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 Numerous studies link toxoplasmosis to a number of psychiatric conditions, including bipolar disorder, schizophrenia, and obsessive-compulsive disorder. This infection is also linked to aspects of behavior and personality, from impulsivity to prolonged feelings of guilt and excessive worry, and may even slow a person's reflexes. Now new research has linked latent toxoplasmosis infection to aggression and impulsivity. Those with a psychiatric condition called intermittent explosive disorder (IED) have a greater likelihood of toxoplasmosis infection (when compared to people without IED). People who have IED typically experience recurrent outbursts of extreme, impulsive anger, such as seen in road rage.

Three groups (normal healthy control group, intermittent explosive disorder (IED) group, and non-IED psychiatric disorders group) were studied, and in all 3 groups those who tested positive for the infection had higher levels of aggression within their group. 22% of the IED group had the infection, 17% in the non-IED psychiatric group, and only 9% in the healthy normal control group.This research suggested that toxoplasmosis infection is linked to aggression and impulsivity (with aggression being stronger).

Toxoplasmosis is caused by an infection with Toxoplasma gondii, a protozoan parasite carried by cats. It can also infect humans, through contact with cat feces, poorly cooked meat or contaminated water, and as many as one-third of the world’s population may be infected. The person may not feel sick, but the parasite may form cysts in the brain where it can remain for the rest of a person’s life. In this study they determined that someone had a latent T. gondii infection by the circulating immunoglobulin G (IgG) antibodies to T. gondii in the blood (IgG > 12 IU). And yes, the infection can be successfully treated with medications. From Science Daily:

People with rage disorder twice as likely to have latent toxoplasmosis parasite infection

Individuals with a psychiatric disorder involving recurrent bouts of extreme, impulsive anger--road rage, for example--are more than twice as likely to have been exposed to a common parasite than healthy individuals with no psychiatric diagnosis. In a study involving 358 adult subjects, a team led by researchers from the University of Chicago found that toxoplasmosis, a relatively harmless parasitic infection carried by an estimated 30 percent of all humans, is associated with intermittent explosive disorder and increased aggression.

"Our work suggests that latent infection with the toxoplasma gondii parasite may change brain chemistry in a fashion that increases the risk of aggressive behavior," said senior study author Emil Coccaro, MD, Ellen. C. Manning Professor and Chair of Psychiatry and Behavioral Neuroscience at the University of Chicago. "However, we do not know if this relationship is causal, and not everyone that tests positive for toxoplasmosis will have aggression issues," Coccaro said, adding that additional studies are needed.

Intermittent explosive disorder (IED) is defined by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, as recurrent, impulsive, problematic outbursts of verbal or physical aggression that are disproportionate to the situations that trigger them. IED is thought to affect as many as 16 million Americans, more than bipolar disorder and schizophrenia combined.....examined possible connections to toxoplasmosis, an extremely common parasitic infection. Transmitted through the feces of infected cats, undercooked meat or contaminated water, toxoplasmosis is typically latent and harmless for healthy adults. However, it is known to reside in brain tissue, and has been linked to several psychiatric diseases, including schizophrenia, bipolar disorder and suicidal behavior.

The research team recruited 358 adult subjects from the U.S., who were evaluated for IED, personality disorder, depression and other psychiatric disorders. Study participants were also scored on traits including anger, aggression and impulsivity. Participants fell into one of three groups. Roughly one third had IED. One third were healthy controls with no psychiatric history. The remaining third were individuals diagnosed with some psychiatric disorder, but not IED. This last group served as a control to distinguish IED from possible confounding psychiatric factors.

The research team found that IED-diagnosed group was more than twice as likely to test positive for toxoplasmosis exposure (22 percent) as measured by a blood test, compared to the healthy control group (9 percent). Around 16 percent of the psychiatric control group tested positive for toxoplasmosis, but had similar aggression and impulsivity scores to the healthy control group. IED-diagnosed subjects scored much higher on both measures than either control group.

Across all study subjects, toxoplasmosis-positive individuals scored significantly higher on scores of anger and aggression. The team noted a link between toxoplasmosis and increased impulsivity, but when adjusted for aggression scores, this link became non-significant. This finding suggests toxoplasmosis and aggression are most strongly correlated. However, the authors caution that the study results do not address whether toxoplasmosis infection may cause increased aggression or IED. "Correlation is not causation, and this is definitely not a sign that people should get rid of their cats," said study co-author Royce Lee, MD.... "We don't yet understand the mechanisms involved..."

 Toxoplasma gondii  tissue cyst in a mouse brain Credit: Jitinder P. Dubey, Wikipedia