Infection by the organism Toxoplasma gondii doubles the chances of suffering a mental condition that triggers bouts of uncontrollable anger, a study found.
The parasite is thought to alter brain chemistry, resulting in “intermittent explosive disorder” (IED).
People with IED are liable to display outbursts of violent rage which are disproportionate to the provocation received — for instance, when annoyed by another driver on the road.
Scientists who assessed a group of 358 US adults found that more than a fifth (22%) of those diagnosed with IED tested positive for T. gondii. In comparison, 9% of non-infected individuals had the condition.
Lead researcher Professor Emil Coccaro, from the University of Chicago, said: “Our work suggests that latent infection with the Toxoplasma gondii parasite may change brain chemistry in a fashion that increases the risk of aggressive behaviour.
“However, we do not know if this relationship is causal, and not everyone that tests positive for toxoplasmosis will have aggression issues.”
Many people are believed to carry the parasite, found in cat faeces and contaminated food, without realising it.
Very often, the parasite causes no obvious symptoms.
However, it can lead to miscarriages and birth defects, and cause serious illness in people with weak immune systems.
It is also known to reside in brain tissue and has already been linked to a number of psychiatric conditions including schizophrenia, bipolar disorder and suicidal behaviour.
The organism can be transmitted through contact with dirty cat litter, or by handling or swallowing contaminated meat, vegetables or drinking water.
Roughly a third of those recruited for the US study had IED, another third were suffering from some other psychiatric condition such as depression or personality disorder, and the remainder were healthy individuals with no history of mental problems.
Across all the participants, toxoplasmosis-positive individuals scored significantly higher on measures of anger and aggression.
Co-author Dr Royce Lee, also from the University of Chicago, said: “Correlation is not causation, and this is definitely not a sign that people should get rid of their cats.
“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.”