Syphilis transmission in US higher among transgender women and Black gay and bisexual men, study finds

Transgender women and Black gay and bisexual men in Chicago are nearly twice as likely to contract syphilis at some point in their lives as white gay men, according to a new study conducted by scientists at Northwestern University.

The study, "Syphilis prevalence, incidence, and demographic differences in a longitudinal study of young sexual and gender minority adults assigned male at birth," is the first to examine syphilis over time among young sexual and gender minorities—a category that encompasses gay and bisexual men, trans women and non-binary individuals. They found meaningful demographic differences in prevalence, across participants' lifetimes and during the six-month period of the study, leading to a more nuanced understanding of risk factors for the under researched group.

The research comes as the Centers for Disease Control and Prevention plan to recommend doxy-PEP, a one-time dose of the antibiotic doxycycline, for use after an unprotected sexual encounter, to gay and bisexual men and trans women. The treatment could help curb cases of syphilis, gonorrhea and chlamydia.

The study was published today (Oct. 10) in The Journal of Infectious Diseases.

Syphilis cases in the U.S. and in Chicago have increased over the past several years, with most new cases occurring among gay and bisexual men. In addition to causing serious health consequences when it is not detected or treated, syphilis is linked to an increased risk of HIV transmission.

Rather than studying only gay men, as with many HIV/STI studies in the past, the study also included gender-diverse individuals from groups often overlooked by prospective research.

"In our sample, transgender populations were the most likely to have a lifetime syphilis diagnosis, but they are rarely included in public health surveillance in a way that allows us to compare to other groups," said Brian Mustanski, the study's lead author and director of the Institute for Sexual and Gender Minority Health and Wellbeing and the Third Coast Center for AIDS Research at Northwestern University Feinberg School of Medicine. "Our findings suggest that preventing and treating syphilis requires outreach to this key population."

A quarter of participants had syphilis at some point in their lifetime and the authors estimate every year 6.7% of them will get a new infection. Forty-five percent of transgender participants had lifetime incidences of syphilis in addition to 42% of Black participants.

Mustanski explained, "My prior research on racial disparities in HIV has shown that they are not attributable to group differences in risk behaviors, but instead are due to social and structural factors such as denser sexual networks and less access to testing and treatment of STIs. Future research should examine if these are the same drivers of disparities in syphilis, and if so, how we can create structural changes that promote greater health equity."

Prospective studies, which track study participants over time, allow scientists to estimate incidence of syphilis. This was among the first U.S.-based prospective study cohort of racially diverse young sexual and gender minorities to observe cases over time and examine risk factors. The study used data from part of a larger study, the Institute's RADAR study, that incorporated social media and social networks to recruit a range of participants.

"Chicago and the U.S. need to buckle down and scale up syphilis testing and treatment among young gay men and trans women, particularly in communities of color," Mustanski said. "Innovative approaches like doxy-PEP deserve further implementation research on how to reach these key populations with this new solution.

"Nationally about half of all syphilis infections are among gay and bisexual men," Mustanski said. "Beyond this worrying statistic, half of syphilis cases among these men occur in men living with HIV. Men who test HIV-negative who are diagnosed with syphilis then have an increased risk of getting HIV because syphilis facilitates HIV transmission. Chicago needs more syphilis prevention efforts to address current transmission rates and curb a potential syphilis epidemic."