The Intersections Between Syphilis and HIV

Syphilis and HIV are very different sexually transmitted infections (STIs). Syphilis is a bacterial infection that is treatable with antibiotics. It can have serious, even fatal consequences if not treated, but syphilis is easy to treat and cure once an infection has been identified. HIV, in contrast, is caused by a virus. It can be treated, quite effectively, with highly active anti-retroviral therapy (called HAART or cART), but it currently can not be fully cured.

Syphilis and HIV infections also have several things in common. In the early stages of infection, they are both difficult to detect without medical intervention. The early stage syphilis sores are painless. If not in a visible location, they are easily missed. Newly acquired HIV infections often have no recognizable symptoms, and HIV can cause no serious symptoms for years or even decades. In addition, each infection has long been known to make a person more susceptible to the other. Syphilis sores provide an easy entry point for HIV; HIV impairs the immune system in ways that make it easier for syphilis to take hold.

Recent research suggests that there may be even more interactions between syphilis and HIV than were previously known. Some are directly related to infections. Others are related to disease treatment and sexual behavior.

Doctor talking to patient
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Linking HIV Treatment and Syphilis Infection?

In 2017, a group of scientists realized that syphilis infection was rising faster than other bacterial STIs among men who have sex with men (MSM). Until that point, most people thought that HIV treatment led to increased STI infection rates among men with HIV because of behavioral factors. In other words, the prevailing belief was that because men knew that effective HIV treatment and prophylaxis reduced HIV risk, they were practicing less safe sex. This, in turn, was increasing their risk of STIs other than HIV. However, if syphilis rates were increasing faster than other STI rates, something else might also be going on.

The researchers hypothesized that HAART might alter the way the immune system responds to infections. Specifically, they thought that those changes might enhance susceptibility to the type of bacteria that causes syphilis. This could, in turn, explain why syphilis rates were rising faster than rates of chlamydia and gonorrhea. That said, the research was quite preliminary, and there were other possible explanations. Among other things, doctors may be less effective about screening men for gonorrhea and chlamydia than HIV and syphilis. Still, it is a question that merits further exploration.

Does that mean that HAART is a bad idea? Absolutely not. Early treatment is important not just for improving individual health, but also for reducing the spread of HIV. It does, however, mean that there needs to be continuing education about the risk of STIs other than HIV, particularly in the context of treatment. There also needs to be more effective and reliable screening and treatment for syphilis.

Linking Syphilis Infection and HIV Prevention

Another important development in understanding the link between syphilis and HIV is the growing recognition that new diagnoses of sexually transmitted infections represent opportunities for prevention. Several groups of researchers have found that MSM who are newly diagnosed with syphilis are at very high risk of then becoming infected with HIV.

The implication? Use new syphilis infections to prioritize getting men access to pre-exposure prophylaxis (PrEP). Providing high-risk men with PrEP can lower their risk of acquiring HIV by treating them in advance of infection. Prioritizing high-risk men can also make PrEP more cost-effective, as it makes certain the treatment first gets to the individuals who need it most. In this case, targeting MSM with syphilis diagnoses identifies a high-risk population who are clearly both being exposed to STIs and not reliably practicing safer sex.

A Word From Verywell

Syphilis and HIV are very different sexually transmitted infections, but their similarities can potentially lead to a number of problematic interactions. As such, it is important to highlight the importance of both screening and prevention. Neither of these diseases is easily recognizable without a doctor's visit. That means that regular STI screening should be a priority for anyone at high risk of developing any ST, including syphilis and HIV. Both of these diseases are preventable if people consistently practice safer sex. Encouraging people to try to do so should be a priority as well. That's true even when they can't be perfect. After all, STIs aren't spread every time someone has sex. That means forgetting to use a condom (external or internal) during one encounter isn't a good reason to avoid using a condom when you see them again.

Still, regular screening and consistent safer sex practices aren't options for everyone. Not everyone has access to affordable medical care. Not everyone has the ability to negotiate safer sex. That's why it's important to recognize the utility of other tools, such as pre-exposure prophylaxis and treatment as prevention. It's also important to accept that no one is perfect in their behavior. Professionals need to help people do what they can to maintain and improve their sexual health. It's more useful than the instinct to chastise them for not doing what doctors and educators think they should.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Rekart ML, Ndifon W, Brunham RC, Dushoff J, Park SW, Rawat S, Cameron CE. A double-edged sword: does highly active antiretroviral therapy contribute to syphilis incidence by impairing immunity to Treponema pallidum? Sex Transm Infect. 2017;93(5):374-378. doi:10.1136/sextrans-2016-052870

  3. Girometti N, Gutierrez A, Nwokolo N, McOwan A, Whitlock G. High HIV incidence  in men who have sex with men following an early syphilis diagnosis: is there room for pre-exposure prophylaxis as a prevention strategy? Sex Transm Infect. 2017;93(5):320-322. doi:10.1136/sextrans-2016-052865

  4. Solomon MM, Mayer KH, Glidden DV, et al. Syphilis predicts HIV  incidence among men and transgender women who have sex with men in a preexposure prophylaxis trial. Clin Infect Dis. 2014;59(7):1020-1026. doi:10.1093/cid/ciu450

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.