Congenital Syphilis in Suzhou, China: Why Late Diagnosis Is the Biggest Risk

Syphilis passed from mother to baby remains a major global health problem. In 2012, an estimated 930,000 syphilis infections in pregnant women worldwide caused 350,000 adverse outcomes:

In China, reported congenital syphilis cases rose from 468 in 2000 to 10,032 in 2013 — an incidence rising nearly 26-fold, from 2.6 to 69.9 per 100,000 live births. This study, by researchers in Nanjing, Guangzhou and Suzhou, looked for the risk factors behind transmission.

The study

From July 2011 through July 2014, the Fifth People's Hospital of Suzhou, in eastern China, enrolled 189 pregnant women newly diagnosed with syphilis, confirmed by both a nontreponemal test (rapid plasma reagin, RPR) and a treponemal test (Treponema pallidum particle agglutination, TPPA). Four had a fetal death or stillbirth and 30 were lost to follow-up, leaving 155 mothers and their 155 live-born babies.

Babies were checked at 3, 6, 9, 12, 15 and 18 months. Because a mother's antibodies can cross to the baby, a positive test at birth doesn't prove infection; those antibodies usually fade by 15 months in an uninfected child. A baby was confirmed to have congenital syphilis if the TPPA test was still reactive at 18 months. Every mother and baby with syphilis was treated immediately under national guidelines.

Selection of participants for the study. Credit: Centers for Disease Control and Prevention.

What they found

27 of the 155 babies (17.4%) had congenital syphilis.

• Late diagnosis was the strongest risk: after adjusting for other factors, mothers diagnosed after 36 weeks were about 25 times as likely to have an infected baby as those diagnosed by 12 weeks.
• Antibody levels mattered. Each doubling of the mother's RPR titer nearly doubled the risk. Of 24 women with RPR titers of 1:16 or higher, 16 (66.7%) passed syphilis to their babies; both women with titers of 1:64 or higher did. Higher TPPA titers were also linked to transmission. High nontreponemal titers suggest early, infectious syphilis.
• Migrant women — living in Suzhou with temporary residence status — had an infected baby far more often: 22 of 82 (26.8%), compared with 5 of 73 (6.8%) of non-migrant women, about five times the odds before adjusting for other factors. Migrant status was not a significant risk on its own after adjustment, but three-quarters of the women diagnosed after 36 weeks were migrants, and about half of the pregnant migrant women were diagnosed that late.
• Fathers: babies were more likely to be infected when the father currently had syphilis — 13 of 40 (32.5%), compared with 12 of 104 (11.5%) when he tested negative.

Why the rate was so high

A 2010 study in Shenzhen found a transmission rate of 5.2%, far below Suzhou's 17.4%. The difference is mostly timing: about a third of the Suzhou mothers (52 of 155) were diagnosed only at delivery, and 21 of their babies were infected. Shenzhen excluded such women. Leave them out in Suzhou, and the rate falls to 5.8% (six of 103).

Late diagnosis means late treatment, or none — and penicillin is highly effective both for treating syphilis in pregnancy and for preventing transmission.

Limitations

• With few cases, the study may have lacked the power to confirm risks found elsewhere, such as the stage of the mother's syphilis.
• It wasn't designed to study transmission, so it has no data on behavior, other sexually transmitted infections, or why women and their partners weren't tested or treated.
• It covers a single city.

What would help

The authors recommend earlier screening, diagnosis and treatment of syphilis in pregnancy; better education for communities and providers, especially about infections with high antibody titers; and better access to prenatal care and screening for migrant women. Understanding why diagnosis comes so late, particularly among migrants, could guide China's policy on preventing congenital syphilis.

Sources

Based on Wang Y, Wu M, Gong X, Zhao L, Zhao J, Zhu C, Gong C, "Risk Factors for Congenital Syphilis Transmitted from Mother to Infant — Suzhou, China, 2011–2014," Morbidity and Mortality Weekly Report 68(10), Centers for Disease Control and Prevention; material in the MMWR series is in the public domain. Original report.