Causes and Risk Factors of Syphilis

In This Article

Syphilis is sexually transmitted infection/disease (STI/STD) caused by a spiral-shaped bacterium called Treponema pallidum. You can get infected by coming into contact with a syphilis sore during sex. It can also be passed from mother to child during pregnancy. People often have misconceptions about syphilis, believing you can only get it by being "promiscuous."

The simple truth is that you can get infected by a single exposure and may be entirely unaware that you're infected for years and even decades.

As concerning as this is, there are simple ways to avoid a syphilis infection. It all starts with learning how the disease is spread and what factors place you, as an individual, at risk.

syphilis causes
Illustration by Joshua Seong. © Verywell, 2018.

Common Causes

Anyone with syphilis either contracted the infection from sexual contact or from their mother in utero.

Adult Transmission

Syphilis infection occurs when skin or mucosal tissue comes into contact with an open, ulcerative sore known as a chancre. The corkscrew shape of the bacteria allows it to burrow into the mucous membranes of the mouth, vagina, or rectum or to enter microscopic breaks in the skin.

In adults and sexually active youth, syphilis is passed almost exclusively through oral, vaginal, or anal sex. In rare cases, an infection may occur as a result of kissing.

If left untreated, syphilis will go through five stages of infection: primary, secondary, early latent, late latent, and tertiary. The risk and mode of transmission can vary by the stage:

  • During primary syphilis, the disease is passed by coming into contact with a chancre.
  • During secondary syphilis, the disease can be passed by coming into contact with the secondary rash.
  • During early latent syphilis, secondary symptoms can sometimes relapse and increase the risk of transmission.
  • During late latent or tertiary syphilis, the disease is considered non-contagious.

Syphilis cannot be passed via toilet seats, casual contact, or the shared use of utensils or personal care items. This is because T. pallidum has a fragile shell that lacks the lipoproteins needed to sustain life outside of the body for very long.

As a result, the object-to-human transmission of syphilis is extremely unlikely.

Perinatal Transmission

The perinatal transmission of syphilis (also known as congenital syphilis) occurs when the syphilis bacteria penetrates the placenta surrounding the developing fetus. While this can happen during any stage of the pregnancy, the likelihood is greatest during the second half. The risk of transmission varies by the mother's stage of infection.

The risk of transmission of syphilis from a mother who is untreated is very high.

Lifestyle Risk Factors

While syphilis can affect anyone, there are a number of risk factors that can increase your likelihood of infection. Some are related to sexual behaviors, while others are associated with characteristics that can place an entire population at risk.

Among the most common risk factors:

  • Inconsistent condom use is the main cause of transmission in all groups. According to a study by the Centers for Disease Control and Prevention (CDC), only around 24 percent of women and 33 percent of men between the ages 15 and 44 use a condom consistently.
  • Multiple sex partners place you at risk simply by providing you a greater opportunity for exposure. This is especially true among anonymous partners who meet on online platforms.
  • Men who have sex with men (MSM) account for approximately 60 percent of syphilis infections in the United States. Physiological vulnerabilities (such as the fragility of rectal tissues) and high rates of HIV place MSM at an inherently increased risk compared to heterosexual peers.
  • Injecting drug use has given rise to a spate of STD outbreaks in recent years (including the 2015 HIV outbreak in Indiana linked to oxycontin). While syphilis is not transmitted by blood-to-blood exposure, injecting drug use can impair judgment and increase the risk of sexual violence or the exchange of sex for drugs.

Risk Attitudes and Behaviors

Perhaps one of the greatest risks factors of syphilis infection is the avoidance of STD screening. 

While it was once thought that the need for screening depended on the number of partners someone had, the CDC now recommends at least once-a-year testing (syphilis, chlamydia, and gonorrhea) for all sexually active gay men, bisexual men, and other men who have sex with men.

Those who have multiple or anonymous sex partners should be screened more often (e.g., at three- to six-month intervals).

Unfortunately, many of these men are either unaware of the guidelines or actively ignore them, either because of stigma or the fear of receiving a concurrent HIV diagnosis. This translates to not only an increased risk of infection but reinfection.

​​A 2015 study from the University of California, Los Angeles reported that between 6 percent and 8 percent of MSM previously infected with syphilis experienced a repeat infection within two years. Many of those who had delayed testing until secondary infection admitted that they either didn't want to know or were afraid to learn the results.

The same has been seen in young African American men who are 62 percent less likely to be tested if they associate STDs with immorality, shame, uncleanliness, or a weakness of character. Today, the rate of syphilis among African Americans is nearly five times that of the white population, largely as a result of these attitudes.

Syphilis infections in the United States are increasing steadily in many ethnic and age groups.

In 2000, fewer than 6,000 cases of primary or secondary syphilis were reported (or 2.1 cases per 100,000 people); by 2017, that number had increased to over 30,000 (or 9.5 cases per 100,000).

Until these attitudes and testing behaviors are changed, the general risk of infection can be expected to rise. Be honest with yourself about your risk and do what you can to protect yourself.

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Article Sources

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