June 16, 2016
Treponema pallidum is the bacterium that causes syphilis, a sexually transmitted infection (STI) with no preventive vaccine. Researchers recently identified a mutated form of T. pallidum, which has become resistant to treatment with the macrolide class of antibiotics, in samples from 25 syphilis patients in Cuba.
Lola Stamm, PhD, associate professor of epidemiology at the UNC Gillings School of Global Public Health, has a long history working with the T. pallidum bacterium. As early as 2000, she conducted research that identified how the germ had made one tiny change to its DNA and thereby rendered itself resistant to erythromycin, a commonly used macrolide antibiotic.
Now, a new study published online by the journal Sexually Transmitted Diseases highlights research performed by Stamm in collaboration with colleagues at the Tropical Medicine Institute. Their investigation aimed to determine the presence of macrolide-resistant T. pallidum subtypes in Havana after an increase of adult syphilis cases occurred in Cuba beginning in 2009.
The researchers tested samples from 41 syphilis patients for the two known T. pallidum 23S ribosomal RNA gene mutations (A2058G and A2059G) that confer resistance to macrolide antibiotics. Twenty-five patients (61 percent) harbored a form of the bacterium with the A2058G mutation, which was present in all eight subtypes that were identified. The A2059G mutation was not detected in any of the samples.
T. pallidum strains with the A2058G mutation are currently widespread, with highest prevalence rates in North America, Europe and Asia. So far, bacteria with the A2059G mutation have only been identified in syphilis patients in the United States, China, Australia, the United Kingdom and the Czech Republic.
The researchers theorize that the high occurrence of macrolide-resistant T. pallidum in Havana may be due to extensive use of the macrolide-class antibiotic azithromycin in the city’s clinical practice. The drug often is prescribed for respiratory and skin infections as well as other STIs.
As Stamm notes in a recent NPR story on syphilis and ‘superbugs,’ penicillin still offers effective treatment for syphilis. Based on her latest study findings, however, azithromycin is not an appropriate second-line drug for syphilis treatment in patients with a penicillin allergy. Additionally, according to the U.S. Centers for Disease Control and Prevention, macrolide antibiotics must not be used to treat syphilis in pregnant women or men who have sex with men.
Last week, a press release from the U.S. Preventive Services Task Force recommended that all patients at high risk for syphilis be screened for infection, even if they have no symptoms. To better inform syphilis treatment guidelines for patients with macrolide-resistant strains of T. pallidum, more research and surveillance will be required.
“Syphilis rates have increased in several countries, including the United States,” Stamm reports. “Syphilis is a major concern to global public health, particularly because the lesions of early syphilis increase the risk of acquisition and transmission of infection with the human immunodeficiency virus (HIV). This study is the first report of azithromycin-resistant T. pallidum in Cuba, and we are hopeful that this preliminary information will help improve the management of Cuban syphilis patients.”