Over the past three turbulent decades, research has profoundly reshaped our understanding of chronic Toxoplasma gondii 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 Toxoplasma 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 Toxoplasma 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 Toxoplasma 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 Toxoplasma infection in the development or exacerbation of mental health disorders has been extensively investigated. Meta-analyses, ecological studies, and large-scale observational studies have demonstrated associations between Toxoplasma infection and an increased risk of disorders such as schizophrenia and obsessive-compulsive disorder. While the precise mechanisms underlying these associations remain under investigation, research suggests that neuroinflammation and alterations in neurotransmitter systems are likely to play a role. Far from being harmless, subclinical toxoplasmosis is increasingly recognized as a hidden factor influencing human health, behavior, and cognitive performance-with implications that extend well beyond the individual to public health at large. Further research is warranted to elucidate the complex interplay between Toxoplasma infection, host physiology, and the development of various physical, cognitive, behavioral, and mental health conditions.

• Keywords
• OCD, RhD, Rhesus D antigen, autism, evolution, intelligence, manipulation hypothesis, mental health, parasite, schizophrenia, subclinical toxoplasmosis, testosterone,
• Publication type
• Journal Article MeSH
• Review MeSH

In this article, I recount the journey of discovering the effects of latent toxoplasmosis on human psychology, behaviour, morphology, and health as I observed it from the closest perspective over the past 30+ years, during which our laboratory has been intensely focused on this research. I trace how we moved from the initial observations of differences between infected and uninfected individuals in certain personality traits to the systematic study of similar differences in behaviour, both in the laboratory and in everyday life, as well as in physiological and even morphological traits. This eventually led us to investigate the causal relationships behind these observed associations and their molecular basis. I describe some of the unexpected discoveries our research revealed - whether it was the impact of toxoplasmosis on the human sexual index, the prenatal and postnatal development, the sexual preferences and behaviour, the modulatory effect of blood Rh factor on toxoplasmosis, or the discovery of sexual transmission of toxoplasmosis. In exploring whether the toxoplasmosis-associated effects were merely side effects of an ongoing latent infection, we gradually uncovered that seemingly asymptomatic toxoplasmosis has profound (and certainly not positive) effects on the mental and physical health of infected individuals. The article also includes three separate boxes that discuss some key methodological challenges we encountered along the way, such as how to distinguish the effect of infection from mere statistical association, or how to differentiate parasitic manipulation from a simple side effect.

• Keywords
• Rh factor, Toxoplasma, behaviour, chronic toxoplasmosis, dopamine, manipulation, mental health, morbidity, parasite., personality, physical health, schizophrenia, sex ratio, testosterone,
• MeSH
• History, 20th Century MeSH
• History, 21st Century MeSH
• Humans MeSH
• Sexual Behavior MeSH
• Toxoplasmosis * psychology   physiopathology   parasitology   MeSH
• Animals MeSH
• Check Tag
• History, 20th Century MeSH
• History, 21st Century MeSH
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Historical Article MeSH
• Review MeSH

Markers of chronic infection Toxoplasma gondii (Nicolle et Manceaux, 1908) have been associated with suicidal self-directed violence (SSDV). We present the results of the first study relating T. gondii IgG serology with suicide attempts and suicidal ideation in United States Veterans, known to have higher suicide rates than members of the general population. We also related T. gondii serology to SSDV risk factors, including valid and reliable measures of trait impulsivity, aggression, self-reported depression, and sleep disturbance. We recruited 407 Veterans enrolled at three Veterans Affairs Medical Centers with mean (S.D.) age = 45.6 (11.6) years; 304 men (74.7%); 203 with a history of SSDV and 204 with no history of any self-directed violence (SDV). Seropositivity and serointensity, categorised as high (top quartile) or low (lower three quartiles), were analysed in relationship to SSDV, suicidal ideation and clinical risk factors using age and gender-adjusted linear and logistic methods, after transformations and nonparametric tests when appropriate. Associations between seropositivity and SSDV and its risk factors were not significant in all groups. High serointensity, while not associated with SSDV or repeat suicide attempts, was positively associated with suicidal ideation, depression, impulsivity, and daytime dysfunction due to sleepiness (p < 0.05), but only in Veterans with a history of SSDV. In Veterans without a history of SDV, no associations were significant. These associations remained significant after adjustment for certain socioeconomic factors (i.e., income, homelessness, military rank). Including education in the model downgraded the statistical significance of suicidal ideation and depression to statistical trends, but the significance of associations with impulsivity and daytime dysfunction due to sleepiness remained. Major limitations include the cross-sectional design, overall low seropositivity within the sample, and potentially spurious results due to multiple comparisons. Thus, the results of this report need to be replicated in larger samples, ideally longitudinally.

