Determinants of STD/HIV risk among adolescents

Adolescence is a unique time of life, when societal, biologic, behavioral, and developmental factors all act in together to increase the rate of STD/HIV contraction.


Biological factors


Several aspects of physical development may be relevant to the high risk of STDs among sexually active adolescents. The histology of the cervix and vagina undergo dramatic changes from childhood through puberty and into adulthood. Newborns show effects of exposure to maternal estrogen, which produces the squamous epithelium lining of the vagina as seen in adults. Soon after birth, these squamous cells are replaced with columnar epithelium. Such epithelial changes may be particularly important at the cervix, since persistence of cervical epithelium in young women appears to significantly increase their their vulnerability to STDs. Although cervical columnar epithelium eventually recedes completely, to be replaced with squamous epithelium, the replacement is a gradual process, continuing well into adulthood. Typically the cervix in the adolescent still displays areas of exposed columnar epithelium, a condition often referred to as ectopy. This is significant because C. trachomatis infects columnar, not squamous, epithelium and thus ectopy may increase exposure of susceptible mucosa to infection.

The presence of ectopy has repeatedly been associated with chlamydial infection, even after adjusting for sexual behavior and other confounders. Although it may be that chlamydial infection causes the appearance of columnar epithelium on the cervix, longitudinal studies have demonstrated that ectopy is associated with increased risk of subsequent infection. However, whether ectopy facilitates the detection of chlamydia is still the question.

The presence of ectopy also appears to increase the risk to other STDs and their adverse outcomes. Neisseria gonorrhoeae attaches preferentially to columnar epithelium rather than squamous tissue. In addition, there is some evidence that ectopy may contribute to HIV acquisition and HIV shedding. The vasculature found with the column epithelium associated with ectopy is more superficial and more easily traumatized than that of squamous epithelium, theoretically permitting HIV-infected cellular elements from the circulation to gain access to mucosal surface, infected monocytes and lymphocytes to reach the circulation. Ectopy may partially account for the high incidence of HIV among adolescent women by increasing susceptibility; the implications of possibly increased infectivity, associated with HIV shedding, are unknown. However, some studies have not found an association between HIV and ectopy. The vaginal flora also changes during puberty. The appearance of Lactobacillus spp results in the reduction of the high vaginal pH levels of childhood to the more acidic pH associated with adulthood. The higher vaginal pH of early adolescence may be associated with a lower prevalence of hydrogen peroxide-producing organisms. However, the casual links between these anatomic or physiologic changes and STD contraction have not yet been demonstrated.

Changes in mucosal anatomy produce changes in mucus production, which is minimal in childhood. Mucus production is greatly increased in early puberty, but the mucus is thinner than that found in older adolescents or adult women. Thinner mucus may permit organisms to penetrate more easily and to attach to mucosal sites or gain access to the upper tract.

Research has suggested  that adolescent females experience a greater decline in IgG levels in cervical secretions during menstrual cycles than adult women, but the implications of such a finding for disease contraction are unknown. There are many aspects of immune protection of the female reproductive tract, such as expression of chemokines and cytokines, or expressions of CCR5 and CXCR4 receptors, that my function differently among young women, but such differences have not yet been documented and need to be elucidated. Unfortunately, there is very little information about how male development affects their risk of STD contraction and transmission.


Psychological and cognitive development


The stages of adolescence have been arbitrarily categorized as “early,” “middle,” and “late,” and have been considered in terms of psychologic, physiologic, and social development. Development in each of these areas is not necessarily parallel. Individuals are often advanced in some categories, but slower than their age-matched peers in others. Futhermore, growth in some of the cognitive areas is strongly influenced by the quality of teaching or role-modeling individuals’ experience. This is particularly relevant for STD prevention, where adults may use indirect methods of educating or rely on “scare tactics” rather than using skills training. Several characteristics of adolescents, particularly in early or middle stages of development, may have important implications for STD risk and prevention. Younger adolescent frequently use a concrete style of reasoning, focusing on the present time, and until they reach middle or late adolescence are unable to conceptualize the long-term impact that current actions may have. Some some STDs (e.g. HIV or chlamydia) may have adverse effects that are not experienced for a decade or more, it should not be surprising that younger adolescents may not take actions needed to avoid such consequences. Furthermore, adolescents may have difficulty correctly implementing complex tasks (such as condom use) involving a series of steps that must be accomplished in a certain sequence to be effective.

