HERPES SIMPLEX VIRUS TYPE 2 (HSV-2) infections are endemic in many populations and studies of seroprevalence indicate that HSV-2 is increasing in parts of the world. 1–2 Being a sexually transmitted disease, HSV-2 is the main cause of genital herpetic ulcers in many countries. 3 Frequent viral shedding from unrecognized symptomatic and asymptomatic carriers is the main cause of continued genital HSV-2 transmission. 4–5 However, HSV-2 may cause painful and recurrent genital lesions, 6 the rare outcome of neonatal herpes 7 and has been suggested to increase the risk of HIV infection. 8
HSV-2 antibody prevalence has been found to be higher among women than men, 2 suggesting that women have a greater risk of acquisition; this finding may be due to anatomical differences in susceptibility of infection. The highest rates of HSV-2 type specific antibodies (79–96%) have been reported among women with high-risk sexual behavior, 9 including having a greater number of sexual partners, 10 paying or being paid for sex, 11 and having sexual activity under the influence of drugs. 12 Epidemiologic studies suggest that HSV-2 seroprevalence peaks and may plateau in the fourth decade of life, 1–2,13 data consistent with HSV-2 antibodies representing a cumulative measurement of exposure.
In industrialized countries, HSV-2 serosurveys have demonstrated an increase in HSV-2 seropositivity of approximately 30% in the past two decades. 1,2 Currently, HSV-2 prevalence is greater than 20% among adults in the United States, 2 and approximately 60% to 90% of female commercial sex workers worldwide have antibodies to HSV-2, 1 including Mexico City. 11,14
In this context, few population-based studies of HSV-2 have been reported using type-specific serologic methods to differentiate HSV-2 and HSV-1 antibodies, particularly in less-developed countries. We present results from a study of women from Mexico City in order to determine HSV-2 type specific seroprevalence and risk factors at the population-level.
This analysis is based on 730 women selected from an age-stratified random population-based sample of residents in the Mexico City metropolitan area, identified from 3694 households in the National Household Sampling Frame. From May 1994 to December 1996, interviewers enumerated selected households, and women ages 15 to 82 years who had been residents of the Mexico City area for at least 1 year were eligible for this study. A limited number of women were selected from each household to participate as control subjects for a cervical cancer case–control study.
All the participants were frequency age-matched to invasive cancer cases ascertained by the cervical cancer registry in Mexico City, as previously described. 15 Of the households visited, 28.4% had no women eligible to participate in the study. Of the women eligible for the study, 86.4% consented to participate. Interviewers administered a standardized questionnaire to collect information on sociodemographic variables including sexual behavior, reproductive and gynecologic history, family planning use, smoking, and current vaginal douching.
After informed consent was obtained from the women, blood samples were taken for the determination of antibodies to HSV-2. By venipuncture, 7 ml of peripheral blood was obtained using Vacutainer SST tubes (Becton-Dickinson, Rutherford, NJ). After clot retraction was allowed to occur at room temperature, the tubes were centrifuged at 700 g for 10 minutes. Sera samples were refrigerated for shipping to the National Institute of Public Health in Cuernavaca, Morelos State, Mexico, where they were kept frozen at −20 °C until tested.
Type-Specific Herpes Simplex Virus Type 2 Serology
Type-specific immunoblot assay was performed blindly to determine HSV-2 serostatus. The antigens were produced in Sf-9 insect cells infected with a recombinant baculovirus expressing the gG-2 gene, 16 as developed at the Centers for Disease Control and Prevention (CDC). The protocol used for these studies has been described elsewhere, 17 and had been used previously for other surveys in Mexico. 11,14
For the evaluation of HSV-2 risk factors, age-adjusted analyses were calculated for all variables. Variables measured on a continuous scale were explored in terms of their original distribution and categories. A three-level index of socioeconomic status was developed using a compilation index based on the following variables: number of persons living in the house, number of rooms (excluding the kitchen and bathroom), availability of drinking water, sanitary conditions, and education of the most recent male sexual partner. To evaluate the association between HSV-2 infection and specific risk factors, odds ratios and 95% confidence intervals were calculated on the basis of multiple logistic regression to control for potential confounding factors (STATA 5.0, Stata Press, College Station, TX). A test for linear trend was performed using the likelihood ratio test under the assumption of a linear relation.
Analysis was performed for all women and stratified by two age categories: younger than 50 years and 50 years of age or older. The prevalence of HSV-2 in the general population was estimated from the prevalence in the sample using a Horvitz-Thompson type of estimating function 18 based on the age distribution of female residents in Mexico City.
