Is Toxoplasma gondii Infection Associated with Sexual Promiscuity? A Cross-Sectional Study

We determined the association between T. gondii seropositivity and a history of sexual promiscuity. The study included 3933 people (mean age: 41.81 ± 14.31 years) who attended public health facilities. Face-to-face interviews were used to collect data. Enzyme immunoassays were used to determine anti-T. gondii IgG and IgM antibodies. Anti-T. gondii IgG antibodies were found in 57 (18.1%) of 315 individuals with sexual promiscuity and in 374 (10.3%) of 3618 individuals without this practice (OR: 1.91; 95% CI: 1.41–2.60; p < 0.0001). High (>150 IU/mL) levels of anti-T. gondii IgG antibodies were found in 29 (9.2%) of the 315 participants with sexual promiscuity and in 167 (4.6%) of the 3618 participants without this history (OR: 2.09; 95% CI: 1.38–3.16; p = 0.0003). The association of sexual promiscuity with T. gondii seropositivity and serointensity was observed in men but not in women. Sexual promiscuity was associated with T. gondii seropositivity in all age groups studied (≤30 years, 31–50 years, and >50 years) and with T. gondii serointensity in two age groups (≤30 years, and >50 years). No difference in the frequencies of anti-T. gondii IgM antibodies among the groups was found. Our findings indicate that T. gondii seropositivity and serointensity are associated with sexual promiscuity.


Introduction
The parasite Toxoplasma gondii (T. gondii) is an intracellular protozoan that can infect, survive, and replicate in nearly all mammalian cells [1]. Nearly one-third of humanity has been infected with T. gondii [2]. Toxoplasmosis, a disease caused by infection with T. gondii, is a zoonosis with medical and veterinary importance [3]. However, this disease is still a neglected parasitic infection [1]. As the most common feline definitive host, cats play a vital role in the transmission of T. gondii [4]. The main route of infection with T. gondii is consumption of raw or undercooked meat containing tissue cysts [2]. Infection with T. gondii can also occur by ingestion of water or food contaminated with oocysts from the feces of infected cats [5]. Most human infections with T. gondii are mild or asymptomatic [1]. However, individuals infected with T. gondii may develop ocular disease or cervical lymphadenopathy [5]. Reactivation of a latent infection in immune deficiency conditions can cause fatal toxoplasmic encephalitis, and primary infection with T. gondii in pregnant women may cause health-threatening sequelae for the fetus or death in uterus [6]. Recent studies suggest that asymptomatic infections with T. gondii may have effects on behavior [7] and might be responsible for a vast array of neuropsychiatric symptoms [8]. T. gondii can persist lifelong in the central nervous system within neurons, modifying their function and structure and leading to specific behavioral changes of the host [9]. Infections with T. gondii have been associated with schizophrenia [10,11], suicide behavior [12][13][14], mixed anxiety and depressive disorder [15], traffic accidents [16], and aggression [17]. In contrast, some studies have shown a lack of association between T. gondii seropositivity and depression in pregnant women [18], completed suicide [19], and bipolar disorder [20]. Reports thus indicate that further research to elucidate the link between T. gondii infection and clinical and behavioral changes are largely needed. It is unclear whether T. gondii infection might be linked to sexual behavior. We hypothesized that sexual promiscuity is associated with T. gondii infection. On the one hand, a group of researchers have hypothesized that toxoplasmosis can be a sexually transmitted infection [21]. If T. gondii can be transmitted by sexual intercourse, persons with sexual promiscuity might have a higher risk for T. gondii infection than persons without this practice. High seroprevalence of T. gondii infection has been reported in population groups with large number of sexual partners or having a sexually transmitted infection including female sex workers [22] and people with human immunodeficiency virus infection [23]. On the other hand, high seroprevalence of infection with T. gondii is found in psychiatric patients [11,24] and sexual promiscuity is a common behavior in patients with psychiatric illnesses. An increase in sexual activity and promiscuity have been found in patients with bipolar depression [25], depressed children [26], patients with borderline personality disorder [27], and psychiatric patients with a history of suicide attempts [28]. In a study of 137 psychiatric inpatients in Mexico, an association between T. gondii seropositivity and sexual promiscuity was found [24]. However, in such a study, the analysis was performed in a quite limited number of psychiatric inpatients (not in their controls), in a single hospital, and the association was obtained without any matching or stratification by age and gender. Therefore, in the present study including a large sample of participants, in several public health facilities, and with a stratification by age and gender, we aimed to determine the association between T. gondii seropositivity and sexual promiscuity.

