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

Trends of syphilis seroprevalence at a tertiary care center in New Delhi, India: A 10-year analysis

Objectives

Syphilis has been called the great mimicker or the great imitator due to its myriad of clinical presentations. Despite strenuous concerted efforts toward the elimination of syphilis, it still remains a goal to be achieved. During recent years, many different trends of syphilis have been reported from around the globe.

Methods

A retrospective observational clinico-investigative study was carried out to analyze the 10-year trends of syphilis at one of the largest public sector hospitals in India.

Results and Conclusion

Both venereal disease research laboratory and Treponema pallidum Hemagglutination assay positivity rates showed a significant declining trend from 2007 to 2016, reflecting the success of the National Sexually Transmitted Infection Prevention and Control Program.

Pathogen transmission modes determine contact network structure, altering other pathogen characteristics

Pathogen traits can vary greatly and heavily impact the ability of a pathogen to persist in a population. Although this variation is fundamental to disease ecology, little is known about the evolutionary pressures that drive these differences, particularly where they interact with host behaviour. We hypothesized that host behaviours relevant to different transmission routes give rise to differences in contact network structure, constraining the space over which pathogen traits can evolve to maximize fitness. Our analysis of 232 contact networks across mammals, birds, reptiles, amphibians, arthropods, fish and molluscs found that contact network topology varies by contact type, most notably in networks that are representative of fluid-exchange transmission. Using infectious disease model simulations, we showed that these differences in network structure suggest pathogens transmitted through fluid-exchange contact types will need traits associated with high transmissibility to successfully proliferate, compared to pathogens that transmit through other types of contact. These findings were supported through a review of known traits of pathogens that transmit in humans. Our work demonstrates that contact network structure may drive the evolution of compensatory pathogen traits according to transmission strategy, providing essential context for understanding pathogen evolution and ecology.

Prevalence of selected sexually transmitted infectious agents in a cohort of asymptomatic soldiers in Austria

BACKGROUND

According to the World Health Organization (WHO), more than one million sexually transmitted infections (STIs) are acquired every day worldwide. Although STIs may be asymptomatic in many cases, they can cause severe symptoms and can also lead to adverse pregnancy outcomes and both male and female infertility. Asymptomatic carriers seem to play an important role in terms of the distribution of STIs; however, studies revealing the prevalence of STIs in asymptomatic individuals are rare.

METHODS

In the current study, 654 leftovers of standard urine samples from healthy, asymptomatic Austrian soldiers were investigated for the prevalence of Trichomonas vaginalis, Chlamydia trachomatis, and genital mycoplasmas (Mycoplasma hominis, Mycoplasma genitalium, Ureaplasma urealyticum, Ureaplasma parvum, and Candidatus Mycoplasma girerdii) by specific PCRs.

RESULTS

We detected T. vaginalis, M. hominis, U. urealyticum, U. parvum, and C. trachomatis in the investigated samples with prevalence of 7.6%, 4%, 2.4%, 5.4%, and 3.2%, respectively; neither M. genitalium nor Ca. Mycoplasma girerdii was found in our sample collection.

CONCLUSIONS

Our study introduces data on STIs of a mainly male cohort, which are scarce because most of the available information on sexually transmitted infectious agents arises from fertility clinics (mainly women) or symptomatic patients.

Strategies for successful designing of immunocontraceptive vaccines and recent updates in vaccine development against sexually transmitted infections - A Review

Background

Objective

Methods

Results

Conclusion

Mycoplasma genitalium incidence, persistence, concordance between partners and progression: systematic review and meta-analysis

Background

Mycoplasma genitalium is increasingly seen as an emerging sexually transmitted pathogen, and has been likened to Chlamydia trachomatis, but its natural history is poorly understood. The objectives of this systematic review were to determine M. genitalium incidence, persistence, concordance between sexual partners and the risk of pelvic inflammatory disease (PID).

