Serologic Follow-Up of Infants Exposed to Maternal Syphilis During Pregnancy

For infants born to those with infectious syphilis during pregnancy, guidelines recommend monitoring nontreponemal titers every 2 to 3 months until nonreactive. The primary objective was to determine when nontreponemal titers become nonreactive in this setting.

This retrospective cohort study included infants born to mothers in Alberta with infectious syphilis during pregnancy from January 1, 2015, to December 31, 2021, who had at least one follow-up rapid plasma reagin (RPR) beyond 1 month of age. Cumulative incidence curves for RPR, enzyme immunoassay, and Treponema pallidum particle agglutination seroreversion were examined.

For 228 infants (46% female), a diagnosis of probable, possible, and less likely congenital syphilis was made in 13%, 32%, and 55%, respectively. Treatment with intravenous penicillin G was administered to 144 infants (63%), and 80 infants (38%) were not treated for congenital syphilis. A follow-up RPR performed at median 103 days of life (interquartile range, 86-143) was nonreactive in 174 infants (76%). Rapid plasma reagin, enzyme immunoassay, and T. pallidum particle agglutination seroreversion were documented in 96%, 40%, and 51% of cases, respectively. Rapid plasma reagin reactivity beyond 6 months of age was uncommon (1 of 80 untreated infants and 8 of 147 treated infants) and was associated with higher birth RPR.

Approximately three-quarters of infants born to persons with infectious syphilis during pregnancy will have RPR seroreversion by 3 months of age. A small percentage of treated and untreated infants will have a persistently reactive RPR at 6 months of age despite presumed adequate treatment.