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.     

Verbal Intercourse and Sexual Communication: Impediments to STD Prevention

This article explores the problems of risky sexual behavior by examining the ways people verbally negotiate sexual interaction regarding sexually transmitted diseases. Based on in-depth interviews with 124 adults (ages 21–63) who are infected with genital herpes, the article shows that knowledge about sexually transmitted diseases is not necessarily related to the action needed for their prevention. People are more afraid of being rejected by a partner than they are of contracting an STD (except, of course, AIDS). By examining difficulties people have with sexual health discourse and showing how these difficulties are related to both the problem of communicating politeness and the problem of representing the self and the sexual other, this article demonstrates that STD prevention programs overlook a very important emotional and communicative issue: the lack of a culturally sanctioned language with which to discuss sexual health with partners, [genital herpes, STD prevention, STD communication, sexually transmitted diseases, STD risk]     

A proposed feedback loop of sexually transmitted diseases and sexual behavior: the Red Queen's Dilemma

(Micro)organisms, such as bacteria, which cause sexually transmitted diseases (STDs) in humans are presented with an interesting ecological challenge. These microorganisms need humans to have sexual contact with each other in order for the microorganisms to spread to other hosts as well as to have subsequent generations of descendants. However, diseases tend to lower the sex drive and to render the host less sexually attractive. It is argued that, over time, selective advantages sculpted organisms which cause STDs to be minimally symptomatic and to indirectly increase the number of sexual partners of the host. Neisseria gonorrhoeae which cause the STD gonorrhea are used as a prototype for these putative sexual dynamics. As a counter to the (micro)organisms' biological adaptations, human cultural innovations emerged and became integrated into the various traditions of social structures.     

The occurrence of strongylid nematodes in the epididymides of wood mice

The recent discovery of a larval nematode in the epididymides of free-living wood mice (Apodemus sylvaticus) suggests a sexual transmission of these parasites. They have been placed within the bursate nematodes (order Strongylida) through 18S rDNA analysis, suggesting that they are undetermined metastrongyloid nematodes. The possibility that these parasites are transmitted sexually opens an intriguing field of research because sexually transmitted metazoan parasites are known to occur mainly in invertebrates, whereas in vertebrates sexually transmitted parasites are usually microparasites such as viruses, bacteria and protozoa.     

Interval estimates for epidemic thresholds in two-sex network models

Epidemic thresholds in network models of heterogeneous populations characterized by highly right-skewed contact distributions can be very small. When the population is above the threshold, an epidemic is inevitable and conventional control measures to reduce the transmissibility of a pathogen will fail to eradicate it. We consider a two-sex network model for a sexually transmitted disease which assumes random mixing conditional on the degree distribution. We derive expressions for the basic reproductive number (R(0)) for one and heterogeneous two-population in terms of characteristics of the degree distributions and transmissibility. We calculate interval estimates for the epidemic thresholds for stochastic process models in three human populations based on representative surveys of sexual behavior (Uganda, Sweden, USA). For Uganda and Sweden, the epidemic threshold is greater than zero with high confidence. For the USA, the interval includes zero. We discuss the implications of these findings along with the limitations of epidemic models which assume random mixing.     

Patterns of sexual behaviour and reported symptoms of STI/RTIs among young people in Croatia--implications for interventions' planning

