Blame Your Road Rage on that Cat Parasite, Says New Study

A new study links to the common "cat parasite" to extreme anger issues. Thomas Peter/Reuters

Cat owners have a reputation for being a little on the kooky side. This is partially because a certain parasite that commonly makes its home in our furry feline friends is linked to psychiatric conditions in humans. It seems like every six months there's another study that suggests cleaning a litter box could get you one step closer to the looney bin.

The latest in the string of "crazy cat lady" studies suggests the toxoplasma gondii (t. gondii) parasite may also be behind intermittent explosive disorder (IED), a condition marked by unsettling behavior such as angry and irrational outbursts, road rage and unjustifiable aggressiveness. Some might say that's simply a long-winded way of describing an unpleasant person, but IED is actually a diagnosable condition. It's listed in the Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5)—the so-called shrink bible—and managed with medication and talk therapy.

According to experts involved in writing the DSM, IED is more common than bipolar disorder and schizophrenia combined, and is thought to affect as much as 16 million Americans (and perhaps one current presidential candidate).

The study, published March 23 in the Journal of Clinical Psychiatry, involved 358 adults who were evaluated for IED, personality disorders, depression and other psychiatric disorders. 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 other psychiatric conditions.

The researchers found people with IED were more than twice as likely to test positive for t. gondii exposure (22 percent) compared with the healthy group (9 percent). They also found the t. gondii-positive group scored higher on all assessments of anger, impulsiveness and 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," senior study author Dr. Emil Coccaro, a chair of psychiatry and behavioral neuroscience at the University of Chicago, said in a statement. "However, we do not know if this relationship is causal, and not everyone who tests positive for toxoplasmosis will have aggression issues," Coccaro said, adding that additional studies are needed.

The t. gondii parasite is a type of protozoan parasite that can infect all warm-blooded mammals. It is incredibly common and usually harmless. Cats are the "definitive host" of this parasite, meaning they are the only hosts in which the parasite can sexually reproduce. But the parasite is also found in undercooked meat, contaminated water and soil.

A human who accidentally ingests the parasite can develop toxoplasmosis, though most people remain asymptomatic. Symptoms of an infection include muscle pain, fever and headache. It is treatable with antibiotics. The acute infection typically resolves within a few weeks. However, the parasite can remain in a human host without symptoms and tends to make its home in brain tissue.

The researchers say they aren't certain why the link exists between this common parasite and conditions such as IED, though Coccaro suggests it may be due to an inflammatory response in a person who becomes infected. A less believable theory posed by the authors is that people who are more aggressive tend to own more cats and eat more undercooked meat.

"It will take experimental studies to see if treating a latent toxoplasmosis infection with medication reduces aggressiveness," Coccaro said. "If we can learn more, it could provide rational to treat IED in toxoplasmosis-positive patients by first treating the latent infection."