These findings are consistent with ongoing research concerning cognitive development. Significant structural changes in the brain continue into the later teenage years, and the development of an “executive suite” f upper level decision-making functions is a lengthy process, requiring time and experience. Pubertal development and its attendant hormonal milieu appear to fire romantic motivations and an appetite for risk taking, independent of chronological age. However, self-regulation and competent decision making while under emotional stress (i.e., in the throes of adolescent passion) develop slowly and continue well after puberty. The occurence of puberty at younger ages is, as described by one researcher, like starting a car engine with an inexperienced driver at the well. The fact that adolescents are not fully prepared to handle the situations they find themselves in is a reason why it is important to provide appropriate “social scaffolding,” and may explain why parental monitoring appears to be an effective prevention strategy.

Finally many parents, educators, and health-care workers do not teach about STD risk or even details of pubertal development until long after many adolescents are at risk for STDs. Therefore, these youths do not have even the basic information to make informed choices.


Sexual behavior


Over the last century, sociocultural and behavior changes have combined with changes in aspects of the developmental physiology of adolescents to increase the risk of STDs among these young people. Biologically, the average age at menarche has decreased (although it has been stable over the last generation). At the same time, societal changes have resulted in increases in the average age at which young men and women marry. As a result, while 100 years ago young men in the US spent approximately 7 years between maturation and marriage, more recently the interval was 13 years, and increasing; for young women, the interval between menarche and marriage has increased from 8 to 14. For this reason alone, it should be expected that premarital sex in the US has increased.

Changes in sexual behavior have placed adolescents at increased risk of STDs, with the longstanding trend to earlier age at first intercourse occurring worldwide. However, in the US, several ongoing, population-based surveys that provide information about the sexual behavior of adolescents show that during the past decade, the proportion of teenagers that have experienced premartial sexual intercourse has declinded. This change in behavior reveres the trend seen over the previous several decades in the United States, in which, the age of first intercourse had steadily decreased. in 1970, only 5% of women in the US had premarital intercourse by age 15, whereas in 1988, 26% had engaged in intercourse by this age. However, in 1988 37% of never married 15-17-year-olds had egaged in intercourse but in 2002, only 30% had. Comparable data from males demonstrated even greater declines-50% of never married 15-17-year-olds reported having had intercourse in 1988, compared with only 31% in 2002. Consistent with these U.S. data, between 1991 and 2000, the pregnancy rate among US females (16-17 years) declined by 33%. Similarly, the percentage of 9-12th graders who reported having had four or more sexual partners also declined from 19% in 1993 to 14% in 2003. This trend is not universal; no such decline was reported in Sweden.

Younger age of sexual “debut” is associated with a greater number of sexual partners–an important determinant of STD risk. In 1988, among American women 15-24 years of age who were sexually active for the same length of time (less than 24 months), over 40% of 15-19-year-olds had two or more partners, compared with only 26% of women 20 years of age or older. Younger age of sexual debut (below age 18) is also associated with ongoing sexual risk among unmarried women, with an increased likelihood of have two or more recent partners. Adolescent relationships are of shorter duration than those of older adults. Duration on average is 15 months, and many relationships are  less than 4 months. It is becoming clear that determinants of STD risk at both an individual and population level include other factors such as concurrency and sexual mixing patterns, which involve assessment of sexual network structure and should be considered regarding adolescent STD risk. Nevertheless, partner acquisition tends to follow a pattern of serial monogamy, with one study finding that fewer than 10% of sexually active adolescents having more than one partner within a 3-month period. In another study, among adolescents reporting activity in the previous 18 months, over 85% indicated that they had either a single or sequential relationships. However, among serially monogamous females of all ages, adolescents are quicker to acquire a new partner than older women; in a national survey, mean gap between partners was 8 months for serially monogamous 15-19-year-old women, compared with 11 months and 18 months for 20-29 and 30-44-year old women, respectively. (Some smaller studies among higher risk cohorts of adolescent have found mean interval between old and new partners to be as short as 21 days). It should be noted, however, that among those adolescents who had more than one partner, concurrency is rather frequent; among adolescents who reported more than one partner in the previous 18 months, over 40% had overlapping sexual relationships.

Although concurrency is expected to have population level effects, implications for individual risk are less clear, since concurrency may or may not be associated with increased individual STD risk. Furthermore, risk for the individual adolescent may be affected by the likelihood that one’s partner has concurrent partners, which appears related to whether one lives in a high-risk community or not.

