New research in Folia Parasitologica provides evidence that certain latent infections, specifically with Toxoplasma gondii and Borrelia spp., are directly related to personality traits without being mediated by overall health deterioration. This research challenges the previously held belief that the behavioral changes observed in infected individuals were merely side effects of compromised health.
Toxoplasma gondii and Borrelia spp. are two microorganisms known for their intriguing, and sometimes insidious, impacts on human health. Toxoplasma gondii, a protozoan parasite, infects a broad range of hosts, including humans, primarily through undercooked contaminated meat, contaminated water, or contact with infected cat feces. In most healthy adults, this infection often goes unnoticed or may cause mild flu-like symptoms.
Borrelia spp., on the other hand, are a group of bacteria best known for causing Lyme disease, transmitted to humans through bites from infected ticks. The initial signs of infection may include a characteristic bull’s-eye rash and flu-like symptoms, which can progress to more serious neurological or cardiac issues if left untreated. Similar to Toxoplasma, Borrelia can also persist in the body long after the initial infection, leading to chronic symptoms even in treated individuals.
Previous research has linked Toxoplasma gondii with various psychiatric disorders, such as schizophrenia, and changes in behavior and personality traits, such as increased risk-taking and aggression. Similarly, while less is known about the long-term behavioral effects of Borrelia infections, emerging evidence suggests potential impacts on cognitive functions and mental health, long after the acute infection phase has resolved.
In their new study, led by Professor Jaroslav Flegr of Charles University in Prague, researchers sought to clarify whether the behavioral and health changes associated with Toxoplasma and Borrelia infections are direct effects of the pathogens themselves or merely consequences of the health deterioration they cause.
The study included data from 4,942 women and 2,820 men who completed an online survey, which was advertised on social media platforms such as Facebook and Twitter. The average age of the women was approximately 43 years, and for men, it was around 40 years. Among these participants, 24% of women and 12% of men reported testing positive for Toxoplasma gondii. For Borrelia infections, 41.6% of women and 30.7% of men reported testing positive.
To assess cognitive and personality traits, a variety of psychometric tests were included in the survey. These included the Cattell 16PF test and a modified Meili test to evaluate memory and intelligence. For psychomotor performance, the researchers used the Stroop test, which measures cognitive flexibility and processing speed. Personality traits were assessed using the Ten Item Personality Inventory and the Czech version of the Three Domain Disgust Scale, which included measures for pathogen disgust, sexual disgust, moral disgust, and an additional scale for injury disgust.
The results indicated that individuals infected with Toxoplasma gondii were more likely to experience a range of health issues, which aligns with previous studies suggesting a link between Toxoplasma infection and various health complications, including psychiatric disorders. Similarly, participants with Borrelia infections reported worse physical health than their uninfected counterparts, although the impact on mental health was less pronounced compared to Toxoplasma.
The researchers found that infection status was associated with changes in several behavioral traits. Both Toxoplasma and Borrelia infections were linked to differences in personality, including lower conscientiousness and changes in traits related to the Dark Triad — Machiavellianism, narcissism, and psychopathy. In particular, Toxoplasma-positive individuals demonstrated lower scores on Machiavellianism, and Toxoplasma-positive women scored lower on narcissism.
Additionally, the researchers identified differences in disgust sensitivity, with infected individuals showing altered reactions to pathogen and injury disgust scenarios. There was a negative correlation between Toxoplasma gondii infection and both pathogen and injury disgust, suggesting that higher levels of these disgust responses might protect against infection
Interestingly, the study also found that these infections were related to cognitive functions, as demonstrated in tasks like the Stroop test. Toxoplasma-infected individuals showed prolonged reaction times and lower precision in their responses, suggesting a direct impact on cognitive processing speed and accuracy.
One of the most crucial aspects of the findings was that the observed changes in personality and cognitive functions were not mediated by the health status of the individuals. This was evidenced by the statistical analyses showing that controlling for physical and mental health did not diminish the association between infection status and behavioral changes. This directly contradicts the hypothesis that the behavioral effects seen in infected individuals are merely side effects of poor health caused by the infection.
But do Toxoplasma gondii and Borrelia spp. infections cause the observed differences in personality and cognitive function? That is still unclear.
The study’s cross-sectional nature limits the ability to draw causal inferences from the data. Although the researchers attempted to control for several variables, there could still be unmeasured confounders that affect both the likelihood of infection and the outcomes studied. For example, certain behavioral traits might make individuals more likely to acquire these infections rather than being a consequence of the infection.
“Our results suggest that the behavioural effects of latent toxoplasmosis and borreliosis are direct effects of the infections rather than side effects of impaired health of the infected subjects,” the researchers concluded. “However, it should be emphasised that neither path analysis nor any other statistical method can conclusively confirm the validity of a model. … For example, no statistical method can determine whether the positive association between intelligence and Toxoplasma or Borrelia seropositivity is the result of a positive (direct or indirect) effect of higher intelligence on the risk of infection or vice versa. Only an experimental study, which is not feasible for ethical reasons, could distinguish between these two fundamentally different models.”
The study, “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,” was authored by Jaroslav Flegr, Jana Ullmann, and Jan Toman.
