Seroprevalence of Entamoeba histolytica Infection among Chinese Men Who Have Sex with Men

Background Men who have sex with men (MSM) were found to be one of the high-risk populations for Entamoeba histolytica (E. histolytica) infection. Accompanied by the prevalence of human immunodeficiency virus (HIV) among MSM, invasive amebiasis caused by E. histolytica has been paid attention to as an opportunistic parasitic infection. However, the status of E. histolytica infection among MSM has been barely studied in mainland China. Methods Seroprevalance of E. histolytica was determined using an enzyme-linked immunosorbent assay based on a cross-sectional study conducted in Beijing and Tianjin, China. Factors potentially associated with E. histolytica infection were identified by logistic regression analysis. Results A total of 602 MSM were included in the study and the laboratory data on serostatus of E. histolytica were available for 599 of them (99.5%). 246 (41.1%) and 51 (8.5%) of the study participants were E. histolytica seropositive and HIV seropositive, respectively. Univariate analyses suggested preferred anal sex behaviors were associated with E. histolytica seropositivity. In multivariate logistic regression analysis, “only has receptive anal sex” (OR: 2.03; 95% CI: 1.22 3.37), “majority receptive anal sex” (OR: 1.83; 95% CI: 1.13, 2.95), and “sadomasochistic behavior (SM)” (OR: 2.30; 95% CI: 1.04, 5.13) were found to be significantly associated with E. histolytica infection. Conclusions High seroprevalence of E. histolytica infection was observed among MSM from Beijing and Tianjin, China. Receptive anal sex behavior and SM were identified as potential predictors. Therefore, E. histolytica and HIV co-infection needs to be concerned among MSM due to their sharing the common risk behaviors.


Introduction
Entamoeba histolytica (E. histolytica) has a worldwide distribution and is endemic in most developing countries. Invasive amebiasis (IA) caused by E. histolytica is a very common human gastrointestinal parasitic disease which affected 50 million people worldwide and caused greater than 100,000 deaths annually. High risk populations for developing IA include infants, travelers from endemic area, and patients who are taking immunosuppressant [1,2]. In mainland China, E. histolytica infection was also very popular in general population. The average prevalence of E. histolytica infection was 0.95%, ranged from 0.01% to 8.12% [3].
In 1967, the association between amebiasis and homosexuality was suggested for the first time [4]. Men who have sex with men (MSM) population had already been found to be a high risk population with E. histolytica infection before 1990. Homosexuality and oral-anal sex have been most frequently reported as potential risk factors for E. Histolytica infection [5][6][7][8][9][10][11][12][13]. Accompanied by the transmission of human immunodeficiency virus (HIV) in MSM population, the prevalence of IA caused by E. histolytica are increasing and getting the attention as an important opportunistic parasitic infection. Recent studies from Australia, Japan, Korea and Taiwan reported increased risks for E. histolytica infection and IA among HIV-positive MSM [14][15][16][17][18]. Hung CC and colleagues recently reviewed the status of E. Histolytica infection in MSM [19]. By the end of 2011, China had about 780,000 people living with HIV/AIDS and 17.4% of them were MSM. The estimated new HIV infections in 2011 are 48,000 and 38.1% were MSM [20]. Case report of IA suggested the risk of E. histolytica prevalence among Chinese MSM, especially in the HIV positives [21]. However, to our knowledge, the prevalence of E. histolytica infection among MSM population has not been investigated in mainland China. The major aim of this study was to assess the seroprevalence of E. histolytica infection and the potential impact factors among MSM from China.

Ethics statement
The study was approved by the Ethics Committees of the Institute of Pathogen Biology, Chinese Academy of Medical Sciences & Peking Union Medical College. All participants were the adults of men who had sex with men. Written informed consent was obtained before the interview and testing.

Study design and participants
A cross-sectional study was conducted in Beijing and Tianjin, China. Six hundred study participants were recruited between March and July 2010, though local non-government organizations (Beijing Rainbow Volunteers Workstation and Tianjin Deep Blue Volunteers Workgroup). Participants' recruitment and inclusion criterion had been addressed in previous study [22].

Data collection
Questionnaire administered by the trained interviewer were performed for each study participant, the data acquired from the questionnaire includes socio-demographic characteristics (e.g., age, income, ethnicity, education, employed, and marital status), sexual orientation and homosexual act, sexual behaviors in the past 6 months, history of STDs and HIV infection. Anilingus behaviors were defined as sexual stimulation involving oral contact with the anus and sadomasochistic behaviors (SM) were defined as behaviors which aimed to enhance sexual gratification from inflicting or submitting to physical and emotional abuse.

