SEXUAL BEHAVIOR PATTERNS within a population influence the spread of sexually transmitted infections (STIs) in that population, and linkages between sexual networks are necessary for the spread of STIs across sexual networks.1 So-called “core groups” appear to be important in the spread of sexually transmitted diseases (STDs) and in their prevention.2–4 Core groups refer to small proportions of persons with an STD who are frequently infected with and transmit the disease, and who sustain the endemic and epidemic transmission of STDs.2 Mixing between members of the “core” and the “periphery” affect the extent to which STDs spread to the general population.5 The sexual transmission of STDs and HIV beyond core groups may depend on persons who have sexual intercourse with members of core groups and with members of the general population-so-called “bridge populations.”1 Studies in Thailand revealed that large proportions of men in certain occupations, such as truck drivers, the police, and the military tend to function as “bridges” between female sex workers and their wives or girlfriends.6,7 This pattern is observed in other diverse settings, particularly in Asia. Building on this literature and contributing further to it, in this issue of Sexually Transmitted Diseases Gorbach and colleagues8 report on the extent and correlates of male sexual bridging among the military, the police, and motodrivers in five cities in Cambodia. They report that 14% to 21% of the population in these groups have unprotected sex with both high-risk and low-risk partners. This behavior was correlated with age, age at sexual debut, having friends who frequent sex workers, and residence in the port city. These correlates include demographic and behavioral attributes and social and social-environmental characteristics. This is an important study that describes the “sexual behavior map” of Cambodia-a country marked by the rapid spread of HIV and STDs9-and identifies social and social environmental factors associated with risk behaviors such as bridging. These risk factors may prove invaluable in the development and targeting of interventions aimed at preventing the spread of STIs. The study by Gorbach et al8 highlights the extent to which studies regarding the behavioral aspects of STDs have evolved, thereby becoming more sophisticated, and future steps that need to be taken in this area. Definitions: Behavioral, Biomedical, or Integrated Many studies of bridge populations and core groups6–8 define these concepts in purely behavioral terms, with little reference to levels of STD prevalence and incidence. In this approach the implicit assumption is that high-risk behaviors, including sexual mixing with high-risk groups, are highly correlated with STIs. Yet in the absence of STIs, risky sexual behaviors may not be associated with the acquisition of STDs. Similarly, if STIs were ubiquitous, any sexual behavior-even those considered very low risk-may lead to the acquisition of an STD, necessitating the independent measurement of sexual behavior and STDs.10 Even in the same society, a particular sexual behavior must be associated with different degrees of STD risk in different subpopulations, depending on the prevalence of STIs in the subpopulation.11 In addition to sexual behaviors (e.g., number of partners), preventive behaviors (e.g., condom use) and health-related behaviors (e.g., periodic screening and treatment) may influence the prevalence and incidence of STDs in populations. The effect of health-related behaviors would be particularly pronounced together with the incidence and prevalence of bacterial STDs; however, in recent years developments in antiviral therapy have increased the relevance of health behaviors for viral STDs. Because the real question is the spread of STIs in a population, it seems important to define core groups and bridge populations in terms that incorporate levels of STI incidence and prevalence. What is called for is a comprehensive mapping of STD levels and levels of sexual behaviors in populations. Integrated surveys that simultaneously collect data on biomedical markers for STDs and sexual behaviors can provide the necessary information for such mapping. Core groups and bridge populations may then be defined empirically. In the past, a major concern with integrated surveys involved the difficulties inherent in the collection and transportation of biomedical specimens. Recent advances in diagnostic technologies have rendered most of these concerns obsolete. Moreover, the acceptability of integrated behavioral surveys for the general population has been established.12 Importance of Context The concept of bridge population may be particularly relevant in given epidemiologic contexts where the distribution of an STD in a population is clustered in distinct subpopulations. Where an STD is evenly distributed throughout a population, there would be less of a role for bridge populations in the spread of disease. Therefore, the relative importance of bridge populations in an epidemic may be closely linked to the role of core groups. The extent to which STDs cluster in core groups appears to vary through the evolution of epidemics.13 We would expect the role played by both core groups and bridge populations to be more pronounced early in the history of epidemics, before the spread of the infection in the general population, and also after the decline phase. Impact of Bridge Populations: Disease Burden An interesting question involves the impact of bridge populations on STD morbidity: what proportion of infections can be attributed to indirect sexual mixing between subpopulations facilitated by bridge populations? The answer probably varies depending on many factors, including the sexual behavior map of the population, the clustering of STDs in subpopulations, and perhaps the phase of the epidemic. In a recent study conducted in Seattle, we found that most of the disease burden for gonococcal and chlamydial infections in both high-prevalence and low-prevalence subpopulations was attributable to mixing within the subpopulation or to direct mixing with members of high-prevalence subpopulations. The proportion of infection attributable to bridge populations was remarkably small.14 As more data on core groups and bridge populations in diverse societies become available, the relative impact of these groups on STD burden and the conditions that limit or magnify that impact will be better identified. Our understanding of STD transmission dynamics and potentially successful prevention approaches is greatly enhanced through research on core groups, bridge populations, and overall distribution of behaviors and infection in populations. The growing number of ongoing integrated behavioral-biomedical surveillance projects will greatly contribute to that understanding.
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Sevgi O. Aral (2000) studied this question.