Evidence for reinfection and persistent carriage of Shigella species in adult males reporting domestically-acquired infection in England.

We analysed national surveillance typing data of Shigella isolated from adult males with domestically-acquired infection (a cohort largely consisting of men who have sex with men, (MSM)) to establish whether multiple isolates from the same individual over time represented persistent carriage or reinfection.We carried out a retrospective cohort study of adults diagnosed with Shigella from 2004-2018. Median time intervals between multiple isolations of Shigella flexneri and S. sonnei were compared. Analysis of whole genome sequencing data provided strain discrimination at the single nucleotide level and was used to quantify the genetic distance among isolates. Maximum likelihood phylogenies were constructed to determine whether persistent carriage (characterised by multiple isolations of the same strain) or reinfection (characterised by multiple isolations of different strains) was best-supported by the phylogenetic analysis. A comparison analysis was carried out using data linked to adult females with domestically-acquired shigellosis.The number of men reporting multiple isolations of Shigella species was 165/4733 (3.5%) compared to 31/2423 (1.3%) females (p<.001). For isolate pairs from men associated with persistent carriage, the isolation time interval range was 6-176 days (median=23.5; IQR 8-70.) and single nucleotide polymorphisms (SNP) distance range was 0-7 SNPs (median=0.5; IQR 0-2). For those associated with reinfection, the isolation time interval was 34-2636 (median=732; IQR 191-1258) and the SNP distance was 10-1462 SNPs (median=120; IQR 29-377).Multiple Shigella isolations in individuals with domestically-acquired infections was more frequently observed in adult males compared to adult females. Following the acute phase of infection, carriage can persist for months, and infection can re-occur within months even with strains belonging to the same species and the same serotype. A combination of multiple sexual partners, persistent carriage following the acute phase of infection and evidence of recurrent reinfection is likely to contribute to sustained transmission in this population.