Are Cats Making Some People Aggressive?

Research probes link between a cat parasite and Intermittent Explosive Disorder

Posted Apr 01, 2016

Cats are generally thought to help people cope with stress and anger, but a provocative new study suggests that there may also be a link between the protozoan parasite toxoplasma gondii (T. gondii) and increased human aggressionT. gondii commonly inhabits cats who then can pass along the infection to humans through cat feces.   While it is thought that 10-20% of people also carry T. gondii, symptoms are usually mild to non-existent for most people, although serious problems can occur rarely, especially among those whose immune systems are compromised.  Concern over transmitting T. gondii to newborns, whose immune systems are not fully developed, is the reason why pregnant women are often advised to stay away from cat litter.

The link between T. gondii and aggression comes from some animal studies as well as a few studies in people.  Recently, researchers from the University of Chicago decided to investigate this association further by studying 358 adults.  Some of them were control subjects with no history of psychiatric disorders while others had a history of Intermittent Explosive Disorder (IED), a poorly studied entity characterized by people who are prone to intense and frequent impulsive aggression. There was also a group of adults who met criteria for different psychiatric disorders other than IED.  These subjects all underwent blood tests to see if the researchers could find the presence of antibodies to T. gondii which would suggest a low lying infection.   The subjects also completed rating scales about their level of aggression and impulsivity.

The study found that people with IED were more than twice as likely to test positive for T. gondii (at 21.8%) than controls (at 9.1%), while the group with other psychiatric disorders was in between (16.7%).  They further found that quantitatively higher overall aggression and impulsivity scores were found for subjects who were seropositive for T. gondii, although no differences were found related to suicide or other types of self-directed aggression. 

This study looked at only one point in time, so the association they found cannot be interpreted as causal.  Nevertheless, the authors did speculate on how T. gondii might actually be related to increased aggression.  Hypotheses included the possibility that infection leads to low grade chronic inflammation in the brain or alterations in how particular brain regions are connected.  There is even the possibility, from animal studies, that T. gondii leads to increased production of testosterone

Because things remain so speculative, the article gives no recommendations about doctors assessing or treating T. gondii as part of regular practice, let alone offer advice about people and their cats.   To be sure, aggression and impulsivity are complicated things caused by many different factors.  Whether or not this might be one of the many will need to wait for further study.

@copyright by David Rettew, MD

David Rettew is author of Child Temperament: New Thinking About the Boundary Between Traits and Illness and a child psychiatrist in the psychiatry and pediatrics departments at the University of Vermont College of Medicine.

Follow him at @PediPsych and like PediPsych on Facebook.