Typhoid: S. Typhi is more drug-resistant

The bacteria causing typhoid fever is becoming increasingly resistant to some of the most important antibiotics for human health, according to a study published in The Lancet Microbe journal.

The largest genome analysis of  Salmonella Typhi (S. Typhi) also shows that resistant strains — almost all originating in South Asia — have spread to other countries nearly 200 times since 1990.

The researchers noted that typhoid fever is a global public health concern, causing 11 million infections and more than 1,00,000 deaths per year.

While it is most prevalent in South Asia — which accounts for 70% of the global disease burden — it also has significant impacts in sub-Saharan Africa, Southeast Asia, and Oceania, highlighting the need for a global response, they said.

Antibiotics can be used to successfully treat typhoid fever infections, but their effectiveness is threatened by the emergence of resistant S. Typhi strains. Analysis of the rise and spread of resistant S. Typhi has so far been limited, with most studies based on small samples.

“The speed at which highly-resistant strains of S. Typhi have emerged and spread in recent years is a real cause for concern, and highlights the need to urgently expand prevention measures, particularly in countries at greatest risk,” said study lead author, Jason Andrews, from Stanford University, United States.

“At the same time, the fact that resistant strains of S. Typhi have spread internationally so many times also underscores the need to view typhoid control, and antibiotic resistance more generally, as a global rather than local problem,” Mr. Andrews said.

In the new study, the researchers performed whole-genome sequencing on 3,489 S. Typhi isolates obtained from blood samples collected between 2014 and 2019 from people in Bangladesh, India, Nepal, and Pakistan with confirmed cases of typhoid fever. A collection of 4,169 S. Typhi samples isolated from more than 70 countries between 1905 and 2018 was also sequenced and included in the analysis.

Resistance-conferring genes in the 7,658 sequenced genomes were identified using genetic databases.

Strains were classified as multidrug-resistant (MDR) if they contained genes giving resistance to classical front-line antibiotics ampicillin, chloramphenicol, and trimethoprim/sulfamethoxazole.

The authors traced the presence of genes conferring resistance to macrolides and quinolones, which are among the most critically important antibiotics for human health. The analysis shows resistant S. Typhi strains have spread between countries at least 197 times since 1990. While these strains most often occurred within South Asia and from South Asia to Southeast Asia, East and Southern Africa, they have also been reported in the U.K., the U.S., and Canada, the researchers said.

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