Methods

We searched Medline, EMBASE, LILACS, IndMed and African Index Medicus from 1 January 1981 until 17 March 2018. Two independent researchers screened studies for inclusion and extracted data. We examined results in forest plots, assessed heterogeneity and conducted meta-analysis where appropriate. Risk of bias was assessed for all studies.

Results

We screened 4634 records and included 18 studies; six (4201 women) reported on incidence, five (636 women) on persistence, 10 (1346 women and men) on concordance and three (5139 women) on PID. Incidence in women in two very highly developed countries was 1.07 per 100 person-years (95% CI 0.61 to 1.53, I² 0%). Median persistence of M. genitalium was estimated from one to three months in four studies but 15 months in one study. In 10 studies measuring M. genitalium infection status in couples, 39%–50% of male or female sexual partners of infected participants also had M. genitalium detected. In prospective studies, PID incidence was higher in women with M. genitalium than those without (risk ratio 1.73, 95% CI 0.92 to 3.28, I² 0%, two studies).

Discussion

Incidence of M. genitalium in very highly developed countries is similar to that for C. trachomatis, but concordance might be lower. Taken together with other evidence about age distribution and antimicrobial resistance in the two infections, M. genitalium is not the new chlamydia. Synthesised data about prevalence, incidence and persistence of M. genitalium infection are inconsistent. These findings can be used for mathematical modelling to investigate the dynamics of M. genitalium.

Registration numbers

CRD42015020420, CRD42015020405

Dynamics of Drug Resistance: Optimal Control of an Infectious Disease

Antimicrobial resistance is a significant public health threat. In the U.S. alone, 2 million people are infected and 23,000 die each year from antibiotic resistant bacterial infections. In many cases, infections are resistant to all but a few remaining drugs. We examine the case where a single drug remains and solve for the optimal treatment policy for an SIS infectious disease model incorporating the effects of drug resistance. The problem is formulated as an optimal control problem with two continuous state variables, the disease prevalence and drug's "quality" (the fraction of infections that are drug-susceptible). The decision maker's objective is to minimize the discounted cost of the disease to society over an infinite horizon. We provide a new generalizable solution approach that allows us to thoroughly characterize the optimal treatment policy analytically. We prove that the optimal treatment policy is a bang-bang policy with a single switching time. The action/inaction regions can be described by a single boundary that is strictly increasing when viewed as a function of drug quality, indicating that when the disease transmission rate is constant, the policy of withholding treatment to preserve the drug for a potentially more serious future outbreak is not optimal. We show that the optimal value function and/or its derivatives are neither C ¹ nor Lipschitz continuous suggesting that numerical approaches to this family of dynamic infectious disease models may not be computationally stable. Furthermore, we demonstrate that relaxing the standard assumption of constant disease transmission rate can fundamentally change the shape of the action region, add a singular arc to the optimal control, and make preserving the drug for a serious outbreak optimal. In addition, we apply our framework to the case of antibiotic resistant gonorrhea.

Genomic Analysis and Comparison of Two Gonorrhea Outbreaks

Gonorrhea is a sexually transmitted disease causing growing concern, with a substantial increase in reported incidence over the past few years in the United Kingdom and rising levels of resistance to a wide range of antibiotics. Understanding its epidemiology is therefore of major biomedical importance, not only on a population scale but also at the level of direct transmission. However, the molecular typing techniques traditionally used for gonorrhea infections do not provide sufficient resolution to investigate such fine-scale patterns. Here we sequenced the genomes of 237 isolates from two local collections of isolates from Sheffield and London, each of which was resolved into a single type using traditional methods. The two data sets were selected to have different epidemiological properties: the Sheffield data were collected over 6 years from a predominantly heterosexual population, whereas the London data were gathered within half a year and strongly associated with men who have sex with men. Based on contact tracing information between individuals in Sheffield, we found that transmission is associated with a median time to most recent common ancestor of 3.4 months, with an upper bound of 8 months, which we used as a criterion to identify likely transmission links in both data sets. In London, we found that transmission happened predominantly between individuals of similar age, sexual orientation, and location and also with the same HIV serostatus, which may reflect serosorting and associated risk behaviors. Comparison of the two data sets suggests that the London epidemic involved about ten times more cases than the Sheffield outbreak.

