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

The nongenomic effects of progesterone in repressing iNOS activation through P38MAPK pathways in gonococci-infected polymorphonuclear leukocytes and the clinical significance

Progesterone has nongenomic effects on inducible nitric oxide synthase (iNOS), which is mediated by mitogen activated protein kinase (MAPK) pathways. This effect is supposed to have some potential association with asymptomatic gonococcal infections in women by immunological depression. In this study, polymorphonuclear leukocytes (PMNs) challenged by gonococci were used to study the nongenomic effects of progesterone. The activation of iNOS was assessed by measuring [(3)H] L-arginine converses to [(3)H] L-citrulline, and the activity of MAPK was detected by Western blot. It was found that the activity of iNOS and the yields of NO were enhanced significantly in gonococci-challenged PMNs compared with the controls (P<0.01). Progesterone could repress the activation of iNOS through P38MAPK pathway within PMNs (P<0.05), which could be blocked by SB203580 (P<0.01), but not by actinomycin D (P>0.05). It was also found subsequently that in the serum specimens collected from gonococci-infected but asymptomatic women, the progesterone level was higher than that in women with severe symptoms (P<0.01). Moreover, the yield of NO had an inverse correlation with progesterone. With these results it suggested that the rapid nongenomic effects of progesterone may inhibit iNOS activation and NO yields mediated by P38MAPK pathways, which were supposed to be concerned with asymptomatic women infected with gonococci.

Age- and gender-specific estimates of partnership formation and dissolution rates in the Seattle sex survey

PURPOSE

Partnership formation and dissolution rates are primary determinants of sexually transmitted infection (STI) transmission dynamics.

METHODS

The authors used data on persons' lifetime sexual experiences from a 2003-2004 random digit dialing survey of Seattle residents aged 18-39 years (N=1,194) to estimate age- and gender-specific partnership formation and dissolution rates. Partnership start and end dates were used to estimate participants' ages at the start of each partnership and partnership durations, and partnerships not enumerated in the survey were imputed.

RESULTS

Partnership formation peaked at age 19 at 0.9 (95% confidence interval [CI]: 0.76-1.04) partnerships per year and decreased to 0.1 to 0.2 after age 30 for women and peaked at age 20 at 1.4 (95% CI: 1.08-1.64) and declined to 0.5 after age 30 for men. Nearly one fourth (23.7%) of partnerships ended within 1 week and more than one half (51.2%) ended within 12 weeks. Most (63.5%) individuals 30 to 39 years of age had not formed a new sexual partnership in the past 3 years.

CONCLUSION

A large proportion of the heterosexual population is no longer at substantial STI risk by their early 30s, but similar analyses among high-risk populations may give insight into reasons for the profound disparities in STI rates across populations.

The role of complement in gonococcal infection of cervical epithelia

Neisseria gonorrhoeae is an exclusive human pathogen that causes the sexually transmitted disease, gonorrhea. The gonococcus has developed an exquisite repertoire of mechanisms by which it is able to evade host innate and adaptive immune responses. Our previous data indicate that the predominately asymptomatic nature ofgonococcal cervicitis may, in part, be attributed to the ability of these bacteria to subvert the normal function of complement to promote cervical disease. Herein we describe the interaction of N. gonorrhoeae with the complement alternative pathway with a particular focus on the importance of this interaction in promoting gonococcal cervicitis.

Functional analysis of NsrR, a nitric oxide-sensing Rrf2 repressor in Neisseria gonorrhoeae

Nitric oxide (NO) has been shown to be an important component of the human immune response, and as such, it is important to understand how pathogenic organisms respond to its presence. In Neisseria gonorrhoeae, recent work has revealed that NsrR, an Rrf2-type transcriptional repressor, can sense NO and control the expression of genes responsible for NO metabolism. A highly pure extract of epitope-tagged NsrR was isolated and mass spectroscopic analysis suggested that the protein contained a [2Fe-2S] cluster. NsrR/DNA interactions were thoroughly analysed in vitro. Using EMSA analysis, NsrR::FLAG was shown to interact with predicted operators in the norB, aniA and nsrR upstream regions with a K(d) of 7, 19 and 35 nM respectively. DNase I footprint analysis was performed on the upstream regions of norB and nsrR, where NsrR was shown to protect the predicted 29 bp binding sites. The presence of exogenously added NO inhibited DNA binding by NsrR. Alanine substitution of C90, C97 or C103 in NsrR abrogated repression of norB::lacZ and inhibited DNA binding, consistent with their presumed role in co-ordination of a NO-sensitive Fe-S centre required for DNA binding.

