SEXUALLY TRANSMITTED INFECTIONS (STIs) are currently considered to be a major public health problem, 1,2 particularly for young people. One of the main STI agents is herpes simplex virus type 2 (HSV-2). As a cause of genital ulcers, 3 this virus can persist throughout the affected person's life with periods of reactivation, producing symptoms and increasing the viral load. 4 However, HSV-2 infection can also behave asymptomatically in population groups with high HSV-1 prevalence, probably because of a cross-immunity phenomenon. 5,6 In the United States, during a 20-year period, 7 a fivefold increase in HSV-2 seroprevalence has been observed among adolescents. It is estimated that close to 5.6% of the young people aged 12–19 years are HSV-2-infected. 8 The strong association between HSV-2 prevalence and sexual behavior in different studies 9 suggests that age-specific HSV-2 prevalence may, in some circumstances, provide a useful marker for sexual behavior.
In Mexico, most studies of HSV-2 prevalence have been conducted in groups that are considered high-risk populations, such as sex workers, employees in bars where sex work takes place, and general populations considered at risk of infection. 10–14
Little research has been done on STIs in adolescents in Mexico. 15,16 In this context, the objective of this study was to quantify HSV-2 seroprevalence and identify possible associated factors in a population-based sample of students aged 11–24 years old in the state of Morelos, Mexico.
Materials and Methods
A cross-sectional population-based study was conducted among students in Morelos State, Mexico, during 2000. We developed a sample framework of junior high, high school, and college facilities. Schools were the sampling unit, all of which had the same selection probability. The sampling framework comprised 260 junior high schools, with a school population of 60,686 students; 92 high schools, with 18,067 students; and a public university with 15 schools and a total of 7452 students. All schools corresponded to the 72 administrative areas of the 33 municipalities in the State of Morelos, in rural, semiurban, and urban areas. From this framework, 13,293 adolescents and young adults, both male and female, aged 11 to 24 years, were randomly selected.
After signing informed consent, the students who had agreed to participate in the global study answered a self-administered questionnaire at school. The response rate was 98.6%. A blood sample was obtained from only 55% of the population under study because of logistic and budgetary reasons.
Finally, 898 sexually active students participated in this survey. They gave background information about their sex life and agreed to provide a blood sample. The participants were not aware of the research hypothesis. The survey included different sections, of which the following were taken into account in this study: (1) sociodemographic characteristics (age, sex, place of residence, school and socioeconomic level); and (2) sexual behavior (person with whom they had their first sexual relation, condom use, number of partners throughout their life, having same-sex relations, and the time elapsed since they initiated sexual activity).
Sociodemographic characteristics and histories of sexual activity were similar (P > 0.05) for students who did and did not have a blood sample obtained (data not shown).
Serum Sample Collection, Transport, and Conservation
After getting informed consent from each participant, a 10-ml blood sample was taken through venous puncture with use of the Vacutainer vacuum tube system with a separation gel (Becton-Dickinson, Cockeysville, MD). The tubes were code bar–labeled, placed in an icebox with gel coolers, and transported every day from the field work sites to the laboratory at the Infectious Diseases Research Center at the National Institute of Public Health. Within the following 4 hours, the samples were centrifuged at 3000 rpm for 10 minutes, and 2-ml serum aliquots were stored at −70 °C until processing took place.
Specific Serology for HSV-2
The Western blot technique to detect the presence of anti-HSV-2 antibodies was used. The source of the test's antigen is a crude extract of insect cells infected with a recombinant baculovirus that expresses specific viral-type glycoprotein gG2. 17 To summarize, after separating the proteins by electrophoresis and transferring them to nitrocellulose filters, we incubated these with the serum samples and negative and positive controls. An additional incubation was carried out with the corresponding enzyme conjugate, and the results were developed with addition of the enzyme substrate. The serum samples were considered positive when they reacted by generating a band indicative of antibodies to HSV-2 glycoprotein gG2 with an apparent molecular weight of 114 kD. According to Sanchez et al., 18 this procedure has 98% sensitivity and specificity and has also been applied to other research studies in Mexico. 10-14
A socioeconomic index was constructed according to the combination of the following variables: number of people living in the house, number of bedrooms, presence of tubed water and drainage, and availability of a house of their own, stationary gas, refrigerator, telephone, television, VCR, and car. The socioeconomic index was configured as a dichotomous variable (low = 0, medium and high = 1).