This paper describes some of the results of the first national-level survey on sexual behaviour and the distribution of risks to sexually transmitted infections (STIs), including human immunodeficiency virus infection (HIV), among youth in Croatia, and the nature and the extent of their vulnerability to these adverse health outcomes. The study was a cross-sectional, probability-based household survey conducted in 2005, and included 1093 respondents aged 18-24. This paper aims to describe the findings related to the knowledge of HIV transmission, key behavioural outcomes relevant for potential transmission of HIV and STIs, and correlates of genital discharge in young men and young women. More than 80% of young people know that the correct use of condoms protects against HIV and that HIV can be transmitted by someone who looks healthy. Fifty-nine percent of young men and 52.4% of young women reported using condoms during the first sexual intercourse, and 59.3% of men and 46.1% of women used condoms during the last sexual intercourse with a casual partner. This points to the gap between knowledge of condom use and the actual use of condoms as a high proportion of risky sexual contact remain unprotected. Having sexual intercourse frequently or regularly while consuming alcohol was reported by 19.2% of men and 7.6% of women. Much lower proportion are using drugs frequently or regularly during sexual intercourse (3.7% of men and 2.1% of women). Among those sexually experienced, 11.8% of men and 44.1% of women reported ever having a genital discharge. Higher presence of genital discharge in women is suggestive of reproductive tract infections that are not necessarily sexually transmitted. In the multivariate analysis, the lack of knowledge of whether chlamydial infections is an STI and having more than five partners in life were correlates of genital discharge in men, while in women the correlates included having more than five partners in life and not using condoms during the first sexual intercourse. Higher burden of STI-related symptoms was found among men who have men as sexual partners, those who paid for sex, and those with concurrent partnerships. These findings point out to the immediate need to strengthen sexual health education among young people and to the necessity for further development of other broad-based interventions to prevent adverse sexual health outcomes among both men and women, as well as those targeted towards more vulnerable subgroups.     

Biomechanical Diversity of Mating Structures among Harvestmen Species Is Consistent with a Spectrum of Precopulatory Strategies

Diversity in reproductive structures is frequently explained by selection acting at individual to generational timescales, but interspecific differences predicted by such models (e.g., female choice or sexual conflict) are often untestable in a phylogenetic framework. An alternative approach focuses on clade- or function-specific hypotheses that predict evolutionary patterns in terms neutral to specific modes of sexual selection. Here we test a hypothesis that diversity of reproductive structures in leiobunine harvestmen (daddy longlegs) of eastern North America reflects two sexually coevolved but non-overlapping precopulatory strategies, a primitive solicitous strategy (females enticed by penis-associated nuptial gifts), and a multiply derived antagonistic strategy (penis exerts mechanical force against armature of the female pregenital opening). Predictions of sexual coevolution and fidelity to precopulatory categories were tested using 10 continuously varying functional traits from 28 species. Multivariate analyses corroborated sexual coevolution but failed to partition species by precopulatory strategy, with multiple methods placing species along a spectrum of mechanical antagonistic potential. These findings suggest that precopulatory features within species reflect different co-occurring levels of solicitation and antagonism, and that gradualistic evolutionary pathways exist between extreme strategies. The ability to quantify antagonistic potential of precopulatory structures invites comparison with ecological variables that may promote evolutionary shifts in precopulatory strategies.     

Fatal or Harmless: Extreme Bistability Induced by Sterilizing,Sexually Transmitted Pathogens

Models of sexually transmitted infections have become a fixture of mathematical epidemiology. A common attribute of all these models is treating reproduction and mating, and hence pathogen transmission, as uncoupled events. This is fine for humans, for example, where only a tiny fraction of sexual intercourses ends up with having a baby. But it can be a deficiency for animals in which mating and giving birth are tightly coupled, and mating thus mediates both reproduction and pathogen transmission. Here, we model dynamics of sterilizing, sexually transmitted infections in such animals, assuming structural consistency between the processes of reproduction and pathogen transmission. We show that highly sterilizing, sexually transmitted pathogens trigger bistability in the host population. In particular, the host population can end up in two extreme alternative states, disease-free persistence and pathogen-driven extinction, depending on its initial state. Given that sterilizing, sexually transmitted infections that affect animals are abundant, our results might implicate an effective pest control tactic that consists of releasing the corresponding pathogens, possibly after genetically enhancing their sterilization power.     

Susceptible-infected-recovered epidemics in dynamic contact networks

Contact patterns in populations fundamentally influence the spread of infectious diseases. Current mathematical methods for epidemiological forecasting on networks largely assume that contacts between individuals are fixed, at least for the duration of an outbreak. In reality, contact patterns may be quite fluid, with individuals frequently making and breaking social or sexual relationships. Here, we develop a mathematical approach to predicting disease transmission on dynamic networks in which each individual has a characteristic behaviour (typical contact number), but the identities of their contacts change in time. We show that dynamic contact patterns shape epidemiological dynamics in ways that cannot be adequately captured in static network models or mass-action models. Our new model interpolates smoothly between static network models and mass-action models using a mixing parameter, thereby providing a bridge between disparate classes of epidemiological models. Using epidemiological and sexual contact data from an Atlanta high school, we demonstrate the application of this method for forecasting and controlling sexually transmitted disease outbreaks.     