The program cost and cost-effectiveness of screening men for Chlamydia to prevent pelvic inflammatory disease in women

BACKGROUND

Because men transmit Chlamydia trachomatis to women, screening men to prevent pelvic inflammatory disease in women may be a viable strategy. However, the cost-effectiveness of this approach requires careful assessment.

METHODS

Data from a demonstration project and longitudinal study that examined screening men for chlamydia were applied to a compartment-based transmission model to estimate the cost-effectiveness of screening men for chlamydia compared with alternative interventions, including expanded screening of women and combining disease investigation specialist-provided partner notification with screening. Cases of pelvic inflammatory disease and quality-adjusted life years lost were the primary outcome measures. A male screening program that screened 1% of men in the population annually was modeled.

RESULTS

A program targeting high-risk men for screening (those with a larger number of partners in the previous year than the general population and a higher chlamydia prevalence) was cost saving compared with using equivalent program dollars to expand screening of lower-risk women. Combining partner notification with male screening was more effective than screening men alone. In sensitivity analyses, the male program was not always cost saving but averaged $10,520 per quality-adjusted life year saved over expanded screening of women.

CONCLUSIONS

Screening men can be a cost-effective alternative to screening women, but the men screened must have a relatively high prevalence compared with the women to whom screening would be expanded (under baseline assumptions, the prevalence in screened men was 86% higher than that of screened women). These modeling results suggest that programs targeting venues that have access to high-risk men can be effective tools in chlamydia prevention.

A metapopulation modelling framework for gonorrhoea and other sexually transmitted infections in heterosexual populations

Gonorrhoea continues to be a public health problem in the UK, and is the second most common bacterial sexually transmitted infection (STI) after chlamydia. In the UK, gonorrhoea is disproportionately concentrated in epidemiologically distinct subpopulations, with much higher incidence rates in young people, some ethnic minorities and inner city subpopulations. The original model of STI transmission proposed by Hethcote and Yorke explained some of these features through the concept of the 'core group'. Since then, several authors have modified the original model approach to include multiple sexual activity classes, but found this modelling approach to be inadequate when applied to low-prevalence settings such as the UK. We present a metapopulation framework for modelling gonorrhoea and other STIs. The model proposes that the epidemiology of gonorrhoea is largely driven by subpopulations with higher than average concentrations of individuals with high sexual risk activity. We show how this conceptualization of gonococcal epidemiology overcomes key limitations associated with some of the prior efforts to model gonorrhoea. We also use the model to explain several epidemiological features of gonorrhoea, such as its asymmetric distribution across subpopulations, and the contextual risk experienced by members of at-risk subpopulations. Finally, we extend the model to explain the distribution of other STIs, using chlamydia as an example of a more ubiquitous bacterial STI.

cis- and trans-acting elements involved in regulation of norB (norZ), the gene encoding nitric oxide reductase in Neisseria gonorrhoeae

The ability of Neisseria gonorrhoeae to reduce nitric oxide (NO) may have important immunomodulatory effects on the host during infection. Therefore, a comprehensive understanding of the regulatory mechanism of the nitric oxide reductase gene (norB) needs to be elucidated. To accomplish this, we analysed the functional regions of the norB upstream region. The promoter contains an extended -10 motif (TGNTACAAT) that is required for high-level expression. Deletion and substitution analysis of the norB upstream region revealed that no sequence upstream of the -10 motif is involved in norB regulation under anaerobic conditions or in the presence of NO. However, replacement of a 29 bp inverted repeat sequence immediately downstream of the extended -10 motif gave high levels of aerobic expression of a norB : : lacZ fusion. Insertional inactivation of gonococcal nsrR, predicted to bind to this inverted repeat sequence, resulted in the loss of norB repression and eliminated NO induction capacity. Single-copy complementation of nsrR in trans restored regulation of both norB transcription and NorB activity by NO. In Escherichia coli, expression of a gonococcal nsrR gene repressed gonococcal norB; induction of norB occurred in the presence of exogenously added NO. NsrR also regulates aniA and dnrN, as well as its own expression. We also determined that Fur regulates norB by a novel indirect activation method, by preventing the binding of a gonococcal ArsR homologue, a second repressor whose putative binding site overlaps the Fur binding site.

