The role of sexual partnership networks in the epidemiology of gonorrhea

HIV prevalence, sexual risk behaviour and sexual mixing patterns among migrants in Amsterdam, The Netherlands

OBJECTIVE

To study (1) HIV prevalence; (2) sexual risk behaviour; (3) sexual mixing patterns; (4) determinants of disassortative (between-group) mixing among migrant groups in Amsterdam, the Netherlands and to gain insight into the potential for heterosexual spread of HIV/sexually transmitted diseases.

DESIGN

Cross-sectional study among 1660 Surinamese, Antilleans and sub-Saharan Africans, mainly recruited on the streets.

METHODS

Saliva was tested for HIV and questions were asked about sociodemographic characteristics, sexual behaviour and the ethnicity of sexual partners. Multivariate logistic regression analysis was used to find predictors for disassortative mixing.

RESULTS

HIV prevalence was 1.1% (95% confidence interval: 0.6-1.7). Compared with the Dutch population in general, our study group reported having multiple partners, concurrent partnerships and a history of sexually transmitted diseases much more frequently. Sex in the country of origin during a visit occurred frequently and there was a considerable degree of sexual mixing between different ethnic groups in the Netherlands. For men, disassortative mixing was associated with hard drug use, recent immigration, a high number of partners, being from Nigerian or Hindu-Surinamese origin, a recent sexually transmitted disease and, for steady relationships, consistent condom use. For women, determinants included: hard drug use, low income, being a-religious and, for Antillean and Ghanaian women, consistent condom use.

CONCLUSION

Our data suggest a potential for heterosexual spread of sexually transmitted diseases within and between ethnic groups in the Netherlands. The potential for HIV spread is however limited by the low HIV prevalence at present among these groups. This situation may change when HIV prevalence increases in the countries of origin, as bridges exist between those countries and the Netherlands. Culturally appropriate AIDS prevention programmes remain important for these groups.

Partner notification in the United States: an evidence-based review

INTRODUCTION

To conduct an evidence-based review of the literature on the effectiveness of partner notification strategies for syphilis, gonorrhea, chlamydia, and human immunodeficiency virus (HIV) in the United States.

METHODS

Systematic literature searches of available databases yielded 212 English language articles on partner notification, 13 of which met the inclusion criteria. These 13 articles were systematically reviewed, abstracted, and rated for quality of study methods, analysis, and generalizability.

RESULTS

Partner notification can newly detect HIV and other sexually transmitted diseases among partners. Of the six high-quality studies, the highest numbers of infections per infected person, 0.23 and 0.24, were detected by provider referral while the lowest number of infections per infected person, 0.03, was detected by self referral. None of the 13 studies examined the consequences of partner notification, such as infections or health consequences averted or changes in behavior and partnerships for infected persons or their partners.

CONCLUSIONS

There is good evidence that partner notification is a means of newly detecting infections. In addition, there is fair evidence that provider referral generally ensures that more partners are notified and medically evaluated than does self referral. More research is needed to improve elicitation and notification procedures and tailor them to specific populations, to assess the effect of new testing technologies on partner notification, and to understand the consequences of partner notification for infected persons and their partners.

Perspective: human contact patterns and the spread of airborne infectious diseases

Networks of social contacts channel the transmission of airborne infections. Emerging insights from fields of science as diverse as mathematics, population biology and the social sciences are beginning to reveal how the contact pattern of the hosts determines the spread and evolution of airborne infectious agents.

Individual causal models and population system models in epidemiology

A group of individuals behaves as a population system when patterns of connections among individuals influence population health outcomes. Epidemiology usually treats populations as collections of independent individuals rather than as systems of interacting individuals. An appropriate theoretical structure, which includes the determinants of connections among individuals, is needed to develop a "population system epidemiology." Infection transmission models and sufficient-component cause models provide contrasting templates for the needed theoretical structure. Sufficient-component cause models focus on joint effects of multiple exposures in individuals. They handle time and interactions between individuals in the definition of variables and assume that populations are the sum of their individuals. Transmission models, in contrast, model interactions among individuals over time. Their nonlinear structure means that population risks are not simply the sum of individual risks. The theoretical base for "population system epidemiology" should integrate both approaches. It should model joint effects of multiple exposures in individuals as time related processes while incorporating the determinants and effects of interactions among individuals. Recent advances in G-estimation and discrete individual transmission model formulation provide opportunities for such integration.

