If you own a cat, it might not be just cleaning its litter box that’s making you angry. A study published in The Journal of Clinical Psychiatry on March 23 associated a parasite commonly found in cat feces to an aggression disorder.

The study found that individuals with Intermittent Explosive Disorder (IED), a disorder characterized by bouts of sudden uninhibited and aggressive behavior, were twice as likely to be infected with the parasite, known as Toxoplasma gondii, or T. gondii, when compared to individuals who do not have the disorder.

The study was co-authored by doctors Emil Coccaro and Royce Lee, faculty members of the Clinical Neuroscience and Psychopharmacology Research Unit (CNPRU), a part of the University’s Department of Psychiatry and Behavioral Neuroscience.

The parasite causes an infection called toxoplasmosis. The infection does not typically generate physical symptoms; however, an increasing amount of research on the parasite is indicating that it is associated with a variety of mental illnesses in humans, including bipolar disorder and schizophrenia. This study is the first to examine its association with IED.

Lee said that the results of the study can be interpreted in a variety of ways. “There are three possibilities. The first is that T. gondii increases inflammation and that this inflammation makes aggressive behavior more likely. The second is that T. gondii directly affects brain function, causing aggressive behavior,” Lee said in an e-mail. For the third possibility, Lee said that the study might not indicate that T. gondii causes IED, but rather that those with IED are more likely to contract the parasite. For example, the study might simply show that individuals with IED are more likely to own one or multiple cats.

T. gondii can live in virtually any animal, including humans, but can only reproduce within the guts of cats. According to the study, about a third of all humans are infected with the parasite. Factors that increase the risk of contracting the parasite include improper hand washing after cleaning a pet cat’s litter box and eating insufficiently cooked meat. After entering the human body, the parasite can reach the human brain, where it hides from the body’s immune response.

The CNPRU decided to investigate T. gondii infections in people with IED because of the growing amount of research suggesting that inflammation plays a role in the development of the disorder. The lab wants to understand what might cause this inflammation. Since previous research indicated that T. gondii changes the way the brain functions in other animals, the lab investigated T. gondii in relation to IED, which had not been previously studied.

According to a New York Times article, T. gondii can alter the brain function of rats and mice. For example, rats and mice have evolved to avoid the scent of predators, like cats. However, the parasite, when present in a rat or mouse, can make its host unafraid or even drawn to a cat’s scent.

The CNPRU hopes to discover the cause of aggressive behavior in people with IED. “We’ve begun to think about the next set of experiments. Our job as scientists and psychiatrists is to show compassion and seriousness of purpose as we gain insights into what makes human aggression happen,” Lee said.