Toxoplasmosis infection significantly alters human personality and intelligence

Beyond Latency: Chronic <i>Toxoplasma</i> Infection and Its Unveiled Behavioral and Clinical Manifestations-A 30-Year Research Perspective.. 2025. https://doi.org/10.3390/biomedicines13071731

Over the past three turbulent decades, research has profoundly reshaped our understanding of chronic <i>Toxoplasma gondii</i> infection-traditionally regarded as harmless in immunocompetent individuals-unveiling its surprising impact on human health, performance, and behavior. This review emphasizes the effects of chronic <i>Toxoplasma</i> infection on physical and mental health, cognitive performance, and behavioral changes, highlighting key findings from studies investigating these domains, with a particular focus on both ultimate and proximate mechanisms underlying the observed effects. To this end, the primary focus will be on human studies; however, animal model studies will also be thoroughly considered when necessary and appropriate, to provide context and additional important information. Research demonstrates that chronic <i>Toxoplasma</i> infection may contribute to a broad spectrum of physical health issues. Ecological studies have revealed correlations between toxoplasmosis prevalence and increased morbidity and mortality from various conditions, including cardiovascular diseases, neurological disorders, and certain cancers. Large-scale cross-sectional studies have further shown that infected individuals report a higher incidence of numerous health complaints and diagnosed diseases, suggesting a significant impact on overall physical well-being. In addition to physical health, lifelong <i>Toxoplasma</i> infection (subclinical toxoplasmosis) has been implicated in cognitive impairments and behavioral changes. Studies have reported associations between infection and poorer performance in areas such as reaction time, processing speed, working memory, and executive function. Many of these behavioral changes likely relate to worsened health and a shift towards a "fast life history strategy." These cognitive deficits can have significant implications for daily functioning and performance. Furthermore, the role of <i>Toxoplasma</i> infection in the development There is also evidence suggesting that Toxoplasma infection compromises cognitive functioning in mice. Infected animals show deficits in learning and memory, including a reduced responsiveness to novel stimuli such as unfamiliar objects [ 15 ]. Piekarski et al. [ 16 ] reported significantly worse Although from both clinical and economic perspectives, the impact of this form of toxoplasmosis on physical and mental health is of particular importance ( Table S1 , which summarizes the meta-analytical studies and reviews used in this review that examine Toxoplasma infection and its relationship with physical health, mental health, and cognition, is included in the ‘Supplementary Tables section’), research in humans has historically progressed in a different order—beginning with its influence on behavior and personality traits, then on cognitive performance, followed by mental health, and only more recently on physical health. Toxoplasma and Cattell’s 16PF Early studies on personality showed that infection with latent toxoplasmosis in humans can result in personality profile shifts that are moderated by sex. Two studies using Cattell’s 16 Personality Factor Questionnaire (16 PF) reported opposite trends of personality shift in some scales in infected men and women. In these studies, infected men scored significantly lower in factor G (Rule-Consciousness: Expedient vs. Rule-bound) and higher in factor L (Vigilance: Trusting vs. Suspicious) and non-significantly lower in factors A (Warmth: Reserved vs. Warm) and Q3 (Perfectionism: Tolerates disorder vs. Perfectionistic) [ 19 , 20 ]. Another study, using the same questionnaire, investigated the association between Toxoplasma infection and personality change in 502 Czech male soldiers and found that infected subjects scored significantly lower in Neuroticism, among other differences [ 72 ]. In this respect, an ecological study, investigating the effects of toxoplasmosis on aggregate personality at the population level, found that aggregate Neuroticism increased significantly with Toxoplasma prevalence [ 73 ]. The reduced intelligence in infected individuals was primarily observed in fluid, numeric, and figural intelligence, while verbal intelligence remained unaffected. In this study, intelligence was found to negatively correlate with the level of anti- Toxoplasma IgG antibodies, suggesting that either the negative effects diminish over time since infection or that they are stronger in individuals who have experienced a more severe or repeated infection, resulting in higher antibody levels. RhD phenotype, commonly known as the Rh factor, has also proved to significantly influence the association between toxoplasmosis and intelligence. Whether Toxoplasma differentially affects these states across sexes, infection stages, or personality traits remains to be investigated. In conclusion, vasopressin is emerging as a critical node in the neurohormonal network influenced by Toxoplasma . Its interaction with testosterone, coupled with its epigenetic regulation in key brain regions, supports its role as a mediator of infection-induced behavioral change—at least in males. While human research in this area is limited, the existing rodent data suggest that vasopressin may be a viable candidate for understanding the socioemotional and psychiatric effects of latent toxoplasmosis. Among the seropositive participants (39 men and 24 women), both testosterone and cortisol levels were significantly elevated compared to seronegative controls. Additionally, stress and anxiety scores were higher across sexes, while depressive symptoms increased only in men. These results suggest that Toxoplasma infection is associated with the hyperactivation of the HPA axis, potentially contributing to mood disturbances. Another human study focused on schizophrenic patients examined the prevalence of latent toxoplasmosis and its association with cortisol, testosterone, and glutathione (GSH) levels [ 305 ].