Suppressive Valacyclovir Therapy: Impact on the Population Spread of HSV-2 Infection

OBJECTIVES

Recent trial results demonstrate that the transmission probability of HSV-2 in monogamous couples is nearly halved by the use of valacyclovir as suppressive therapy.

GOAL

The goal of this study is to understand the potential impact of suppressive valacyclovir therapy on the transmission of HSV-2 within a population.

STUDY DESIGN

A mathematical model of HSV-2 epidemiology was developed which included suppressive therapy with the efficacy observed in the clinical trial. The model represented HSV-2 spread in an age and sexual activity stratified population where rates of viral shedding declined based on time since infection. The model tested the impact of a range of suppression coverage levels.

RESULTS

Suppressive therapy reduces the population incidence of HSV-2. With coverage rates of 3.2%, the incidence of HSV-2 would be reduced by between 1.8% and 2.8%. Higher coverage rates were estimated to reduce the incidence of new cases up to 13%. Starting suppression closer to the time of infection also reduces the incidence of new cases.

CONCLUSION

The impact of suppressive therapy on the HSV-2 epidemic is modest at current coverage levels but could be substantially increased with higher rates of diagnosis and a focus on coverage soon after infection.

Modelling the impact of migration on the HIV epidemic in South Africa

OBJECTIVE

To use observed data to develop a mathematical model that estimates the impact of migration on the spread of HIV in South Africa.

METHODS

A deterministic mathematical model was designed to evaluate the dynamic interactions between mobility, sexual behaviour, HIV, and sexually transmitted infections. The model was based on a population study of 488 adults, which included male migrants, male non-migrants and their rural partners in KwaZulu/Natal, South Africa.

RESULTS

The model predicted that the impact of migration depends upon the epidemic's stage and the pattern of migration. Early in the epidemic, frequent migration between populations with different HIV prevalence rates accelerated HIV spread; however, local sexual risk behaviour determined the eventual scale of the epidemic. If migration is coupled with increased sexual risk behaviour by migrant men, as has been reported in the South African communities studied, HIV prevalence would increase 10 times among migrants' female partners (1.8 to 19%). In contrast, if migration were to occur infrequently, with migration-associated risk behaviour assumed to be at current levels, the predicted epidemic would be one fifth that currently observed (2.8 versus 15.1%).

CONCLUSIONS

Migration primarily influences HIV spread by increasing high-risk sexual behaviour, rather than by connecting areas of low and high risk. Frequent return of migrants is an important risk factor when coupled with increased sexual risk behaviour. Accordingly, intervention programmes in South Africa need to target the sexual behaviour of short-term migrants specifically, even though these individuals may be more difficult to identify.

The role of compensatory mutations in the emergence of drug resistance

Pathogens that evolve resistance to drugs usually have reduced fitness. However, mutations that largely compensate for this reduction in fitness often arise. We investigate how these compensatory mutations affect population-wide resistance emergence as a function of drug treatment. Using a model of gonorrhea transmission dynamics, we obtain generally applicable, qualitative results that show how compensatory mutations lead to more likely and faster resistance emergence. We further show that resistance emergence depends on the level of drug use in a strongly nonlinear fashion. We also discuss what data need to be obtained to allow future quantitative predictions of resistance emergence.

Determinants of nitric oxide steady-state levels during anaerobic respiration by Neisseria gonorrhoeae

