High diversity amongst African Treponema pallidum genomes provides a window into global transmission dynamics of syphilis: A genomic epidemiology study

This genomic epidemiology study reveals that African *Treponema pallidum* strains exhibit remarkable local diversity and limited global sharing compared to high-income countries, with significantly lower macrolide resistance, underscoring the need for intensive local sampling to inform targeted surveillance, treatment guidelines, and vaccine design.

The Gist

The Big Picture: A Genetic Detective Story

Imagine syphilis (caused by a tiny bacteria called Treponema pallidum) as a massive, global family reunion. For years, scientists have been taking photos of the guests at this reunion, but almost all the photos were taken at parties in wealthy countries (like the US, UK, and Europe). They noticed that most of the guests at these parties looked almost identical—they were all wearing the same "global brand" outfits.

This new study is like sending a camera crew to a different set of parties: Africa. The researchers wanted to see if the guests there were wearing the same outfits or if they had their own unique, local styles.

The Main Discovery: Africa Has Its Own "Fashion"

The team took genetic "photos" (genomes) of the bacteria from patients in Botswana, Ghana, Uganda, Zimbabwe, and South Africa. They compared these to the thousands of photos they already had from the rest of the world.

The Surprise:

• The Rest of the World: The bacteria here are like a global fast-food chain. Whether you are in New York, London, or Tokyo, you get the exact same burger. The bacteria are very similar everywhere.
• Africa: The bacteria here are like a collection of unique, local artisanal bakeries. Each country, and even different cities within a country, has its own special recipe.
  • They found 56 different "sub-families" (sublineages) of the bacteria.
  • 20 of these families exist ONLY in Africa. You won't find them anywhere else on Earth.
  • 84% of the syphilis cases in Africa are caused by these local, unique strains. They aren't just copies of the global strains; they are homegrown.

The Analogy:
Think of the global syphilis strains as Starbucks. You go to any Starbucks, and the coffee tastes the same. The African strains are like local coffee shops in different villages. One village might brew a spicy tea, another a sweet herbal blend. They are all coffee (or tea), but they are distinct, local creations that haven't been exported to the rest of the world yet.

The "Imported" Problem: When Global Strains Arrive

While most of the bacteria in Africa are local, the study found that some "global" strains have been sneaking in.

• South Africa is like a busy international airport. Because it has high travel and connectivity, it has seen many "imports" of the global strains.
• The Danger: The global strains are often drug-resistant. Specifically, they are resistant to a common antibiotic called macrolides (like azithromycin).
• The Result: In South Africa, the "imported" resistant strains are taking over, causing resistance rates to skyrocket. However, in neighboring countries like Zimbabwe and Botswana, the "local" strains are still mostly sensitive to the drugs, meaning the standard treatments still work there.

The Analogy:
Imagine a virus that is immune to a specific shield (antibiotic).

• In South Africa, the "immune virus" has been flown in from abroad and is spreading fast.
• In Zimbabwe, the local virus is still vulnerable to the shield. If doctors in Zimbabwe start using the same treatment plan as the US (which assumes the virus is immune), they might be over-treating or using the wrong drugs. They need to know their local "enemy" is different.

Why This Matters: No "One-Size-Fits-All"

The study highlights a major problem with how we usually fight diseases: We assume the world is uniform.

• Vaccines: If scientists design a vaccine based only on the "global" strains (the Starbucks burgers), it might not work on the unique African strains (the local bakeries). We need to know the local recipes to make a vaccine that works for everyone.
• Treatment: Doctors need to know which "flavor" of bacteria is circulating in their specific country before prescribing antibiotics. In some African countries, the old, cheap drugs still work perfectly. In others, they don't.

The "Missing Map" Problem

The researchers also realized they are only seeing a small part of the African map.

• They only sampled 6 countries.
• Their data shows that for every new country they sample, they find new types of bacteria.
• The Lesson: Africa is a continent of huge diversity. To understand the full picture, we need to take many more "photos" from many more countries. We can't just look at South Africa and assume we know what's happening in Uganda or Ghana.

Summary in One Sentence

This study reveals that syphilis in Africa is a diverse patchwork of unique, local strains that are mostly sensitive to current drugs, but these are being threatened by the arrival of drug-resistant "global" strains, meaning we need local data to treat patients correctly and design better vaccines.

Technical Summary

1. Problem Statement

Global syphilis rates have risen dramatically since the early 2000s. While genomic epidemiology has been instrumental in tracking transmission and antimicrobial resistance (AMR) in High-Income Countries (HICs), there is a critical data gap regarding Treponema pallidum subspecies pallidum (TPA) in Low- and Middle-Income Countries (LMICs), particularly in Africa.

• Knowledge Gap: Previous studies suggest global homogeneity of TPA strains, but African data is scarce. It is unknown if African syphilis is driven by locally circulating, unique strains or by the importation of global lineages.
• Clinical Relevance: First-line treatment (benzathine penicillin G) remains effective, but macrolide resistance is rising globally. Understanding the origin of resistance (local emergence vs. global importation) and the antigenic diversity of African strains is crucial for updating WHO treatment guidelines and designing effective vaccines.

2. Methodology

The study employed a comprehensive population genomics approach combining prospective surveillance with retrospective data analysis.

