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Toxoplasma Gondii: The Brain Hitchhiker

Posted by Rebecca James on Nov 22, 2019 1:00:00 PM

kitty3Toxoplasmosis is a widely known disease that results from infection with the Toxoplasma gondii parasite, one of the world's most common parasites. Infection usually occurs by eating undercooked contaminated meat, exposure from infected cat feces, or mother-to-child transmission during pregnancy.  Estimates suggest that about 1/3 of the entire human population of the Western world is harboring the parasite in the form of “benign cysts” concentrated mostly in the brain.

The parasite can form cysts anywhere almost: hearts, lungs and eyeballs are common hiding places, but the brain is the preferred hangout for this organism.  If you are a cat lover, chances are high you are already infected, courtesy of your cat.  Cat brains are a natural repository for the adult protists, which reproduce inside our feline friends and spread from cat feces.  This is not to say that won’t infect any warm-blooded animal that they are capable of, but the only hosts truly necessary for their continued survival are felids, according to the CDC website.  Once infected, they usually infect you for your entire lifespan, mostly asymptomatically.toxo

The most interesting thing about the infection, though, is that the cysts hijack our brains, producing subtle but measurable and influences on behavior.  Scientists have suspected that the success of Toxoplasma is due in part to its’ ability to change rats and mice behavior, and causing them to stop fearing cats.

In a now famous study, it was demonstrated that mice, which have a morbid fear of cats and a strong fear reaction to cat odor, tend to lose that fear and unequivocally show a preference for cat odor after being infected with the parasite. 

Mice aren’t the only ones whose brains can be hijacked, though: there’s a good chance that the parasite in your head may be at least partly responsible for the shots you call, too.  A growing body of research indicates that the manipulation theory, as it is called in medical and biological journals, extends to human in several ways.  Latent toxoplasmosis in humans has been associated with serious neurological disorders, including schizophrenia, intermittent explosive (rage) disorder and suicide.

In addition, research shows infection by Toxoplasma gondii, directly affects the production of dopamine, a key chemical messenger in the brain.  Dopamine is a natural chemical which relays messages in the brain controlling aspects of movement, cognition and behaviour. It helps control the brain's reward and pleasure centres and regulates emotional responses such as fear. The presence of a certain kind of dopamine receptor is also associated with sensation-seeking, whereas dopamine deficiency in humans results in Parkinson's disease.

Infection changes the way that your brain processes information, slowing reaction times (more traffic accidents) and changing your preferences for a great many things, from risk aversion to tidiness to extraversion.  The effects of Toxoplasma infection on an individual depend on genetics, with some genotypes immune to infection and less likely to experience effects, and gender.  To take one example, the effect on personality has been summed up after analyzing multiple studies, each with well over a hundred (and often several hundred) subjects.

According to Effects of Toxoplasma on Human Behavior, significant differences in personality factors were found between Toxoplasma-infected and -uninfected subjects in 9 of 11 studies, and these differences were not the same for men and women. After using the Bonferroni correction for multiple tests, the personality of infected men showed lower rule consciousness and higher vigilance. Thus, the men were more likely to disregard rules and were more expedient, suspicious, jealous, and dogmatic. The personality of infected women, by contrast, showed higher warmth, suggesting that they were more warm hearted, outgoing, conscientious, persistent, and moralistic.

Even sexual characteristics and preferences might be affected.  In one photograph study, men who harbor infections are consistently rated as being more dominant and masculine looking then men who don’t have infections – on the basis of photographs alone.  Toxoplasma increases expression of the genes coding for testosterone in men, and the effect is large enough to be seen by the naked eye and borne out by physical measurements, including a noticeable 3 cm boost in average height.

Although the mechanisms by which this parasite seems to influence its’ hosts are still being elucidated, its’ clear that it touches our lives in ways we never before imagined.  How much of “them” is really “us”?

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