Across Australia during the past decade, incident infections of syphilis and HIV have increased, especially among gay men. Syphilis epidemics also have resurged, coincidentally with increases in HIV diagnoses, in other industrialized settings. Noting that sexually transmitted infections (STI) "are a biologically plausible cofactor for increasing HIV transmission," the authors undertook this study to learn whether "strategies purely targeting syphilis also have an indirect impact on HIV incidence."
The researchers created an agent-based computer model to simulate the transmission and progress of HIV and syphilis in a population of sexually active gay men; the model was calibrated to reflect the epidemics in Victoria. Detailed behavioral data from a variety of sources informed the model, which was used to study the potential epidemiological impact of different public health interventions.
Assuming that syphilis could be a biological cofactor for HIV transmission, ranging from no effect to a five-fold increase in risk, the model suggests that the effective implementation of Australia's syphilis action plan could cut the number of HIV infections "by up to 48 percent over the next decade in the absence of any specific HIV interventions," the authors reported.
"It is plausible that effective implementation of interventions targeting syphilis epidemics can have an indirect effect of mitigating the spread of HIV," the researchers concluded. "The possible effects of STI should be considered in the design, implementation and evaluation of public health strategies and programs."