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The role of the parasite Toxoplasma gondii on human behavior: Causation or correlation?

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Before becoming a graduate student, I knew little to nothing about the neglected microscopic parasite, Toxoplasma gondii. Once I began my studies, I quickly realized that unless you were a healthcare worker, vet, or pregnant, the awareness of Toxoplasma was close to nonexistent. Despite prevalence among one-third of the world’s population, infected immunocompetent individuals remain largely asymptomatic.

In the last few years, several academic research groups have reported on the association between Toxoplasma gondii and “mind control”. I originally found this claim very far-fetched. However, after reviewing the scientific literature,  I could now see why news articles and short communication pieces about Toxoplasma gondii include phrases such as “mind control”, “brain parasite”, and “mind altering microbe”. What I fear is being miscommunicated is that these data largely lean onto the correlative side. Correlation implies that there is a positive or negative relationship between two variables. Causation, on the other hand, is more than a relationship. It is the direct dependence of two variables on one another. I want to ensure that the general public is made aware that these studies are only correlative. 

Definitive host of Toxoplasma gondii. “Cat” by joshuascottphotos is licensed under CC BY 2.0

Toxoplasma gondii is a highly efficient parasite. The parasite reaches full maturation in the cat, its definitive host, while intermediate hosts can be any type of warm-blooded organism, including humans. Humans can become infected by Toxoplasma gondii in two major ways: 1) ingesting infected undercooked meat, and 2) congenitally, where an infected pregnant mother passes the parasite to the fetus. Within the human host, the parasite will begin to multiply in the gut. From there it can travel via the circulatory system and make its way to the liver, spleen, lungs and the brain. Keep in mind that most healthy individuals that become infected display mild flu-like symptoms despite the parasite persisting in different areas of the body. The “mind control” claims are related to risky behaviors including a range of mental illnesses, aggression and risky decision-making. Below I discuss the results from three different studies that are aimed to support the potential link between abnormal behavioral patterns and Toxoplasma infection. We will dive into whether the data can categorize Toxoplasma gondii as a “mind controlling parasite” or if this claim is as far-fetched as I originally thought?

Image of extracellular Toxoplasma gondii tachyzoites. Shown in blue is the nucleus and green is the endoplasmic reticulum. Image used with permission by Abigail Calixto.

Alcohol consumption 

In 2017, a study done in the Medical University of Warsaw in Poland, analyzed and tested the serum of 538 corpses for Toxoplasma gondii. The cause of death was divided into three main categories: 1) risky behavior, 2) inconclusively risky behavior 3) and control groups. Of these, Toxoplasma infection was found in 51% of those that died due to risky behavior, 48% of those that died due to inconclusively risky behavior and in 38% of the control group (death most likely due to another disease). From the deaths characterized as “risky behavior”, 60% of them contained alcohol in their system. This led the researchers “to associate Toxoplasma infection with alcohol consumption” and even more, risky behavior. Additional studies have arrived to similar conclusions with other types of “risky behavior” including suicide and impulsivity.

Entrepreneurial behavior 

During my second year of graduate school, a largely controversial study was published, stating that individuals infected with Toxoplasma gondii were likelier to major in business. How could anyone reach this conclusion? The study began by testing 1293 undergraduates for Toxoplasma gondii across the Arts and Science, Business and Engineering colleges at an unmentioned US university. 22% of non-business majors were infected with Toxoplasma gondii, while 31% of business majors tested positive. This 9% difference led the researchers to conclude that there must be a link between infection and entrepreneurial behavior. They went as far as to interpret this link by referencing another study that showed that “entrepreneurs tend to score high in risk taking”. This single study made headlines in Business Insider and Forbes magazine.  

Schizophrenia 

Although the association of different mental illnesses and Toxoplasma gondii has been investigated, one of the most commonly studied is Schizophrenia. Whether this link is simply correlative, or causative remains up for debate.  A study in 2018 in the FondaMental Expert center in France found Toxoplasma gondii in 75% of patients with Schizophrenia (N=250). Another study in Russia was done comparing two populations, one containing individuals that had Schizophrenia and another that didn’t. In the Schizophrenia positive group, 40% of the patients were positive for Toxoplasma gondii, while in the control group 25% of the individuals were positive. Many more studies in different countries show similar results including in Iran, Mexico, Turkey and China. The data certainly supports a connection between Schizophrenia and parasite burden; however, these results are strictly correlative. 

Conclusion

From these three studies, it can certainly appear like Toxoplasma gondii affects behavior. But, numerous factors are not being tested such as an individual’s access to healthcare, proper education, nutrition/diet and location and lifestyle. And although studies are conducted worldwide, different parasite types exist globally, and several factors can affect the infection rate in a population. Additionally, there are studies that contradict some of these positive associations. In 2020, a group of scientists conducted an analysis of 13 published studies on Toxoplasma gondii infection in schizophrenic patients. They observed no association between infection and schizophrenic symptoms. However, further analyses did show an association between infection and individuals under the age of 35 and had exhibited symptoms for over 10 years. 

I think one of the most important skills we learn as scientists is the ability to dissect primary literature. Another skill that is just as important, but maybe less practiced, is effective science communication. Our community and overall, the general public can only benefit from proper dissemination of our research progress. So, all in all, if you are a relatively healthy individual and not pregnant, no need to practice extra caution when cleaning out your cat’s litter box.

Feature Image: “The Human Mind” by Psychology Pictures is licensed under CC BY-NC-ND 2.0

About the Author

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My name is Abby and I am a Microbiology graduate student in UGA. My PhD focuses on characterizing different aspects of the calcium-signaling pathway in the apicomplexan parasite, Toxoplasma gondii. Outside of lab, I have a strong interest in graduate student advocacy and peer mentoring, with hopes of pursuing a career in science policy. I hope the skills I gain from participating with the ASO will not only allow me to support the graduate student body voice, but also provide me with skills I can use in my future career.

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