Sample collection and laboratory tests
Blood samples were collected for E. histolytica and HIV serology. Serum samples were stored at 280uC until tested. Each study participant was assigned a unique identification number that was used to link the questionnaire and specimens. The HIV infection status was screened by an enzyme immunoassay (Wantai Biological Medicine Company, Beijing, China), and positive tests were confirmed by HIV-1/2 Western blot assay (HIVBlot 2.2 WB; Genelabs Diagnostics, Singapore). Qualitative screenings of serum immunoglobulin G (IgG) antibodies to E. histolytica were retrospectively performed using the commercial enzyme-linked

Statistical analysis
Questionnaires were double entered and compared with EpiData software (EpiData 3.02 for Windows, The Epi Data Association Odense, Denmark). After cleaning, the data were then converted and analyzed using Statistical Analysis System (SAS 9.2 for Windows; SAS Institute Inc., NC, USA).
Study population was characterized by site with respect to age, ethnicity, education, marriage status, and current status of HIV. Differences between sites in these variables were assessed with Pearson chi-square test. The associations of E. histolytica infection with the characteristics of demographics, sexual behaviors, diagnosed STDs including HIV, and current status of HIV were estimated using Pearson chi-square test. Variables related with E. histolytica serology (p,0.1) in the univariate analyses were included in a multiple logistic regression model additionally adjusted for age, site, and HIV infection status. The Cochran-Armitage test was used to evaluate the trend of OD value in E. histolytica antibody test associated with anal sex behaviors categories.

Demographic characteristics and its association with E. histolytica infection
A total of 607 participants were interviewed and signed the informed consent, 6 of them were excluded (3 did not complete sample collection and 2 Vietnamese living in Beijing). Finally, 602 were used for the analyses (302 from Beijing and 300 from Tianjin). Laboratory data on serostatus of E. histolytica and HIV were available for 599 (99.5%) and 598 (99.3%) of the study participants respectively. Age, ethnicity, education, marriage status, HIV-1 serostatus and E. histolytica serostatus of the study population were compared by site (Table 1). No significant difference was found for any characters (p.0.05). Therefore, participants from the two sites were pooled together for further association analyses. Age of the participants ranged from 16 to 72 years, with a mean age of 27.968.3 years. The participants showed the following characteristics: 95% (570/600) were Han nationality; 51.5% (309/600) had more than 12 year's education;

Sexual behaviors and its association with E. histolytica infection
Homosexual men accounted for 71.0% (427/602), and bisexual men for 24.3% (146/602) of the study population. 73.6% (181/ 246) of participants with E. histolytica seropositive were homosex-uals. Age of the first homosexual act ranged from 5 to 55 years old, with a mean age of 21.465.0 years. The median number of their homosexual partners was eleven and 43.1% of participants had stable homosexual partners before baseline survey. In the past one year, sixty one participants (10.2%) had group sex, thirty eight participants (6.3%) had ever received money for sex with male partners, and twenty three (3.8%) had ever provided money for sex with male partners. In the past 6 months, 133 (22.2%) participants reported had sexual behavior less than 1 time per month and only 1.3% (8/602) of participants insisted on using condoms in the process of insertive or receptive anal sex and oral sex. 22.5% of participants had ever been diagnosed sexually transmitted diseases. The association between sex behaviors and E. histolytica infection were also assessed. Self-reported preferred anal sex behaviors were classified to four types (only has insertive anal sex, majority insertive anal sex, majority receptive anal sex, and only has receptive anal sex). Univariate analyses suggested preferred anal sex behaviors were associated with E. histolytica infection. HIV infection was not found associated with E. histolytica infection ( Table 3).
The variables associated with E. histolytica infection in the univariate analyses (P,0.1) were included in a multivariate logistic regression model. Age, site, and HIV infection status were fixed in the model as well. In the multivariate logistic regression model, only has receptive anal sex (OR: 2.03; 95% CI: 1.22, 3.37), majority receptive anal sex (OR: 1.83; 95% CI: 1.13, 2.95), and SM (OR: 2.30; 95% CI: 1.04, 5.13) were found to be significantly associated with E. histolytica infection after adjusted for the other variables ( Table 4).
The OD in ELISA test was used for further analysis. Five hundred and eighty three participants reported their preferred anal sex behavior. As shown in figure 1, the OD values increased from the group of only had insertive anal sex (median OD, 0.036), majority insertive anal sex (median OD, 0.049), majority receptive anal sex (median OD, 0.117) to only had receptive anal sex (median OD, 0.131). The trend of increasing was significant (p,0.001), which was consistent with the result of multivariate analysis.