Characteristics of the Population
Altogether, 730 women with a median age of 46 years (range, 15–82 years) participated in this study. Table 1 shows that most of the women surveyed were married (70.4%), had less than 6 years of education (74.5%), and reported only one lifetime sexual partner (82.6%). Women reported a median age at first intercourse of 19 years. Almost all the participants (97.4%) reported a previous pregnancy and participants had a median of four children. A smaller proportion of participants had high socioeconomic status (16.8%) or reported a history of smoking (19.9%), oral contraceptive use (23.6%), or condom use (3.8%).
Herpes Simplex Virus Type 2 Prevalence in Relation to Age
The HSV-2 seroprevalence among participating women was 29.8%. Figure 1 shows that there was a marked trend of increasing HSV-2 prevalence with each higher level of age (P < 0.001). Women 60 years of age or older (45%) were more than four times as likely to be HSV-2 seropositive than women younger than 30 years of age (10.3%;P < 0.001). The prevalence of HSV-2 antibody did not differ significantly when women 50 to 59 years of age were compared with women 60 years of age or older (P = 0.4).
To determine whether the higher HSV-2 seropositivity among older women could result from potential confounding factors, multiple logistic regression was used to compare the prevalence of HSV-2 antibodies among stratified age groups after controls were used for the following potential confounders: marital status, the lifetime number of sexual partners, and current vaginal douching (Table 2). Women ages 50 to 59 years were four times more likely to be HSV-2 positive than women younger than 40 years (OR, 4; 95% CI, 2.4–6.8), and women 60 years of age or older had approximately a fivefold risk (OR, 5.3; 95% CI, 3.2–9.0). The trend in odds ratios for HSV-2 seropositivity was statistically significant (P < 0.001).
Risk Factors for Herpes Simplex Virus Type 2 Seropositivity
The multivariate analyses for the sociodemographic risk factors of HSV-2 seropositivity are shown in Table 3. Stratified analyses by age (younger than years and 50 years or older) were conducted to determine whether HSV-2 risk factors differed by age. Women who reported cohabitation with their partners were at a higher risk for HSV-2 seropositivity than married women (OR, 2.5; 95% CI, 1.3–4.7). This association was present among women younger than 50 years (OR, 3.4; 95% CI, 1.6–7.5), but was not observed among women 50 years of age or older. Neither a woman’s reported level of education attainment nor her socioeconomic status was significantly associated with HSV-2 seropositivity. Women who reported a history of smoking were not at an increased HSV-2 risk. In the age-stratified analysis, women younger than 49 years with a history of smoking had a 90% increased risk of HSV-2 seropositivity, as compared with women who had never smoked (95% CI, 1.1–3.4), although women ages 50 years or older with a history of smoking had no increased HSV-2 risk (OR, 0.8; 95% CI, 0.4–1.6).
Table 4 shows the sexual and reproductive risk factors for HSV-2 seropositivity, as stratified by age. As shown, HSV-2 antibody prevalence was not significantly associated with a woman’s age at first intercourse and number of live births. However, HSV-2 seropositivity was significantly associated with other markers of sexual behavior. Women with a history of two or more sexual partners before their first pregnancy had a higher HSV-2 risk than women who reported only one partner (OR, 2.3; 95% CI, 1.4–3.7). This association was strong among women younger than 49 years (OR, 4.6; 95% CI, 1.4–14.4), but was not observed in women 50 years of age or older (OR, 1.3; 95% CI, 0.7–2.6). A woman’s lifetime number of sexual partners was a predictor of HSV-2 antibodies, both among the participants as a whole (OR, 2.2; 95% CI, 1.4–3.4) and in the age-stratified analyses. Women who reported current vaginal douching had a higher seropositivity (OR, 1.7; 95% CI, 1.2–2.6). This association was observed in women younger than 49 years (OR, 1.9; 95% CI, 1.1–3.3), but did not reach statistical significance among the small number of women 50 years of age or older (OR, 1.6; 95% CI, 0.9–3.0). Previous reported use (ever/never) of oral contraceptives, condom, or intrauterine devices (IUDs) was not significantly associated with HSV-2 seropositivity (data not shown). Finally, a significant correlation was found between a women’s lifetime number of sexual partners and the following two variables: age at first intercourse (r = 0.6) and the number of sexual partners before first pregnancy (r = 0.9).
Estimation of Herpes Simplex Virus Type 2 Seroprevalence in the Mexican General Population
The estimated overall “point” prevalence of HSV-2 in the general Mexican population was 35.8% (95% CI, 34.8–36.8).
