Have you ever felt like your reaction time isn’t as good as it should be? Or, if you’re male, have you ever had people comment that you look particularly masculine? These traits might seem unrelated, but they actually can be due to the same thing — infection by a very common parasite called Toxoplasma gondii (which infects people through exposure to cats and eating undercooked meat). A little while ago we talked about the Toxoplasma parasite, how it’s thought that about one in three people in the U.S. (and other developed countries) are infected with it, and that it makes mice less afraid of cat urine. Infection in people has been typically considered asymptomatic, but it turns out that this isn’t really the case — surprisingly, and concerningly, studies are showing that infection can affect human development, immune response, personality, and much more.
A paper published in 2013 took a look at studies done over the past two decades on the effects of Toxoplasma infection in humans — primarily college students, pregnant women, and military personnel — and made some interesting conclusions. It was found that infection with the parasite not only affects our behavior (which is not too surprising considering the effect of infection on mice), but also our immune response, which has far-reaching consequences, such as potentially changing which embryos are selected by a woman’s body for pregnancy.
Infection by the Toxoplasma parasite may potentially affect a person’s personality and behavior in several ways (and these are only the ways we’re aware of so far — there are likely many more that we have yet to learn about). Specifically, it’s been found that people infected with the parasite have increased reaction times (i.e., it takes them longer to respond to a stimulus). It’s thought this impairment is why it’s been found that infected individuals are more often involved in traffic accidents and work accidents. Additionally, infected men have been found to have clothes that are less tidy and be less sociable (whereas infection may cause women to become more sociable). And, perhaps most concerningly, it’s thought that infection with the parasite may help severe schizophrenia get started — infection is clearly more common in patients with schizophrenia (possibly due to increased dopamine levels in certain areas of the brain). The behavior effects of infection are also thought to increase in severity the longer it’s been since the person became infected.
Toxoplasma infection has also been found to suppress a person’s immune system. It’s thought that this might affect which embryos are chosen for a woman’s pregnancy by making the selection process less stringent, and this impairment — which may allow embryos to be used that would normally be aborted due to developmental defects — may be why children of infected mothers do not develop as well during their first 18 months, and why mothers of children with Down syndrome are more likely to be infected (84% of them have the parasite) than mothers of children who don’t have Down syndrome (where only about 32% have it). This parasite-induced suppression of the immune system may also be why infected women are more likely to give birth to boys than to girls.
Other subtle side-effects of Toxoplasma infection have also been found. For example, infected male college students were found to be 3 centimeters taller on average, and male college students appeared more masculine, which may be due to increased testosterone.
So the next time you find yourself at a loss for explaining somebody’s odd behavior, or even their physical appearance, it may be the all-too-common parasite Toxoplasma that you’ll want to blame.
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