Recent studies provide some insight into sexual (and romantic) network structures among adolescents. As part of the AddHealth, data were obtained from all students in a large high school and all the romantic and sexual relationships among those students identified. The relationship structure was described by the researchers as a “spanning tree,” similar to “rural phone lines running from a long trunk to individual house.” Such a structure does not reflect random partner selection, but rather a situation in which there are rules about who can be involved with whom. In addition, although there were few “short cycles” (i.e., cyclical pattern which promptly leads back to the same individual), there was a large network component that linked over half of the adolescents involved in romantic relationships. Such a structure would allow efficient spread of an STD across the network, and, as the authors point out, demonstrates that STD risk in not simply defined by the number of partners. Moreover, the structure has implications for STD prevention. Given the existence of a “spanning tree” network, effective prevention efforts need to target individuals in the “core.” A prevention approach that reaches all adolescents-and thus breaks the “spanning tree” into smaller, unconnected clusters-might be even more effective. Of course, this is a single study, performed in a middle-sized town, in the Midwest among working-class population, with limited diversity; and the network analysis did not include individuals who were not high-school attendees; therefore the extent to which its results are generalizable is unknown.

Population-based data demonstrate that condom use has increased substantially in the United States, but that use is not consistent. Surveys indicate that adolescents are more likely to use condoms than older individuals. In 2002, 66% of male teens reported using condoms at last intercourse compared with 53%, 45%, and 29% of males 20-24 years, 25-29 years, and 30-44 years, respectively. In addition, 67% of US women reported condom use at first intercourse if it occurred during 1990-2002, while only 36% of women 15-44 years of age reported condom use at first intercourse that occurred before 1990. Condom use among adolescents in general has increased. In 2002, among never married sexually active 15-19-year-old females, 54% used condoms at last intercourse, compared to 38% and 31% in 1995 and 1998, respectively. Similar increases are reported by adolescent males; in 2002, 71% of sexually active, never married 15-19-year-old males reported use of condoms at last intercourse, compared with 64% and 53% in 1995 and 1988, respectively.

However, follow-up data indicate that as males get older and as the duration of the existing relationship increases, condoms are likely to be replaced by forms of contraception that offer less protection against STDs, particularly oral contraceptives. This pattern has also been reported from other countries, such as Australia, Canda, and New Zealand, It is noteworthy that the combined effect of serial monogamy and diminishing use of condoms over the duration of a relationship may be particularly important in exposure to and ongoing transmission of organisms such as HSV-2, chlamydia, HPV, and HIV, which are associated with chronic and often asymptomatic infection.

Although more people are using condoms, few people, including adolescents, use them consistently. Among unmarried US women, 28% of 15-19-year-olds report using condoms consistently (i.e., all the time) over the past 12 months; 48% of 15-19-year-old males report doing so. Not surprisingly, both teenagers and older individuals use condoms less frequently with partners in an ongoing relationship, than in sexual encounters outside that relationship. Furthermore, the young people most at risk for STDs appear to use condoms least. A survey among adolescent males indicated that those who were substance abusers or had paid for sex were among the least likely to have used condoms at last intercourse. Young males were less likely than older males to use condoms at first intercourse with partners they perceived to be at higher STD risk.  Among young people with two or more partners, individuals with a greater number of partners were less likely to use condoms consistently with either primary or secondary partners. A similar pattern has been observed in other countries.

Use of condoms is complex behavior, but we can make some generalizations about determinants of condom use upon which to base prevention strategies; most parallel determinants noted among adults. Many studies, but not all, indicate that use is associated with perceived risk of HIV infection. Youths who think their peers use condoms are more likely to use them, and adolescent males who are more able to communicate with their parents and with peers. Adolescents are often mistaken about what their partners believe, however, with females overestimating the resistance and negative attitudes that males have about condom use. In addition, when condoms are used, adolescents frequently experience errors in their use. In one study, among adolescent females who used condoms in the prior 3 months, at least one condom error was reported by 71%. National survey data indicates that misconceptions about condom use (i.e., need for space at tip; use of vaseline; protection with lambskin vs. latex condoms) are quite prevalent, with up to 50% of adolescents being mistaken about some of these issues.

Studies have noted that self-efficacy, perceived risk, and partner support are important factors in increased condom use. However, many young females who feel confident that they could get their partner to use condoms, are not motivated to use them. An important factor appears to be the extent to which young people underestimate their partners’ risk of infection. Young homosexual males believe that they are safe if they have sex with younger partners. Heterosexual females often feel that they have little or no risk of acquiring HIV from their male partners and believe their boyfriends’ statements of fidelity, often despite have a history of STD themselves. Another major concern is the belief that their partners, particularly their steady partners, would view the request to use a condom as indicating a lack of trust. Conversely, if the request for use is made by the male, the female may assume he is dating outside the relationship.  Approaches to reconciling these issues are complex and require skillful and practiced communication, as well as interventions suitable for sexually active adolescents who are in the formative phases of social skill development. Other barriers to condom use that are unique to adolescents include lack of ready availability. Embarrassment about purchasing condoms may be a particular obstacle for girls.

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