Individual causal models and population system models in epidemiology

A group of individuals behaves as a population system when patterns of connections among individuals influence population health outcomes. Epidemiology usually treats populations as collections of independent individuals rather than as systems of interacting individuals. An appropriate theoretical structure, which includes the determinants of connections among individuals, is needed to develop a "population system epidemiology." Infection transmission models and sufficient-component cause models provide contrasting templates for the needed theoretical structure. Sufficient-component cause models focus on joint effects of multiple exposures in individuals. They handle time and interactions between individuals in the definition of variables and assume that populations are the sum of their individuals. Transmission models, in contrast, model interactions among individuals over time. Their nonlinear structure means that population risks are not simply the sum of individual risks. The theoretical base for "population system epidemiology" should integrate both approaches. It should model joint effects of multiple exposures in individuals as time related processes while incorporating the determinants and effects of interactions among individuals. Recent advances in G-estimation and discrete individual transmission model formulation provide opportunities for such integration.

Sexual mixing patterns in the spread of gonococcal and chlamydial infections

OBJECTIVES

This study sought to define, among sexually transmitted disease (STD) clinic attendees, (1) patterns of sex partner selection, (2) relative risks for gonococcal or chlamydial infection associated with each mixing pattern, and (3) selected links and potential and actual bridge populations.

METHODS

Mixing matrices were computed based on characteristics of the study participants and their partners. Risk of infection was determined in study participants with various types of partners, and odds ratios were used to estimate relative risk of infection for discordant vs concordant partnerships.

RESULTS

Partnerships discordant in terms of race/ethnicity, age, education, and number of partners were associated with significant risk for gonorrhea and chlamydial infection. In low-prevalence subpopulations, within-subpopulation mixing was associated with chlamydial infection, and direct links with high-prevalence subpopulations were associated with gonorrhea.

CONCLUSIONS

Mixing patterns influence the risk of specific infections, and they should be included in risk assessments for individuals and in the design of screening, health education, and partner notification strategies for populations.

The transmission dynamics of gonorrhoea: modelling the reported behaviour of infected patients from Newark, New Jersey

A survey of the sexual behaviour of gonorrhoea patients in Newark was undertaken to evaluate parameters within a model of gonorrhoea transmission. Modelling work aimed to explain observed epidemiological patterns and to explore the potential impact of interventions. Reported behaviours, along with values derived from the literature, were used within a standard deterministic model of gonorrhoea transmission, where the population was stratified according to sex and rates of sex-partner change. The behaviours reported, particularly among women, are insufficient by themselves to explain the continued existence of gonorrhoea within the population. The majority of symptomatic patients seek treatment within a few days, and report that they do not have unprotected sex while symptomatic. The proportion of patients with low numbers of sex partners suggests that sexual mixing between people categorized according to sexual behaviour is near random. To explain the persistence of gonorrhoea, there must be some patients who, when infected, do not seek care in public clinics. In addition, gonorrhoea incidence in the model is sensitive to change, such that very small reductions in risk behaviour could lead to its elimination. This does not accord with the observed failure of many interventions to eliminate infection, suggesting that the modelled infection is too sensitive to change. The model, which has been influential in gonorrhoea epidemiology, is not consistent with the observed epidemiology of gonorrhoea in populations. Alternative models need to explore the observed stability of gonorrhoea before robust modelling conclusions can be drawn on how best to control infection. However, the current results do highlight the potential importance of asymptomatic infections and infections in those who are diseased and do not attend public health services. Screening and contact-tracing to identify asymptomatic infections in both men and women will be more effective in reaching those who maintain the infection within the community rather than simply treating symptomatic cases.

Concurrent sex partners and risk for sexually transmitted diseases among adolescents

BACKGROUND/OBJECTIVES

A large percentage of sexually active adolescents have multiple sex partners and are at high risk of acquiring sexually transmitted diseases (STDs). Little is known about adolescents' patterns of sexual partnerships (e.g., concurrent versus serial) and how these patterns influence STD risk.

GOAL OF THE STUDY

To determine the frequency with which adolescents have concurrent partners during a main relationship and the association between having concurrent partners and STD risk.

