Toxoplasma gondii (T. gondii), a protozoan parasite that persists in host tissues, including brain, has been associated with several psychiatric disorders and with suicidal behavior. We sought to test the hypothesis that latent T. gondii infection, as manifest by circulating immunoglobulin G (IgG) antibodies to T. gondii, is associated with both categorical and dimensional measures of aggression.
IgG antibodies to T. gondii were collected between 1991 and 2008 from 358 adult subjects with DSM-5 intermittent explosive disorder (IED), non-IED psychiatric disorders (psychiatric controls), or no evidence of any psychiatric diagnosis (healthy controls). Assessments of aggression, anger, and impulsivity, as well as state/trait anger, depression, and anxiety were completed. T. gondii seropositive status (IgG > 12 IU) was the primary outcome measure for this study.
T. gondii seropositive status (IgG > 12 IU) was associated with higher aggression (P = .022) and impulsivity (P = .05) scores. When both aggression and impulsivity scores were controlled for, however, only aggression scores were higher in seropositive subjects (P = .011). In addition, T. gondii seropositive status and marginal mean ± SE aggression scores increased from healthy controls (9.1% and -0.66 ± 0.05) to psychiatric controls (16.7% and -0.27 ± 0.05) to subjects with IED (21.8% and 1.15 ± 0.06; P ≤ .05). These findings were not accounted for by the presence of other syndromal/personality disorders or by states or traits related to depressed or anxious moods.
These data are consistent with previous studies suggesting a relationship between T. gondii and self-directed aggression (ie, suicidal behavior) and further add to the biological complexity of impulsive aggression both from a categorical and a dimensional perspective.