An estimated 22.5% of the U.S. population aged 12 and older has been infected with toxoplasmosis, which is caused by the parasite Toxoplasma gondii (T. gondii).1 Though cats are the target hosts of T. gondii, any vertebrate can be infected and serve as an intermediate host. For example, the parasite causes infected rodents to lose their fear of cats, making them more likely to be eaten by its ultimate host.2
Humans can become infected by coming in contact with infected cats, which shed them in microscopic oocyte form through their waste. It is estimated that 60 million Americans are latently infected with toxoplasmosis, but most people don’t show obvious symptoms because they have healthy immune systems.3 Treatment with antibiotics is typically indicated only for people with compromised immune systems or other vulnerabilities.4
But why should mental health clinicians be concerned about T. gondii?
A growing body of research suggests the parasite should be considered more of a threat than it is, especially in the realm of mental health. The link between T. gondii and schizophrenia, in particular, has been well established. A 2012 paper from the journal Schizophrenia Research reported that patients with schizophrenia are 2.7 times more likely to be infected with T. gondii than the general population.5 And a study released last year suggested the parasite could be involved in one-fifth of schizophrenia cases.6
“Results of several studies even suggest that the Toxoplasma infection is a cause of some forms (probably the more serious forms) of schizophrenia,” Jaroslav Flegr, PhD, a researcher and professor of biology at Charles University in Prague, explained to Psychiatry Advisor. “The clinical picture of schizophrenia in Toxoplasma-infected patients is worse and some morphological changes originally attributed to schizophrenia disease, such as a decrease of grey matter density in certain regions of brain, can be detected only in Toxoplasma-infected schizophrenia patients.
“It seems to me now that at least two different forms of schizophrenia exist and the most severe one is probably caused — in predisposed subjects — by Toxoplasma,” he added.
There are several mechanisms by which T. gondii may influence behavior and the development of mental disorders, but “one of the most studied at the moment is the increase in dopamine in the brains of people with toxoplasmosis,” Guillaume Fond, MD, a psychiatrist and researcher at the Schizophrenia Expert Center in Créteil, France, told Psychiatry Advisor. The neurotransmitter plays an important role in the brain’s reward system and functions like motivation, cognition, and fear regulation.
Though less is known about the relationship between T. gondii and other mental disorders, researchers have recently begun broadening the investigation. A study conducted by the Stanley Research Program at Sheppard Pratt Health System in Baltimore found that T. gondii infection may increase the risk of developing mania. Compared with non-psychiatric controls and participants with other psychiatric disorders, patients with mania had higher levels of antibodies to T. gondii,7 which the researchers suggest are part of an immune response to exposure or reactivation of T. gondii.