Discussion
This pilot study investigated E. histolytica seroprevelance in MSM from China; potential factors associated with E. histolytica infection were evaluated as well. In a total of 602 study participants, E. histolytica seroprositivity was found to be 41.1%. Types of preferred anal sex behavior (only has receptive anal sex and majority receptive anal sex) and SM were identified as  significant predictors for E. histolytica infection. In addition, significant different antibody levels were observed between subgroups with respect to the preferred anal sex behavior. The first observation of a relation between enteric protozoan infections and sexual behavior was reported in 1968 [4]. Epidemiological studies conducted in the developed countries showed homosexuals or MSM had significant higher risk of E. histolytica infection. Using microscopy for diagnosis, the prevalence varied from 20% to 32% among MSM without gastrointestinal symptoms [5,7,8,[23][24][25][26][27]. However, microscopy is not sensitive or specific enough for the detection of E. histolytica in clinical specimens, especially for the differentiation E. histolytica from E dispar and E moshkovskii in the epidemiological studies with a large sample size. Therefore, serological tests were used to detect the infection though measuring anti-E. histolytica antibodies and seroprevalence ranging from 0.2% to 21% using ELISA test among HIV negative MSM were reported in several developed countries [18,19]. Our results, for the first time, suggested a high prevalence (41.1%) of E. histolytica infection among MSM community from China. A recently published study conducted among general population in seven provinces in China showed that the seroprevalence of E. histolytica infection varied from 6% to 11% [28]. Although antibody test could not distinguish the past or current infection status and maybe overestimated the epidemic status, the fact that amebic liver abscess and latent infection had become one of the common opportunistic infection diseases among Chinese MSM AIDS patients reminds us to pay attention to the co-infection of E. histolytica and HIV. [21,29].
In the present study, homosexual behaviors were mostly classified according to participants' tendency. Receptive anal sex behavior was found to be related to higher prevalence of E. histolytica infection. This finding and its underlying mechanisms should be further studied in the future. Homosexuals and history of anilingus had been demonstrated to be the risk factors of E. histolytica infection. In 1978, a study from the New York city reported that 20% of eighty nine sexually active homosexual men had amebiasis and the presence of infections associated with history of anilingus [5]. Another study from a venereal-diseaseclinic population compared the prevalence of E. histolytica infections in homosexual men, bisexual men and heterosexual men. Homosexuality and oral-anal sex were found to be the most important risk factors for E. histolytica infection [7]. However, such an association was not observed in our study population. Interestingly, SM was found to be associated with E. histolytica infection in the present study. The data on the specific behaviors during the process of SM has not been well studied in China due to the potential issues of social culture and discrimination. Several published studies had revealed that people who had engaged in SM were more likely to have experienced oral-anal sex and other sexual risk practice [30][31][32]. In addition, fecal-oral contamination in these sexual behaviors maybe occurs and increases the opportunity of pathogen infection. Keystone JS' study showed cleaning of anus before anal sex was associated with a significant lower prevalence of infection [23]. But it is difficult to explore the factors linked SM behaviors to the infection susceptibility in our present study because only 30 participants (5.0%) reported SM behaviors. Developing targeted prevention and control strategies, such as developing sanitary habit before sexual behavior, may decrease the opportunity of pathogen infection. Therefore, it is necessary to further study potential risky behaviors associated with health problems among Chinese MSM.
Several limitations of this study should be kept in mind. First, potential bias due to the inaccurate response to the questionnaire, especially to the questions on sexual behaviors, could not be excluded completely. Second, our study participants might not represent the general MSM population due to the potential limitation of enrollment methods. Therefore, potential selection bias should be considered when interpret our results. Third, serology could not clearly identify the infection status as current infection or past infection, potential bias caused by such misclassification could not be excluded. Although statistically significant difference of antibodies levels was observed with respect to the preferred anal sex behaviors (p,0.001), however, the smaller sample size in each subgroups and a broad range of OD value should be considered. Further studies are needed to explore the underlying mechanisms for the observed relation between receptive anal sex behaviors and E. histolytica infection. Fourth, cross-sectional study design has its limitation on association analysis. Therefore, our results need confirmation by further large-scale case-control studies or prospective studies.
In conclusion, high prevalence of E. histolytica infection was observed among MSM from Beijing and Tianjin, China. Receptive anal sex behaviors and SM were identified as significant predictors for E. histolytica infection. Prevention and control of E. histolytica infection among Chinese MSM should be concerned because this special population confronted with high risk of HIV infection.