The recent increases in gonorrhea incidence and antibiotic resistance are cause for public health concern. Successful intervention requires a better understanding of transmission patterns, which is not uncovered by traditional molecular epidemiology techniques. Here we studied two outbreaks that took place in Sheffield and London, United Kingdom. We show that whole-genome sequencing provides the resolution to investigate direct gonorrhea transmission between infected individuals. Combining genome sequencing with rich epidemiological information about infected individuals reveals the importance of several transmission routes and risk factors, which can be used to design better control measures.

Improving Control of Antibiotic-Resistant Gonorrhea by Integrating Research Agendas Across Disciplines: Key Questions Arising From Mathematical Modeling

The rise in gonococcal antibiotic resistance and the threat of untreatable infection are focusing attention on strategies to limit the spread of drug-resistant gonorrhea. Mathematical models provide a framework to link the natural history of infection and patient behavior to epidemiological outcomes and can be used to guide research and enhance the public health impact of interventions. While limited knowledge of key disease parameters and networks of spread has impeded development of operational models of gonococcal transmission, new tools in gonococcal surveillance may provide useful data to aid tracking and modeling. Here, we highlight critical questions in the management of gonorrhea that can be addressed by mathematical models and identify key data needs. Our overarching aim is to articulate a shared agenda across gonococcus-related fields from microbiology to epidemiology that will catalyze a comprehensive evidence-based clinical and public health strategy for management of gonococcal infections and antimicrobial resistance.

The theoretical impact and cost-effectiveness of vaccines that protect against sexually transmitted infections and disease

Sexually transmitted diseases, a source of widespread morbidity and sometimes mortality, are caused by a diverse group of infections with a common route of transmission. Existing vaccines against hepatitis B virus (HBV) and human papilloma virus 16, 18, 6 and 11 are highly efficacious and cost effective. In reviewing the potential role for other vaccines against sexually transmitted infections (STIs) a series of questions needs to be addressed about the burden of disease, the potential characteristics of a new vaccine, and the impact of other interventions. These questions can be viewed in the light of the population dynamics of sexually transmitted infections as a group and how a vaccine can impact these dynamics. Mathematical models show the potential for substantial impact, especially if vaccines are widely used. To better make the case for sexually transmitted infection vaccines we need better data and analyses of the burden of disease, especially severe disease. However, cost effectiveness analyses using a wide range of assumptions show that STI vaccines would be cost effective and their development a worthwhile investment.

Reaching the unreachable: providing STI control services to female sex workers via mobile team outreach

BACKGROUND

As part of a community-randomized trial of a multicomponent intervention to prevent sexually transmitted infections, we created Mobile Teams (MTs) in ten intervention cities across Peru to improve outreach to female sex workers (FSW) for strengthened STI prevention services.

METHODS

Throughout 20 two-month cycles, MTs provided counseling; condoms; screening and specific treatment for Neisseria gonorrhoeae (NG), Chlamydia trachomatis (CT), and vaginal Trichomonas vaginalis (TV) infections; and periodic presumptive metronidazole treatment for vaginal infections.

RESULTS

MTs had 48,207 separate encounters with 24,814 FSW; numbers of sex work venues and of FSW reached increased steadily over several cycles. Approximately 50% of FSW reached per cycle were new. Reported condom use with last client increased from 73% to 93%. Presumptive metronidazole treatment was accepted 83% of times offered. Over 38 months, CT prevalence declined from 15.4% to 8.2%, and TV prevalence from 7.3% to 2.6%. Among participants in ≥ 9 cycles, CT prevalence decreased from 12.9% to 6.0% (p <0.001); TV from 4.6% to 1.5% (p <0.001); and NG from 0.8% to 0.4% (p = 0.07).

