Conquering sexually transmitted diseases

Sexually transmitted diseases (STDs) are a major public-health problem and also a significant financial burden on the economy. Past and ongoing attempts to create vaccines against sexually transmitted pathogens have met with varying success. This article highlights some of the public-health and social problems that are associated with STDs and the technical and ethical challenges in treating them, and raises several questions that need to be addressed if STDs are to be conquered.     

Immunodiagnosis of sexually transmitted disease

Methods for detecting microbial antigens in clinical specimens offer an alternative to culture in the diagnosis of some sexually transmitted diseases. Developers of the immunologic methods are faced with a number of problems in evaluating the new tests. Traditionally, these tests are compared to culture as the "gold standard." Unfortunately, culture for Neisseria gonorrhoeae or Chlamydia trachomatis--the two agents most commonly sought--is considerably less sensitive than 100 percent. Immunologic methods may appear to produce false positives when the paired specimens are actually false-negative cultures. Another source of discordant results is sampling variation. These considerations, however, will not account for all false-positive results. Even the best non-culture methods have a low rate of false-positive results. If a new test has a specificity of 97 percent, it, by definition, yields approximately 3 percent false-positive reactions. In low-prevalence settings this false-positive rate will create problems in interpreting the results. For example, in a population with 3 percent prevalence of infection, a positive result in a 97 percent specificity test could only have a predictive value of 50 percent. Most testing for STD agents is performed in low-prevalence settings. None of the currently available immunodiagnostic procedures has a performance profile that suggests it will be satisfactory for diagnostic use in the low-prevalence setting.     

Epidemiological models for sexually transmitted diseases

The classical models for sexually transmitted infections assume homogeneous mixing either between all males and females or between certain subgroups of males and females with heterogeneous contact rates. This implies that everybody is all the time at risk of acquiring an infection. These models ignore the fact that the formation of a pair of two susceptibles renders them in a sense temporarily immune to infection as long as the partners do not separate and have no contacts with other partners. The present paper takes into account the phenomenon of pair formation by introducing explicitly a pairing rate and a separation rate. The infection transmission dynamics depends on the contact rate within a pair and the duration of a partnership. It turns out that endemic equilibria can only exist if the separation rate is sufficiently large in order to ensure the necessary number of sexual partners. The classical models are recovered if one lets the separation rate tend to infinity.This work has been supported by Deutsche Forschungsgemeinschaft     

Male circumcision and sexually transmitted infections in Botswana

This study set out to investigate the influence of male circumcision and other factors on sexually transmitted infections in Botswana. A syndromic approach, which diagnoses a sexually transmitted infection based on the presence of urethral discharge or genital ulcers rather than on laboratory tests, was used. The data were from the 2001 Botswana AIDS Impact Survey where a nationally representative, randomly selected sample of men and women aged 10-64 years were interviewed in both urban and rural areas. The sample selected for this study consisted of 216,480 men aged 15-64 years who had ever had sexual intercourse. The logistic regression technique was executed to examine the association between male circumcision and self-reported urethral discharge or genital ulcers, while controlling for all other independent variables in the analysis. The main finding of this study was that among men who are circumcised, the odds for self-reported urethral discharge or genital ulcers are significantly lower than for those men who are not circumcised in both urban and rural Botswana. The analysis also showed that the odds in favour of self-reported urethral discharge or genital ulcers, for men who drink alcohol, are twice as large as those for men who do not drink alcohol, controlling for all other independent variables in the analysis. Religion and ethnicity also came through as factors exerting a protective influence against self-reported symptoms of sexually transmitted infections. The conclusion is that while male circumcision appears to be significantly associated with the risk for self-reported urethral discharge or genital ulcers, it is man's behaviour, irrespective of ethnicity or religious dictates, that continues to play a vital role in protection against self-reported symptoms of sexually transmitted infections in Botswana.     

The influence of sexually active non-reproductive groups on persistent sexually transmitted diseases

We describe several population models exposed to a mild life-long sexually transmitted disease, i.e. without significant increased mortality among infected individuals and providing no immunity/recovery. We then modify these models to include non-reproductive groups consisting of those isolated from sexual contact and those who are sexually active but infertile due to choice, medical or other reasons. We analyse the potential effect on the dynamics of the population. We are interested in how the isolated class may curb the growth of the infected group while keeping the healthy population at acceptable levels. We also analyse the difference between being sexually active and abstained within the non-reproductive class and its impact on the epidemic reproductive number and the nature of the bifurcation around the disease-free equilibrium. We provide a comparison with our models introduced in a previous article, which include only the isolated from sexual contact class.     

The influence of sexually active non-reproductive groups on persistent sexually transmitted diseases

We describe several population models exposed to a mild life-long sexually transmitted disease, i.e. without significant increased mortality among infected individuals and providing no immunity/recovery. We then modify these models to include non-reproductive groups consisting of those isolated from sexual contact and those who are sexually active but infertile due to choice, medical or other reasons. We analyse the potential effect on the dynamics of the population. We are interested in how the isolated class may curb the growth of the infected group while keeping the healthy population at acceptable levels. We also analyse the difference between being sexually active and abstained within the non-reproductive class and its impact on the epidemic reproductive number and the nature of the bifurcation around the disease-free equilibrium. We provide a comparison with our models introduced in a previous article, which include only the isolated from sexual contact class.     

Monogamous networks and the spread of sexually transmitted diseases

Patterns of sexual mixing and heterogeneity in the number of sexual partners can have a huge effect on the spread of a sexually transmitted disease (STD). The sexual mixing network identifies all partnerships within a population over a given period and is a powerful tool in the study of such infections. Previous models assumed all links within the network to be concurrent active partnerships. We present a novel modelling approach in which we adapt the notion of a sexual contact network to a monogamous population by allowing the nature of the links to change. We use the underlying network to represent potential sexual partnerships, only some of which are active at any one time. Thus serial monogamy can be modelled while maintaining the patterns of mixing displayed by the population.     

Cost benefit analysis of sexually transmitted diseases.

Suggested measures to prevent transmission and sequelae of sexually transmitted diseases are almost universally agreed. But the important question to be answered is whether the interventions are worth the resources spent. To facilitate the decision making on resource allocation, the best possible information should be available concerning the relationships between health intervention programs and changes in the health status. Cost benefit analysis is the main approach used to assess the efficiency of any intervention. If the economic benefits of the program are greater than the costs, the program should be seriously considered. Although information about the costs of management of STDs is scarce in developing countries because of the complicated behavioral, social and economic issues involved, it is widely held that making treatment available for curable sexually transmitted diseases represents one of the most cost-effective ways to improve the health in the world.