Effects of Toxoplasma on Human Behavior - PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC2526142/

s, several differences were found. Other studies have demonstrated reduced psychomotor performance in affected individuals. Possible mechanisms by which T. gondii may affect human behavior include its effect on dopamine and on testosterone. Keywords: personality test, reaction time, dopamine, testosterone Introduction Toxoplasma gondii is the most common protozoan parasite in developed nations. Following the initial acute phase of infection, the parasite assumes a latent form. Up to 80% of the population may be infected, depending on eating habits and exposure to cats. 1 The dormant form of T. gondii is found predominantly in nervous and muscle tissues in infected hosts. Until recently, latent infections in humans were assumed to be asymptomatic. Results of animal studies and recent studies of personality profiles, behavior, and psychomotor performance, however, have led to a reconsideration of this assumption. Personality Profile and Behavior Since 1992, a series of studies have been carried out in the Czech Republic comparing the personality characteristics of individuals who have anamnestic antibodies to T. gondii , and are thus assumed to have a latent infection, and those without such antibodies. The personality questionnaires used in these studies have been Cattell's 16-personality factor (16PF) questionnaire 2 – 6 and Cloninger's Temperament and Character Inventory (TCI) personality test. 7 , 8 The subjects tested with Cattell's 16PF have included students and faculty in the Department of Biology at Charles University ( n = 243, 200, 107, and 255 in various studies), military conscripts ( n = 475), blood donors ( n = 55, 268, 190), individuals known to have had symptomatic toxoplasmosis in the past ( n = 190 and 230), and women tested for toxoplasmosis during pregnancy ( n = 191). Consistent and significant differences in Cattell's personality factors were found between Toxoplasma -infected and -uninfected subjects in 9 of 11 studies, and these differences we Published by Oxford University Press on behalf of the Maryland Psychiatric Research Center. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org. PMC Copyright notice PMCID: PMC2526142  PMID: 17218612 Abstract Although latent infection with Toxoplasma gondii is among the most prevalent of human infections, it has been generally assumed that, except for congenital transmission, it is asymptomatic. The demonstration that latent Toxoplasma infections can alter behavior in rodents has led to a reconsideration of this assumption. When infected human adults were compared with uninfected adults on personality questionnaires or on a panel of behavioral tests, several differences were found. Other studies have demonstrated reduced psychomotor performance in affected individuals. Possible mechanisms by which T. gondii may affect human behavior include its effect on dopamine and on testosterone. Keywords: personality test, reaction time, dopamine, testosterone Introduction Toxoplasma gondii is the most common protozoan parasite in developed nations. Following the initial acute phase of infection, the parasite assumes a latent form. Up to 80% of the population may be infected, depending on eating habits and exposure to cats. 1 The dormant form of T. gondii is found predominantly in nervous and muscle tissues in infected hosts. Until recently, latent infections in humans were assumed to be asymptomatic. Results of animal studies and recent studies of personality 7 , 8 In general, differences in personality factors were greater in subjects in older age groups. In order to ascertain whether there was any correlation between personality change and duration of infection, personality test (16PF) data were available on 190 men and 230 women in whom acute toxoplasmosis had been diagnosed in the previous 14 years. After the age of subjects was controlled for, superego strength (factor G) was found to significantly decrease in men ( P = 0.017, t -test, 1 tailed) 3 and increase in women ( P = 0.010, t -test, 1 tailed) 6 in relation to the duration of infection. gondii infections are exerting effects on human personality characteristics and behavior, what is the possible mechanism? It is known that T. gondii increases dopamine in rodents 19 and also that treating the rodents with a selective dopamine uptake inhibitor differentially alters the behavior of the infected and uninfected rodents. 20 Also the observed low level of novelty seeking in humans infected with Toxoplasma or cytomegalovirus is supposedly associated with high dopamine levels in the ventral midbrain. 7 , 8 The mechanism of the dopamine increase by T. gondii is not known but may involve the inflammatory release of dopamine by increasing cytokines such as interleukin-2. Cent Eur J Public Health. 1998;6:45–50. [ PubMed ] [ Google Scholar ] 5. Flegr J, Havlicek J. Changes in the personality profile of young women with latent toxoplasmosis. Folia Parasitol (Praha) 1999;46:22–28. [ PubMed ] [ Google Scholar ] 6. Flegr J, Kodym P, Tolarova V. Correlation of duration of latent Toxoplasma gondii infection with personality changes in women. Biol Psychol. 2000;53:57–68. doi: 10.1016/s0301-0511(00)00034-x. 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