Results
Of the 3933 people surveyed, 315 (8.0%) had a history of sexual promiscuity and 3618 (92.0%) did not have this history. Anti-T. gondii IgG antibodies were found in 57 (18.1%) of the 315 individuals with a history of sexual promiscuity and in 374 (10.3%) of the 3618 individuals without this history (OR: 1.91; 95% CI: 1.41-2.60; p < 0.0001). Logistic regression analysis showed that the association between T. gondii infection and sexual promiscuity remained significant when an adjustment by age, gender, and residence area (urban vs. rural) was performed (OR: 1.54; 95% CI: 1.11-2.14; p = 0.009). Table 1 shows a stratification by age and gender and seroprevalence of T. gondii infection in individuals with and without a history of sexual promiscuity.
Men with a history of sexual promiscuity had a significantly higher (30/94: 31.9%) seroprevalence of T. gondii infection than men without this history (104/748: 13.9%) (OR: 2.90; 95% CI: 1.79-4.69; p < 0.0001). Participants of the age groups ≤30 years, 31-50 years, and >50 years with a history of sexual promiscuity had a significantly higher seroprevalence of T. gondii infection than those of the same age group without sexual promiscuity (p = 0.004; p = 0.02; and p = 0.01, respectively). High (>150 IU/mL) levels of anti-T. gondii IgG antibodies were found in 29 (9.2%) of the 315 participants with a history of sexual promiscuity and in 167 (4.6%) of the 3618 participants without this history (OR: 2.09; 95% CI: 1.38-3.16; p = 0.0003). Table 2 shows a stratification by sex and age groups and the association between high (>150 IU/mL) anti-T. gondii IgG antibody levels and sexual promiscuity. The rate of high (>150 IU/mL) levels of anti-T. gondii IgG antibodies was higher in men with sexual promiscuity than in men without this practice (OR: 3.36; 95% CI: 1.77-6.35; p < 0.0001). Participants of the age groups ≤30 years and >50 years with a history of sexual promiscuity had a significantly higher rate of high (>150 IU/mL) levels of anti-T. gondii IgG antibodies than those of the same age groups without sexual promiscuity (p = 0.02; and p = 0.04, respectively).

