People who display frequent bouts of extreme, impulsive anger, such as road rage, are more than twice as likely to be infected with a common parasite than are individuals who do not exhibit such explosive behavior, according to a new study.
The findings, reported in the Journal of Clinical Psychiatry, suggest that the parasitic infection toxoplasmosis could lead to increased aggressiveness and associated mental illness in some people.
People who repeatedly fly off the handle with little provocation and who overreact to stress are often diagnosed with a condition known as intermittent explosive disorder.
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"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," senior author Emil Coccaro of the University of Chicago said in a press release.
"However, we do not know if this relationship is causal, and not everyone that tests positive for toxoplasmosis will have aggression issues," Coccaro added, saying that additional studies are needed.
Nevertheless, the researchers did find - via blood tests - that individuals diagnosed with intermittent explosive disorder were more than twice as likely to test positive for toxoplasmosis exposure compared to healthy people with no such prior diagnosis. The study involved 358 adult subjects.
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The researchers also evaluated the test subjects' levels of anger and aggression, and found that toxoplasmosis-positive individuals scored significantly higher than did the other participants.
It is believed that around 30 percent of all humans carry the Toxoplasma gondii parasite, but most show no symptoms at all. The parasite seems to be similar to prevalent pathogens like staphylococcal bacteria, in that it is very common and does very minor, if any, damage to most people. When the immune system weakens or conditions somehow otherwise become more favorable for the pathogens, they can strengthen in numbers and pose significant health threats.
Prior research has shown that the Toxoplasma gondii parasite can break through the blood-brain barrier and may then reside in brain tissue. Infection with the parasite has been linked to several psychiatric diseases, including schizophrenia, bipolar disorder and suicidal behavior.
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Transmission can occur through many different ways, such as drinking contaminated water, eating undercooked meat (especially pork, lamb, or venison), or accidentally swallowing the parasite through contact with cat feces and/or dirt. It can also be transmitted through an organ transplant or a transfusion, although this is rare.
"We don't yet understand the mechanisms involved–it could be an increased inflammatory response, direct brain modulation by the parasite, or even reverse causation where aggressive individuals tend to have more cats or eat more undercooked meat," co-author Royce Lee of the University of Chicago said. "Our study signals the need for more research and more evidence in humans."