A new study creating lots of both media attention and puzzled looks explores the link between psychiatric disorders, particularly those that involve aggression and irritability, and latent infection with the protozoan parasite toxoplasma ghondii (T gondii). In hearing about this study, the first reaction of many might be why anyone bothered to look at this link in the first place. Cats are known hosts of the parasite, but infection can occur in humans from infected cat feces in addition to eating
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undercooked meat or drinking contaminated water. Human infection is common but usually not dangerous, unless someone is immunosupressed. While not an agent with a huge literature behind it in terms of behavioral health, there have been previous associations found between latent toxoplasmosis infection and psychiatric symptoms, including a larger study in 2015. There also exists a literature on the behavioral effects of T gondii in animals. Thus, this study did not just come totally out of the blue.
For this investigation, the authors assessed three groups of adults (total n=358), including healthy controls, a group with well diagnosed intermittent explosive disorder (IED: characterized by episodes of severe impulsive aggression), and another group who met criteria for DSM-5 psychiatric disorders other than IED. Aggression and impulsivity were assessed with various known rating scales, and toxoplasmosis status was assessed using a blood test for IGG antibodies.
The overall rate of T. Gondii seropositive subjects was 15.9% which evidently (and a little unsettlingly to be honest) is close to national norms. Statistically higher overall scores on aggression and impulsivity scores were found among subjects who were seropositive for T. Gondii, while, contrary to some previous work, no differences were found related to suicide or self-directed aggression. In terms of the three diagnostic groups, the percentage of subjects with IED who were seropositive for T. Gondii was 21.8%, which was significantly higher than controls (9.1%) but not psychiatric controls (16.7%).
The authors concluded that their data support others that suggest a link between T. Gondii and aggression. (Actually, in the abstract they say it supports the link with self-directed aggression but I think that is a mistake).
In the discussion section, the authors go into some speculation about why this link might exist, including possible low grade chronic inflammation in the brain, alterations of corticolimbic circuits, or even via increased production of testosterone, based on some animal studies. Other than once mentioning that T Gondii infections are treatable, the article gives no advice about assessing or treating the infection in aggressive people, likely because their cross sectional data require further investigation using other designs. I haven’t heard any official response from the cat lobby.
Reference
Coccaro EF, et al. Toxoplasma gondii infection: Relationship with aggression in psychiatric patients. J Clin Psychiatry 2016; 77:334-341
Tags: aggression, Intermittent Explosive Disorder, toxoplasmosis