CONCLUSIONS

Mobile outreach to FSW reached many FSW not utilizing government clinics. Self-reported condom use substantially increased; CT and TV prevalences declined significantly. The community-randomized trial, reported separately, demonstrated significantly greater reductions in composite prevalence of CT, NG, TV, or high-titer syphilis serology in FSW in these ten intervention cities than in ten matched control cities.

Current concepts in bacterial sexually transmitted diseases

Sexually transmitted diseases (STDs) are the most common infectious diseases worldwide, with over 350 million new cases occurring each year, and have far-reaching health, social, and economic consequences. Failure to diagnose and treat STDs at an early stage may result in serious complications and sequelae. STDs are passed from person to person primarily by sexual contact and are classified into varied groups. Some cause mild, acute symptoms and some are life-threatening. They are caused by many different infectious organisms and are treated in different ways. Syphilis and gonorrhea are ancient afflictions. Now, however, Chlamydia is prevalent and has become the most common bacterial STD. Antimicrobial resistance of several sexually transmitted pathogens is increasing, rendering some regimens ineffective, adding to therapeutic problems. A standardized treatment protocol for STDs is recommended to ensure that all patients receive adequate treatment. Appropriate treatment of STDs is an important public health measure.

Sexually transmitted disease core theory: roles of person, place, and time

The authors' purpose was to expand sexually transmitted disease core theory by examining the roles of person, place, and time in differentiating geographic core areas from outbreak areas. The authors mapped yearly census-tract-level syphilis rates for San Francisco, California, based on new primary and secondary syphilis cases reported to the San Francisco City sexually transmitted disease surveillance program between January 1, 1985, and December 31, 2007. SaTScan software (Information Management Services, Inc., Silver Spring, Maryland) was used to identify geographic clusters of significantly elevated syphilis rates over space and time. The authors graphed epidemic curves for 1) core areas, 2) outbreak areas, 3) neither core nor outbreak areas, and 4) noncore areas, where noncore areas included outbreaks, and stratified these curves according to demographic characteristics. Five clusters of significantly elevated primary and secondary syphilis rates were identified. A 5-year threshold was useful for differentiating core clusters from outbreak clusters. Epidemic curves for core areas, outbreak areas, neither core nor outbreak areas, and noncore areas were perfectly synchronized in phase trends and wavelength over time, even when broken down by demographic characteristics. Between epidemics, the occurrence of syphilis affected all demographic groups equally. During an epidemic, a temporary disparity in syphilis occurrence arose and a homogeneous core group of cases could be defined.

Modelling patterns of clearance of HPV-16 infection and vaccination efficacy

A model representing carcinogenic HPV infection transmission dynamics and cervical cancer natural history was adapted to assess the consequences of introducing vaccination against HPV-16 infection. Alternative scenarios either allowing repeated infections with the HPV-16 (i.e. SIS scenario) or assuming that clearance of infection occurs through the development of a long lasting, specific immune response which protects against re-infection (i.e. SIR scenario) were investigated. The difference in reduction in lifetime cervical cancer achieved through vaccination of 12-year-old girls, between SIS and SIR scenarios, was up to 25% of expected cases in an unscreened population. This difference increased to 30% when vaccination of 12-year-old boys was also included as an intervention. The role of SIS or SIR dynamics should be accounted for in the assessment of model-based projections of the effectiveness of vaccination programmes, until available data about the transmission dynamics support the accuracy of model predictions.

Current crisis or artifact of surveillance: insights into rebound chlamydia rates from dynamic modelling

Background

After initially falling in the face of intensified control efforts, reported rates of sexually transmitted chlamydia in many developed countries are rising. Recent hypotheses for this phenomenon have broadly focused on improved case finding or an increase in the prevalence. Because of many complex interactions behind the spread of infectious diseases, dynamic models of infection transmission are an effective means to guide learning, and assess quantitative conjectures of epidemiological processes. The objective of this paper is to bring a unique and robust perspective to observed chlamydial patterns through analyzing surveillance data with mathematical models of infection transmission.

