Toxoplasmosis and psychosis: What evidence supports a link?
Until recently, cerebral infections, congenital infections, and eye disease were thought to be the main clinical problems associated with toxoplasmosis. However, accumulating evidence suggests that psychosis should be added to this list. Five lines of evidence support this.
1. T. gondii can cause psychotic symptoms. It has been known for decades that T. gondii can cause delusions, auditory hallucinations, and other psychotic symptoms.1 In one of the earliest publications (1966), Ladee10 concluded “The literature not infrequently focuses attention on psychosis with schizophrenia or schizophreniform features that accompany chronic toxoplasmosis.” Among the cases Ladee10 described was a laboratory worker who became infected with T. gondii and developed delusions and hallucinations.10
2. Patients with schizophrenia who are infected with T. gondii have more severe psychotic symptoms. This finding has been reported in at least 7 studies.1 Holub et al11 evaluated 251 patients with schizophrenia who were treated in Prague Psychiatric Centre between 2000 and 2010. Overall, 57 participants were infected with T. gondii and 194 were not infected. Compared to those who were not infected, the infected group:
- had significantly more severe symptoms (P = .032) as measured on the Positive and Negative Symptom Scale
- were prescribed higher doses of antipsychotic medications
- had been hospitalized longer.11
3. Compared with controls, patients with psychosis are significantly more likely to have antibodies against T. gondii, indicating previous infection. To date there have been approximately 100 such studies, of which at least three-fourths reported a positive association. In a 2012 meta-analysis of 38 such studies, Torrey et al12 reported an odds ratio (OR) of 2.7—compared to persons who have not been infected, those who have been infected with T. gondii were 2.7 times more likely to have schizophrenia.12 This study replicated the findings of a previous meta-analysis of 23 antibody studies, which also found an OR of 2.7.13
4. Compared with controls, individuals with schizophrenia or bipolar disorder are significantly more likely as a child to have lived in a home with a cat. Since 1995, 10 such studies have been published; 7 were positive, 2 were negative, and 1 was inconclusive.1 Torrey et al14 reviewed 2,025 individuals with schizophrenia or bipolar disorder and 4,847 controls and found that 51% of the cases and 43% of the controls had owned a cat before age 13; this difference was highly significant (P < .001). In fact, it is surprising that any study can find a statistically significant association between cat ownership and childhood psychosis. This is because a child who did not own a cat could become infected in many locations where cats have been present, including sandboxes at school, a babysitter’s or friend’s house, or a public park. And even if a child became infected at home, they would not necessarily have owned a cat, since the neighbor’s cat could have been responsible for the oocyst contamination.
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