Comparison of sexual mixing patterns for syphilis in endemic and outbreak settings

Sexual mixing in bisexual activity in male-male partnerships in Melbourne, Australia

Background The patterns of sexually transmitted infections (STIs) in populations may be influenced by the sexual mixing within the population. We aimed to investigate the assortative sexual mixing pattern by bisexuality in male-male partnerships. Methods This was a retrospective repeated cross-sectional study of men with mostly regular male partners attending the Melbourne Sexual Health Centre between 2011 and 2019. Data on sexual practices, including their sexual practices, presence of other male/female sex partners and the gender of sexual partners in the previous 3 and 12months, were collected using computer-assisted self-interview. We calculated the proportion of male partnerships where at least one man in the partnership reported a female sex partner. Results A total of 2056 male-male partnerships (i.e. 4112 individuals) with a median age of 29 years (IQR 25 to 35) were included. Overall, in 94.4% (1941/2056) of male-male partnerships both men had male partners only; however, in 5.5% (113/2056) of partnerships, one man had both male and female partners, and in 0.1% (2/2056) partnerships, both men had both male and female partners. No assortative relationship was found on the sexual mixing by bisexuality in male-male partnerships due to the low assortativity coefficient (r =0.006, 95% CI: -0.004 to 0.016). Conclusion One in 20 male-male partnerships had at least one man who had both male and female partners within the preceding year. Individuals were not selective by bisexuality, suggesting that partnerships of bisexual individuals are mixed proportionately to the distribution of their characteristics. Still, these sexual mixing practices may affect STI transmission dynamics.

Assortative Sexual Mixing by Age, Region of Birth, and Time of Arrival in Male-Female Partnerships in Melbourne, Australia

BACKGROUND

Patterns of sexual mixing may be influenced by demographic factors where individuals show a preference for partners with particular traits and may have different levels of risk for HIV and sexually transmitted infection transmission. We aimed to explore age differences and mixing by region of birth among male-female partnerships.

METHODS

Male-female partnerships who presented to Melbourne Sexual Health Centre in Australia between 2015 and 2019 were investigated. Age and country of birth of sexual partners were collected. We calculated the age differences between partners and created tables demonstrating partnership mixing by age groups and by international region of birth.

RESULTS

A total of 2112 male-female partnerships (i.e., 4224 individuals) were included. The median age was 27 years (interquartile range, 23-31 years). Between men and women in partnerships, the median age difference was 1 year. Nearly half of all individuals (49.1% [2072 of 4224]) were in a partnership with another individual in the same 5-year interval age group as their own, and a majority of individuals (58.5% [2334 of 3988]) were in a partnership with another individual from the same region of birth when compared with other regions.

CONCLUSIONS

There is a strong assortative sexual mixing pattern by age and region of birth among male-female partnerships in Melbourne, Australia. These results may have applications in further research to understand sexually transmitted infection transmission among clients attending sexual health centers, particularly those born overseas.

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.

Human papillomavirus vaccine coverage in male-male partnerships attending a sexual health clinic in Melbourne, Australia

We aimed to investigate the sexual mixing by human papillomavirus (HPV) vaccination status in male-male partnerships and estimate the proportion of male-male partnerships protected against HPV. We analyzed male-male partnerships attending the Melbourne Sexual Health Center between 2018 and 2019. Data on self-reported HPV vaccination status were collected. Newman’s assortativity coefficient was used to examine the sexual mixing by HPV vaccination status. Assortativity refers to the tendency of individuals to have partners with similar characteristics (i.e. same vaccination status). Of 321 male-male partnerships where both men reported their HPV vaccination status, 52.6% (95% CI: 47.0–58.2%) partnerships had both men vaccinated, 32.1% (95% CI: 27.0–37.5%) partnerships had only one man vaccinated, and 15.3% (95% CI: 11.5–19.7%) had both men unvaccinated. The assortativity on HPV vaccination status was moderate (assortativity coefficient = 0.265, 95% CI: 0.196–0.335). There were about 15% of male-male partnerships where both men were not protected against HPV. Interventions targeting vaccinated individuals to encourage their unvaccinated partners to be vaccinated might increase the HPV vaccine coverage.

Being fast or cautious? Sociocultural conditions influencing the sexual pathways of Black females in the United States

BACKGROUND

Black females in the United States face unique sociocultural conditions that impact their sexual development and increase their risk for sexually transmitted infections (STI), including but not limited to chlamydia, gonorrhea, and HIV. Research has not adequately explained how sociocultural conditions contribute to this increased risk. The purpose of our investigation was to explore the sociocultural conditions that influence Black cisgender females risk for STI.