Methods

This study integrated 25-year testing volume data from the Canadian province of Saskatchewan with one susceptible-infected-treated-susceptible and three susceptible-infected-treated-removed compartmental models. Calibration of model parameters to fit observed 25-year case notification data, after being combined with testing records, placed constraints on model behaviour and allowed for an approximation of chlamydia prevalence to be estimated. Model predictions were compared to observed case notification trends, and extensive sensitivity analyses were performed to confirm the robustness of model results.

Results

Model predictions accurately mirrored historic chlamydial trends including an observed rebound in the mid 1990s. For all models examined, the results repeatedly highlighted that increased testing volumes, rather than changes in the sensitivity and specificity of testing technologies, sexual behaviour, or truncated immunological responses brought about by treatment can, explain the increase in observed chlamydia case notifications.

Conclusions

Our results highlight the significant impact testing volume can have on observed incidence rates, and that simple explanations for these observed increases appear to have been dismissed in favor of changes to the underlying prevalence. These simple methods not only demonstrate geographic portability, but the results reassure the public health effort towards monitoring and controlling chlamydia.

A Bayesian approach to uncertainty analysis of sexually transmitted infection models

OBJECTIVES

To propose a Bayesian approach to uncertainty analysis of sexually transmitted infection (STI) models, which can be used to quantify uncertainty in model assessments of policy options, estimate regional STI prevalence from sentinel surveillance data and make inferences about STI transmission and natural history parameters.

METHODS

Prior distributions are specified to represent uncertainty regarding STI parameters. A likelihood function is defined using a hierarchical approach that takes account of variation between study populations, variation in diagnostic accuracy as well as random binomial variation. The method is illustrated using a model of syphilis, gonorrhoea, chlamydial infection and trichomoniasis in South Africa.

RESULTS

Model estimates of STI prevalence are in good agreement with observations. Out-of-sample projections and cross-validations also show that the model is reasonably well calibrated. Model predictions of the impact of interventions are subject to significant uncertainty: the predicted reductions in the prevalence of syphilis by 2020, as a result of doubling the rate of health seeking, increasing the proportion of private practitioners using syndromic management protocols and screening all pregnant women for syphilis, are 43% (95% CI 3% to 77%), 9% (95% CI 1% to 19%) and 6% (95% CI 4% to 7%), respectively.

CONCLUSIONS

This study extends uncertainty analysis techniques for fitted HIV/AIDS models to models that are fitted to other STI prevalence data. There is significant uncertainty regarding the relative effectiveness of different STI control strategies. The proposed technique is reasonable for estimating uncertainty in past STI prevalence levels and for projections of future STI prevalence.

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.

Sexual and reproductive health: a matter of life and death

Despite the call for universal access to reproductive health at the 4th International Conference on Population and Development in Cairo in 1994, sexual and reproductive health was omitted from the Millennium Development Goals and remains neglected (panel 1). Unsafe sex is the second most important risk factor for disability and death in the world's poorest communities and the ninth most important in developed countries. Cheap effective interventions are available to prevent unintended pregnancy, provide safe abortions, help women safely through pregnancy and child birth, and prevent and treat sexually transmitted infections. Yet every year, more than 120 million couples have an unmet need for contraception, 80 million women have unintended pregnancies (45 million of which end in abortion), more than half a million women die from complications associated with pregnancy, childbirth, and the postpartum period, and 340 million people acquire new gonorrhoea, syphilis, chlamydia, or trichomonas infections. Sexual and reproductive ill-health mostly affects women and adolescents. Women are disempowered in much of the developing world and adolescents, arguably, are disempowered everywhere. Sexual and reproductive health services are absent or of poor quality and underused in many countries because discussion of issues such as sexual intercourse and sexuality make people feel uncomfortable. The increasing influence of conservative political, religious, and cultural forces around the world threatens to undermine progress made since 1994, and arguably provides the best example of the detrimental intrusion of politics into public health.