Discussion
There is scarce information about the link between T. gondii infection and sexual behavior. Therefore, in this survey, we determined the association between T. gondii exposure and sexual promiscuity in a large sample of people attending public health facilities in Durango, Mexico. We found that individuals with sexual promiscuity had a significantly higher seroprevalence of T. gondii infection than individuals without this practice. This higher seroprevalence of T. gondii infection was observed in all age groups studied. In addition, the rate of high levels of anti-T. gondii IgG antibodies was significantly higher in participants with sexual promiscuity than in those without this practice. This higher rate of high anti-T. gondii IgG antibodies was observed in two of the three age groups studied. Therefore, our findings suggest that T. gondii seroprevalence and serointensity is associated with sexual promiscuity. Our results are in line with the one reported in a seroepidemiology study of 137 psychiatric inpatients in Durango City, where an association between T. gondii seropositivity and sexual promiscuity was found [24]. However, in such a study, no stratifi-cation by age and sex was performed, and no association between sexual promiscuity and T. gondii serointensity or IgM seropositivity was assessed. In the present study, we performed a stratification by age groups in order to determine whether the observed association was uniform regardless of the age of participants or whether it occurred only in a certain age group. Seropositivity to T. gondii was associated with sexual promiscuity in all age groups, whereas T. gondii serointensity was associated with sexual promiscuity in people aged ≤30 years and >50 years but not in individuals aged 31-50 years. It is unclear why the T. gondii serointensity association was not uniformly observed in all age groups. It is possible that unknown factors linked to T. gondii infection and/or sexual promiscuity present in individuals aged 31-50 years differed from those present in younger and older individuals. Anti-T. gondii IgG antibodies peak at 3 months after infection and then remain at a plateau level for six months and after one year start to decrease [29]. A study of mice showed that increased IgG levels correlated with proliferation of tachyzoites in the brain during the chronic stage of infection, suggesting that high anti-T. gondii IgG antibody levels may indicate an occurrence of cerebral tachyzoite growth in immunocompetent individuals chronically infected with T. gondii [30]. Thus, in the present study, a considerable number of participants with high anti-T. gondii IgG antibody levels and sexual promiscuity might have had a recent infection or a reactivation of the infection. Anti-T. gondii IgM antibodies usually appear after one week of infection and disappear after 9 months but can remain for two years or more in some individuals [29]. Thus, the lack of association between T. gondii IgM seropositivity and sexual promiscuity found in the current study suggests that the association between T. gondii infection and sexual promiscuity occurred in individuals with chronic infections. A recent review on advances in T. gondii behavioral biology showed that infection with T. gondii alters host hormones and neurotransmission within the host brain, and that T. gondii effects could be mediated through neuroinflammation [31]. In another review, researchers showed that several factors interact and combine to alter the likelihood and outcome of clinical and neuropsychiatric disease including host genetics, parasite genotype, seroprevalence, and coinfecting neurotropic agents [32]. Intriguingly, in the present study, stratification by gender showed that the association between T gondii seropositivity and serointensity was present in men but not in women. It is not clear why this association was found only in men, but it raises the question whether testosterone might be playing any role in the association. Reports indicate that T. gondii increases the synthesis of testosterone as shown in a recent review [31], and this effect might lead to sexual promiscuity in T. gondii-infected individuals. In a recent article, researchers argued that the presence of tissue cysts in the host brain is merely incidental to the behavioral change and proposed a neuroendocrine loop to explain the role of gonadal steroids in the parasitized host in mediating the behavioral manipulation [33]. Some researchers have suggested that a higher level of testosterone could be responsible for at least some of the toxoplasmosis-associated shifts in human and animal behavior [34]. In a study on the association of latent toxoplasmosis and levels of serum testosterone in humans, researchers found a higher testosterone concentration in T. gondii seropositive individuals than in T. gondii seronegative ones [35]. In a study of students, researchers found that T. gondiiinfected men had a higher concentration of testosterone and T. gondii-infected women had a lower concentration of testosterone than T. gondii-free controls [36]. In addition to testosterone, a possible mechanism by which T. gondii may affect human behavior is its effect on dopamine [37]. T. gondii contains two genes encoding tyrosine hydroxylase that catabolize phenylalanine to tyrosine and tyrosine to L-DOPA, and L-DOPA is the precursor to dopamine [38]. However, a study showed that infection with T. gondii did not affect host dopamine levels in vitro or in vivo [39]. Variation in the human dopamine D4 receptor gene has been associated with both sexual promiscuity and infidelity [40]. On the other hand, experiments in male rats have shown that T. gondii reprograms host brains to change fear to sexual attraction [41,42]. The results of the above-mentioned studies in humans and animals suggest that there is a link between T. gondii infection and sexual behavior. The results of our study may mean that infection with T. gondii could lead to sexual promiscuity, especially in men, or that people with sexual promiscuity could have risk factors that favor infection with T. gondii. Sexual promiscuity has been linked to mental health problems in men [43], and infection with T. gondii has been linked to mental disorders [10][11][12][13][14][15]. Therefore, sexual promiscuity might be a manifestation of mental disorders in some individuals exposed to T. gondii. In fact, in a previous report, T. gondii seropositivity was associated with sexual promiscuity in psychiatric inpatients [24]. On the other hand, whether T. gondii could be transmitted by sexual contact in people with sexual promiscuity is unclear. Sexual transmission of T. gondii is a matter of controversy. A study showed that transmission of T. gondii through the sexual route did not have epidemiological significance for men and animals [44]. In another study, a poor concordance of serological and molecular markers of T. gondii infection among heterosexual couples was found [45]. In contrast, a study showed that female sex workers had a higher seroprevalence of T. gondii infection than their controls, and injuries during sex work were associated with T. gondii seropositivity [22]. In a recent study of couples, the prevalence of toxoplasmosis in women with infected male partners was higher than in women with uninfected male partners, suggesting a sexual transmission of T. gondii from men to women [46]. The present study has limitations: (1) we performed a logistic regression analysis with adjustment of only few variables, and additional research with a more extensive analysis of variables should be conducted; and (2) we did not perform molecular analyses. It is of interest to determine the T. gondii genotypes in people in Mexico. It raises the question whether any T. gondii genotype might be linked to behavioral changes including sexual promiscuity. Further research to determine the association between T. gondii infection and sexual promiscuity using a case-control study design is needed. In addition, studies to determine the seroepidemiology of T. gondii infection in people with sexual promiscuity including analysis of sociodemographic, housing, clinical, and behavioral characteristics of participants should be conducted.