METHODS

This grounded theory study involved in-depth audio-recorded interviews with 20, primarily heterosexual, Black females ages 19-62.

RESULTS

Findings informed a conceptual model that builds on previous theory about the sexual development of Black females and explains how sociocultural conditions impact two, participant identified, sexual pathways: Fast and Cautious. Movement on these sexual pathways was not always a linear trajectory; some participants shifted between pathways as their sociocultural contexts changed (i.e., sexual assault, STI, and level of protection). The Fast sexual pathway often led to greater STI risk.

CONCLUSIONS

This model may inform future research designed to prevent STI/HIV and promote the sexual health of Black females across the life course.

Predicting Emergence of Primary and Secondary Syphilis Among Women of Reproductive Age in US Counties

BACKGROUND

Syphilis, a sexually transmitted infection that can cause severe congenital disease when not treated during pregnancy, is on the rise in the United States. Our objective was to identify US counties with elevated risk for emergence of primary and secondary (P&S) syphilis among women of reproductive age.

METHODS

Using syphilis case reports, we identified counties with no cases of P&S syphilis among women of reproductive age in 2017 and 1 case or more in 2018. Using county-level syphilis and sociodemographic data, we developed a model to predict counties with emergence of P&S syphilis among women and a risk score to identify counties at elevated risk.

RESULTS

Of 2451 counties with no cases of P&S syphilis among women of reproductive age in 2017, 345 counties (14.1%) had documented emergence of syphilis in 2018. Emergence was predicted by the county's P&S syphilis rate among men; violent crime rate; proportions of Black, White, Asian, and Hawaiian/Pacific Islander persons; urbanicity; presence of a metropolitan area; population size; and having a neighboring county with P&S syphilis among women. A risk score of 20 or more identified 75% of counties with emergence.

CONCLUSIONS

Jurisdictions can identify counties at elevated risk for emergence of syphilis in women and tailor prevention efforts. Prevention of syphilis requires multidisciplinary collaboration to address underlying social factors.

Lost and found: applying network analysis to public health contact tracing for HIV

Infectious disease surveillance is often case-based, focused on people diagnosed and their contacts in a predefined time window, and treated as independent across infections. Network analysis of partners and contacts joining multiple investigations and infections can reveal social or temporal trends, providing opportunities for epidemic control within broader networks. We constructed a sociosexual network of all HIV and early syphilis cases and contacts investigated among residents of 11 contiguous counties in North Carolina over a two-year period (2012-2013). We anchored the analysis on new HIV diagnoses ("indexes"), but also included nodes and edges from syphilis investigations that were within the same network component as any new HIV index. After adding syphilis investigations and deduplicating people included in multiple investigations (entity resolution), the final network comprised 1470 people: 569 HIV indexes, 700 contacts to HIV indexes who were not also new cases themselves, and 201 people who were either indexes or contacts in eligible syphilis investigations. Among HIV indexes, nearly half (48%; n = 273) had no located contacts during single-investigation contact tracing, though 25 (9%) of these were identified by other network members and thus not isolated in the final multiple investigation network. Constructing a sociosexual network from cases and contacts across multiple investigations mitigated some effects of unobserved partnerships underlying the HIV epidemic and demonstrated the HIV and syphilis overlap in these networks.

Factors Associated With Human Immunodeficiency Virus Infections Linked in Genetic Clusters But Disconnected in Partner Tracing

BACKGROUND

Successful partner notification can improve community-level outcomes by increasing the proportion of persons living with human immunodeficiency virus (HIV) who are linked to HIV care and virally suppressed, but it is resource intensive. Understanding where HIV transmission pathways may be undetected by routine partner notification may help improve case finding strategies.

METHODS

We combined partner notification interview and HIV sequence data for persons diagnosed with HIV in Wake County, NC in 2012 to 2013 to evaluate partner contact networks among persons with HIV pol gene sequences 2% or less pairwise genetic distance. We applied a set of multivariable generalized estimating equations to identify correlates of disparate membership in genetic versus partner contact networks.

RESULTS

In the multivariable model, being in a male-male pair (adjusted odds ratio [AOR], 16.7; P = 0.01), chronic HIV infection status (AOR, 4.5; P < 0.01), and increasing percent genetic distance between each dyad member's HIV pol gene sequence (AOR, 8.3 per each 1% increase, P < 0.01) were all associated with persons with HIV clustering but not being identified in the partner notification network component. Having anonymous partners or other factors typically associated with risk behavior were not associated.

