A systematic review of antimicrobial resistance in Salmonella enterica serovar Typhi, the etiological agent of typhoid

Abstract

Background

The temporal and spatial change in trends of antimicrobial resistance (AMR) in typhoid have not been systematically studied, and such information will be critical for defining intervention, as well as planning sustainable prevention strategies.

Methodology and findings

To identify the phenotypic trends in AMR, 13,833 individual S. Typhi isolates, reported from 1973 to 2018 in 62 publications, were analysed to determine the AMR preponderance over time. Separate analyses of molecular resistance determinants present in over 4,000 isolates reported in 61 publications were also conducted. Multi-drug resistant (MDR) typhoid is in decline in Asia in a setting of high fluoroquinolone resistance while it is on the increase in Africa. Mutations in QRDRs in gyrA (S83F, D87N) and parC (S80I) are the most common mechanisms responsible for fluoroquinolone resistance. Cephalosporin resistant S. Typhi, dubbed extensively drug-resistant (XDR) is a real threat and underscores the urgency in deploying the Vi-conjugate vaccines.

Conclusion

From these observations, it appears that AMR in S. Typhi will continue to emerge leading to treatment failure, changes in antimicrobial policy and further resistance developing in S. Typhi isolates and other Gram-negative bacteria in endemic regions. The deployment of typhoid conjugate vaccines to control the disease in endemic regions may be the best defence.

Author summary

Typhoid is an invasive bacterial disease causing 26 million illness episodes globally, each year particularly in South Asia and sub-Saharan Africa afflicting children and poorer sections of society disproportionally. AMR is increasingly recognized among S. Typhi lineages spreading from South Asia to Africa, with resistance to first line antibiotics (co-trimoxazole, ampicillin and chloramphenicol), and fluoroquinolones and, of concern, cephalosporins which contribute to treatment failure. AMR in typhoid is not uniform globally and has evolved at different rates in various endemic regions. These trends have not been systematically analysed previously and the objectives of this study included reviewing the phenotypic and genetic determinants of AMR globally over time. The significance of this study revolves around identifying the different trends and mechanisms of AMR and planning interventional strategies accordingly, particularly in light of the Vi-conjugate vaccine candidate which recently received SAGE recommendation and WHO pre-qualification.