What are the factors contributing to the increasing incidence of syphilis among sexually active adults, particularly in young adults and high-risk groups such as men who have sex with men?

Why Syphilis is Increasing

The resurgence of syphilis, particularly among men who have sex with men (MSM), is driven by a convergence of behavioral changes including decreased condom use due to HIV treatment optimism, increased methamphetamine use facilitating high-risk sexual encounters, internet-based partner seeking enabling anonymous contacts, and suboptimal screening practices that fail to detect infections early enough to interrupt transmission chains. 1, 2, 3

Primary Drivers of the Epidemic

Behavioral and Social Factors in MSM

• HIV treatment optimism has fundamentally altered risk perception—the advent of highly active antiretroviral therapy (HAART) rendering HIV-infected individuals noninfectious and the availability of pre-exposure prophylaxis (PrEP) has substantially reduced concern about HIV transmission, resulting in higher levels of condomless sex among MSM 4, 1, 2

• Methamphetamine use is strongly associated with increased numbers of sexual partners and consequently increased risk of infectious syphilis, creating a pharmacologically-driven pathway to high-risk encounters 4, 3

• Internet and app-based partner seeking has transformed sexual networking by facilitating anonymous sexual encounters and increasing the absolute number of partners, making traditional contact tracing more difficult 3

• MSM inherently have structural factors favoring transmission: they tend to have more sexual partners than heterosexuals and engage in anal sex, which is highly efficient for syphilis transmission and makes primary chancres in the rectum difficult to detect 2

Screening Failures and Healthcare Access Barriers

• Screening remains suboptimal despite clear guidelines—studies demonstrate that even high-risk MSM populations are not being screened at recommended frequencies 4

• Modeling studies indicate that more frequent screening of MSM every 3-6 months is more cost-effective than increased breadth of coverage, yet this intensive approach is not being implemented consistently 4

• Societal rejection and anticipated discrimination in healthcare settings lead many sexually active MSM to delay needed screening and treatment, allowing them to remain infectious to partners for longer periods 2

• Economic marginalization among black MSM decreases access to preventive services and treatment, contributing to the disproportionate concentration of syphilis in this population 2

Biological and Epidemiological Synergies

• The bidirectional relationship between syphilis and HIV creates a vicious cycle—syphilis promotes increased HIV viral shedding, while HIV-infected persons may have atypical presentations that delay diagnosis 4

• Rising STD rates among MSM indicate both ongoing risky behavior and increased potential for HIV transmission through the synergistic effect of STDs on HIV infectivity and susceptibility 4

• Assortative mixing patterns, particularly black MSM being more likely to have other black sex partners, concentrate infection within networks and amplify transmission 2

Recent Epidemic Trends

• From 2019 to 2023, US syphilis cases increased by 61% overall, with diagnoses among females increasing by 112% and congenital syphilis cases increasing by 106% 5

• Despite declining syphilis prevalence in the general U.S. population, sustained outbreaks among MSM continue to occur, with MSM comprising one-third (32.7%) of all males with primary and secondary syphilis in 2023 4, 5

• In San Francisco, early syphilis increased from 44 cases in 1999 to 522 cases in 2003, with cases more likely in gay or bisexual men, those with HIV infection, those with anonymous partners, and those who met partners on the Internet 3

Critical Clinical Pitfalls Contributing to Spread

• Atypical presentations are being missed—primary syphilitic chancres may be multiple and painful, contradicting the classic teaching of a single painless lesion, leading to delayed diagnosis 4

• Oral swab samples can be positive for T. pallidum even in the absence of symptoms or oral lesions, highlighting that asymptomatic individuals may be infectious 4

• Some laboratories report incomplete nontreponemal titers (e.g., reporting RPR as >1:32 without specifying the end titer), making it impossible to manage patients effectively and track treatment response 4

What This Means for Prevention

• The CDC recommends routine screening at least annually for all sexually active MSM, with screening every 3-6 months for those with multiple partners, anonymous partners, or sex in conjunction with illicit drug use 4, 6

• Doxycycline post-exposure prophylaxis (200 mg within 72 hours after sex) is now recommended for MSM and transgender women with a history of bacterial STI in the past year 7, 5

• Partner notification and presumptive treatment remain critical—persons exposed within 90 days of a partner's diagnosis should receive benzathine penicillin G 2.4 million units IM regardless of their serologic status 8