CONCLUSIONS

Based on genetic networks, partnerships which may be stigmatized, may have occurred farther back in time or may have an intervening partner were more likely to be unobserved in the partner contact network. The HIV genetic cluster information contributes to public health understanding of HIV transmission networks in these settings where partner identifying information is not available.

Addressing sexually transmitted infections in the sociocultural context of black heterosexual relationships in the United States

RATIONALE

Black girls and women are disproportionately affected by sexually transmitted infections (STIs) and human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS). Research shows sexual networks, sexual concurrency, and assortative mixing impacting racial disparities in STI/HIV. However, the underlying sociocultural conditions of these phenomenon have yet to be fully explored within a framework of Black girls' and women's sexual development.

OBJECTIVE

This grounded theory study investigated the sociocultural conditions and processes of becoming a sexual Black woman in order to understand the sociocultural drivers of STI/HIV rates among this group.

METHOD

We used theoretical sampling to select and interview 20 Black women aged 19-62 years old from a Midwestern community.

RESULTS

This study revealed sociocultural conditions related to Black heterosexual relationships and STI/HIV risk. Protecting Black men, silencing Black girls and women, cultural norms and messaging about sexuality, and gendered societal expectations and sexual stereotypes contribute to STI/HIV risk in Black girls and women.

CONCLUSIONS

Our findings demonstrate how the intersection of social and systemic structures (i.e.,history, incarceration, unemployment) shape the context of Black heterosexual relationships. We suggest STI/HIV prevention efforts address these systemic, cultural, and societal factors in order to effectively reduce racial disparities in STI/HIV risk.

Déjà vu? A Comparison of Syphilis Outbreaks in Houston, Texas

BACKGROUND

After reaching an all-time low in 2000, syphilis incidence in the United States has increased as the burden shifted from heterosexuals to men who have sex with men (MSM). Houston, Texas, experienced 2 outbreaks of syphilis during this transformation in trends. Further evaluation is necessary to determine if these outbreaks occurred among the same subpopulations.

METHODS

Surveillance data collected on all reported infectious syphilis cases in Houston from 1971 to 2013 were analyzed. Trends in incidence among MSM and human immunodeficiency virus-positive Houston residents were examined. Peak syphilis years subsequent to 1999, years 2007 and 2012, were compared to determine if outbreaks arose in distinctive subpopulations. Categorical variables between these years were compared using chi-square and Fisher's exact tests, whereas further associations between the years were evaluated using multivariable logistic regression.

RESULTS

Incidence among MSM was 20.9 to 32.1 times higher than other men from 2005 to 2013. After adjusting for covariates, cases in 2012 were significantly more likely to be Hispanic (adjusted odds ratio [AOR] = 1.61; 95% confidence interval [95% CI], 1.03-2.53), reported meeting partners via the Internet (AOR, 1.74; 95% CI, 1.18-2.58), and engaged in anonymous sex (AOR, 1.92; 95% CI, 1.40-2.63) in comparison to cases in 2007.

CONCLUSIONS

We found marked disparities of syphilis by subpopulation in Houston. Herein, we present evidence that outbreaks have been distinct in a major southern city with a high burden of syphilis.

Disassortative mixing patterns of drug-using and sex networks on HIV risk behaviour among young drug users in Yunnan, China

OBJECTIVES

The dominant mode of HIV transmission in China has changed from injection drug use to sexual contact. The objectives of this study were to describe the disassortative and assortative mixing patterns of drug-using and sex networks among young drug users in China.

STUDY DESIGN

Cross-sectional study.

METHODS

Respondent-driven sampling (RDS) was used to recruit young drug users in an egocentric network study in Yunnan, China. Egos were categorized as having disassortative mixing network patterns if they reported both sex and drug-using networks. Egos who only had a sex network (no drug-using network), or only a drug-using network (no sex network) were categorized as having assortative mixing network patterns. Multiple logistic regression was performed to analyze the relationships between disassortative patterns with risky sexual behaviour and drug-using practices.

RESULTS

A total of 426 participants were recruited into the study. Two hundred forty-two egos reported disassortative mixing patterns and 139 egos had assortative patterns. The RDS-adjusted proportion of having a disassortative pattern was 53.2%. Participants with disassortative patterns were more likely to engage in HIV risk behaviour compared to those with assortative patterns. Specifically, drug users with disassortative patterns reported more multiple sex partners (31.4% vs 19.6%), concurrent partnerships (52.1% vs 39.0%), non-regular sex partners (12.0% vs 4.3%), and sex partners who were IDUs (24.9% vs 12.5%). Consistent condom use with regular or non-regular partners was low (between 18.9% and 47.2%) regardless of the mixing pattern. However, parenteral risk for HIV transmission was relatively low in both groups.