• Keywords
• United States, depression, impulsivity, sleep impairment, suicidality,
• MeSH
• Adult MeSH
• Middle Aged MeSH
• Humans MeSH
• Suicide, Attempted * statistics & numerical data   MeSH
• Antibodies, Protozoan blood   MeSH
• Risk Factors MeSH
• Suicidal Ideation * MeSH
• Toxoplasma * isolation & purification   MeSH
• Toxoplasmosis * psychology   epidemiology   complications   MeSH
• Veterans * psychology   MeSH
• Check Tag
• Adult MeSH
• Middle Aged MeSH
• Humans MeSH
• Male MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Geographicals
• United States epidemiology   MeSH
• Names of Substances
• Antibodies, Protozoan MeSH

Humans infected by Toxoplasma gondii express no specific symptoms but manifest higher incidence of many diseases, disorders and differences in personality and behavior. The aim of this study was to compare the political beliefs and values of Toxoplasma-infected and Toxoplasma-free participants. We measured beliefs and values of 2315 responders via an online survey (477 Toxoplasma-infected) using the Political Beliefs and Values Inventory (PI34). This study showed Toxoplasma-infected and Toxoplasma-free participants of our cross-sectional study differed in three of four factors of PI34, scoring higher in Tribalism and lower in Cultural liberalism and Anti-Authoritarianism. We found sex differences in political beliefs associated with Toxoplasma infection. Infected women scored higher in tribalism and lower in cultural liberalism, compared with the Toxoplasma-free control group, while infected men scored higher in economic equity. These results fit with sexual differences in behavior and attitude observed after toxoplasmosis infection. Controlling for the effect of worse physical health and mental health had little impact, suggesting that impaired health did not cause these changes. Rather than adaptation to prevalence of parasites, as suggested by parasite-stress theory, the differences might be side-effects of long-term mild inflammatory reaction. However, to get clear picture of the mild inflammation effects, more research focused on different infectious diseases is needed.

• Keywords
• Toxoplasma gondii, infectious diseases, manipulation hypothesis, parasite threat, pathogen avoidance, political beliefs, stress,
• MeSH
• Humans MeSH
• Personality MeSH
• Cross-Sectional Studies MeSH
• Sexual Behavior MeSH
• Toxoplasma * MeSH
• Toxoplasmosis * complications   epidemiology   parasitology   MeSH
• Check Tag
• Humans MeSH
• Male MeSH
• Female MeSH
• Publication type
• Journal Article MeSH

BACKGROUND: Latent toxoplasmosis, i.e. a lifelong infection with the protozoan parasite Toxoplasma gondii, affects about a third of the human population worldwide. In the past 10 years, numerous studies have shown that infected individuals have a significantly higher incidence of mental and physical health problems and are more prone to exhibiting the adverse effects of various diseases. METHODS: A cross-sectional internet study was performed on a population of 4499 (786 Toxoplasma-infected) participants and looked for factors which positively or negatively affect the risk of SARS-CoV-2 infection and likelihood of a severe course of COVID-19. RESULTS: Logistic regression and partial Kendall correlation controlling for sex, age, and size of the place of residence showed that latent toxoplasmosis had the strongest effect on the risk of infection (OR = 1.50) before sport (OR = 1.30) and borreliosis (1.27). It also had the strongest effect on the risk of severe course of infection (Tau = 0.146), before autoimmunity, immunodeficiency, male sex, keeping a cat, being overweight, borreliosis, higher age, or chronic obstructive pulmonary disease. Toxoplasmosis augmented the adverse effects of other risk factors but was not the proximal cause of the effect of cat-keeping on higher likelihood of COVID infection and higher severity of the course of infection because the effect of cat-keeping was also observed (and in particular) in a subset of Toxoplasma-infected respondents (Tau = 0.153). Effects of keeping a cat were detected only in respondents from multi-member families, suggesting that a cat could be a vector for the transmission of SARS-CoV-2 within a family. CONCLUSIONS: Toxoplasmosis is currently not considered a risk factor for COVID-19, and Toxoplasma-infected individuals are neither informed about their higher risk nor prioritised in vaccination programs. Because toxoplasmosis affects a large segment of the human population, its impact on COVID-19-associated effects on public health could be considerable.

