Explain the pathogenesis of vertical transmission of syphilis.

Pathogenesis of Vertical Transmission of Syphilis

Overview

Congenital syphilis results primarily from the transplacental passage of Treponema pallidum subspecies pallidum from an infected mother to her fetus during pregnancy¹. Less frequently, neonatal infection occurs through direct contact with maternal syphilitic lesions at the time of delivery². The vertical transmission represents a significant global health burden, with an estimated 700,000 to 1.5 million cases reported annually between 2016 and 2023³.

Mechanism of Transplacental Transmission

The pathogenesis of vertical transmission involves several key steps:

1. Maternal Dissemination and Placental Invasion
The in-utero transmission typically occurs during maternal disseminated bloodstream infection, which results in invasion of the placenta by T. pallidum, followed by transmission across the placental barrier⁴. The placenta normally maintains separation between maternal and fetal compartments; however, T. pallidum overcomes this barrier through mechanisms that remain partially unknown⁴,⁵.

2. Fetal Hematogenous Dissemination
Once across the placental barrier, T. pallidum enters the umbilical vein, leading to hematogenous systemic infection in the fetus⁶. Unlike adult syphilis, where the organism initially establishes a local lesion, congenital syphilis involves direct release of T. pallidum into the fetal bloodstream, causing spirochetemia with early spread to multiple organs including bones, kidneys, spleen, liver, and heart⁶.

3. Immune Evasion
T. pallidum possesses a small genome with limited outer membrane protein expression, which renders the organism essentially undetectable by the fetal immune system after exposure, leading to persistent fetal infection¹. This immune evasion capability is critical for the establishment and maintenance of congenital infection¹.

Molecular Mechanisms of Placental Barrier Breach

Recent research has identified specific molecular mechanisms by which T. pallidum traverses the placental barrier:

Adhesion and Colonization
The surface lipoprotein Tp0954 functions as a placenta-targeted adhesin. Its tetratricopeptide repeat (TPR) domain mediates specific interactions with host tissues, particularly glycosaminoglycans such as dermatan sulfate, heparin, and heparan sulfate⁷. This interaction facilitates binding to placental trophoblast cells and enhances adhesion efficiency by more than 50%⁷.

Disruption of Intercellular Junctions
Tp0954 promotes vertical transmission by disrupting intercellular junction structures, representing a fundamental mechanism in the pathogenesis of congenital syphilis⁷. Additionally, T. pallidum Tp0751 alters the expression of tight junction proteins by promoting cell apoptosis and IL-6 secretion, further compromising barrier integrity⁵.

Placental Inflammation
The placentas in fetuses with maternal syphilis become significantly enlarged due to localized inflammatory response⁶. Histological examination reveals enlarged hypercellular villi, necrotizing funisitis (“barber’s pole” appearance), proliferative vascular changes, and acute and chronic villitis⁶. Over 75% of neonates born with a placenta heavier than the 90th percentile for birth weight have been found to have congenital syphilis⁶.

Risk Factors and Timing of Transmission

Transmission may occur at any time during pregnancy, with the risk varying by maternal disease stage:

Maternal Stage Transmission Risk
Primary/Secondary (untreated, 3rd trimester) 60–100%⁸
Early latent 40%⁸
Late latent <8%⁸

The risk to the fetus is 50–70% in pregnancies complicated by early syphilis but decreases to approximately 15% if maternal syphilis was contracted more than a year before pregnancy¹. Worse outcomes (prematurity, spontaneous abortion, stillbirths) are associated with early transmission during the first trimester⁶.

Clinical Consequences

After placental infection occurs, T. pallidum is consistently present in amniotic fluid⁴. Clinical manifestations in the neonate range from asymptomatic infection (in up to 70% of cases) to severe outcomes including stillbirth, hydrops fetalis, preterm delivery, low birth weight, hepatosplenomegaly, osteolytic bone lesions, pseudoparalysis, and central nervous system infection³,⁶.

References

1. Peeling RW, Mabey D, Kamb ML, et al. Syphilis. Nat Rev Dis Primers. 2017;3:17073. doi:10.1038/nrdp.2017.73

2. Bowen V, Su J, Torrone E. Increase in incidence of congenital syphilis — United States, 2012–2014. MMWR Morb Mortal Wkly Rep. 2015;64(44):1241-1245.

3. Newman L, Kamb M, Hawkes S, et al. Global estimates of syphilis in pregnancy and associated adverse outcomes: analysis of multinational antenatal surveillance data. PLoS Med. 2013;10(2):e1001396. doi:10.1371/journal.pmed.1001396

4. Arora N, Sadovsky Y, Dermody TS, Coyne CB. Microbial vertical transmission during human pregnancy. Cell Host Microbe. 2017;21(5):561-567. doi:10.1016/j.chom.2017.04.007

5. Lu S, Li Y, Wang Q, et al. Treponema pallidum Tp0751 alters the expression of tight junction proteins by promoting bEnd3 cell apoptosis and IL-6 secretion. Int J Med Microbiol. 2022;312(6):151568. doi:10.1016/j.ijmm.2022.151568

6. Sankaran D, Partridge E, Lakshminrusimha S. Congenital syphilis—an illustrative review. Children (Basel). 2023;10(8):1310. doi:10.3390/children10081310

7. Primus S, Rocha SC, Giacani L, Parveen N. Identification and functional assessment of the first placental adhesin of Treponema pallidum that may play critical role in congenital syphilis. Front Microbiol. 2020;11:621654. doi:10.3389/fmicb.2020.621654

8. Tuddenham S, Hamill MM, Ghanem KG. Diagnosis and treatment of sexually transmitted infections: a review. JAMA. 2022;327(2):161-172. doi:10.1001/jama.2021.23487