anger toxoplasmosis
People with IED are more likely to be suffering from parasitic disease toxoplasmosis. Istock

A parasite found in raw meat and cat faeces has the potential to cause rage disorders in humans. Researchers found people with Intermittent Explosive Disorder – episodes of sudden rage – could actually be suffering from a latent toxoplasmosis infection.

Their study, published in the Journal of Clinical Psychiatry, shows that this relatively harmless parasitic disease may be associated with changes in the brain, which can make people more aggressive. Those diagnosed with an intermittent explosive disorder are twice as likely as healthy individuals to test positive for toxoplasmosis exposure.

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Intermittent explosive disorder manifests itself through repeated, sudden episodes of impulsive, and violent behaviours, episodes of rage and abusive verbal outbursts.

Road rage, domestic abuse, and throwing objects can be signs of this chronic disorder which can affect the person throughout her lifetime. However, frequency may decrease as the person grows in age. IED is more common in people under 40 years old.

These episodes are generally a disproportionate response to the trigger situation and may leave the person feeling remorse and tiredness.

Psychotherapy can help patients learn to control their tantrums and their violent outbursts. People with a history of having been physically abused or who have other psychiatric issues may be more at risk of developing the disorder.

Scientists believe causes are due to a mix of environmental, cognitive and genetic factors. Differences in brain chemistry is one of the most promising leads.

Caused by the Toxoplasma gondii parasite, toxoplasmosis is transmitted to humans through the faeces of infected cats, contaminated water or undercooked meat.

The majority of people will never know they carried the disease. Its impact on physical health is generally minimal, except for pregnant women, as it can cause miscarriages, or for people with weak immune systems.

The parasite is also known to stay in brain tissues, and it had been previously associated with a range of psychiatric diseases, including schizophrenia.

The scientists, from the University of Chicago, suspected that a link could also be found between intermittent explosive disorder, and toxoplasmosis.

They decided to verify this hypothesis, in the hope it could help improve diagnosis and treatment for IED patients.

22% IED patients with toxoplasmosis

The scientists recruited 358 adults whom they tested for different psychiatric disorders, including IED. The participants were also given scores for anger, aggression and impulsivity. Based on these results, the researchers made three groups: a third of volunteers were diagnosed with IED, another with any other psychiatric disorder, and the last third was considered "healthy".

Blood tests to check for the presence of Toxoplasma gondii revealed that 22% of IED patients were infected, roughly twice more than the healthy group, where only 9% had toxoplasmosis.

Amongst the people with psychiatry issues, but not IED, 16% tested positive for the parasitic disease. Their scores for aggression and impulsivity where much lower than those in the IED group.

Experimental research needed

The study's findings therefore suggest toxoplasmosis and aggression are most strongly correlated. The authors believe this is due to chemical changes within the brain that occur under the action of the parasite, but are not yet able to describe these modifications.

"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. "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. Our study signals the need for more research and more evidence in humans."

The way forward, the scientists say, is to carry out experimental studies to see if the treatment of latent toxoplasmosis infections in people with IED can reduce aggressive behaviours and rage fits.

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