Distinct sexual networks, defined geographically or by sexual preference, for syphilis transmission have been discovered by scientists using genomics. The networks also show a resistance to drug treatments in a majority of cases.
By grouping closely related strains of treponema pallidum, the bacterium that causes syphilis, scientists have demonstrated how a large number of cases are linked together. Researchers from the Wellcome Sanger Institute and their collaborators at the UK Health Security Agency (UKHSA) sequenced 237 whole genome samples and integrated this with epidemiological data to map the bacterium’s evolution and spread through a population. They show distinct transmission chains between individuals, as well as significant resistance to a commonly prescribed class of antibiotics in England.
Here in Ireland, latest reporting shows that 652 cases of syphilis have been diagnosed this year up to September 9, an increase of 15 on the same period in 2022. More than nine in ten cases diagnosed this year occurred in men, while almost six-in-ten (59 per cent) were diagnosed in patients aged between 25 and 40. While cases fell in 2020 due to the pandemic, the number of cases of early infectious syphilis here in the first ten months of 2022 was equal to the total case count for all of 2019.
“The COVID-19 pandemic has reimagined what scale is possible in genomic surveillance and this study capitalises on that, providing important background information on how fast the genomes of T. pallidum evolve as syphilis spreads through a population,” said Dr Mathew Beale, first author of the study and senior staff scientist at the Wellcome Sanger Institute.
“We should explore with future sampling work whether these evolutionary baselines are representative, and if the approach can be used robustly in settings outside of England. Syphilis genome diversity is poorly understood in countries where STI control programmes are most needed.”
Dr Helen Fifer, senior author and lead microbiologist for bacterial sexually transmitted infections at the UK Health Security Agency, added: “We are seeing record levels of STIs including syphilis. Genomics provides yet another tool in our toolbox for understanding chains of transmission of syphilis and predicting response to treatments.
“We must also focus on readily available prevention strategies and STI services, such as condoms, including information about their limitations, effective follow-up of people with new STI diagnoses and self-monitoring for symptoms when necessary.”
The findings were published in the journal Lancet Microbe.