Nitric oxide (NO) is an important host defence molecule that varies its immune stimulatory effects depending on the concentrations at which it is produced, with low concentrations (< 1 microM) promoting an anti-inflammatory host response while higher concentrations (>1 microM) lead to inflammatory responses. Neisseria gonorrhoeae grows anaerobically by anaerobic respiration using nitrite reductase (Nir) to convert nitrite to NO and nitric oxide reductase (Nor) to convert NO to nitrous oxide. As N. gonorrhoeae can both produce and degrade NO, we have begun a study of NO metabolism in this bacterium to understand how gonococcal manipulation of NO concentration may influence the inflammatory response during infection. N. gonorrhoeae has an apparent Nir Km of 33 microM nitrite and an apparent Nor Km of 1.2 microM NO. The maximum specific activities for Nir and Nor were 135 nmoles nitrite reduced per minute per OD600 (pH 6.7) and 270 nmoles NO reduced per minute per OD600 (pH 7.5) respectively. N. gonorrhoeae established a steady-state concentration of NO after nitrite addition that was dependent on the nitrite concentration until saturation at 1 mM nitrite. The NO steady-state level decreased as pH increased, and the ratio of activities of Nir and Nor correlated to the NO steady-state level. When the NO donor DETA/NO was used to simulate host NO production, N. gonorrhoeae also established a NO steady-state level. The concentration of NO at steady state was found to be a function of the concentration of NO generated by DETA/NO, with N. gonorrhoeae reducing the NO from proinflammatory (>1 microM) to anti-inflammatory (approximately 100 nM) concentrations. The implications of the ability of N. gonorrhoeae to maintain an anti-inflammatory NO concentration is discussed in relation to asymptomatic infection in women.

Patterns of uptake of treatment for self reported sexually transmitted infection symptoms in rural Zimbabwe

OBJECTIVES

To determine the extent of self reported symptoms perceived to be related to sexually transmitted infections and the patterns of subsequent treatment seeking behaviour in a predominantly rural population of Zimbabwe.

METHODS

A population based survey of 4331 men and 5149 women was conducted in rural Zimbabwe during 1998-2000. Structured confidential interviews collected data on self reported sexually transmitted infection symptoms, treatment seeking behaviour, sociodemographic characteristics, and sexual behaviour.

RESULTS

25% of men aged 17-54 years report experiencing genital sores and 25% of men report experiencing urethral discharge; 30% of women aged 15-44 years report experiencing vaginal discharge. The lifetime number of sexual partners, age, and years of sexual activity were all significant predictors of symptoms for both men and women (all p values <0.001). 92% of men and 62% of women had sought treatment for their symptoms in the past year (p value <0.001). Men and women were equally likely to have sought treatment at a local hospital or clinic, but women were much less likely than men to have sought treatment at a different hospital or clinic. Among those who had sought treatment, men sought treatment faster than women and were more likely to report being "very satisfied" with their treatment than women.

CONCLUSIONS

The gender differences in treatment seeking are of major concern for control efforts and further work on determining the reasons for these should be a priority. This would inform the likely impact of both increasing availability of local services and further reducing the stigma faced by those wishing to access such services.

Patterns of Sex Worker–Client Contacts and Their Implications for the Persistence of Sexually Transmitted Infections

Sex workers (SWs) and their clients are often identified as being central in transmission of sexually transmitted infections (STIs). Little is known about how patterns of contact between SWs and their clients influence the persistence of STIs. We developed an individual-based simulation model to explore how variation in number of client contacts per SW, whether clients repeatedly visited the same SW, and the relative sizes of the SW and client populations influence the endemic prevalence of gonorrhea and herpes simplex virus type 2 infection. Persistence of either infection was more likely if clients visited many different SWs, regardless of variation in the SW-client contact rate, and also resulted in a higher endemic prevalence in both populations and a greater likelihood of persistence of infection at lower levels in the general population. The size of the SW population (relative to the total population) was found to be most important in determining the overall prevalence of infection.

Host immunity and synchronized epidemics of syphilis across the United States

A central question in population ecology is the role of 'exogenous' environmental factors versus density-dependent 'endogenous' biological factors in driving changes in population numbers. This question is also central to infectious disease epidemiology, where changes in disease incidence due to behavioural or environmental change must be distinguished from the nonlinear dynamics of the parasite population. Repeated epidemics of primary and secondary syphilis infection in the United States over the past 50 yr have previously been attributed to social and behavioural changes. Here, we show that these epidemics represent a rare example of unforced, endogenous oscillations in disease incidence, with an 8-11-yr period that is predicted by the natural dynamics of syphilis infection, to which there is partially protective immunity. This conclusion is supported by the absence of oscillations in gonorrhoea cases, where a protective immune response is absent. We further demonstrate increased synchrony of syphilis oscillations across cities over time, providing empirical evidence for an increasingly connected sexual network in the United States.

