Genomic epidemiology and antimicrobial resistance transmission of Salmonella Typhi and Paratyphi A at three urban sites in Africa and Asia

Background Enteric fever is a serious public health concern. The causative agents, Salmonella enterica serovars Typhi and Paratyphi A, are frequently antimicrobial resistant (AMR), leading to limited treatment options and poorer clinical outcomes. We investigated the genomic epidemiology, resistance mechanisms and transmission dynamics of these pathogens at three urban sites in Africa and Asia. Methods Bacteria isolated from febrile children and adults at study sites in Dhaka, Kathmandu, and Blantyre were sequenced and AMR determinants identified. Phylogenomic analyses incorporating globally-representative genome data, and ancestral state reconstruction, were used to differentiate locally-circulating from imported pathogen variants. Findings S . Paratyphi A was present in Dhaka and Kathmandu but not Blantyre. S . Typhi genotype 4.3.1 (H58) was common in all sites, but with different dominant variants (4.3.1.1.EA1 in Blantyre; 4.3.1.1 in Dhaka; 4.3.1.2 in Kathmandu). Resistance to first-line antimicrobials was common in Blantyre (98%) and Dhaka (32%) but not Kathmandu (1.4%). Quinolone-resistance mutations were common in Dhaka (99.8%) and Kathmandu (89%) but not Blantyre (2.1%). AcrB azithromycin-resistance mutations were rare (Dhaka only; n=5, 1.1%). Phylogenetic analyses showed that (a) most cases derived from pre-existing, locally- established pathogen variants; (b) nearly all (98%) drug-resistant infections resulted from local circulation of AMR variants, not imported variants or recent de novo emergence; (c) pathogen variants circulated across age groups. Most cases (67%) clustered with others that were indistinguishable by point mutations; individual clusters included multiple age groups and persisted for up to 2.3 years, and AMR determinants were invariant within clusters. Interpretation Enteric fever was associated with locally-established pathogen variants that circulate across age groups. AMR infections resulted from local transmission of resistant strains. These results form a baseline against which to monitor the impacts of control measures. Funding Wellcome Trust, Bill & Melinda Gates Foundation, European Union’s Horizon 2020, NIHR. Evidence before this study Current knowledge of the enteric fever pathogen populations in Dhaka, Kathmandu, and Blantyre comes from retrospective analysis of isolates captured from routine diagnostics or treatment trials. Due to these study designs, most focus on either adult or paediatric cohorts, which complicates assessment of pathogen variant transmission across age groups. Many studies report prevalence of antimicrobial resistance (AMR) and associated mechanisms amongst enteric fever cases. Genomic studies at these sites and elsewhere have identified the spread of AMR clones, and a recent genomic study quantified the inter- and intra-continental spread of resistant S . Typhi between countries. However, PubMed search of “(typhoid OR (enteric fever)) AND (genom*)” identified no studies quantifying the relative proportion of resistant infections that is attributable to local transmission of resistant variants vs imported strains or de novo emergence of AMR. Added value of this study We estimate the vast majority (98%) of drug-resistant enteric fever cases identified in our study resulted from local circulation of resistant variants. Further, we show genetically indistinguishable pathogen variants (either resistant or susceptible) persisting for up to 2.3 years and causing infections across all age groups (under 5 years; 5-15 years; ≥15 years). Implications of all the available evidence While inter-country transfer of resistant enteric fever pathogens does occur and is concerning, the burden of drug-resistant enteric fever at the study sites is currently caused mainly by transmission of locally-established variants, and transmits across age groups. These data confirm assumptions made in models of vaccine impact regarding heterogeneity of pathogen variants and AMR across age groups, and support that childhood immunisation programmes can be expected to reduce the overall burden of resistant infections in endemic settings. ### Competing Interest Statement Andrew J. Pollard is Chair of DHSCs Joint Committee on Vaccination and Immunisation (unpaid) and a member of WHOs SAGE until 2022 (unpaid). Virginia E. Pitzer is a member of WHO IVIR-AC (unpaid) and has received support from Merck and Pfizer (<$500 each) to attend Scientific Input Engagement on RSV. The remaining authors have declared no competing interests. ### Funding Statement This research was funded by the Wellcome Trust [STRATAA grant number 106158/Z/14/Z and core funding to the Wellcome Sanger Institute, grant number 206194] and the Bill & Melinda Gates Foundation [grant number OPP1141321]. ZAD received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Actions grant agreement No 845681. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Research Ethics Committee approval for a joint study protocol across all three surveillance sites was obtained within each country (International Centre for Diarrhoeal Disease Research, Bangladesh Institutional Review Board PR-15119; Malawi National Health Sciences Research Committee 15/5/1599; Nepal Health Research Council 306/2015) as well as from the Oxford Tropical Research Ethics committee (Oxford Tropical Research Ethics Committee 39-15). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Raw sequence data have been deposited in the European Nucleotide Archive under project PRJEB14050 (accessions in Table S1). All other data produced in the present work are contained in the manuscript.