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[ISLAMABAD] The emergence and spread of an extensively drug-resistant (XDR) strain of Salmonella enterica serovar Typhi (S. Typhi), the bacterium responsible for typhoid fever, highlights the need for urgent measures against the disease, according to a study by researchers in Pakistan and the UK.
While multidrug resistant (MDR) S. Typhi strains have become common, this is the first large-scale spread of an XDR clone, according to the study, which was published in 20 February in mBio. It reports that over 300 cases have been recorded in Pakistan’s Sindh province since November 2016, although local health authorities say the case count has already reached 800. One travel-associated XDR typhoid case has also surfaced in the UK.
Rumina Hasan, professor of pathology at the Aga Khan University (AKU) and an author of the study, tells SciDev.Net that the new XDR typhoid strain has mutated to gain additional DNA, or plasmids, that encode for resistance to multiple antibiotics.
The genetic roots that confer the XDR strain with resistance to five classes of antibiotic drugs were uncovered through genome sequencing of typhoid samples from Pakistan — work done by researchers at the Wellcome Sanger Institute (WSI) and their collaborators at Public Health England and the AKU.
The researchers believe the bacteria likely picked up the plasmid from E. Coli (Escherichia Coli), a bacterium commonly associated with food poisoning through faecal contamination. They also warned that treatment options for typhoid fever are running out, and preventive strategies are urgently needed.
“The multidrug strain, identified as H58, is resistant to three first-line drugs for typhoid — ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole. While MDR typhoid can be treated with second-line medicines like ceftriaxone and fluoroquinolone, the new XDR strain is showing resistance to these drugs as well,” Hasan tells SciDev.Net.
Elizabeth Klemm of the WSI, who co-led the study, says that while sporadic cases of XDR have been reported in a few countries in Asia and Africa, it is for the first time that “such high levels (large-scale cases) of drug-resistance in typhoid disease have been observed”.
Typhoid fever, a highly contagious, life-threatening bacterial infection, spreads in areas with poor sanitation, resulting in contamination of food or drinking water. Symptoms include fever, stomach pain, headache and constipation or diarrhoea.Sergey Eremin, a medical officer for the Antimicrobial Drug Resistance, Infection Control and Publications Unit at the World Health Organization, says the case underscores the fact that S. Typhi can quickly acquire new resistance mechanisms via plasmids and spread resistance genes within and even between different species of bacteria.
More importantly, the study shows the ease with which S. Typhi can transform from MDR to XDR by acquiring a plasmid, and the ability of XDR clones to spread globally, Eremin tells SciDev.Net.
In addition to vaccination campaigns, he suggests that Pakistan should strengthen surveillance for antimicrobial resistance, and improvement of sanitation and access to safe drinking water.