The findings and conclusions in this report are those of the authors and do not necessarily represent views of the Centres for Disease Control and Prevention.

Background Incident syphilis among HIV-infected persons indicates ongoing behavioural risk for HIV transmission. Detectable viral loads among co-infected cases may amplify this risk.

Methods Primary and secondary (P&S) cases reported during 2009–2010 from four US sites were cross-matched to local HIV surveillance registries to identify syphilis case-persons infected with HIV prior to or shortly after the syphilis diagnosis. We examined HIV viral load and CD4 results collected within 6 months before or after syphilis diagnosis for the co-infected cases identified. Independent correlates of detectable viral loads (≥200 copies/mL) were determined.

Results We identified 1675 cases of incident primary or secondary syphilis among persons with HIV. Median age was 37 years, 99.5% were male, 41.1% were African American, 24.5% Hispanic, and 79.9% of the HIV diagnoses were made at least one year prior to syphilis diagnosis. Among those co-infected, there were no viral load results reported for 188 (11.2%); of the 1487 (88.8%) with reported viral load results, 809 (54.4%) had a detectable viral load (median 25,101 copies/mL, range 206–3,590,000 copies/mL). Detectable viral loads were independently correlated with syphilis diagnosed at younger age, at an STD clinic, and closer in time to HIV diagnosis.

Conclusion More than half of syphilis case-persons identified with HIV had a detectable viral load collected within 6 months of the syphilis diagnosis. This suggests virologic as well as active behavioural risk for transmitting HIV.