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.

Role of Herd Immunity in Determining the Effect of Vaccines against Sexually Transmitted Disease

BACKGROUND

Vaccination programs provide both direct protection to those immunized and herd immunity, which is indirect protection of those who remain susceptible, owing to a reduced prevalence of infections.

METHODS

The well-understood impact of vaccination against ubiquitous childhood infections is compared with that of vaccination against sexually transmitted infections (STIs), and theoretical insights are derived from a review of mathematical modeling studies.

RESULTS

Typically, a large fraction of cases of STIs are acquired by those with modest risk, and these cases could be prevented by low-efficacy vaccines. If coverage is good, vaccination of only one sex can protect the other sex. Candidate vaccines against human papillomavirus (HPV) and genital herpes are in the final stages of testing. The former is likely to be highly efficacious for a limited number of disease-causing HPV types, and the latter has provided protection against disease in women who initially were seronegative for both herpes simplex virus (HSV) type 1 and HSV-2, with 73% efficacy. In models, this vaccine had a substantial impact when infectiousness was assumed to be reduced along with incidence of disease.

CONCLUSION

With such vaccines on the horizon, the requirements for vaccine delivery need to be considered, particularly who should be vaccinated and at what age.

Mathematical tools for planning effective intervention scenarios for sexually transmitted diseases

BACKGROUND

We studied the dynamics of sexually transmitted diseases in structured populations, deriving analogies from the theory of metapopulations.

GOAL

The goal was to study the impact of potential interventions, such as reducing the probability of transmission (e.g., by condom use), reducing the duration of infectiousness (e.g., by early diagnosis and treatment), or reducing the number of new contacts.

STUDY DESIGN

A structured SIS model was used for the study.

RESULTS

We extended the concept of core groups to the concept of nodal epidemiologic contribution (NEC) by considering the contribution that an individual in a particular (social, spatial, or other such) group makes to a target quantity, such as the threshold condition for disease persistence, the equilibrium prevalence, or the number of disease transmissions in the long term. The measures for NEC derived here are analogous to patch values in metapopulation theory, which measure the contributions of particular habitat patches to various-aspects of metapopulation dynamics.

CONCLUSION

Our results highlight that intervention measures should be targeted especially promptly if the goal is the complete eradication of the disease, as the variance in the contributions made by the individuals is the highest for this case.

Asymptomatic sexually transmitted diseases: the case for screening

BACKGROUND

The United States has the highest prevalence of sexually transmitted diseases in the developed world. Control strategies should address the most frequent reasons why curable sexually transmitted diseases are not treated.

METHODS

We approached 1,631 persons ages 18-29 in various sites and offered them screening for gonorrhea and chlamydial infection and surveys regarding past genitourinary symptoms. For those with past symptoms we abstracted medical records or conducted additional interviews. From these data we estimated the total number of persons who had gonorrhea or chlamydial infections in the previous year, the proportion treated, and the primary reasons for nontreatment.

RESULTS

The prevalence of gonorrhea was 2.3% and that of chlamydial infection was 10.1%. We estimate that 45 and 77% of all cases of gonorrhea and chlamydial infection, respectively, were never symptomatic and that 86 and 95% of untreated cases of gonorrhea and chlamydial infection, respectively, were untreated because they were never symptomatic. The remaining 14 and 5% of untreated cases of gonorrhea and chlamydia, respectively, were not treated because persons did not receive medical care for symptoms.

CONCLUSIONS

The primary reason that gonorrhea and chlamydial infections are untreated is that infected persons never have symptoms. The most effective method to control these sexually transmitted diseases is routine screening at high-volume sites.

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.