STUDY DESIGN

Adolescents seeking care at a public STD clinic were recruited from March, 1996, to May, 1998. Demographic and behavioral data were obtained during an interviewer-administered questionnaire. Sexually transmitted disease testing and physical exams were performed by clinicians.

RESULTS

Of those adolescents who reported having at least one main partner during the previous 6 months (n = 245), 110 (44.9%) had multiple partners, and 76 (31%) had at least one concurrent partner during a main relationship. Greater number of concurrent partners was associated with STD diagnosis/exposure after controlling for number of sex partners (OR = 1.6; 95% CI, 1.1-2.4).

CONCLUSIONS

A significant percentage of adolescents have concurrent partners during a main relationship, and having concurrent partners increases STD risk.

Subsequent sexually transmitted infections among adolescent women with genital infection due to Chlamydia trachomatis, Neisseria gonorrhoeae, or Trichomonas vaginalis

OBJECTIVE

To identify factors associated with subsequent sexually transmitted infection (STI) (within 1 year of initial infection) due to Chlamydia trachomatis, Neisseria gonorrhoeae or Trichomonas vaginalis.

DESIGN

Prospective cohort study.

SETTING

A sexually transmitted diseases clinic and four community-based primary care clinics for adolescents.

PARTICIPANTS

Female patients (ages 15 to 19 years) with initial diagnosis of chlamydia, gonorrhea, or trichomonas.

MAIN OUTCOME MEASURES

Subsequent infection by chlamydia, gonorrhea, or trichomonas.

RESULTS

More than 40% of subjects were subsequently infected by at least one STI. Reinfection was common, but infections with sexually transmitted organisms other than the initial infecting organism were also common. Predictors of subsequent infection were black race, gonorrhea as the initial infection, two or more sex partners in the previous 3 months, and inconsistent condom use.

CONCLUSIONS

Subsequent STI frequently follow an initial STI, but there is substantial variation in the causal organism. These data suggest the importance of comprehensive STI prevention programs for adolescents rather than organism-specific interventions.

The basic reproductive rate of infection and the course of HIV epidemics

The basic reproductive rate is a measure of the potential for growth of an infectious disease epidemic and depends on the pattern of infectious contacts within the host population, the likelihood of infection being transmitted during a contact, and the duration of infectiousness. These three variables are reviewed along with the surveillance data that records the progress of the epidemic, with an emphasis on the HIV-1 epidemic in heterosexual populations in developing countries. The problems with sentinel surveillance data for HIV infection mean that our knowledge of HIV incidence is poor. However, what is clear is that HIV has spread widely in heterosexual populations in sub-Saharan Africa. The scale of the demographic impact seen here will depend on the local AIDS incubation period with a shorter period generating a more acute demographic impact. Heterogeneity in sexual behaviour is vital to generate a high sexual activity "core group" within which HIV spreads rapidly. How far out of this core group the virus will spread depends on the patterns of mixing within populations. Interventions to reduce the incidence of HIV work through reducing the reproductive rate of the virus. To be efficient, these interventions have to be targeted at those most likely to spread the virus.

The natural history of syphilis. Implications for the transmission dynamics and control of infection

BACKGROUND

Syphilis remains a significant cause of morbidity in many developing countries and in some areas within North America and Europe. Mathematical models of the transmission dynamics of sexually transmitted infections have provided insights of relevance both to the interpretation of observed epidemiological patterns and to the design of control programs. Their use for the study of syphilis has been limited to date. GOALS AND STUDY DESIGN: The authors investigated the transmission dynamics of syphilis against a template based on the natural history of infection in individual patients with the aim of (1) identifying gaps in our understanding of the biology of infection, and (2) providing insights of relevance to the design of control policies.

RESULTS

Analyses reveal that Treponema pallidum has a moderate to high probability of transmission during contact between susceptible and infectious sexual partners. This, combined with questions over the existence of any immunity to reinfection, helps to ensure the long-term persistence of syphilis within "core" activity groups. Patterns of treatment in North America are shown to have significantly altered the relative frequency of individuals in the different stages of disease.

CONCLUSIONS

The analyses emphasize the benefits to be gained from treating infected people early in the primary stage of infection to reduce the effective period during which infected people can transmit to others. This form of treatment is beneficial for both the individual and the community. Treatment has greatly altered the incidence of different disease stages, but the full implications of treatment depend on whether immunity is present.