Common Parasite Linked To Aggression, Intermittent Explosive Disorder

Parasite May Play Role in Some Schizophrenia Cases
Parasite May Play Role in Some Schizophrenia Cases
Researchers from multiple US universities investigated the previously observed association between latent T gondii infection and measures of aggression and impulsivity.

Latent infection with Toxoplasma gondii (T. gondii), a common protozoan parasite, is associated with elevated scores on measures of aggression, according to new findings published in the Journal of Clinical Psychiatry.1

T gondii is transmitted via consumption of undercooked contaminated meat and contaminated water, as well as by contact with cats or exposure to their litter. T gondii can also be passed from an infected mother to her child in utero or via transplants or transfusions with infected organs or blood, though those scenarios are relatively rare. The parasite travels to the brain, where it “hides within neurons and glial cells, forming characteristic cystic intracellular structures under the pressure of the immune system,” the authors wrote.

While cats are the ultimate host of T gondii, it also infects other warm-blooded animals as intermediate hosts, including an estimated one-third of humans worldwide. It is the second leading cause of death related to foodborne illness in the United States.2 The parasite is known to make mice “suicidal” in that they lose their fear of cats when infected, thereby facilitating its ultimate transmission to the targeted host. In humans, research has linked T gondii with suicidal behavior, schizophrenia, bipolar disorder, and personality disorders.

In the current study, researchers from multiple US universities investigated the previously observed association between latent T gondii infection and measures of aggression and impulsivity. Between 1991 and 2008, the researchers collected blood samples from 358 healthy adult participants to test plasma levels of immunoglobulin G (IgG) antibodies to the parasite. Additionally, they administered numerous psychometric assessments of impulsivity and aggression, suicidal behavior and self-injury, and anxiety and depression.

Based on diagnostic assessment, participants represented 3 different groups: one group (n=110) met DSM-5 criteria for intermittent explosive disorder (IED); another group (n=138) had psychiatric disorders other than IED; and participants in the third group had no known psychiatric disorders (n=110). The only demographic difference between the 3 groups pertained to age: the age difference was less than 5 years between the IED group and the healthy control group and it was less than 3 years between the IED group and psychiatric control group.  

The researchers tested the stored plasma samples in 2014 and found T gondii seropositivity in 15.9% of participants, and it “did not differ as a function of sex, race, or socioeconomic status,” the authors reported. While significant associations were not observed for the other tested variables, the findings show that participants with plasma IgG antibodies to T gondii >12 IU had higher scores on measures of aggression and impulsivity (but primarily aggression). While certain aggression scores were 9.1% in healthy controls, they were 16.7% in psychiatric controls and 21.8% in participants with IED.

The results were not accounted for by comorbid disorders or depressed or anxious mood. These findings align with other human and animal studies showing a link between T gondii and aggression and “further add to the biological complexity of impulsive aggression both from a categorical and a dimensional perspective,” the authors concluded.


1. Coccaro EF, Lee R, Groer MW, Can A, Coussons-Read M, Postolache TT. Toxoplasma gondii infection: relationship with aggression in psychiatric subjects. J Clin Psychiatry. 2016;77(3):334-341.

2. Centers for Disease Control and Prevention. Parasites–Toxoplasmosis (Toxoplasma Infection). Available at: Accessed March 27, 2016.