A bad case of toxoplasmosis? Getty Images

There are many ways your cat might be filling you with rage. Scratching your furniture, sitting on your computer keyboard – or giving you parasites that may cause explosive anger.

Infection with Toxoplasma gondii, a protozoan parasite carried by cats, has been linked to a human psychiatric condition called intermittent explosive disorder. People who have IED typically experience disproportionate outbursts of aggression, like road rage. T. gondii is already known to change the behaviour of the organisms it infects. By making rodents bolder and more adventurous, the parasite makes them more likely to be caught and eaten by a cat, allowing the parasite to complete its life cycle.

It can also infect humans, through contact with cat faeces, poorly cooked meat or contaminated water, and as many as one-third of the world’s population may be infected. The protozoan doesn’t make us feel sick, but forms cysts in the brain where it can remain for the rest of a person’s life. Such infections have been linked to psychiatric conditions including schizophrenia, bipolar disorder and suicidal behaviour. People infected with T. gondii also have slower reaction times and are more likely to be involved in car accidents.


To see if T. gondii is associated with aggression, Emil Coccaro at the University of Chicago and his colleagues examined 358 adults. These fell into three groups: people with IED, people with other psychiatric conditions, and controls who had not been diagnosed with any psychiatric condition.

They found that people with IED were more than twice as likely to test positive for exposure to T. gondii as the control group. Those with other conditions were also more likely to have the parasite. Across all three groups, those who tested positive tended to rank more highly in tests measuring aggression.

Enhanced aggression

Coccaro thinks the parasite may boost aggression by altering neurotransmitters in the brain, either by overstimulating neurons in the amygdala, the part of the brain that controls response to threats, or by impairing one of the forebrain’s functions – as a braking mechanism on such behaviour.

“The findings make sense,” says Paul Ewald, an evolutionary biologist at the University of Louisville in Kentucky. “A mouse that is preoccupied with attacks on another mouse may be easy prey for a cat.” In this way, enhancing aggression could help spread T. gondii.

But Coccaro also says his evidence only shows a correlation between infection and aggression – T. gondii doesn’t necessarily cause the explosive rage. It’s possible that aggressive people are more likely to catch the parasite, because they may be less likely to fully cook their meat or to wash their hands.

One way to test the link would be to treat the infection in the people with IED who tested positive for the parasite, and see if it has any effect on their behaviour. The only problem is that treatment can take a long time. “These parasites are so bloody difficult to kill,” says Coccaro.

Journal reference: Journal of Clinical Psychiatry, DOI: 10.4088/JCP.14m09621

Read more: The cat made me do it: Is your pet messing with your mind?