Sexual networks: implications for the transmission of sexually transmitted infections

The structures of sexual networks are essential for understanding the dynamics of sexually transmitted infections. Standard epidemiological models largely disregard the complex patterns of intimate contacts. Social network analysis offers important insight into how to conceptualize and model social interaction and has the potential to greatly enhance the understanding of disease epidemics.     

Profile of young people's advice clinic in reproductive health, 1988-93.

For an unprepared sexually active teenager the 1990s present a minefield of potential problems in addition to the most obvious problems of unplanned pregnancy and sexually transmitted disease. Promoting the importance of a healthy sexual and reproductive life to young people requires openness, better sex education, realistic discussion of related issues, and provision of contraception, as well as support if things go wrong. The Woodhouse Park Clinic in south Manchester opened in October 1988 with the aim of providing weekly advice sessions to young people up to the age of 18. Advice is given on topics such as substance misuse and smoking as well as on family planning. Attendance has steadily increased over the years; from April to June 1993 the average number of attenders per session was 39, one third of whom were young men.     

A new design of stochastic partnership models for epidemics of sexually transmitted diseases with stages

A problem of importance in modelling epidemics of sexually transmitted diseases is the development of mathematical structures accommodating sexual and other contacts among members of a population. Because these models may be complex, it is often necessary to use computer intensive methods in their analysis, which raises questions on the design of computer models. In this paper a new approach to designing models sexual contacts is presented within the context of a stochastic model accommodating the formation and dissolution of partnerships in heterosexual populations. Emphasis will be placed on the development of algorithms with a view towards developing software to implement computer intensive methods. Unlike previous formulations, rather than using rejection methods in Monte Carlo simulations to impose necessary constraints on random functions describing partnership formation, in the new formulation all constraints are satisfied with probability one.     

SEXUALLY TRANSMITTED DISEASE AND PARASITE-MEDIATED SEXUAL SELECTION

Few studies have investigated the consequences of parasite-mediated sexual selection on the parasites involved. In some cases parasite-mediated sexual selection could lead to increased virulence, but I develop a simple model that shows that, if a parasite is sexually transmitted (i.e., is a sexually transmitted disease, or STD) and if mating success of the host is adversely affected by the parasite, then less virulent STDs will be selected for because transmission of the STD depends on the mating success of the host. This selection for reduced virulence could have important consequences for the role of STDs in sexual selection.     

Sexual dimorphism in the Japanese cranial base: a Fourier-wavelet representation

An approach, computational shape analysis (CSA), is presented here which utilizes a Fourier-wavelet representation to numerically describe shape features of biological forms. Two elements are involved: 1) elliptical Fourier functions (EFFs), to provide estimates of global aspects, and 2) continuous wavelet transforms (CWTs) to generate an objective estimate of localized features. EFFs are computed, using a set of pseudohomologous points, to create a precise analog of the boundary. This computed contour is then normalized by scaling and rotated in two-dimensional space to insure a representation that is invariant with respect to starting point, size, and orientation. The predicted point coordinates derived from the EFFs are submitted to CWT for further processing. Wavelet coefficients are then computed to identify localized features, localization being a subjective process with EFFs. The advantage of wavelets is that they eliminate the inevitable subjectivity inherent in the choice of measurements. To test the usefulness of CSA, a sample of cranial base (CB) lateral radiographic outlines was available. Five archaeological periods, Yayoi, Kofun, Kamakura, Edo, and Modern, were utilized (n = 297). Statistically significant differences in sex and archaeological age were found. Although archaeological age differences were present, they were small and random in character, suggesting stability in the CB structures. In contrast, sexually dimorphic differences were present in every group from the Yayoi to the Modern period. This presence of sexually dimorphic differences in shape was consistent with earlier studies of M. nemestrina, G. gorilla, and P. troglodytes. Consequently, it is suggested that the pattern of sexual dimorphism documented in the Japanese CB is a primate pattern with an ancient evolutionary history. The results demonstrate, both visually and numerically, that CSA is a powerful approach for describing both global and localized features of craniofacial structures such as the CB.     

