The emergence of sexually transmissible Shigella flexneri serotype 1b between 2019 and 2024 in England: a descriptive epidemiological study

<b>Background.</b> <i>Shigella</i> species are pathogenic bacteria that cause gastrointestinal symptoms ranging from mild watery diarrhoea to bacillary dysentery. Transmission is faecal-oral and historically associated with international travel. Recently, sexual transmission has been documented among gay, bisexual and other men who have sex with men (GBMSM). Through routine surveillance, we observed an increase in notifications of <i>S. flexneri</i> serotype 1b. We investigated the emergence of this serotype and examined possible drivers of transmission.<b>Methods.</b> We used historical data and whole-genome sequencing data from <i>S. flexneri</i> 1b isolates submitted to the United Kingdom Health Security Agency (UKHSA) to determine the relatedness of isolates and describe the population structure using phylogenetics. We tested for associations with possible epidemiological, biological and genetic drivers.<b>Results.</b> Between 1 January 2004 and 30 June 2024, 1,672 isolates of <i>S. flexneri</i> 1b were identified. Prior to 2019, there was a median of 12.5 [interquartile range (IQR) 10-17] notifications per quarter, rising to a median of 39.5 (IQR 23-58) notifications from 2019 to 2024. The rise was predominantly among adult males, consistent with patterns seen in prior sexually transmitted shigellosis epidemics among GBMSM. Unlike previous outbreaks of shigellosis among GBMSM, the emergence of <i>S. flexneri</i> 1b showed no evidence of an association with the acquisition of antimicrobial resistance determinants.<b>Conclusions.</b> Shigellosis can have severe clinical outcomes, and the repeated emergence of <i>Shigella</i> variants among GBMSM highlights the significance of the sexual transmission pathway. Continued surveillance of <i>Shigella</i> subtypes is necessary to inform public health interventions aimed at preventing sexual transmission of enteric pathogens in the GBMSM community.