This is the first population-based study of type-specific HSV-2 seroprevalence among women in Mexico. Approximately one third (29.8%) of the 730 middle-age women surveyed in Mexico City had HSV-2 type-specific antibodies, demonstrating an estimated HSV-2 population-based overall seroprevalence in Mexico City of 35.8%. Previous epidemiologic studies have shown that HSV-2 seropositivity varies widely, largely in relation to the geographic location of the study, the type of participants surveyed, and the specificity of the HSV-2 serologic assay used. 19–21
Few population-based surveys have been performed to determine HSV-2 type-specific seroprevalence and risk factors in Latin America. In a population-based study among Costa Rican women, HSV-2 seroprevalence was reported to be 39.4%22 among women with a mean age of 41 years, which is similar to the 35.8% overall population-based prevalence obtained in this study of middle-age Mexican women. Fleming et al 2 reported a lower overall HSV-2 seropositivity of 25.7% among Mexican Americans surveyed in a population-based study in the United States, although their reported HSV-2 prevalence among women ages 40 to 49 (33%) was comparable with the current findings among women with a median age of 46 years. Among these middle-age Mexican women, HSV-2 seroprevalence reached a plateau between 50 and 59 years of age, later than among Costa Rican women or Mexican American women, among whom HSV-2 seropositivity leveled off between 40 and 49 years of age in these countries.
In Mexico City, HSV-2 seroprevalence was notably high among predominantly monogamous women. Given that HSV-2 antibodies persist over a lifetime and represent a cumulative measurement of past exposure, this data suggest that Mexican population-based HSV-2 prevalence is high. This is consistent with the notion that HSV-2 is circulating widely in this Mexico City population, in contrast to nonendemic areas where HSV-2 seroprevalence is low among similar low-risk groups. 23,24
Generally, genital HSV-2 infections are transmitted by an asymptomatic sexual partner, who may have acquired his or her herpetic infection many years before. Therefore, the risk of HSV-2 transmission may be unexpectedly high even in long-term monogamous relationships. 6 The notably elevated HSV-2 prevalence among the older monogamous women in the current study may be partially related to the high-risk sexual behavior of their male sexual partners. 25 However, no information on male sexual behavior was obtained in this study.
Independent risk factors for HSV-2 seropositivity among all the participating women from Mexico City included the following: older age, a greater number of lifetime sexual partners or more sexual partners before first pregnancy, cohabitation with their partners, and current vaginal douching. The findings showed HSV-2 seroprevalence to be significantly higher among women with the history of two or more sexual partners. These results are similar to those obtained from studies of college students, 26 women attending primary care clinics, 27 and voluntary blood donors, 28 among whom a increase in HSV-2 seroprevalence correlated well with the greater number of lifetime partners.
Among women younger than 50 years, HSV-2 seroprevalence was significantly associated with cohabitation rather than marriage. Women who reported cohabitation with their partners did not have a significantly higher number of lifetime sexual partners than other women in the sample.
Current vaginal douching also was associated with an increased risk of HSV-2 after controls were used for potential confounders, notably among women younger than 50 years of age. Given the unclear temporality of events, it is possible that vaginal douching may be a consequence of a genital herpetic infection rather than a risk factor as such. Consistently, vaginal douching has been correlated with gynecologic symptoms and has been proved more frequent among women with sexually transmitted diseases. 29,30
Similar to other reports, the current data did not show an association between HSV-2 and a younger age at first intercourse. 31 However, HSV-2 was strongly associated with having two or more sexual partners before first pregnancy, with more significant association among women younger than 50 years of age.
The prevalence the condom use among Mexican women was notably low (2.4%). Condom use was not significantly associated with HSV-2 seropositivity in the multivariate model, although HSV-2 seropositivity was lower among female condom users (17.9%) than among nonusers (29.8%;P = 0.04). Previous reports have suggested that condom use may decrease the risk of acquiring HSV-2 infections. 32 However, the cross-sectional nature of this study made it impossible to determine whether condoms reduce HSV-2 risk. Given that condoms reduce the risk of HIV and other sexually transmitted diseases, promoting condom use also may be an appropriate means of reducing HSV-2 transmission on a population basis.
An unexpected result indicated that smoking was independently associated with HSV-2 infection among healthy women younger than 49 years of age. Although this association may represent residual confounding by sexual behavior, it has been reported that smoking may suppress the immune response in HIV-positive individuals. 33
In summary, population-based HSV-2 seroprevalence is endemically high among middle-age women in Mexico City and clearly is correlated with higher-risk sexual behavior. Genital herpetic infections are characterized by asymptomatic shedding throughout life, causing unrecognized illness that generally is not treated. Therefore, the elevated HSV-2 seroprevalence among women in Mexico City may be attributable to unrecognized HSV-2 viral transmission throughout life. As an adjunct to the potential use of prophylactic vaccines or antiviral therapy for genital herpes prevention efforts, sexually transmitted disease prevention efforts should be strengthened to improve access to information on unrecognized HSV-2 transmission. In addition, capacity should be built for the detection and clinical management of genital herpetic infections, which currently are not being monitored in Mexico.
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