For analysis of the characteristics of sexual behavior in this population group, we developed a condom use index (Do you use the condom? Answers: always, sometimes, never, both with your stable partners and with occasional partners). The condom use variable was grouped dichotomously (yes = consistent condom use, no = inconsistent condom use or condom never used).
A nonconditional multivariate logistic regression analysis was used to evaluate risk factors associated with HSV-2 infection. The odds ratio (OR) and its 95% confidence interval (CI) were estimated and adjusted for age and same-sex sexual relations. Stata 6 software was used for the statistical analysis (Stata Corporation, College Station, TX). 19
The sample included 898 sexually active students. Mean ages of males and females were 18.1 and 17.5, respectively, with an age interval of 11 to 24 years. Table 1 shows this and other characteristics. Men constituted 57.6% of the sample, and women, 42.4%. Most participants (54.5%) were ≥18 years old; 428 (51.1%) were high school students; 544 (60.6%) lived in an urban area in the state of Morelos; and 492 (51.8%) were classified as having a low socioeconomic status.
Students who reported having initiated sexual activity with their boyfriend or girlfriend or their husband or wife accounted for 62.4%. Mean age at which sexual activity was started was 15.4 years, with a range of 10 to 21 years. Those students who reported having had one single sex partner accounted for 54.3%. Likewise, 77.4% did not use the condom consistently or practiced sex without ever using the condom. Of the population under study, 4.9% had had same-sex sexual relations, which was twice as high among men than among women. The prevalence of anti-HSV-2 antibodies among those surveyed was 5.7% and was lower among men (4.1%) than among women (7.9%).
Sexual Behavior by Education Level
There were significant differences in age at first intercourse in relation to schooling level (Table 2). In this regard, students in junior high school reported an earlier age at first intercourse than did students in high school and at university (P < 0.001). In contrast, the number of sex partners increased with age (P < 0.001), and the prevalence of consistent condom use was related to higher education level, especially for female partners (females, P < 0.001; males, P > 0.05). Likewise, the HSV-2 seroprevalence was higher among the youngest female students (9.5% in junior high school versus 3.3% in university; P < 0.001); this difference was not statistically significant for male students (6.1% in junior high school versus 3.0% in university; P > 0.05).
Participants who had ≥1 year of sexual experience had a 2.8 times higher risk of HSV-2 seropositivity (95% CI, 1.2–6.4), regardless of their educational level (data not shown).
HSV-2 Seroprevalence by Education Level
As can be seen in Figure 1, there was an inversely proportional relation between HSV-2 infection and schooling level (P < 0.01). When students were stratified by sex, HSV-2 seroprevalences were lower among males.
Sociodemographic Data and Sexual Behavior Associated with HSV-2 Seropositivity
Table 3 shows data from the multivariate analysis that includes the following factors: sociodemographic data, sexual behavior, and HSV-2 seropositivity. The sample was stratified by sex, with adjustments for age and having had same-sex sexual relations, in order to determine whether these HSV-2 risk factors differed or not. HSV-2 seropositivity presented a significant association with females: the possibility that they would be positive for HSV-2 was 2.2 times higher than for males (95% CI, 1.2–3.9). The presence of anti-HSV-2 antibodies also had a significant association with lower schooling levels. The possibility that adolescents in junior high school would be infected with HSV-2 was 2.5 times higher than that for undergraduate students (95% CI, 1.1–6.0).
In an analysis of markers for sexual behavior associated with HSV-2 seropositivity, students who reported having had same-sex sexual relations had a greater possibility of being HSV-2-positive than did those who reported having had no intercourse of this kind (OR, 3.5; 95% CI, 1.4–8.9). Furthermore, in analysis of this variable by sex, the possibility of infection was five times higher among males (95% CI, 1.7–14.7).
Sexual Behavior Factors Associated with HSV-2, Stratified by Schooling Level
Table 4 shows sexual behavior factors associated with HSV-2 seropositivity, stratified by schooling level. Females had a greater possibility of being infected than did males, particularly females in high school (OR, 2.6; 95% CI, 1.1–5.9). Having had same-sex sexual relations was a risk factor for having anti-HSV-2 antibodies, both for junior high students (OR, 5.1; 95% CI, 0.9–29.0) and for undergraduate students (OR, 8.3; 95% CI, 1.5–46.7), in comparison with those who did not have relations of this kind. Junior high adolescents who used condoms inconsistently or never used them and had begun having sexual intercourse within the previous 2 years had 9.1% HSV-2 seroprevalence, whereas anti-HSV-2 seroprevalence was 0% among consistent condom users. Of these subjects, 19 had started having sexual intercourse within the previous couple of years and the remaining 16 had started at least 2 years earlier.