Study Design and Study Population
This cross-sectional study was conducted in Durango State, Mexico from June 2014 to May 2018. Individuals aged ≥15 years, of any gender, who attended 7 public health facilities in Durango City and rural Durango were invited to participate in the study. In total, 3933 people with a written informed consent were included in the survey. Of them, 842 were men and 3091 were women. The mean age of participants was 41.81 ± 14.31 (range 15-93) years. Face-to-face interviews were used to collect survey data. General data and a history of sexual promiscuity from participants were obtained.

Serology Tests
A blood sample was obtained from each participant. Sera were obtained from blood samples and stored at −20 • C until testing. Sera were tested for the presence of anti-T. gondii IgG antibodies using a commercially available enzyme immunoassay "Toxoplasma gondii IgG" kit (Diagnostic Automation/Cortez Diagnostics, Inc., Woodland Hills, CA, USA). This test was run in qualitative and quantitative formats. The manufacturer's suggested positive cut-off of 8 IU/mL was used. According to the manufacturer's insert, this IgG test has a sensitivity of 95.3%, a specificity of 100%, and an intra-run precision (C.V.) between 2.0% and 23.3%. High anti-T. gondii IgG antibody levels may indicate an acute or recent infection, or in chronic infections, reactivation or reinfection. Measuring the level of IgG can provide information about an on-going or resumed replication of the pathogen [47]. Sera that were positive for anti-T. gondii IgG antibodies were tested for detection of anti-T. gondii IgM antibodies by a commercially available enzyme immunoassay "Toxoplasma gondii IgM" kit (Diagnostic Automation/Cortez Diagnostics, Inc.). According to the manufacturer's insert, this IgM test has a sensitivity of 100%, a specificity of 100%, an intra-run precision (C.V.) between 1.5% and 6.0%, and an inter-run precision (C.V.) between 8.0% and 12.3%. The good sensitivity, specificity, and precision of the IgG and IgM tests indicate that these tools are reliable for the diagnosis of T. gondii infection. All assays were performed according to the manufacturer's instructions.

Statistical Analysis
Statistical analyses were performed using the software SPSS version 15 and EPIDAT 3.1. Based on an expected frequency of T. gondii exposure of 6.1% [48], a population size of 1,238,564 people aged 15 years and older in Durango, confidence limits of 1%, and a confidence level of 95%, the total sample size was calculated at 2197 individuals. The Pearson's chi-square test was used to observe the association between two categorial variables. Logistic regression analysis with adjustment by age, gender, and residence area was performed to determine the association between T. gondii seropositivity and sexual promiscuity. Odd ratios (ORs) and 95% confidence intervals (CIs) were calculated, and statistical significance was set at p value < 0.05.

Conclusions
The results of this survey indicate that T. gondii seropositivity and serointensity are associated with sexual promiscuity. Further research on the association between T. gondii exposure and sexual promiscuity should be conducted.