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© 2015 Elsevier Ltd. Background: Shigellosis is an acute, severe bacterial colitis that, in high-income countries, is typically associated with travel to high-risk regions (Africa, Asia, and Latin America). Since the 1970s, shigellosis has also been reported as a sexually transmitted infection in men who have sex with men (MSM), in whom transmission is an important component of shigellosis epidemiology in high-income nations. We aimed to use sophisticated subtyping and international sampling to determine factors driving shigellosis emergence in MSM linked to an outbreak in the UK. Methods: We did a large-scale, cross-sectional genomic epidemiological study of shigellosis cases collected from 29 countries between December, 1995, and June 8, 2014. Focusing on an ongoing epidemic in the UK, we collected and whole-genome sequenced clinical isolates of Shigella flexneri serotype 3a from high-risk and low-risk regions, including cases associated with travel and sex between men. We examined relationships between geographical, demographic, and clinical patient data with the isolate antimicrobial susceptibility, genetic data, and inferred evolutionary relationships. Findings: We obtained 331 clinical isolates of S flexneri serotype 3a, including 275 from low-risk regions (44 from individuals who travelled to high-risk regions), 52 from high-risk regions, and four outgroup samples (ie, closely related, but genetically distinct isolates used to determine the root of the phylogenetic tree). We identified a recently emerged lineage of S flexneri 3a that has spread intercontinentally in less than 20 years throughout regions traditionally at low risk for shigellosis via sexual transmission in MSM. The lineage had acquired multiple antimicrobial resistance determinants, and prevailing sublineages were strongly associated with resistance to the macrolide azithromycin. Eight (4%) of 206 isolates from the MSM-associated lineage were obtained from patients who had previously provided an isolate; these serial isolations indicated atypical infection patterns (eg, reinfection). Interpretation: We identified transmission-facilitating behaviours and atypical course(s) of infection as precipitating factors in shigellosis-affected MSM. The intercontinental spread of antimicrobial-resistant shigella through established transmission routes emphasises the need for new approaches to tackle the public health challenge of sexually transmitted infections in MSM. Funding: Wellcome Trust (grant number 098051).

The trends underlying the rapid and substantial increases in HIV diagnoses among heterosexual people in the United Kingdom are complex and sometimes misunderstood. Although the number of people becoming infected with HIV through heterosexual intercourse in the United Kingdom is rising steadily, most of the overall rise in HIV diagnoses among heterosexuals is among people who originate from and were infected abroad, mainly in Africa.1 We describe HIV diagnoses where infection was probably acquired through heterosexual intercourse in the United Kingdom. The Health Protection Agency Centre for Infections receives voluntary and confidential reports of HIV diagnoses in England, Wales, and Northern Ireland.2 For all reports, the centre collects the probable route of infection and the probable country of infection for patients who were infected through heterosexual intercourse. A research nurse/counsellor follows up where data are incomplete. Where more than one possible country of infection is given, the country with the highest prevalence of HIV is allocated by an epidemiologist or a research nurse/counsellor. If an individual was probably infected in the United Kingdom, further information concerning the partner's probable route and country of infection are recorded. If it is uncertain but possible that the partner was infected in the United Kingdom, …