Syphilis infections are rising more steeply among men who have sex with women (MSW) and women who have sex with men (WSM) than among men who have sex with men (MSM), according to new research from Japan.

“Young women appear to be at particular risk, and with increased reports of congenital syphilis, syphilis prevention and control is currently a public health priority in Japan,” Dr. Takuri Takahashi of the National Institute of Infectious Diseases

Syphilis has re-emerged as a global public health issue in recent years, Dr. Takahashi and colleagues note in their November 21 report in Sexually Transmitted Diseases.

The increase in syphilis cases in Japan occurred first among MSM, but incidence has risen more dramatically among MSW and WSM in recent years. “The magnitude of the increase in WSM and MSW in Japan has been remarkable,” the researchers note.

To explore the epidemiology of syphilis and identify at-risk groups, Dr. Takahashi et al. looked at 10,997 syphilis cases reported from 2012 to 2016 (7,116 men, 2,363 women). Of the men, 49% were MSW, 36% were MSM, 1% were recorded as both MSW and MSM, and 14% had an unknown sexual preference.

Nearly 90% of the women were WSM, 0.7% were women who have sex with women, 0.1% were recorded as both, and 9.4% had an unknown preference.

The % of cases that were primary- or secondary-stage syphilis (rather than asymptomatic or late-stage disease) rose during the study period, reaching nearly 70% by 2016. The number of reports also rose, more than quintupling from 2012 (0.7 cases per 100,000) to 2016 (3.6 per 100,000).

Men accounted for the majority of cases, but the percentage of males peaked at 86.7% in 2013 and fell to 74.0% in 2016. Median age was 37 for men and 26 for women, while the age distribution of primary and secondary cases was wider in men than in women. Among cases of age 25, women had a higher reporting rate than men.

Forty-five cases of congenital syphilis were reported during the study period. Cases per 100,000 live births rose from 0.4 in 2012-2013 to 1.4 in 2016.

“While reports are fortunately still few (potentially due to the universal screening policy for syphilis for all pregnant women in the first trimester, covered by the national health insurance scheme), such trends highlight the need for targeted STI prevention strategies for young women,” the study authors report.

They conclude: “Following the MSM experience, we have initiated a case-control study to assess potential risk factors for syphilis infection among MSW and WSM. The study, currently actively underway, will help shed light on the epidemiology of syphilis acquisition, which will be important in identifying core risk groups and behaviors to better target prevention and control measures against syphilis.”