5. Epidemiologically, there is a close temporal correlation between the rise of cats as pets and the rise of psychosis. This can be illustrated most clearly in England, where the rise of cat ownership has been documented by writers and where there is data on the rise of psychosis, especially in the 18th and 19th centuries.1
How many cases of psychosis might be caused by T. gondii?
In 2014, using data from the antibody studies discussed above,12,13 Smith15 sought to discover how many cases of psychosis might be caused by T. gondii. He concluded that 21% of cases of schizophrenia might have been caused by T. gondii. Based on the annual incidence of schizophrenia in the United States, this would mean an estimated >10,000 new cases of schizophrenia each year are attributable to this parasite.
Some researchers have found links between T. gondii and several nonpsychiatric diseases and conditions, including epilepsy and brain cancer (Box1,16-19).
Box
Toxoplasma gondii and nonpsychiatric conditions
As interest in Toxoplasma gondii (T. gondii) has increased, researchers have looked for associations between this parasite with other diseases and conditions. Based on the literature, the following are of most interest:
Epilepsy. Since 1995, 16 studies1 have explored the relationship between T. gondii and epilepsy. A recent meta-analysis reported a statistically significant association between T. gondii and epilepsy.16
Brain cancer. Authors in 2 of 3 studies of meningiomas and 4 of 5 studies of gliomas reported statistically significant associations between these brain tumors and infection with T. gondii.1,17
Rheumatoid arthritis. Eight studies reported an increased prevalence of T. gondii antibodies in individuals with rheumatoid arthritis.1,18
Motor vehicle accidents. Infection with T. gondii is known to decrease motor reaction times in humans. At least 11 studies1 have examined whether infected individuals are more likely to have been involved in motor vehicle accidents. The results are mixed; the largest study reported a weak but statistically significant association.19
Clinical implications: What to tell patients about cats
What do these studies of toxoplasmosis imply for psychiatric care? As mental health professionals, part of our job is to educate our patients. Anything that appears to be a risk factor for the development of psychosis is thus of interest. Consider discussing the following with your patients.
Are cats safe? Cats that are kept exclusively indoors are safe pets because they are unlikely to become infected with T. gondii. However, cats that are allowed to go outdoors may not be safe, especially for children and young adults. What is needed is an effective vaccine that could be given to newborn kittens to prevent infection, but development of this type of vaccine has never been prioritized. At the community level, programs to decrease the number of stray and feral cats would also decrease the risk of infection.
Continue to: How to decrease risk