Unrecognized Shedding of Treponema pallidum Among MSM With Syphilis

Blood samples with HIV and syphilis positive
A team of investigators sought to determine the frequency of Treponema pallidum shedding in men who have sex with men.

Unrecognized shedding of Treponema pallidum has been found to be occurring in men who have sex with men (MSM) who have early syphilis and more frequently in patients with secondary syphilis, according to study results published in The Lancet Infectious Diseases, suggesting that secondary syphilis is the most infectious stage.

Men aged 18 years and older who had reported engaging in sex with men during the past 12 months and who had a laboratory-confirmed primary, secondary, or early latent syphilis diagnosis were recruited from a sexual health center in Melbourne, Australia. Both primary and secondary syphilis lesions were swabbed, and samples from nonlesion sites such as oral rinse, oral cavity swab, anal canal swab, urine, and semen were collected.

A total of 200 men were included in the study. Of those, 54 (27%) had primary syphilis, 93 (47%) had secondary syphilis, and 53 (27%) had early latent syphilis. Positive oral samples were obtained from 48 of 200 (24.0%; 95% CI, 18.3-30.5) men, 24 of whom did not have oral lesions. Positive results for T pallidum from anal, perianal, rectal lesion, or anal canal samples occurred in 45 of 196 (23.0%; 95% CI, 17.3-29.5) men, 10 of whom had no anal lesions. Urine from 12 of 198 men (95% CI, 6.1%; 3.2-10.3) and semen from 6 of 50 men (12.0%; 95% CI, 4.5-24.3) also tested positive for T pallidum.

Detection of T pallidum shedding by oral swab, rinse, or both was more frequent during secondary syphilis than during primary or early latent syphilis, occurring in 41 of 93 (44%) men with secondary syphilis vs7 of 107 (7%) men with other stages of disease (P <.0001). Oral detection was also more common among men with a rapid plasma reagin titre of 1/64 or higher than in those with lower titres (37 [32%] of 117 vs 11 [13%] of 83; P =.0026). There were no differences in detection from anal samples between secondary syphilis and other stages of the disease. Overall, T pallidum was detected from any site in 69 of 93 (74%) men with secondary syphilis and at more than 1 site in 24 (26%). Among the 54 men with primary syphilis, 49 (91%) were positive at any site, and 11 (20%) were positive at 2 or more separate sites. Of the 53 men with early latent syphilis, 4 (8%) were positive for T pallidum at any site, and none were positive at more than a single site.

Several study limitations were noted, including that the results are not applicable outside of men having sex with men, semen samples were collected after treatment for syphilis, and it is unclear whether T pallidum DNA detected via polymerase chain reaction is from viable or infectious bacteria.

According to the investigators, “To our knowledge, this study is the first in which lesion and [nonlesion] sites have been systematically sampled for T pallidum in both HIV-positive and HIV-negative MSM across all stages of early syphilis infection.” They report that “…oral and anal shedding of T pallidum was most frequent during secondary syphilis, often in the absence of overt syphilis lesions, and that this shedding occurred in HIV-positive and HIV-negative men.” Shedding of T pallidum may therefore be a factor in sustaining syphilis transmission, but further research confirming the viability of the bacteria collected from these sites is needed.


Towns JM, Leslie DE, Denham I, et al. Treponema pallidum detection in lesion and non-lesion sites in men who have sex with men with early syphilis: a prospective, cross-sectional study. Lancet Infect Dis. Published online April 22, 2021. doi:10.1016/S1473-3099(20)30838-0