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Road rage and toxoplasmosis: Return of the parasite

road rage

It was a clear case of a false alarm, toxoplasmosis, a parasite that infects mice and cats was thought to have an effect on humans. However, after a thorough review of the data it was off the hook, or so we thought.  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 a common parasite than healthy individuals with no psychiatric diagnosis.

In a study involving 358 adult subjects, 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.

“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,” said senior study author Emil Coccaro, MD.

“However, we do not know if this relationship is causal, and not everyone that tests positive for toxoplasmosis will have aggression issues,” Coccaro said, adding that additional studies are needed.

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 pioneering 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. Transmitted through the feces of infected cats, undercooked meat or contaminated water, toxoplasmosis is typically latent and harmless for healthy adults. However, it is known to reside in brain tissue and 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.

The research team 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. The team 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.

“Correlation is not causation, and this is definitely not a sign that people should get rid of their cats,” said study co-author Royce Lee, MD.

“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 further examining the relationship between toxoplasmosis, aggression and IED. If better understood, this connection may inform new strategies to diagnose or treat IED in the future.

“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.”

We should probably mention once more, that just because there is a correlation, there is no need to panic. Fluffy’s lovely parasite isn’t trying to take over your mind for sure, we’re still uncertain, at this point anyway. After all, there maybe a corrilation to the parasite and all the cat videos online.

cat on a rumba

Coccaro, E., Lee, R., Groer, M., Can, A., Coussons-Read, M., & Postolache, T. (2016). Toxoplasma gondii Infection: Relationship With Aggression in Psychiatric Subjects The Journal of Clinical Psychiatry, 334-341 DOI: 10.4088/JCP.14m09621

4 responses

  1. Alex P

    I really liked this article and agree with Emil Coccaro that “we do not know if this relationship is causal, and not everyone that tests positive for toxoplasmosis will have aggression issues,” Additional studies need to be preformed to determine if toxoplasmosis is linked with Intermittent explosive disorder. It is a very interesting correlation but should not alarm people that they are more likely to be aggressive individuals. There is an emerging view that the toxoplasmosis parasite is active to some extent during what was previously regarded as a purely “dormant phase”. For example, rats infected with the parasite lose their fear of cats, and are even attracted by their scent, making them easy prey. Scientists have suggested this is how the parasite assures its own survival and propagation: the cats eat the infected rats, shed more parasite through their feces, and that in turn helps to infect more rats. People who suffer from toxoplasmosis are more likely to engage in risky behavior such as road rage and I can see where this could be related to aggression. The study results do not address whether toxoplasmosis infection may cause increased aggression or IED and I think that another study would be beneficial to see the differences!


    March 31, 2016 at 10:07 am

    • Wow, thank you for the thoughtful comment! I agree more studies are needed. Our little parasitic friend isn’t off the hook yet when it comes to the effect it may have in humans and honestly it would be more interesting if it did have an effect because it could lead to other discoveries about how the human brain works.


      April 1, 2016 at 11:25 am

  2. Aaron

    Alex P, I agree with you on the fact that it should not be alarming news. Toxoplasmosis in present in every country with infection rates ranging from 10% to 90%. The main goal of the parasite is not to infect rats, it is to reach the cats intestines where it can reproduce sexually. Dr. Jekyll, there are other ways in which toxoplamosis infection can affect behavior in humans. For example, Latent Toxoplasmosis is directly associated with immunosuppression, which can explain the probability of giving birth to a boy instead of a girl. I think that the parasite can be far more manipulative that we give it credit for.


    April 6, 2016 at 11:52 am

    • I agree that something is probably going on, it’s hard since toxoplasmosis was never meant to infect humans the way it does. While it is probably doing something, it’s just hard to tell what that is at the moment. What would be scary is if (probably when) it ever evolved enough to have very noticeable effects on human behavior.


      April 6, 2016 at 12:41 pm

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