This is the first study in Mexico addressing the school youth population that has analyzed risk factors associated with seropositivity against HSV-2.
In different low-risk 7 and high-risk 20 populations of adolescents, HSV-2 seroprevalence has been significantly higher among females than males, as was shown among these students in Morelos State, Mexico. Prevalence by sex (males, 4.1%; females, 7.9%) showed women had a greater possibility of being HSV-2-positive, which is consistent with findings reported in the literature. 21 It has been seen that adolescents, particularly female, tend to have older sex partners, a circumstance which implies a greater possibility of becoming infected. 22 This suggests that contact with a partner who acquired the infection in the past, possibly in an asymptomatic way, permits transmission of infection even if the relationship is monogamous. 23 In addition, it has been hypothesized that female adolescents show a greater vulnerability to HSV-2 infection because of biologic characteristics related to the epithelium along the lower female genital tract, which increases the likelihood that exposure to an infectious agent may lead to transmission of the disease. 24 In this context, it is important to note that in Latin America and the Caribbean, 15% of all adolescents aged 15–19 years have acquired an STI. 24
In contrast with other reports that the risk of HSV-2 infection increases with age, this study showed that particularly among female students in Morelos State, Mexico, the HSV-2 seroprevalence was higher in younger people. This finding can be explained by a cohort effect of more exposition to behaviors risks. Thus, the students in junior high school were younger at age of first intercourse (mean, 13.3 years) than were high school students (15.6 years) and university students (16.9 years). In addition, the prevalence of consistent use of condoms was very low among female partners (6.3%) of the youngest students, in comparison with that among the youngest male students (25.2%; P < 0.001); this could possibly explain the higher HSV-2 seroprevalence among young females. In contrast, the number of sex partners increased with age. Actually, students with five or more lifetime sex partners had a 3.8 times higher risk of HSV-2 seroprevalence (95% CI, 1.6–8.8) in a multivariate analysis (data not shown).
Adolescents and young adults participating in this study tended to have initiated sex at an increasingly earlier age; they reported a mean initiation age of 15.1 years (males) and 15.8 years (females), data which might be subject to controversy since other studies in Latin American countries have shown age at first intercourse to be between 16 and 17 years. 23 This study on the state of Morelos showed 95% of the participants of both sexes initiated sexual activity before reaching age 20 years, whereas 10 years ago in the United States, 86% of the men and 77% of the women had begun sexual activity before that age. 25
In Morelos State, Mexico, HSV-2 seroprevalence was lower among female students who reported consistent condom use than among those who reported never using condoms. This fact suggests that condoms may have a protective effect against the sexual transmission of HSV-2. 26
A high percentage (77.4%) of the population under study reported not having used condoms or to have used them inconsistently. This figure is similar to that in a report on technical and professional students in Cuba, where the prevalence of condom nonuse was 82.6%. 27 This prevalence in this mostly rural region of Mexico was higher than that reported in urban areas of Mexico City, where the prevalence of condom nonuse among high school and undergraduate students was 57.8% for males and 64.5% for females. 28 It has been suggested that condom use might reduce the risk of acquiring HSV-2 infections. Promoting condom use might be an appropriate measure to reduce transmission of this infection among adolescents in Morelos State. 29
This study did not find a dose-effect response between sexual activity and risk of HSV-2, as reported in other studies. 30,31 Previous reports have shown that in communities with high rates of prevalence of HSV-2, individual risk behavior is not an important predictor of STI acquisition, given that the probability of having an infected partner is very high, even among those with one partner. 8
It has been established that most people become infected with HSV-2 through penis-vagina contact. However, oral-genital contact and anal sexual intercourse may be other means of transmitting STIs, particularly HIV/AIDS. 32 In this study in the state of Morelos, HSV-2 seroprevalence was lower among participants who reported never having had same-sex sexual relations (5.3% versus 13.6%).