Investigating ethnic inequalities in the incidence of sexually transmitted infections: mathematical modelling study

OBJECTIVES

To investigate ethnic differences in rates of gonorrhoea using empirical sexual behaviour data in a simple mathematical model. To explore the impact of different intervention strategies in this simulated population.

METHODS

The findings from cross sectional studies of gonorrhoea rates and sexual behaviour in three ethnic groups in south east London were used to determine the parameters for a deterministic, mathematical model of gonorrhoea transmission dynamics, in a population stratified by sex, sexual activity (rate of partner change), and ethnic group (white, black African, and black Caribbean). We compared predicted and observed rates of infection and simulated the effects of targeted and population-wide intervention strategies.

RESULTS

In model simulations the reported sexual behaviours and mixing patterns generated major differences in the rates of gonorrhoea experienced by each subpopulation. The fit of the model to observed data was sensitive to assumptions about the degree of mixing by level of sexual activity, the numbers of sexual partnerships reported by men and women, and the degree to which observed data underestimate female infection rates. Interventions to reduce duration of infection were most effective when targeted at black Caribbeans.

CONCLUSIONS

Average measures of sexual behaviour in large populations are inadequate descriptors for the epidemiology of gonorrhoea. The consistency between the model results and empirical data shows that profound differences in gonorrhoea rates between ethnic groups can be explained by modest differences in a limited number of sexual behaviours and mixing patterns. Targeting effective services to particular ethnic groups can have a disproportionate influence on disease reduction in the whole community.

Frequency-dependent incidence in models of sexually transmitted diseases: portrayal of pair-based transmission and effects of illness on contact behaviour

We explore the transmission process for sexually transmitted diseases (STDs). We derive the classical frequency-dependent incidence mechanistically from a pair-formation model, using an approximation that applies to populations with rapid pairing dynamics (such as core groups or non-pair-bonding animals). This mechanistic derivation provides a framework to assess how accurately frequency-dependent incidence portrays the pair-based transmission known to underlie STD dynamics. This accuracy depends strongly on the disease being studied: frequency-dependent formulations are more suitable for chronic less-transmissible infections than for transient highly transmissible infections. Our results thus support earlier proposals to divide STDs into these two functional classes, and we suggest guidelines to help assess under what conditions each class can be appropriately modelled using frequency-dependent incidence. We then extend the derivation to include situations where infected individuals exhibit altered pairing behaviour. For four cases of increasing behavioural complexity, analytic expressions are presented for the generalized frequency-dependent incidence rate, basic reproductive number (R0) and steady-state prevalence (i infinity) of an epidemic. The expression for R0 is identical for all cases, giving refined insights into determinants of invasibility of STDs. Potentially significant effects of infection-induced changes in contact behaviour are illustrated by simulating epidemics of bacterial and viral STDs. We discuss the application of our results to STDs (in humans and animals) and other infectious diseases.

The potential epidemiological impact of a genital herpes vaccine for women

BACKGROUND

In two phase III vaccine trials immunisation of women previously uninfected by herpes simplex virus provided protection against genital herpes disease. In deciding policy, an evaluation of the epidemiological impact of the partial protection provided by the vaccine should be considered.

METHODS

A sex and sexual activity stratified deterministic differential and partial differential equation model of the natural history of herpes simplex virus type 2 (HSV-2) and the impact of vaccination is developed and analysed. To explore the role of vaccination, the pattern of viral shedding and the transmission of infection during sexual acts within sexual partnerships are described.

RESULTS

Using literature derived estimates of parameter values and assuming efficacy in only 40% of women the impact of the vaccine depends on assumptions made about its action. The vaccine has a limited impact if it only prevents disease but a more substantial impact if it reduces asymptomatic viral shedding, which it could do indirectly by preventing infection or directly by modifying the biology of the infection. Concern over the implications of a vaccine that prevents disease but has no impact on viral shedding was addressed in a worst case scenario. Here there is a modest increase in the incidence of infection in both men and women but an increase in disease prevalence in men alone, since the virus directly protects some women from disease.

