Syphilis has long been difficult to eradicate — and it’s having another resurgence. In the United States alone, more than 171,000 cases of the sexually-transmitted infection were reported in 2021, up 68 percent since 2017, according to preliminary data from the Centers for Disease Control and Prevention. Especially concerning, doctors say, are cases of congenital syphilis, where the infection is passed from a pregnant person to their fetus during pregnancy, which have nearly tripled in the same time period.
The trend isn’t entirely new. After a sharp decrease in syphilis cases in the 1940s and 1950s in the United States and the United Kingdom, credited to the availability of penicillin and widespread screening, there have been cycles of rises and declines. Over the course of the 1990s, cases dropped, but they began to rise again in 2000. Although the exact cause of the current resurgence is not well understood, Caroline Cameron, a professor of biochemistry and microbiology at the University of Victoria in Canada, suggested it may be partly due to an increase in condomless sex linked to new HIV/AIDS prophylactics, illicit drug-use, and the proliferation of dating apps.
Despite being one of the oldest known sexually transmitted infections, with possible origins in the 14th century — as well as intense public health initiatives over the past many decades — syphilis receives little attention from researchers. In fact, “only a handful of labs in the world are currently working on this important disease and its causative agent,” Sheila Lukehart, a professor emeritus of medicine and global health at the University of Washington, wrote in an email to Undark.
This is in part because the bacterium that causes syphilis, Treponema pallidum, “is fragile to work with,” said Cameron. “So, you can’t use regular experimental methods to work with it.”
Because of that fragility, researchers have been limited in their ability to develop new syphilis diagnostics, treatments, and preventive measures such as vaccines. Effective treatments are additionally challenging, experts say, because of T. pallidum’s ability to evolve resistance to antibiotics. Left untreated, in about 15 to 30 percent of infected people, the disease can permanently damage the brain, heart, and other organs and be life-threatening. Congenital cases can cause birth defects, stillbirth, and premature death.
New techniques to grow the bacteria in the lab may make it easier to study syphilis. But doing so will require more researchers focused on the disease. “There is an entire generation of clinicians and researchers who may have never seen or thought about syphilis,” said Ina Park, a professor of family community medicine at the University of California, San Francisco. Now, she added, “we need to catch up.”
The main limitation for syphilis research has been an inability to grow the delicate bacteria that causes it in the lab. Humans are the only natural host, although in research studies, scientists have been able to infect other animals with T. pallidum.
There were many false starts to growing T. pallidum outside of human and animal bodies, but it wasn’t until the 1970s that researchers found that the bacteria needed both mammalian cells and low oxygen levels for a chance to survive. There was little success, however, in getting the bacteria to thrive in these conditions long enough for an experiment. This changed in 1981, whe