A review of the scientific literature on adolescents and young adults who have had same-sex sexual relations did not show earlier studies that would allow an evaluation of the consistency of the study in the state of Morelos, since most research has been conducted among populations at risk of HIV infection or AIDS. 33
There have been reports of high seroprevalences of HSV-2 among adolescents with alcohol use disorders. 34 However, among students in Morelos State, the prevalence of alcohol use disorders (9%) and drug consumption (2%) was low; for this reason, the seroprevalence of HSV-2 was not associated with addictions.
Within this study's limitations, and because of its cross-sectional design, we interpreted the results as associations between study variables only. Therefore, causality cannot be established. Another limitation was the use of self-reported data. This could underestimate the characteristics of sexual behavior among adolescents and young adults. Possible factors of confusion were controlled, including variables related to HSV-2 infection in a multivariate model. The findings of this study show that the seroprevalence of HSV-2 infection is similar to that reported in populations considered to be at low risk and that it is clearly related to sexual behavior.
Finally, adolescence is a stage in which often sexual activity is initiated. It is therefore a high-risk period for acquiring HSV-2 and other STIs when protective measures are not taken during sex. The risk of acquiring an STI, particularly HIV infection or AIDS, is one of the most important problems observed in adolescent health. Our study consistently showed that regular condom use can reduce HSV-2 risk, at least in a junior high school population. It is thus necessary to promote healthy lifestyles from an early age, particularly in school settings. Longitudinal studies of immune responses to HSV-2 infection in different age groups are warranted.
1. DiClemente RJ. Preventing sexually transmitted infections among adolescents. JAMA 1998; 279: 1574–1575.
2. Magnani RJ, Seiber EE, Gutierrez ZE, et al. Correlates of sexual activity and condom use among secondary-school students in urban Peru. Stud Fam Plann 2001; 32: 53–66.
3. Buchacz K, McFarland W, Hernandez M, et al. Prevalence and correlates of herpes simplex simple type 2 infections in California: The Young Women's Survey Team. Sex Transm Dis 2001; 27: 393–400.
4. Corey L, Handsfield H. Genital herpes and public health: addressing a global problem. Special communication. JAMA 2000; 283: 791–794.
5. Nahmias AJ, Lee FK, Beckmhn-Nahmias, et al. Seroepidemiological patterns of herpes simplex virus infections in the world. Scand J Infect Dis 1990; 69: 19–36.
6. Langenderg AG, Corey L, Ashley RL, et al. A prospective study of new infections with herpes simplex virus type and type 2. Chiron HVS Vaccine Study Group. N Engl J Med 1999; 341: 1432–1438.
7. Fleming DT, McQuillan GM, Johnson RE, et al. Herpes simplex virus type 2 in the United States 1976–1994. N Engl J Med 1997; 337: 1105–1111.
8. Sucato G, Celum C, Dithmer D, Ashley R, Wald A. Demographic rather than behavioral risk factors predict herpes simple virus type–2 infection in sexually active adolescents. Pediatr Infect Dis J 2001; 20: 422–426.
9. Cowan FM, Johnson AM, Ashley RL, Corey L, Mindel A. Antibody to herpes simplex virus type 2 as serological marker of sexual lifestyle in populations. BMJ 1994; 309: 1325.
10. Lazcano-PE, Smith JS, Munoz N, Conde-GC, et al. High prevalence of antibodies to herpes simplex virus type 2 among middle-aged women in Mexico City, Mexico: a population-based study. Sex Transm Dis 2001; 28: 270–276.
11. Uribe F, Hernández M, Juárez L, et al. Risk factors for herpes simplex virus type 2 infection among female commercial sex workers in Mexico City. Int J STD AIDS 1999; 10: 105–111.
12. Conde-González C, Juárez-Figueroa L, Uribe-Salas F, et al. Analysis of herpes simplex virus type and 2 infection in women with high-risk sexual behavior in Mexico. Int J Epidemiol 1999; 28: 571–576.
13. Uribe-Salas F, Conde-González C, Hernández-Ávila M, et al. Low prevalences of HIV infections and sexually transmitted disease among female commercial sex workers in Mexico City. Am J Public Health 1997; 87: 1012–1015.
14. Uribe-Salas F, Hernández-Girón C, Conde-González C, et al. Características relacionadas con ETS/VIH de hombres que trabajan en bares de la ciudad de México donde se ejerce la prostitución femenina. Salud Publica Mex 1995; 385–394.
