Do Cats Cause Schizophrenia?

by Beth Thompson, VMD

Flawed Conclusion Fingers Cats as Cause of Mental Illness
originally published in Compendium Vol 27(9) Sept 2005 pp 648-649

The August issue of National Geographic published a column by Joel Achenbach with the subtitle, “Your Cat Could Make You Crazy.”

He’s not talking about getting you up in the middle of the night, begging for food and then refusing to eat, or other quirky cat behaviors that make us go “Arggh!”

He’s saying, “Some scientists suspect cats can cause mental illness in people… Toxoplasma gondii, commonly found in cat feces, contaminated water, and undercooked meat, has been implicated in some cases of schizophrenia. Alan S. Brown, a Columbia University professor of psychiatry and epidemiology, has found a 2.6 times greater prevalence of schizophrenia in people exposed to ‘toxo’ in utero.”

Interestingly enough, Mr. Achenbach did not reveal that the conclusions in this study fell slightly short of significant, although the study concludes that prenatal exposure to toxoplasmosis is a plausible risk factor for schizophrenia.

The author lists other infectious diseases that have been linked with schizophrenia, psychosis, and bipolar disorder. His point? There may be a biologic cause of mental disease. The column concludes that current research is based on statistical correlations rather than any clear pathway from infection to disease but says, “We’ll still keep an eye on those crazy cats.”


I shudder at the misguided suggestion that people can “get” toxoplasmosis from their cats, and I worry about the obvious (mis)conclusion that the way to keep children from contracting this debilitating disorder is to get rid of the family pet. I was curious about other recent public health information on toxoplasmosis, schizophrenia, and any links between the two.

Research and Statistics

Here’s what I found when I did a search of recent literature regarding toxoplasmosis:

  • T. gondii infection is one of the most common zoonoses: Approximately 30% to 40% of human adults in the world test seropositive for the organism.
  • Worldwide, two to four of every 1,000 live births are affected. The risk levels in developing countries are significantly higher than in developed nations.
  • One study in the United Kingdom estimated that the lifetime risk of ocular symptoms associated with toxoplasmosis is 18 per 100,000. The risk level for West Africa is 100-fold higher.
  • In the United States, toxoplasmosis is not a nationally reportable disease and no reliable data about the number of cases diagnosed per year are available at the national level. However, since the 1960s, rates of infection in the United States appear to be declining. Similar downward trends have been observed in France and Sweden.
  • A report conducted by the USDA’s Economic Research Service concluded that one-half of toxoplasmosis cases in the United States are caused by eating contaminated meat, and the US Public Health Service has identified T. gondii as one of the top 10 culprits of foodborne illness.
  • More than 2 million people in the United States have schizophrenia. Individuals with a family history of the disease are about seven times more likely to be schizophrenic. Suggested risk factors for children include herpesvirus, influenza virus, rubella virus, cytomegalovirus, Toxoplasma infection, high levels of interleukin-8, exposure to painkillers, time of year, birth intervals, urban versus rural settings, glutens, leaded gasoline, and vitamin D deficiency in pregnant women.
  • More men are affected than women. Women tend to contract the disease later, have less severe symptoms, and respond better to medication.
  • In developing countries, the prevalence is lower and the gender bias reversed.

Statistically, what is the relationship between schizophrenia and there being a viral cause?

Arguments for an infectious (viral) or environmental cause include:

  • There are parallels between the rise of schizophrenia and infectious disease, starting with the Industrial Revolution.
  • Manifestations of schizophrenia in general have become less severe since World War II, suggesting it is less “virulent”.
  • There is an overall decline of schizophrenia, along with infectious disease, in industrialized countries.

Is there a connection between toxoplasmosis and schizophrenia?

What’s the connection between toxoplasmosis and schizophrenia? Since 1953, 19 studies of T. gondii antibodies in persons with schizophrenia and other severe psychiatric disorders and in controls have been reported: 18 studies reported a higher percentage of antibodies in affected persons; in 11 studies, the difference was statistically significant. Two other studies found that exposure to cats in childhood was a risk factor for development of schizophrenia. In addition, some medications used to treat schizophrenia inhibit the replication of T. gondii in cell culture.

Is there a link between contact with cats and toxoplasmosis?

What about the link between contact with cats and toxoplasmosis? Exposure to cats, per se, does not appear to subject an individual to increased risk of infection. In general, veterinary health care providers are no more likely than the general population to test seropositive for T. gondii infection. In one case–control study of pregnant women, there was no association between primary toxoplasmosis and having a cat or kitten at home, litterbox cleaning, or owning a cat that hunts. The major routes of infection, depending on geography, are undercooked or raw meat, unwashed fruits or vegetables, water, and direct contact with contaminated soil. That’s no excuse not to be careful with cat excrement; proper disposal minimizes risk. Touching individual cats is an extremely unlikely way to acquire the disease. In fact, cats are so fastidious that bioassays failed to detect oocysts on the fur of experimental cats 7 days after they were shedding millions of oocysts.


In conclusion, experts agree that pet cats (especially indoor ones) pose little threat of toxoplasmosis transmission. Toxoplasmosis is one of many infectious agents being examined as a causative factor of schizophrenia and, like others, deserves further research. So, while I support Mr. Achenbach’s efforts to educate the public on potential medical breakthroughs, I think he may have inadvertently dealt a low blow to cats in the name of “catchy” copy.


  1. Achenbach J: Your Cat Could Make You Crazy. Accessed August 2005 at
  2. Feline Zoonoses Guidelines from the American Association of Feline
    Practitioners: Accessed August 2005 at
  3. Schlundt J, Toyofuku H, Jansen J, Herbst SA: Emerging food-borne
    zoonosis. Rev Sci Tech 23(2): 513–533, 2004.
  4. Preventing congenital toxoplasmosis. MMWR Recommendations and Reports
    49(RR02):57–75, 2000. Accessed August 2005 at
  5. Partnership for Food Safety Education: Ten Least Wanted Foodborne
    Pathogens. Accessed August 2005 at
  6. Infectious Schizophrenia. Accessed August 2005 at
    7. Sorensen HJ, Mortensen EL, Reinisch JM, Mednick SA: Association between
    prenatal exposure to analgesics and risk of schizophrenia. Br J Psychiatry
    185:366–371, 2004.
  7. Interview with Professor John McGrath. Accessed August 2005 at (search John McGrath schizophrenia).
  8. Torrey EF, Yolken RH: Toxoplasma gondii and schizophrenia. Emerg Infect
    Dis 9(11):1375–1380, 2003.

Additional Resources

  • Brown AS, Schaefer CA, Quesenberry Jr CP, et al: Maternal exposure to
    toxoplasmosis and risk of schizophrenia in adult offspring. Am J Psychiatry
    162: 767–773, 2005.
  • Leweke FM, Gerth CW, Koethe D, et al: Antibodies to infectious agents in
    individuals with recent onset schizophrenia. Eur Arch Psychiatry Clin
    Neurosci 254(1): 4–8, 2004.
  • Pet Therapy Helps Schizophrenia, Jan 15, 2005. Accessed August 2005 at

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