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

Bacteria Borrelia burgdorferi s. l. and even more the protozoan Toxoplasma gondii are known to affect the behavior of their animal and human hosts. Both pathogens infect a significant fraction of human population, and both survive in the hosts body for a long time. The resulting latent infections used to be considered clinically asymptomatic. In the last decade, however, numerous studies have shown that this view may be wrong and both infections can have various adverse effects on human health. Their specific behavioral effects may thus be merely side effects of the general impairment of patients health. We tested this hypothesis on a cohort of 7,762 members of internet population using a two-hour-long survey consisting of a panel of questionnaires and performance tests. Our findings confirmed that subjects infected with Toxoplasma were in worse physical and mental health and those infected with Borrelia were in worse physical health than corresponding controls. The infected and noninfected subjects also differed in several personality traits (conscientiousness, pathogen disgust, injury disgust, Machiavellianism, narcissism, tribalism, anti-authoritarianism, intelligence, reaction time, and precision). Majority of the behavioral effects associated with Borrelia infection were the same as those associated with Toxoplasma infection, but some dramatically differed (e.g., performance in the Stroop test). Path analyses and nonparametric partial Kendall correlation tests showed that these effects were not mediated by impaired health of the infected individuals. The results thus contradict predictions of the side effects hypothesis.