CONCLUSIONS

Results suggest that a herpes vaccine should be used universally and that a vaccine that only protects HSV-1-/2- women can paradoxically have a significant epidemiological impact, the scale of which depends upon changes in patterns of viral shedding.

Changes in the transmission dynamics of the HIV epidemic after the wide-scale use of antiretroviral therapy could explain increases in sexually transmitted infections: results from mathematical models

BACKGROUND

Recent increases in bacterial sexually transmitted infections (STI) and risk behavior have coincided with the introduction of antiretroviral therapy (ART) in homosexual communities of industrialized countries. The reasons for these increases are not fully understood.

GOAL

The goal of this study was to understand the various effects of ART on risk behaviors and STI.

OBJECTIVE

The objective of this study was to assess the independent impact of the change in the transmission dynamics of HIV/AIDS as a result of the wide-scale use of ART on a bacterial STI.

STUDY DESIGN

We developed a mathematical model of bacterial STI and treated/untreated HIV/AIDS infection for an open homosexual population. At the individual level, we assume that susceptible and healthy HIV-positive individuals do not increase their risk behavior as a result of ART over time. However, individuals with AIDS, who are successfully treated with ART, can resume sexual activity. The impact of the wide-scale use of ART on risky behavior, STI, and HIV/AIDS was evaluated over a wide range of assumptions on treatment use, ART efficacy, and population characteristics.

RESULTS

Over 10 years, 0% to 55% new bacterial STI could be attributed to the wide-scale use of ART as a result of more modest increases (0-25%) in risky sex occurring at the population level rather than at the individual level. These increases have a negative impact on HIV if coverage is too low. Increasing treatment coverage helps to prevent more HIV infections despite larger increases in risky sex and STI that is predicted to ensue.

CONCLUSION

Taking the differential impact of wide-scale use of ART into account helps to interpret recent behavioral and STI trends. Our results have implications for prevention strategies and for the formulation of public health policies. A better understanding of the differential impact of ART on sexual network over time is required.

Robust parameter estimation techniques for stochastic within-host macroparasite models

We present a stochastic model of the within-host population dynamics of lymphatic filariasis, and use a simulated goodness-of-fit (GOF) method to estimate immunological parameters and their confidence intervals from experimental data. A variety of deterministic moment closure approximations to the stochastic system are explored and compared with simulation results. For the maximum GOF parameter estimates, none of the methods of closure accurately reproduce the behaviour of the stochastic model. However, direct analysis of the stochastic model demonstrates that the high levels of variation observed in the data can be reproduced without requiring parameters to vary between hosts. This indicates that the observed aggregation of parasite load may be dynamically generated by random variation in the development of an effective immune response against parasite larvae.

Stochastic effects on endemic infection levels of disseminating versus local contacts

The effects of two levels of mixing on endemic infection levels are shown to differ for identically conformed deterministic compartmental (DC) and stochastic compartmental (SC) models. Both DC and SC models give similar endemic levels when populations are large, immunity is short lived, and mixing is universal. But local transmissions and/or transient immunity decrease overall population infection levels in SC but not in DC models. DC models also fail to detect the greater effects of eliminating disseminating transmissions in comparison to eliminating local transmissions shown by SC models. These differences in model behavior arise because localities that encounter few infections from distant sites and that have stochastically low infection levels have decreased infection rates while localities with stochastically high levels of infection do not decrease the rate at which they lose infection. At the extreme this generates local stochastic die out with subsequent build up of susceptibility in SC but not DC models. This phenomenon should act upon all endemic infections that have changing geographic or social foci of infection. Neither standard epidemiological investigations nor sufficient-component cause models can capture these effects because they occur in the absence of differences between individuals.

Sexual mixing patterns and sex-differentials in teenage exposure to HIV infection in rural Zimbabwe

BACKGROUND

HIV-1 prevalence typically rises more rapidly at young ages in women than in men in sub-Saharan Africa. Greater susceptibility to infection on exposure in women is believed to be a contributory factor as is greater exposure to previously infected sexual partners of the opposite sex. We investigated the latter hypothesis using data from a field study in rural Manicaland, Zimbabwe.

METHODS

Quantitative data on onset and degree of sexual activity, numbers of partners, concurrent partnerships, condom use, and partner characteristics were used in conjunction with epidemiological data on age and sex specific prevalence of HIV infection to do statistical analyses of association between key variables. Mathematical models and qualitative data were used to aid analysis and interpretation.