15. Loranca SE, Martínez MAJ, González GAG, et al. Enfermedades transmitidas por contacto sexual en la edad pediátrica. Infectología (Mex) 1990; 14: 203–207.
16. García AJL, Reyes NEL, Román CG, et al. Enfermedades transmitidas sexualmente (ETS) en un grupo de adolescentes que asisten a un Instituto de Atención Perinatal. Gac Med Mex 1993; 129: 75–79.
17. SánchezMartinez D, Pellett PK. Expression of HSV-1, HSV-2 glycoprotein G in insect cells by using a novel baculovirus expression vector. Virology 1991; 182: 229–238.
18. Sánchez Martinez D, Schmid S, Whittington W, et al. Evaluation of a test based on baculovirus expressed glycoprotein g for detection of herpes simplex type specific antibodies. J Infect Dis 1991; 164: 1196–1199.
19. Stata Corporation. Stata Statistical Software: Release 6.0. Collage Station, Texas: Stata Corporation, 1999.
20. Suligoi B, Tchamgmena O, Sarmati L, et al. Prevalence and risk factors for herpes simplex virus type 2 infection among adolescents and adults in northern Cameroon. Sex Transm Dis 2001; 28: 690–693.
21. Noell J, Rohde P, Ochs L, et al. Incidence and prevalence of chlamydia, herpes, and viral hepatitis in a homeless adolescent population. Sex Transm Dis 2001; 28: 4–10.
22. Bachanas PJ, Morris MK, Lewis-Gess JK, et al. Psychological adjustment, substance use, hiv knowledge, risky sexual behavior in at-risk minority females: developmental differences during adolescence. J Pediatr Psychol 2002; 27: 373–384.
23. Corey L, Handsfield H. Genital herpes and public health: addressing a global problem. Special communication. JAMA 2000; 283: 791–794.
24. Morris L. Young adults in Latin America and the Caribbean: their sexual experience and contraceptive use. Int Fam Perspect 1988; 14: 153–158.
25. Centers for Disease Control and Prevention. Premarital sexual experience among adolescent women in USA, 1970–1988. MMWR Morb Mortal Wkly Rep 1991; 39: 929–932.
26. Obasi A, Mosha F, Quingley M, et al. Antibody to herpes simplex virus type 2 as a marker of sexual risk behavior in rural Tanzania. J Infect Dis 1999; 179: 16–24.
27. Cortés AA, Sordo RM, Kumba AC, García RR, Fuentes AJ. Comportamiento sexual y enfermedades de transmisión sexual en adolescentes de secundaria básica de ciudad de la Habana, 1995–1996. Rev Cubana Hig Epidemiol 2000; 38: 53–59.
28. Micher Camarena MJ, Bustillos SJJ. Nivel de conocimientos y prácticas de riesgo para Enfermedades de Transmisión Sexual. Rev SIDA ETS (Mex) 1997; 3: 68–73.
29. Blake SM, Ledsky R, Lehman T, Goodenow C, Sawyer R, Hack T. Preventing sexual risk behaviors among gay, lesbian, and bisexual adolescents: the benefits of gay-sensitive HIV instruction in schools. Am J Public Health 2001; 91: 940–946.
30. Huerta K, Berkelhamer S, Klein J, Ammerman S, Chang J, Prober CG. Epidemiology of herpes simplex virus type 2 infections in a high-risk adolescent population. J Adolesc Health 1996; 18: 384–386.
31. Bunnell RE, Dahlberg L, Rolfs R, et al. High prevalence and incidence of sexually transmitted diseases in urban adolescents females despite moderate risk behaviors. J Infect Dis 1999; 180: 1624–1631.
32. Alfaro CA, Roche GR, Gutierrez MP, et al. Sida, adolescencia y riesgos. Rev Cub Gen Integr 2000; 16: 253–260.
33. Valleroy L, Mackellar DA, Karon JM, et al. HIV prevalence and associated risks in young men who have sex with men: Young Men's Survey Study Group. JAMA 2000; 284: 198–204.
34. Cook R, Pollock N, Rao A, Clark D. Increased prevalence of herpes simplex virus type 2 among adolescent women with alcohol use disorders. J Adolesc Health 2002; 30: 169–174.