Syphilis and HIV are two sexually transmitted infections (STIs). When either one is untreated, serious health problems can develop.

It’s also possible to have both syphilis and HIV at the same time. In fact, there are several links between these two infections.

Below, we’ll explore more about how HIV and syphilis are related, what can happen if someone has both, and more.

According to data from the Centers for Disease Control and Prevention (CDC), the rate of syphilis in the United States has increased each year for the past 20 years. This is true across all regions, sexes, and racial and ethnic groups.

It’s also possible to have both HIV and syphilis. When this happens, it’s called a coinfection.

Living with both HIV and syphilis is connected to increased HIV transmission, higher viral loads, and lower syphilis treatment effectiveness.

Let’s take a deeper dive into how HIV and syphilis are linked.

Having syphilis makes it easier to contract HIV

If someone is HIV-negative, having syphilis can make it easier to contract HIV. This is because sores or inflammation due to syphilis can make it easier for HIV to enter the body.

A 2020 meta-analysis evaluated 22 studies involving a total of 65,232 participants. It found that HIV incidence doubled in people with syphilis compared to those without syphilis.

This association may also work the other way around. A 2020 study of 4,907 HIV-positive individuals found that repeat syphilis infections increased over 11 years of follow-up. Factors associated with repeat syphilis in this study included:

  • younger age
  • being assigned male at birth
  • having a previous history of STIs

Both syphilis and HIV share risk factors when it comes to sex, such as having sex without a condom or having many sexual partners.

Having syphilis can increase HIV viral load

Viral load refers to the amount of HIV viral particles per milliliter of blood. It can be used as an indicator of HIV progression as well as how effective antiretroviral therapy is.

Having syphilis is associated with increases in viral load. This effect on viral load can also increase the risk of HIV transmission events.

A 2012 study compared males with an HIV and syphilis coinfection with HIV-positive males that didn’t have syphilis. Over a period of 6 months, researchers found that:

  • Increases in viral load were seen in 27.3 percent of those with coinfections, compared to 16.6 percent of those with HIV alone.
  • Viral load increases were also observed in those with coinfections receiving antiretroviral drugs.
  • Those with coinfections had a drop in CD4 count while they had syphilis, but this returned to previous levels after treatment.

Read this for more information about HIV.

Syphilis can progress more quickly in some HIV-positive individuals

If left untreated, syphilis can cause a variety of serious health problems. One of these is neurosyphilis.

Neurosyphilis is a serious type of syphilis that affects the nervous system. It can happen during any stage of syphilis and can lead to symptoms like severe headache, paralysis, and dementia.

Some people with HIV may be at an increased risk of developing neurosyphilis. Research from 2013 found that the following factors were associated with neurosyphilis in HIV-positive individuals:

  • viral load that’s not managed through antiretroviral drugs
  • CD4 count lower than 500 cells per microliter
  • symptoms like headache and visual disturbances

Read this for more information about syphilis.

Syphilis treatment can be less effective in people with HIV

Having HIV may also make syphilis harder to treat. This may be due to HIV’s impact on the immune system.

A 2017 cohort study assessed syphilis treatment in 510 people with and without HIV, finding treatment was ineffective in 10.94 percent of people. Being HIV-positive was associated with a three-fold increase of ineffective syphilis treatment or syphilis reinfection.

A 2013 retrospective review looked at the medical records of 560 people with both HIV and syphilis. While the type of syphilis treatment used didn’t impact the outcome, several other factors associated with ineffective syphilis treatment were found, including:

  • low levels of antibodies in a rapid plasma reagin (RPR) syphilis test
  • a previous history of syphilis
  • CD4 count lower than 350 cells per milliliter

The process for diagnosing syphilis in people living with HIV uses the same types of tests as for HIV-negative individuals. Let’s explore this further.

Blood tests

Syphilis diagnosis involves the use of two different types of blood tests. According to the CDC, these types of tests are typically accurate and reliable for diagnosing syphilis in individuals living with HIV.

Syphilis blood tests use a sample of blood taken from a vein in your arm. They include two types of tests:

Using both types is vital for diagnosing syphilis, particularly in HIV-positive individuals. This is because unusual blood test results have been documented in some people living with HIV.

Microscopy

If you still have a syphilis sore, your doctor may also take a sample of fluid from it. This can then be examined under a microscope to check for the presence of syphilis bacteria.

CSF tests

If a person has signs of neurosyphilis, a doctor may wish to test a sample of cerebrospinal fluid (CSF). This is collected using a procedure called a lumbar puncture.

Examples of syphilis tests that can also be used with a CSF sample include the VDRL test and the FTA-ABS test.

As with diagnosis, the treatment of syphilis in HIV-positive people is similar to that for HIV-negative people.

Treatment of the early stages of syphilis typically involves a single injection of the antibiotic penicillin. Later stages of syphilis can involve additional injections or a course of intravenous (IV) antibiotics.

Because having HIV is associated with the potential risk of ineffective syphilis treatment, careful follow-up is important. This typically involves repeat syphilis blood tests in the months after treatment to verify that treatment was effective.

Antibiotics can often effectively treat syphilis in those with and without HIV. When identified and treated promptly, the outlook for syphilis in people with HIV is generally good.

Taking antiretroviral drugs as directed can also help to improve syphilis outlook in people living with HIV. In addition, sticking to treatment can slow HIV disease progression and lower the risk of HIV transmission.

In some cases, syphilis may progress faster or be harder to treat in those living with HIV. This appears to be more likely in people with a higher viral load and lower CD4 count.

After treatment, it’s possible to get syphilis again in the future. This can be prevented using many of the same strategies used to prevent HIV, such as always using a condom during sex, getting tested regularly for STIs, and asking sexual partners to be tested too.

Rates of syphilis in the United States have been increasing over the past 20 years. It’s also possible to have both syphilis and HIV at the same time. Syphilis and HIV coinfection is particularly prevalent in MSM.

Having syphilis can increase a person’s risk of contracting HIV. In those with HIV, syphilis may progress faster and be harder to treat, although these effects are more common in people with high viral load and low CD4 count.

Syphilis is diagnosed and treated in HIV-positive people in much the same way as in HIV-negative people. Treatment typically involves a one-time injection of penicillin. Careful follow-up is needed to make sure treatment was effective.

It’s possible to contract syphilis another time. Because of this, it’s important to take precautions such as using a condom during sex and receiving regular STI screenings. Be sure to see a doctor if you notice any signs of syphilis.