FINDINGS

Older age of sexual partner was associated with increased risk of HIV-1 infection in men (odds ratio 1.13 [95% CI 1.02-1.25]) and women (1.04 [1.01-1.07]). Young women form partnerships with men 5-10 years older than themselves, whereas young men have relationships with women of a similar age or slightly younger. Greater number of lifetime partners is also associated with increased risk of HIV (1.03 [1.00-1.05]). Young men report more partners than do women but infrequent coital acts and greater use of condoms. These behaviour patterns are underpinned by cultural factors including the expectation that women should marry earlier than men. A strong gender effect remains after factors that affect exposure to infected partners are controlled for (6.04 [1.49-24.47]).

INTERPRETATION

The substantial age difference between female and male sexual partners in Manicaland is the major behavioural determinant of the more rapid rise in HIV prevalence in young women than in men. Theoretical studies have suggested that this difference is an important determinant of observed epidemiological patterns but the study reported in this paper provides clear empirical evidence of association.

An introduction to mathematical models in sexually transmitted disease epidemiology

Mathematical models serve a number of roles in understanding sexually transmitted infection epidemiology and control. This article seeks to provide the non-mathematician with a description of their construction and use and presents illustrative examples from sexually transmitted infection epidemiology.

Mathematical models of the transmission and control of sexually transmitted diseases

BACKGROUND

The development of mathematical models to describe and interpret the epidemiology of sexually transmitted infections has involved the incremental addition of various forms of biological and behavioral complexity to simple mathematical templates.

GOAL

To review simple and complex models used in study of observed epidemiologic pattern.

STUDY DESIGN

An overview of modeling in sexually transmitted disease epidemiology identifies the function of different types of models.

RESULTS

Simple models have the advantage of transparency and analytical tractability and can illustrate the relative merits of different intervention options. However, real life is replete with complexities that can have effects that are difficult to predict in the absence of a mathematical framework.

CONCLUSIONS

Research should increasingly be based on robust parameterization of model structures and try to capture individual behaviors. Progress will be most rapid by interdisciplinary work where the clinician, epidemiologist, and mathematician work collaboratively to help improve our knowledge of how to best control infection and disease.

Risks of acquiring and transmitting sexually transmitted diseases in sexual partner networks

BACKGROUND

A person's risk for acquiring infection and their role in continued transmission has traditionally been assessed on the basis of individual characteristics. Recently, network studies have attempted to relate individual risks to position in the wider network.

GOAL

To assess the importance of local and global network structures in assessing the risk of acquiring and transmitting infection.

STUDY DESIGN

An individual-based simulation model was used to construct a variety of potential network structures and track the transmission of infection over time. Logistic and Poisson regression were used to identify which measures of network position influence a person's risk for acquiring and transmitting infection.

RESULTS

Measures of local centrality were more important to risk of acquisition, whereas global centrality mattered more to transmission. Continuous snowball sampling, rather than a fixed number of waves, better estimates a person's risks.

CONCLUSIONS

There is an asymmetry regarding the risk of acquiring and transmitting infection.

More realistic models of sexually transmitted disease transmission dynamics: sexual partnership networks, pair models, and moment closure

BACKGROUND

Mathematical models of sexually transmitted disease transmission have proven powerful tools for interpreting observed epidemiologic pattern. However, the most commonly used formulation of such models largely fail to capture the effect of partnership concurrency and contact network structure on transmission.

GOAL

The development of a compartmental model of partnership formation and dissolution that includes approximations for the influence of the sexual-partner network.

STUDY DESIGN

Theoretical analysis of ordinary differential equation models for sexually transmitted disease transmission within sex-partner networks.

RESULTS

The approach developed advances earlier pair models, allows for the influence of concurrent sexual partnerships, and illustrates the importance of concurrency to the persistence of diseases with relatively short durations of infectiousness. The authors also illustrate that heterogeneity in risk is possible even in model populations in which all individuals follow the same behavioral rules.

CONCLUSION

Deterministic extended pair models offer a powerful approach to modelling sexually transmitted disease transmission that usefully complement computationally intensive microsimulation models.