• Sample Collection:
  • Prospective (2022–2023): 1,198 participants with genital ulcers were recruited from primary care clinics in Botswana, Ghana, Uganda, and Zimbabwe (Multi-Country Aetiology of Genital Ulcer Study - MAGUS).
  • Retrospective (2006–2023): 276 TPA-positive samples were retrieved from national surveillance programs in South Africa.
  • Contextual Data: The study integrated 167 previously published African genomes (from Madagascar, Malawi, Zimbabwe, South Africa) and 1,062 publicly available genomes from 24 non-African countries (Asia, Europe, Americas, Oceania).
• Sequencing and Bioinformatics:
  • Whole Genome Sequencing (WGS) was performed on Illumina NovaSeq 6000 after sequence capture.
  • Quality Control: 147 novel high-quality genomes (≥75% reference sites with ≥5X coverage) were recovered.
  • Phylogenetics: A recombination-masked maximum likelihood phylogeny was constructed for the final dataset of 1,376 genomes.
  • Sublineage Definition: Fine-scale sublineages were defined using rPinecone, clustering genomes within a 10-SNP distance from a common ancestral node.
  • Analysis: Ancestral state reconstruction (ASR) was used to infer continental origins and dates of most recent common ancestors (MRCAs). Accumulation curves were generated to estimate sublineage discovery rates.

3. Key Contributions

• First Genomic Data: Provided the first TPA genomes from Botswana and Uganda, and significantly expanded the dataset for South Africa and Zimbabwe.
• Pan-African Scope: Created the most comprehensive genomic analysis of African syphilis to date, integrating data from six African nations and comparing it against global diversity.
• Methodological Framework: Established a robust framework for distinguishing between locally circulating African strains and globally imported lineages using high-resolution phylogenomics.

4. Key Results

A. High Genomic Diversity and Local Circulation

• Distinct Sublineages: The study identified 56 sublineages. Crucially, 20 sublineages were exclusive to Africa, while 31 were found globally, and only 5 were shared between Africa and the rest of the world.
• Limited Sharing: Only 16.2% of African genomes belonged to globally shared sublineages. Conversely, 83.8% of African syphilis cases were caused by locally circulating sublineages not found elsewhere.
• Accumulation Curves: Unlike global datasets where sublineage discovery is plateauing, the accumulation curve for Africa is still steep, indicating that current sampling has not yet captured the full diversity of African TPA.
• Phylogeography: African strains are characterized by deep, locally circulating clades (e.g., Clade a1746 in SS14 lineage) with MRCAs dating back to the 1980s, suggesting long-term endemic transmission.

B. Transmission Dynamics

• Intra-continental Spread: Sublineage sharing between African countries is rare. Only a few sublineages (e.g., 2, 4, 13, 42, 50, 61) were found in multiple countries, often suggesting specific regional transmission networks (e.g., Sublineage 4 circulating across South Africa, Zimbabwe, Malawi, and Botswana).
• Importation Events: Globally dominant sublineages (specifically Sublineage 1 and Sublineage 2) have been independently imported into South Africa multiple times. In South Africa, the proportion of genomes belonging to globally shared sublineages rose from 5.0% (2007–2012) to 56.1% (2013–2023).
• Sampling Requirements: Rarefaction analysis suggests that at least three geographically distinct sampling locations are required to capture ~50% of the novel diversity in a new country.

C. Antimicrobial Resistance (AMR) Patterns

• Low Resistance in Africa (Excluding SA): Macrolide resistance (A2058G/A2059G mutations) was low in most African countries:
  • Botswana: 50% (small sample size, n=2)
  • Malawi: 2.5%
  • Zimbabwe: 16.4%
  • Uganda: 0% (n=1)
  • Global Average: 68.6%
• Resistance Linked to Importation: The high resistance rate in South Africa (41.2%) was strongly linked to the importation of globally circulating, resistant sublineages (1 and 2). These imported strains accounted for 63.5% of all macrolide resistance observed in the African dataset, despite representing only 16.2% of the total African genomes.
• Emerging Local Resistance: A novel, South Africa-exclusive sublineage (Sublineage 16) showed signs of emerging macrolide resistance, suggesting potential de novo selection or local transmission of resistance independent of global importation.

D. Demographics and Transmission Networks

• Gender Dynamics: In HICs, syphilis is often driven by men who have sex with men (MSM). In the African dataset, transmission appears predominantly heterosexual, with a significant proportion of samples coming from women (e.g., 50.9% in Zimbabwe).
• Network Structure: The spread of globally imported, resistant strains in South Africa was primarily observed in male populations, potentially reflecting differences in care-seeking behavior or bridging from specific networks, whereas local strains circulated widely across genders.

5. Significance and Implications

• Vaccine Design: The high diversity of African TPA strains, which are antigenically distinct from global strains, implies that vaccines designed based solely on HIC data may lack efficacy in Africa. Cataloging this diversity is essential for broad-spectrum vaccine development.
• Treatment Guidelines: The finding that macrolide resistance in Africa is primarily driven by imported strains (except in South Africa) suggests that azithromycin may still be a viable alternative treatment in many African regions, provided local surveillance is maintained. However, the rapid rise of resistance in South Africa and the emergence of local resistance (Sublineage 16) warrant caution.
• Surveillance Strategy: The study highlights that "one-size-fits-all" global surveillance is insufficient. Effective genomic surveillance in Africa requires intense, localized sampling across multiple regions within countries to capture the full spectrum of diversity.
• Public Health Policy: The data supports the need for region-specific treatment guidelines and the development of regional surveillance systems modeled after successful gonorrhea monitoring programs.

Conclusion: African syphilis is characterized by a complex landscape of locally circulating, diverse strains that are distinct from global lineages. While global transmission of resistant strains is occurring (notably in South Africa), the majority of African syphilis remains sensitive to macrolides and driven by local transmission networks. Ignoring this diversity in genomic surveillance and vaccine design poses a significant risk to global syphilis control efforts.