CONCLUSIONS

The transition of the HIV epidemic in China from injection drug use to sexual contact may be attributed to disassortative mixing in drug-use and sexual networks. HIV programs should consider disassortative mixing patterns when designing new behavioural interventions.

Associations between blood lead level and substance use and sexually transmitted infection risk among adults in the United States

The effects of low-level lead exposure on neuropsychological status in the United States (US) general adult population have been reported, and the relationship between neuropsychiatric dysfunction and health risk behaviors including substance use and sexual risk taking is well established. However, the potential influence of lead exposure on risk-taking behavior has received little attention. Using the National Health and Nutrition Examination Survey (NHANES) 2005-2010, we estimated multivariable logistic regression models to measure odds ratios (ORs) and 95% confidence intervals (CIs) for the cross-sectional associations between blood lead level and risk behaviors including binge drinking, drug use, and indicator of sexually transmitted infection (STI) risk. STI indicators included past 12 month sexual risk behaviors (age mixing with partners who were at least five years younger or older and multiple partnerships), self-reported STI, and biologically-confirmed herpes simplex virus type 2 (HSV-2) infection. Dose-response like relationships were observed between blood lead and substance use, age mixing with younger and older partners, self-reported STI, and HSV-2. In addition, participants with lead levels in highest quartile versus those with levels in the lowest quartile had over three times the odds of binge drinking and over twice the odds of injection drug or cocaine use in the past 12 months, while being in one of the top two quartiles was significantly associated with 30-70% increased odds of multiple partnerships, sex with older partners, and self-reported and biologically confirmed STI. Results from this study suggested that lead exposure may contribute to substance use, sexual risk-taking, and STI. However, given limitations inherent in the cross-sectional nature of the study, additional studies that use longitudinal data and measure detailed temporal information are warranted.

Assortativity coefficient-based estimation of population patterns of sexual mixing when cluster size is informative

OBJECTIVES

Population sexual mixing patterns can be quantified using Newman's assortativity coefficient (r). Suggested methods for estimating the SE for r may lead to inappropriate statistical conclusions in situations where intracluster correlation is ignored and/or when cluster size is predictive of the response. We describe a computer-intensive, but highly accessible, within-cluster resampling approach for providing a valid large-sample estimated SE for r and an associated 95% CI.

METHODS

We introduce needed statistical notation and describe the within-cluster resampling approach. Sexual network data and a simulation study were employed to compare within-cluster resampling with standard methods when cluster size is informative.

RESULTS

For the analysis of network data when cluster size is informative, the simulation study demonstrates that within-cluster resampling produces valid statistical inferences about Newman's assortativity coefficient, a popular statistic used to quantify the strength of mixing patterns. In contrast, commonly used methods are biased with attendant extremely poor CI coverage. Within-cluster resampling is recommended when cluster size is informative and/or when there is within-cluster response correlation.

CONCLUSIONS

Within-cluster resampling is recommended for providing valid statistical inferences when applying Newman's assortativity coefficient r to network data.

Structural bridging network position is associated with HIV status in a younger Black men who have sex with men epidemic

Younger Black men who have sex with men (BMSM) ages 16-29 have the highest rates of HIV in the United States. Despite increased attention to social and sexual networks as a framework for biomedical intervention, the role of measured network positions, such as bridging and their relationship to HIV risk has received limited attention. A network sample (N = 620) of BMSM respondents (N = 154) and their MSM and transgendered person network members (N = 466) was generated through respondent driven sampling of BMSM and elicitation of their personal networks. Bridging status of each network member was determined by a constraint measure and was used to assess the relationship between this bridging and unprotected anal intercourse (UAI), sex-drug use (SDU), group sex (GS) and HIV status within the network in South Chicago. Low, moderate and high bridging was observed in 411 (66.8 %), 81 (13.2 %) and 123 (20.0 %) of the network. In addition to age and having sex with men only, moderate and high levels of bridging were associated with HIV status (aOR 3.19; 95 % CI 1.58-6.45 and aOR 3.83; 95 % CI 1.23-11.95, respectively). Risk behaviors observed including UAS, GS, and SDU were not associated with HIV status, however, they clustered together in their associations with one another. Bridging network position but not risk behavior was associated with HIV status in this network sample of younger BMSM. Socio-structural features such as position within the network may be important when implementing effective HIV prevention interventions in younger BMSM populations.

