Antibiotic-resistant strains of typhoid identified
An international team of scientists has found that the bacteria that cause typhoid fever are becoming increasingly resistant to some of the most important antibiotics for human health. The results of the study, published in The Lancet Microbe, show that resistant strains have crossed from South Asia to other countries about 200 times since 1990.
The authors performed whole genome sequencing of 3489 Salmonella enterica serovar 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. The DNA of 4,169 S. typhi samples isolated from over 70 countries between 1905 and 2018 has also been sequenced.
Strains were classified as multidrug-resistant if they contained genes conferring resistance to classical first-line antibiotics: ampicillin, chloramphenicol, and trimethoprim/sulfamethoxazole. The authors also identified genes conferring resistance to macrolides and quinolones, which are among the most critical antibiotics for human health.
It turned out that resistant strains of S. typhi had been spread between countries at least 197 times since 1990. Although these strains were most common in South Asia, from where they entered Southeast Asia, East and South Africa, they have also been reported in the UK, USA and Canada. Since 2000, the occurrence of multidrug-resistant bacteria has been declining in Bangladesh and India, but they are now being replaced by strains that are resistant to other antibiotics.
Gene mutations leading to quinolone resistance have emerged and spread at least 94 times since 1990, with almost all of them (97 percent) originating in South Asia. Quinolone-resistant strains accounted for over 85 percent of S. typhi in Bangladesh by the early 2000s, and by 2010 increased to over 95 percent in India, Pakistan and Nepal. Mutations that cause resistance to azithromycin, a widely used low-toxicity antibiotic, have appeared at least seven times in the past 20 years. In Bangladesh, strains containing these mutations appeared around 2013, and their population size has steadily increased since then.
Typhoid fever is estimated to cause 11 million infections and over 100,000 deaths per year. The results of the study highlight the need to tackle typhoid and antibiotic resistance as a global, not a local problem.