II.     Possible Transmission of T. gondii from Cats to Humans, Causing Schizophrenia

As noted previously, T. gondii can be transmitted from cats to humans in many different ways. Being bitten by a cat, however, apparently does not cause the transmission of T. gondii (Westling K, Jorup-Ronstrom C, Evengard B, Toxoplasmosis not transmitted by cat bite, but high prevalence of antibodies to Toxoplasma gondii in patients bitten by their own cat, Scand J Infect Dis 2010;42:687–690). Some of the means of transmission require no contact whatever between cats and humans, e.g., through tissue cysts in undercooked lamb, drinking water infected with oocysts, oocysts deposited by a neighborhood cat in your garden. For this reason, attempts to show a correlation between having antibodies to T. gondii and past contact with cats have yielded very inconsistent results.

A review of 30 such studies reported that half of them found a correlation, but half did not (Hall S, Ryan M, Buxton D, The epidemiology of toxoplasma infection, in Joynson DHM, Wreghitt TG, eds,Toxoplasmosis: A Comprehensive Clinical Guide, Cambridge: Cambridge University Press, 2001, pp. 85–91). Those studies that were negative were more likely to have been studies of adults, e.g., pregnant women who were asked if they presently owned a cat. Those studies that were positive were more likely to have included children and teenagers, such as studies done in Costa Rica and Panama (Sousa OE, Saenz RD, Frenkel JK, Toxoplasmosis in Panama: a 10-year study, Am J Trop Med Hyg 1988;38:315–322; Frenkel JK, Ruiz A, Human toxoplasmosis and cat contact in Costa Rica, Am J Trop Med Hyg 1980;29:1167–1180). The results varied depending on how the question was asked, with cat ownership in childhood more likely to yield a positive correlation with T. gondii antibodies than cat ownership in adulthood. The complexity of studying human–cat contact was also illustrated by a Norwegian study that asked about cat contact in great detail. Becoming infected with T. gondii was not statistically related to “living in a neighborhood with a cat” (p=0.71) or “living in a household with a cat” (p=0.13) but was statistically significantly related to “living in a household with a kitten less than 1 year old” (p=0.04) (Kapperud G, Jenum PA, Stray-Pedersen et al., Risk factors for Toxoplasma gondii infection in pregnancy: results of a prospective case-control study in Norway, Am J Epidemiol 1966;144:405–412). An American study reported that living in a household with one or two kittens was not a significant risk factor for becoming infected with T. gondii, but living in a household with three or more kittens was a highly significant risk factor (OR 35.4) (Jones JL, Dargelas V, Roberts J et al., Risk factors for Toxoplasma gondii infection in the United States, Clin Infect Dis 2009;49:878–884).

In view of the above, it is of interest that two studies that have assessed cat contact during childhood reported that it was significantly more common in individuals with schizophrenia than in controls. In the first study of 165 parents of individuals with schizophrenia and bipolar disorder, 51 percent reported that they owned a cat during pregnancy or during the first 10 years of life of the affected individuals, compared to 38 percent among matched controls (p=0.02, chi square; however, this was not corrected for the number of questions asked, which would require a p<0.01 using a Bonferroni correction). The question was asked for four different periods, and the results were as follows:

Thus, the largest difference in the ages for cat ownership was for ages 6–10. Dog or other pet ownership was not included in this questionnaire (Torrey EF, Yolken RH, Could schizophrenia be a viral zoonosis transmitted from house cats, Schizophr Bull 1995;21:167–171).

The second study included 264 mothers of individuals with schizophrenia or bipolar disorder and 528 matched controls and included questions on both cat and dog ownership as follows:

Families in which the individual later developed schizophrenia or bipolar disorder were significantly more likely to have owned a cat, but not a dog, between birth and age 13 (p=0.0072) but not during the pregnancy (Torrey EF, Rawlings R, Yolken RH, The antecedents of psychoses: a case-control study of selected risk factors, Schizophr Res 2000;46:17–23).

Given the mixed results of previous studies of antibodies to T. gondii and history of cat contact, the results of these two studies suggest that if T. gondii is associated etiologically with some cases of schizophrenia, then transmission of the protozoa is most likely to be via oocysts, not tissue cysts, and to take place during childhood.

A few of the T. gondii antibody studies (see section V) have also collected information on cat exposure. For example, a Turkish study of 73 patients with first-episode schizophrenia and 40 controls reported that 15/73 (28%) of the patients had a cat in their house compared to 1/40 (3%) of the controls (p=0.006) (Tanyuksel M, Uzun O, Araz E et al., Possible role of toxoplasmosis in patients with first-episode schizophrenia, Turk J Med Sci 2010;40:399–404).