• Keywords
• COVID-19, Cat, Pets, Risk factors, SARS-CoV-2, Symptoms, Zoonosis,
• MeSH
• COVID-19 epidemiology   parasitology   physiopathology   MeSH
• Incidence MeSH
• Humans MeSH
• Cross-Sectional Studies MeSH
• Surveys and Questionnaires MeSH
• Risk Factors MeSH
• Social Media * MeSH
• Severity of Illness Index MeSH
• Toxoplasma immunology   pathogenicity   MeSH
• Toxoplasmosis complications   MeSH
• Check Tag
• Humans MeSH
• Male MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Geographicals
• Czech Republic epidemiology   MeSH
• Slovakia epidemiology   MeSH

Latent infection of the globally spread parasite Toxoplasma gondii in humans has been associated with changes in personality and behavior. Numerous studies have investigated the effect of toxoplasmosis on depression, but their results are inconsistent. Our study focused on the effect of latent toxoplasmosis on depression in men and women in association with their fertility. In 2016-2018, we recruited clients (677 men and 664 women) of the Center for Assisted Reproduction and asked them to complete a standardized Beck Depression Inventory-II. In women without fertility problems, we found higher depression scores in Toxoplasma-positive than in Toxoplasma-negative (p = 0.010, Cohen's d = 0.48). Toxoplasma-positive infertile men, on the other hand, had lower depression scores than Toxoplasma-negative infertile men (p ≤ 0.001, Cohen's d = 0.48). Our results are consistent with the previously described effects of latent toxoplasmosis, which seem to go in opposite directions regarding the effect on personality and behavior of men and women. Our results could be explained by gender-contrasting reactions to chronic stress associated with lifelong infection. This suggests that due to gender differences in the impact of latent toxoplasmosis, future studies ought to perform separate analyses for women and men.

• Keywords
• Beck Depression Inventory, Toxoplasma gondii, depression score, fertility,
• Publication type
• Journal Article MeSH

Many studies show that keeping cats and dogs has a positive impact on humans' physical and mental health and quality of life. The existence of this "pet phenomenon" is now widely discussed because other studies performed recently have demonstrated a negative impact of owning pets or no impact at all. The main problem of many studies was the autoselection-participants were informed about the aims of the study during recruitment and later likely described their health and wellbeing according to their personal beliefs and wishes, not according to their real status. To avoid this source of bias, we did not mention pets during participant recruitment and hid the pet-related questions among many hundreds of questions in an 80-minute Internet questionnaire. Results of our explorative study performed on a sample of 10,858 subjects showed that liking dogs has a weak positive association with quality of life. However, keeping pets, especially cats, and even more being injured by pets, were strongly negatively associated with many facets of quality of life. Our data also confirmed that infection by the cat parasite Toxoplasma had a very strong negative effect on quality of life, especially on mental health. However, the infection was not responsible for the observed negative effects of keeping pets, as these effects were much stronger in 1,527 Toxoplasma-free subjects than in the whole population. Any cross-sectional study cannot discriminate between a cause and an effect. However, because of the large and still growing popularity of keeping pets, the existence and nature of the reverse pet phenomenon deserve the outmost attention.

• MeSH
• Pets * MeSH
• Adult MeSH
• Internet MeSH
• Cats * microbiology   MeSH
• Quality of Life * MeSH
• Humans MeSH
• Mental Healing MeSH
• Human-Animal Bond MeSH
• Cross-Sectional Studies MeSH
• Surveys and Questionnaires MeSH
• Dogs * microbiology   MeSH
• Wounds and Injuries etiology   MeSH
• Toxoplasma MeSH
• Toxoplasmosis etiology   MeSH
• Animals MeSH
• Check Tag
• Adult MeSH
• Cats * microbiology   MeSH
• Humans MeSH
• Male MeSH
• Dogs * microbiology   MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH

BACKGROUNDS: The prevalence of toxoplasmosis is higher in schizophrenics than in the general population. It has been suggested that certain symptoms of schizophrenia, including changes in olfactory functions, are in fact symptoms of toxoplasmosis that can be easily detected in schizophrenics only due to the increased prevalence of toxoplasmosis in this population. Schizophrenics have impaired identification of odors and lower sensitivity of odor detection, however, no information about these parameters of non-schizophrenic Toxoplasma-infected subjects is available. METHODS: Here we searched for differences in olfactory functions between 62 infected and 61 noninfected non-schizophrenic subjects using the case-controls experimental design. RESULTS: The infected men scored better than the non-infected controls in the standard odor-identification test. The infected women rated all smells as more intensive while the infected men rated nearly all smells as less intensive. Infected women rated the pleasantness of the smell of the cat urine as higher than the non-infected women and the opposite was true for the men-in contrast, higher pleasantness of odor in infected men and lower in infected women were observed and described in the 2011 study. Toxoplasmosis, Rh, and toxoplasmosis-Rh interaction were not associated with the rated pleasantness of the smell of other stimuli. However, our sample contained only 17 Rh negative men and 30 Rh negative women. Therefore, all results concerning the main effects of Rh factor and the interaction with Rh factor must be considered only preliminary. CONCLUSIONS: Our results suggest that latent toxoplasmosis is associated with changes in the olfactory functions in humans; however, the observed changes differ from those observed in schizophrenics.