Epidemiology and control and curable sexually transmitted diseases: opportunities and problems

BACKGROUND

Despite the availability of safe and effective treatment, infection with bacterial sexually transmitted diseases persists at a high prevalence in many populations.

GOAL

To review the difficulties of parameter estimation when a cure is readily available and to explore the impact of different treatment and screening strategies that might maximize the benefits of using available treatments.

STUDY DESIGN

A standard deterministic model for the spread of a bacterial sexually transmitted disease that causes symptomatic and asymptomatic infections, in which the population is stratified according to sex and sexual activity, is further stratified into two host groups to enable the modeling of different treatment and screening strategies.

RESULTS

In the presence of a core group, if an infection has a high transmission probability, then screening for asymptomatic infections has a short-lived benefit. Repeated screening is slightly better if it is not restricted to a fraction of the at-risk population, but targeting of high-risk groups should be effective. Screening to treat asymptomatic infections in men could be beneficial if a substantial fraction of cases remain asymptomatic.

CONCLUSIONS

After the initial gains achieved through treating symptomatic infections, further reductions in the prevalence of infections can be achieved by finding asymptomatic infections. However, these gains are difficult to achieve, especially in the case of gonorrhea. Because men are likely to have an asymptomatic chlamydial infection, screening of men for chlamydia should be worthwhile.

Trichomonas vaginalis epidemiology: parameterising and analysing a model of treatment interventions

BACKGROUND

Trichomonas vaginalis, which affects at least 170 million individuals globally, may increase the risk of transmission of HIV and predispose pregnant women to premature rupture of membranes and early labour.

OBJECTIVE

To more clearly define the epidemiology of trichomoniasis and to develop a mathematical model of disease transmission dynamics in order to explore various treatment strategies.

DESIGN

A deterministic model of trichomoniasis was constructed. Parameter values were set to fit the model to known endemic prevalence levels of Trichomonas vaginalis. Two treatment interventions ("screening" and "syndromic management") were simulated.

RESULTS

The age specific prevalence of the disease was seen to differ from other STDs in a number of studies. Parameter fitting indicates that the average duration of infection in women is at least 3-5 years and approximately 4 months for men. "Syndromic management" (that is, treating only those with symptoms of disease) had minimal effect upon the endemic prevalence of disease even at high levels of coverage. "Screening" (that is, identification of individuals with both symptomatic and asymptomatic infection) was shown to be the most efficient method of control, but was sensitive to the screening interval.

CONCLUSIONS

The control of trichomoniasis seems to have been a success in developed countries because of the regular access to health care, whereas it has remained endemic in many developing countries where control may only be possible by regular screening and treatment. However, without a large investment in services, success in controlling trichomoniasis is likely to be transitory.

Gonorrhea in male adolescents and young adults in Newark, New Jersey: implications of risk factors and patient preferences for prevention strategies

BACKGROUND

Although the national rate of gonorrhea declined 68% from 1975 to 1995, rates remain high in many inner-city areas. In 1995, the gonorrhea rate in Newark, NJ, was five times the US rate.

OBJECTIVE

To determine factors associated with acquisition of gonorrhea by men in Newark.

STUDY DESIGN

A case-control study conducted at the sexually transmitted disease (STD) clinic in Newark, comparing males 15 to 29 years with culture-confirmed gonorrhea to controls with no STD.

RESULTS

Compared with controls, males with gonorrhea more frequently reported at least 1 casual sex partner within the preceding month (adjusted odds ratio [OR], 3.2; 95% CI, 1.8-5.7), sex after using marijuana during the preceding month (OR, 2.4; 95% CI, 1.1-5.3), and a history of incarceration (OR, 2.1; 95% CI, 1.2-3.7). Of males with casual partners, having a new casual sexual relationship (onset within the past month) was particularly risky for gonorrhea (OR, 3.9; 95% CI, 1.2-12.7). Incorrect condom use was highly prevalent for both cases and controls. Many persons with gonorrhea reported that they were not willing to consistently use condoms or to have only one partner.

CONCLUSIONS

Sex with casual partners is associated with gonorrhea in males, and may be a difficult practice to change. Condoms are often used incorrectly, if at all, in this population. Prevention strategies, in addition to the promotion of condom use and monogamy, may be necessary.