Does core area theory apply to sexually transmitted diseases in rural environments?

BACKGROUND

Our objective was to determine the extent to which geographical core areas for gonorrhea and syphilis are located in rural areas as compared with urban areas.

METHODS

Incident gonorrhea (January 1, 2005-December 31, 2010) and syphilis (January 1, 1999-December 31, 2010) rates were estimated and mapped by census tract and quarter. Rurality was measured using percent rural and rural-urban commuting area (rural, small town, micropolitan, or urban). SaTScan was used to identify spatiotemporal clusters of significantly elevated rates of infection. Clusters lasting 5 years or longer were considered core areas; clusters of shorter duration were considered outbreaks. Clusters were overlaid on maps of rurality and qualitatively assessed for correlation.

RESULTS

Twenty gonorrhea core areas were identified: 65% were in urban centers, 25% were in micropolitan areas, and the remaining 10% were geographically large capturing combinations of urban, micropolitan, small town, and rural environments. Ten syphilis core areas were identified with 80% in urban centers and 20% capturing 2 or more rural-urban commuting areas. All 10 (100%) of the syphilis core areas overlapped with gonorrhea core areas.

CONCLUSIONS

Gonorrhea and syphilis rates were high for rural parts of North Carolina; however, no core areas were identified exclusively for small towns or rural areas. The main pathway of rural sexually transmitted disease (STI) transmission may be through the interconnectedness of urban, micropolitan, small town, and rural areas. Directly addressing STIs in urban and micropolitan communities may also indirectly help address STI rates in rural and small town communities.

Sexual networks, surveillance, and geographical space during syphilis outbreaks in rural North Carolina

BACKGROUND

Sexually transmitted infections (STIs) spread along sexual networks whose structural characteristics promote transmission that routine surveillance may not capture. Cases who have partners from multiple localities may operate as spatial network bridges, thereby facilitating geographical dissemination. We investigated how surveillance, sexual networks, and spatial bridges relate to each other for syphilis outbreaks in rural counties of North Carolina.

METHODS

We selected from the state health department's surveillance database cases diagnosed with primary, secondary, or early latent syphilis during October 1998 to December 2002 and who resided in central and southeastern North Carolina, along with their sex partners and their social contacts irrespective of infection status. We applied matching algorithms to eliminate duplicate names and create a unique roster of partnerships from which networks were compiled and graphed. Network members were differentiated by disease status and county of residence.

RESULTS

In the county most affected by the outbreak, densely connected networks indicative of STI outbreaks were consistent with increased incidence and a large case load. In other counties, the case loads were low with fluctuating incidence, but network structures suggested the presence of outbreaks. In a county with stable, low incidence and a high number of cases, the networks were sparse and dendritic, indicative of endemic spread. Outbreak counties exhibited densely connected networks within well-defined geographic boundaries and low connectivity between counties; spatial bridges did not seem to facilitate transmission.

CONCLUSIONS

Simple visualization of sexual networks can provide key information to identify communities most in need of resources for outbreak investigation and disease control.

Determinants of sexual network structure and their impact on cumulative network measures

There are four major quantities that are measured in sexual behavior surveys that are thought to be especially relevant for the performance of sexual network models in terms of disease transmission. These are (i) the cumulative distribution of lifetime number of partners, (ii) the distribution of partnership durations, (iii) the distribution of gap lengths between partnerships, and (iv) the number of recent partners. Fitting a network model to these quantities as measured in sexual behavior surveys is expected to result in a good description of Chlamydia trachomatis transmission in terms of the heterogeneity of the distribution of infection in the population. Here we present a simulation model of a sexual contact network, in which we explored the role of behavioral heterogeneity of simulated individuals on the ability of the model to reproduce population-level sexual survey data from the Netherlands and UK. We find that a high level of heterogeneity in the ability of individuals to acquire and maintain (additional) partners strongly facilitates the ability of the model to accurately simulate the powerlaw-like distribution of the lifetime number of partners, and the age at which these partnerships were accumulated, as surveyed in actual sexual contact networks. Other sexual network features, such as the gap length between partnerships and the partnership duration, could-at the current level of detail of sexual survey data against which they were compared-be accurately modeled by a constant value (for transitional concurrency) and by exponential distributions (for partnership duration). Furthermore, we observe that epidemiological measures on disease prevalence in survey data can be used as a powerful tool for building accurate sexual contact networks, as these measures provide information on the level of mixing between individuals of different levels of sexual activity in the population, a parameter that is hard to acquire through surveying individuals.