Lesbian health care. What a primary care physician needs to know.

Many primary care physicians take care of lesbians and women sexually active with women without being aware of their patients'' sexual orientation. These women have unique medical and psychosocial needs that each physician must consider. Lesbian identity or being sexually active exclusively with women influences care in areas such as sexually transmitted diseases, risk of human immunodeficiency virus infection, counseling, cancer risk, screening, parenting, depression, alcohol use, and violence. We review an approach to taking a history with all women that facilitates open, comfortable communication with lesbians. We also review specific medical and psychosocial areas of primary care in which caring for lesbians is different from caring for other women. Further research is needed on lesbian health issues to provide appropriate guidelines to clinicians.     

Is Food Insecurity Associated with HIV Risk? Cross-Sectional Evidence from Sexually Active Women in Brazil

Background

Understanding how food insecurity among women gives rise to differential patterning in HIV risks is critical for policy and programming in resource-limited settings. This is particularly the case in Brazil, which has undergone successive changes in the gender and socio-geographic composition of its complex epidemic over the past three decades. We used data from a national survey of Brazilian women to estimate the relationship between food insecurity and HIV risk.

Methods and Findings

We used data on 12,684 sexually active women from a national survey conducted in Brazil in 2006–2007. Self-reported outcomes were (a) consistent condom use, defined as using a condom at each occasion of sexual intercourse in the previous 12 mo; (b) recent condom use, less stringently defined as using a condom with the most recent sexual partner; and (c) itchy vaginal discharge in the previous 30 d, possibly indicating presence of a sexually transmitted infection. The primary explanatory variable of interest was food insecurity, measured using the culturally adapted and validated Escala Brasiliera de Segurança Alimentar. In multivariable logistic regression models, severe food insecurity with hunger was associated with a reduced odds of consistent condom use in the past 12 mo (adjusted odds ratio [AOR] = 0.67; 95% CI, 0.48–0.92) and condom use at last sexual intercourse (AOR = 0.75; 95% CI, 0.57–0.98). Self-reported itchy vaginal discharge was associated with all categories of food insecurity (with AORs ranging from 1.46 to 1.94). In absolute terms, the effect sizes were large in magnitude across all outcomes. Underweight and/or lack of control in sexual relations did not appear to mediate the observed associations.

Conclusions

Severe food insecurity with hunger was associated with reduced odds of condom use and increased odds of itchy vaginal discharge, which is potentially indicative of sexually transmitted infection, among sexually active women in Brazil. Interventions targeting food insecurity may have beneficial implications for HIV prevention in resource-limited settings. Please see later in the article for the Editors'' Summary     

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.     

Are we failing our teenagers? Value of a family planning service for teenagers within the sexually transmitted disease clinic.

Out of 100 teenage girls attending a sexually transmitted disease clinic for the first time, 77 were found to be using a reliable method of contraception and had similar characteristics to teenage girls attending a family planning clinic. The 23 girls not using any reliable contraception exhibited a different pattern of sexual behaviour and were at high risk of unplanned pregnancy. Subsequently, another group of 23 girls not using contraception when seen at the sexually transmitted disease clinic were actively encouraged to attend a family planning clinic. Their risk of unplanned pregnancy was much reduced, although their pattern of sexual activity was unchanged. Ready availability of contraceptive advice for unprotected teenagers in sexually transmitted disease clinics would reduce their high risk of unplanned pregnancy.     

On the importance of risky behavior in the transmission of sexually transmitted diseases

We consider a population with two types of individuals: those who engage in risky sexual behavior (the core) and those who do not (the majority). We are interested in the situation where the majority would not be able to sustain an epidemic without the presence of a core. We show that even if the core is randomly spread out in the general population and is very infectious there will be no epidemic if the proportion of individuals in the core is below a certain threshold (that depends on the infection rates). Risky behavior does put the whole population at risk but only if the behavior is widespread enough. This gives support to the idea of concentrating resources to fight sexually transmitted diseases on the core group in order to lower the proportion of individuals in the core. We show our results for two models: one with total mixing and the other with limited mixing.