A case report published yesterday in Antimicrobial Stewardship & Healthcare Epidemiology describes the identification of a novel strain of extensively drug-resistant Shigella in Los Angeles.
The strain of XDR Shigella sonnei was isolated separately from two patients in Los Angeles with no known epidemiologic connection or transmission route a year apart, researchers from the University of California Los Angeles (UCLA) David Geffen School of Medicine reported. One patient was a 34-year-old man with advanced HIV and a history of substance abuse, the other a 33-year-old immunocompromised woman with high-grade B-cell lymphoma.
Shigellosis is a highly infectious gastrointestinal condition caused by one of the four species of Shigella bacteria: S sonnei, S flexneri, S boydii, or S dysenteriae.It’s transmitted by the oral-fecal route; via contaminated fomites, food, and water; or by direct person-to-person contact. Symptoms range from mild diarrhea to severe dysentery.
Isolates from both patients were resistant to several oral antibiotics commonly used to treat shigellosis, along with other antibiotic classes. Molecular analysis identified the resistance gene blaDHA-1, which confers resistance to beta-lactam antibiotics,in isolates from both patients.
Isolates from the immunocompromised patient also carried the extended-spectrum beta-lactamase gene blaCTX-M-15, which has been found in XDR S sonnei strains acquired through sexual transmission among men who have sex with men (MSM) in US cities. A previous study by the same researchers identified a unique strain of XDR S sonnei carrying the CTX-M gene spreading among MSM in Los Angeles. Recent studies have also documented the spread of XDR Shigella strains in other high-risk populations, such as the unhoused.
The authors of the study say XDR Shigella isolates carrying both resistance genes have not been reported before.
“The clinical implications of this are significant given that dual cephalosporinase producers may exhibit higher resistance than the more typical CTX-M–associated XDR Shigella,” they wrote.
Concerns about broader spread
Most shigellosis cases are self-limiting, but antibiotics are frequently used to treat more-severe symptoms and to reduce shedding, which can help limit transmission. In 2023, however, the Centers for Disease Control and Prevention (CDC) warned in a health advisory that cases of XDR Shigella were rising in the United States. CDC data showed that the percentage of XDR Shigella isolates collected in the United States rose from 0% in 2015 to 5% in 2022.
Another significant finding from the report was that molecular analysis showed that isolates from both patients had a close genetic relationship and a common ancestor, even though the two patients lived 80 miles from one another, and their visits did not overlap at the institution. There were also no known outbreaks of foodborne Shigella at the time the two cases were identified.
Corresponding report author Shangxin Yang, PhD, who directs the UCLA Molecular Microbiology and Pathogen Genomics Laboratory, said the biggest message of the report is that XDR Shigella appears to be spreading beyond MSM and other high-risk populations.
“Based on my genomic analysis it seems to be a different strain just emerging recently, meaning there two different kinds of XDR Shigella circulating simultaneously, with one strain still actively circulating among the MSM population, but the new strain (remotely related to the MSM-circulating strain) spreading in a more general population,” Yang told CIDRAP News.
That’s a concern, Yang said, because broader spread could lead to more treatment failure, since the strains are “pretty much resistant to all oral drugs.” Both patients in the report required intravenous treatment with carbapenem antibiotics, which are a last-resort option for invasive infections.
Yang and his colleagues say the situation warrants close monitoring and continuous genomic surveillance.
