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Condomless Sex, Younger Age Tied to Rising Syphilis Rate in Swiss HIV Group

June 26, 2017

Sex without condoms and younger age raised chances of contracting syphilis in an analysis of the Swiss HIV Cohort Study (SHCS). Antiretroviral therapy (ART) and resulting immune reconstitution did not protect against syphilis.

After years of waning syphilis incidence, the sexually transmitted infection (STI) has reemerged as a public health threat, particularly in people with HIV infection and primarily in men who have sex with men (MSM). Studies on interactions between ART and syphilis incidence or treatment have yielded divergent results. To address these issues, SHCS investigators analyzed syphilis incidence and risk factors in their cohort.

The SHCS is an ongoing prospective cohort study that collects clinical, laboratory and sociodemographic data on participants every six months. Annual syphilis testing resumed in the SHCS in 2004. This 2004-2014 analysis included cohort members with a baseline negative syphilis test and a later syphilis screening result after at least one year of follow-up. The researchers defined an incident case as one with positive treponemal and nontreponemal results. They used parametric interval censored models to assess immunologic, behavioral and ART-related factors associated with incident syphilis in three HIV risk groups: MSM, heterosexuals and injection drug users (IDUs).

Among 3575 cohort members, 207 had 226 incident syphilis cases, including 207 cases (92%) in MSM, 15 (6%) in heterosexuals and four (2%) in IDUs. Incidence per 1000 person-years stood at 26.8 in MSM, 1.8 in heterosexuals and 1.3 in IDUs. Syphilis incidence rose over the 2004-2014 study period (Pearson correlation between annual incidence and year 0.9, P < .01), doubling from 2004 to 2012.

Analysis of incident syphilis risk factors focused solely on MSM. Older age was independently associated with lower syphilis incidence (hazard ratio [HR] 0.75 for every five years, 95% confidence interval [CI] 0.69 to 0.81). This analysis also linked treatment with nevirapine (Viramune), but no other antiretroviral or antiretroviral classes, to lower syphilis incidence (HR 0.45, 95% CI 0.21 to 0.96). This association held true only in people with an undetectable viral load while on nevirapine. Univariable analysis saw no association between taking ART and syphilis incidence (HR 1.2, 95% CI 0.8 to 1.9). Neither nadir CD4 count nor time-updated CD4 count affected syphilis incidence.

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Three factors independently predicted higher syphilis incidence: later calendar year (HR 1.22, 95% CI 1.14 to 1.31), syphilis testing more than the recommended once yearly (HR 1.80, 95% CI 1.28 to 2.53) and condomless sex with an occasional partner (HR 3.74, 95% CI 2.61 to 5.36).

Univariable analysis linked IDU (versus no IDU) with incident syphilis (HR 11.6, 95% CI 2.1 to 65). But the researchers did not include IDU in the multivariable analysis because only nine people reported injecting drugs.

The SHCS investigators believe the lack of an association between taking ART and syphilis incidence "implies that immune reconstitution does not provide significant protection against syphilis." They note that the 26.8 per 1000 syphilis incidence in Swiss MSM lies below estimates of 42, 46 and 62 per 1000 in MSM in Canada, the Netherlands and the United States. Because of rising syphilis incidence in MSM driven mainly by condom-free sex, the authors propose that "more frequent screening targeted for MSM is needed to limit syphilis spread (and other STIs) and the probably consequent increase of HIV infections."

Mark Mascolini writes about HIV infection.


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