with psychiatric diagnoses, 655 who had attempted suicide or died of suicide and 2,724 individuals who had had traffic accidents.

Through blood samples, researchers analyzed the existence of immunoglobin G antibodies against T. gondii and the common herpes virus cytomegalovirus.

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Compared to the control group, individuals with the T. gondii infection were nearly 50 percent more likely to be diagnosed with schizophrenia compared to uninfected participants. When the researchers accounted for temporality— or for participants who had T. gondii in their blood but had not yet been diagnosed with schizophrenia—the link was even “stronger.”

“The association was even stronger when accounting for temporality and considering only the 28 cases who were diagnosed with a schizophrenia disorder after the date of blood collection,” authors wrote, emphasizing the findings are not definitive proof of causation but merely an association.

They also acknowledged a possible limitation: The study “did not control for socio-economic factors, which may influence the probability of pathogen infection [and] development of psychiatric disorders” but maintain their findings add to growing evidence of an association between exposure to the parasite and schizophrenia or related disorders.

Read the full study at

For worried cat owners, the CDC offers some safety precautions to avoid T. gondii exposure:

  • Change the litter box daily as the parasite becomes infectious only 1-5 days after it is shed in cat feces.
  • Pregnant women or those with compromised immune systems should avoid changing cat litter, keep cats indoors and avoid adopting or handling stray cats, especially kittens.
  • Avoid raw or undercooked meat for cat food. Feed your cats canned, dried commercial food or well-cooked table food instead.
  • Make sure your outdoor sandboxes are covered.
  • Speak with your veterinarian if you're worried.

Learn more about the parasite at

The medical community still doesn't know what exactly causes schizophrenia, a serious mental illness that, if left untreated, can affect every area of life, according to the Mayo Clinic. Researchers believe a multitude of factors may contribute to the disorder, including genetics, brain chemistry and environment. They hope learning more about risk factors for the schizophrenia may lead to early diagnosis and treatment.