It might be hard to believe, but adorable kittens like the one in the photo above might be harboring a parasite that has been linked to explosive bouts of rage in humans.

That parasite is called Toxoplasma gondii (T. gondii), and it is found throughout the world. An estimated 60 million people in the United States and 30-50 percent of the world population has been exposed and may be chronically infected.

T. gondii can sexually reproduce only within cats, so they are defined as the definitive host of the parasite. All other hosts – hosts in which only asexual reproduction can occur – are defined as intermediate hosts, including humans, cats, and birds.

Very few people realize they are infected because in healthy individuals, symptoms are rare because the immune system prevents T. gondii from causing illness.

However, for infants, people with HIV/AIDS, and others with weakened immunity, infection can cause serious and occasionally fatal illness (toxoplasmosis).

When a person becomes infected with T. gondii, the parasite forms cysts that can affect almost any part of the body – often the brain and muscles, including the heart.

There are several ways humans can become infected with this sneaky parasite. Cat fecal matter is particularly dangerous: Just one cyst consumed by a cat can result in thousands of oocysts (eggs), and a few days after a cat is infected, it will start to shed millions of eggs in its feces.

Cats who hunt or who are fed raw meat are most likely to harbor T. gondii. This is why doctors often tell pregnant or ill people to avoid cleaning the cat’s litter box.

Other ways humans can contract T. gondii include:

  • By consuming raw or undercooked meat containing T. gondii tissue cysts. Raw or undercooked lamb, pork, and venison are the most risky, but it is possible, though unlikely, to ingest the parasite through other products. Fresh meat is most risky since freezing meat for several days will kill most tissue cysts.
  • By ingesting water, soil, fruits, vegetables, or anything contaminated with oocysts shed in the feces of an infected animal.
  • By transplacental transmission from mother to fetus, particularly when T. gondii is contracted during pregnancy.
  • By organ transplant or blood transfusion, but this is rare.


Several studies have suggested that subtle behavioral or personality changes may occur in infected humans, and infection with T. gondii has recently been associated with a number of neurological disorders, particularly schizophrenia.

A new study found that individuals with a psychiatric disorder involving recurrent bouts of extreme, impulsive anger – road rage, for example – are more than twice as likely to have been exposed to T. gondii than healthy individuals with no psychiatric diagnosis.

In a study involving 358 adult subjects, a team led by researchers from the University of Chicago found that toxoplasmosis, a relatively harmless parasitic infection carried by an estimated 30 percent of all humans, is associated with intermittent explosive disorder and increased aggression.

The findings were published in the Journal of Clinical Psychiatry on March 23, 2016.

Senior study author Emil Coccaro, MD, Ellen. C. Manning Professor and Chair of Psychiatry and Behavioral Neuroscience at the University of Chicago, said of the findings:

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.

However, we do not know if this relationship is causal, and not everyone that tests positive for toxoplasmosis will have aggression issues.

Intermittent explosive disorder (IED) is defined by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, as recurrent, impulsive, problematic outbursts of verbal or physical aggression that are disproportionate to the situations that trigger them. IED is thought to affect as many as 16 million Americans, more than bipolar disorder and schizophrenia combined.

As part of their research to improve diagnosis and treatment for IED and impulsive aggression, Coccaro and his colleagues examined possible connections to toxoplasmosis, an extremely common parasitic infection. Toxoplasmosis is typically latent and harmless for healthy adults, but it is known to reside in brain tissue. The infection has been linked to several psychiatric diseases, including schizophrenia, bipolar disorder and suicidal behavior.

The research team recruited 358 adult subjects from the U.S., who were evaluated for IED, personality disorder, depression, and other psychiatric disorders. Study participants were also scored on traits including anger, aggression, and impulsivity. 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 possible confounding psychiatric factors.

They found that IED-diagnosed group was more than twice as likely to test positive for toxoplasmosis exposure (22 percent) as measured by a blood test, compared to the healthy control group (9 percent).

Around 16 percent of the psychiatric control group tested positive for toxoplasmosis, but had similar aggression and impulsivity scores to the healthy control group. IED-diagnosed subjects scored much higher on both measures than either control group.

Across all study subjects, toxoplasmosis-positive individuals scored significantly higher on scores of anger and aggression. Researchers noted a link between toxoplasmosis and increased impulsivity, but when adjusted for aggression scores, this link became non-significant. This finding suggests toxoplasmosis and aggression are most strongly correlated.

However, the authors caution that the study results do not address whether toxoplasmosis infection may cause increased aggression or IED.

Study co-author Royce Lee, MD, Associate Professor of Psychiatry and Behavioral Neuroscience at the University of Chicago, elaborated:

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.

Coccaro and his team are now continuing to study the relationship between toxoplasmosis, aggression, and IED. If better understood, this connection may inform new strategies to diagnose or treat IED in the future, Coccaro said:

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

To reduce your chance of becoming infected with T. gondii:

  • Cook meat and poultry thoroughly.
  • Freeze meat for several days at sub-zero (0° F) temperatures before cooking.
  • Peel or thoroughly wash fruits and vegetables before eating.
  • Do not eat raw or undercooked oysters, mussels, or clams.
  • Do not drink unpasteurized goat’s milk.
  • Wash cutting boards, dishes, counters, utensils, and hands with hot soapy water after contact with raw meat, poultry, seafood, or unwashed fruits or vegetables.
  • Wear gloves when gardening and during any contact with soil or sand because it might be contaminated with cat feces.
  • Wash hands with soap and warm water after gardening or contact with soil or sand.


If you own a cat, you can reduce the risk of infection by following these guidelines from International Cat Care:

  • People in high risk groups (eg, pregnant women, immunosuppressed individuals, young children) should not have contact with or handle the cat’s litter tray.
  • Empty litter boxes daily, so that oocysts do not have sufficient time to become infective while in the litter box.
  • Wear gloves when handling cat litter and wash hands thoroughly after cleaning the litter box.
  • Use litter box liners if possible and periodically clean the litter box with detergent and very hot water (which kills oocysts).
  • Dispose of cat litter safely. For example, seal it in a plastic bag before putting it with other household waste.
  • Cover children’s sandboxes when not in use to prevent cats using them as litter boxes.
  • Feed only properly cooked food or commercial cat food to your cat to avoid infection.

Comments are closed.