• MeSH
• Adult MeSH
• Cats MeSH
• Rh-Hr Blood-Group System metabolism   MeSH
• Humans MeSH
• Young Adult MeSH
• Urine MeSH
• Odorants MeSH
• Schizophrenia metabolism   parasitology   physiopathology   MeSH
• Case-Control Studies MeSH
• Toxoplasma pathogenicity   MeSH
• Toxoplasmosis metabolism   physiopathology   MeSH
• Animals MeSH
• Check Tag
• Adult MeSH
• Cats MeSH
• Humans MeSH
• Young Adult MeSH
• Male MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Rh-Hr Blood-Group System MeSH

Several recent studies have demonstrated the association of cat-related injuries with major depression and with depressiveness in the general population. It was suggested that cat-scratch disease, the infection with the bacterium Bartonella henselae, can be responsible for the observed association. However, no direct evidence for the role of the Bartonella infection in this association has been published until now. In this preregistered case-controls study performed on 250 healthy subjects tested earlier for the presence of anti-Toxoplasma IgG antibodies, we searched for the positive association between presence of anamnestic anti-Bartonella IgG antibodies and depressiveness measured with Beck II inventory, depression subscale of neuroticism measured with N-70 questionnaire, and self-reported health problems. We found that that Bartonella seropositivity was positively correlated with Beck depression only in Toxoplasma-seronegative men and negatively correlated with health in Toxoplasma-seronegative women. Bartonella seropositivity expressed protective effects against Toxoplasma seropositivity-associated increased neuroticism in men while Toxoplasma-seropositivity expressed protective effects against Bartonella seropositivity-associated health problems in women. A comparison of the patterns of association of mental and physical health problems with Bartonella seropositivity and with reported cat-related injury suggests that different factor, possibly infection with different pathogen transmitted by cat related-injuries than the B. henselae, is responsible for the observed association of cat related-injuries with depressiveness and major depression. The existence of complex interactions between Bartonella seropositivity, Toxoplasma seropositivity, and sex also suggest that the effect of symbionts on the host's phenotype must by always studied in the context of other infections, and separately for men and women.

• Keywords
• Bartonella, Toxoplasma, animal-related injuries, bartonellosis, cat-scratch disease, depressiveness, major depression, toxoplasmosis,
• Publication type
• Journal Article MeSH

BACKGROUND: Toxoplasma, a protozoan parasite of cats, infects many species of intermediate and paratenic hosts, including about one-third of humans worldwide. After a short phase of acute infection, the tissue cysts containing slowly dividing bradyzoites are formed in various organs and toxoplasmosis proceeds spontaneously in its latent form. In immunocompetent subjects, latent toxoplasmosis was considered asymptomatic. However, dozens of studies performed on animals and humans in the past twenty years have shown that it is accompanied by a broad spectrum of specific behavioural, physiological and even morphological changes. In human hosts, the changes often go in the opposite direction in men and women, and are mostly weaker or non-existent in Rh-positive subjects. METHODS: Here, we searched for the indices of lower endurance of the infected subjects by examining the performance of nearly five hundred university students tested for toxoplasmosis and Rh phenotype in two tests, a weight holding test and a grip test. RESULTS: The results confirmed the existence of a negative association of latent toxoplasmosis with the performance of students, especially Rh-negative men, in these tests. Surprisingly, but in an accordance with some already published data, Toxoplasma-infected, Rh-positive subjects expressed a higher, rather than lower, performance in our endurance tests. DISCUSSION: Therefore, the results only partly support the hypothesis for the lower endurance of Toxoplasma infected subjects as the performance of Rh-positive subjects (representing majority of population) correlated positively with the Toxoplasma infection.

• MeSH
• Physical Endurance * MeSH
• Immunoglobulin G blood   MeSH
• Immunoglobulin M blood   MeSH
• Rh-Hr Blood-Group System blood   MeSH
• Humans MeSH
• Young Adult MeSH
• Antibodies, Protozoan blood   MeSH
• Hand Strength * MeSH
• Toxoplasma immunology   MeSH
• Toxoplasmosis physiopathology   MeSH
• Check Tag
• Humans MeSH
• Young Adult MeSH
• Male MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Immunoglobulin G MeSH
• Immunoglobulin M MeSH
• Rh-Hr Blood-Group System MeSH
• Antibodies, Protozoan MeSH