Drugs used to treat HIV could affect how the body responds to syphilis, inadvertently contributing to a current outbreak.

Syphilis cases in several countries have risen sharply in recent years, primarily affecting men who have sex with men. In this study, researchers investigating the outbreak hypothesize that medications used in highly active antiretroviral therapy (HAART), the gold standard for treating HIV infection, could reduce the body’s immune responses to particular diseases, including syphilis.

“After reading about the astounding increase in syphilis cases, I also noticed a huge gap between syphilis and other sexually transmitted infections like chlamydia and gonorrhea,” said Michael Rekart, lead author and clinical professor in UBC’s school of population and public health. “That led me to believe there must be something else going on.”

Until now, research and clinical communities have suggested that the outbreak could be a result of more risky sexual behaviour, such as people having sex without condoms, springing from the perception that the risk of HIV infection is low due to the availability and effectiveness of HAART.


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However, the study points to evidence showing that many men with HIV do not practice high-risk sexual behaviour and that new syphilis cases are commonly observed in people on HAART. It also suggests there are plausible biological explanations for HAART effects on the body’s immune response to certain diseases, says Caroline Cameron, a microbiologist at the University of Victoria who was part of the study team.

Rekart and his colleagues used mathematical modeling to predict syphilis rates based on the premise that risky sexual behaviour was the sole cause, and calculated a rate of increase lower than today’s actual rate. When they considered both behaviour changes and possible immunity effects, their model predicted a rate that closely approximated the actual incidence of syphilis in B.C. and several other countries. Researchers have not yet compared data of people on HAART who contract syphilis with those not on HAART who contract the infection to establish a link.

Read the full press release here.