Folia Parasitologica 70:020 (2023) | DOI: 10.14411/fp.2023.020

Parasitic manipulation or side effects? The effects of past Toxoplasma gondii and Borrelia spp. infections on human personality and cognitive performance are not mediated by impaired health

Jaroslav Flegr ORCID...1*, Jana Ullmann ORCID...1, Jan Toman ORCID...1
Charles University, Faculty of Science, Department of Philosophy and History of Sciences, Laboratory of Evolutionary Biology, Prague, Czech Republic *Address for correspondence: Laboratory of Evolutionary Biology, Department of Philosophy and History of Sciences, Faculty of Science, Charles University, Viničná 7, 128 00, Prague, Czech Republic. E-mail: flegr@cesnet.cz

Bacteria Borrelia burgdorferi s. l. and even more the protist Toxoplasma gondii Nicolle et Manceaux, 1908, are known to affect the behaviour and mental health of their animal and human hosts. Both pathogens infect a significant fraction of human population, both are neurotropic and survive in the host's body for a long time. While latent infections were thought to be clinically asymptomatic, recent studies suggest otherwise, revealing adverse effects on human health. It was hypothesised that the specific behavioural effects of these pathogens may be side effects of general health impairments in infected individuals. This hypothesis was tested using about one hour-long survey consisting of questionnaires and performance tests on a cohort of 7,762 members of the internet population. Results showed that individuals infected with T. gondii reported worse physical and mental health, and those infected with Borrelia spp. reported worse physical health than uninfected controls. Furthermore, infected and noninfected individuals differed in several personality traits, including conscientiousness, pathogen disgust, injury disgust, Machiavellianism, narcissism, tribalism, anti-authoritarianism, intelligence, reaction time, and precision. While the majority of behavioural effects associated with Borrelia infection were similar to those associated with Toxoplasma infection, some differences were observed, such as performance in the Stroop test. Path analyses and nonparametric partial Kendall correlation tests showed that these effects were not mediated by impaired health in infected individuals, contradicting the side effects hypothesis.

Keywords: toxoplasmosis, borreliosis, manipulation hypothesis, behaviour, personality, side effects hypothesis

Received: June 13, 2023; Revised: October 1, 2023; Accepted: October 31, 2023; Published online: November 28, 2023  Show citation

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Flegr, J., Ullmann, J., & Toman, J. (2023). Parasitic manipulation or side effects? The effects of past Toxoplasma gondii and Borrelia spp. infections on human personality and cognitive performance are not mediated by impaired health. Folia Parasitologica, 70, Article 2023.020. https://doi.org/10.14411/fp.2023.020
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