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September 01, 2022

3 min read

The authors report no relevant financial disclosures.

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More than 90% of cases identified in a Shigella sonnei outbreak in the United Kingdom were drug resistant, prompting calls for better stewardship of STI treatment, researchers wrote recently in The Lancet Infectious Diseases.

“The study was prompted as routine surveillance at the U.K. Health Security Agency detected an increase in S. sonnei initially, and that this increase was among men who have sex with men (MSM),” Hannah Charles, MSc, senior HIV/STI surveillance and prevention scientist at the agency, told Healio.

This strain of S. sonnei identified in an outbreak among MSM is extensively drug resistant, with high levels of resistance to ciprofloxacin, azithromycin and ceftriaxone. Source: Adobe Stock

Hannah Charles

According to the CDC, 77,000 antibiotic-resistant cases of S. sonnei are reported in the United States each year. Although anyone can be infected, gay and bisexual MSM are at increased risk, along with international travelers and people with weakened immune systems, including those with HIV or who are undergoing chemotherapy treatment.

“Whole-genome sequencing found that these recent cases of S. sonnei were highly genetically related and were extensively drug resistant,” Charles said.

Charles said that isolates within this outbreak consistently expressed a genomic marker for ceftriaxone resistance, which researchers had not seen among S. sonnei isolates before. Ceftriaxone is a second-line oral treatment for S. sonnei, and losing its efficacy could make treatment of the infection more difficult.

The finding prompted an outbreak control team to gather epidemiological, demographic and clinical data to understand the outbreak and identify potential interventions to reduce transmission.

According to the study, routine laboratory surveillance showed S. sonnei clade 5, which was first detected in September 2021. Cases from this clade were subsequently reported from Scotland, Wales and Northern Ireland. The researchers used whole-genome sequencing with single nucleotide polymorphism (SNP) typing to identify genomic clusters and antimicrobial-resistance determinants.

In total, 72 cases belonging to the 10 SNP single linkage cluster of S. sonnei clade 5 were identified between Sept. 4, 2021, and Feb. 9, 2022. These isolates were predominantly extensively drug resistant, with 92% harboring blaCTX-M-27 — a plasmid-mediated gene for production of extended-spectrum beta-lactamases.

Of the 33 cases with clinical data available, 58% received antibiotics and 24% were hospitalized. Additionally, 78% of the 27 cases with completed outbreak questionnaires were HIV-negative MSM who were using HIV PrEP. These patients also reported sexual contacts in the U.K. and Europe within the incubation period.

“This strain of S. sonnei identified to cause this outbreak among MSM is extensively drug resistant, displaying high levels of resistance to ciprofloxacin, azithromycin and ceftriaxone, leaving very limited therapeutic options available if required and the potential for treatment failures,” Charles said. “Shigellosis can be severe, and interviews with cases involved in this outbreak revealed that the impact on people’s life and well-being can be significant.”

In a related commentary, Marcus Y. Chen, FRCP, FAChSHM, PhD, adjunct professor at the Melbourne Sexual Health Centre and Central Clinical School Monash University, and Deborah A. Williamson, MD, PhD, clinical and public health Microbiologist and director of microbiology at the Royal Melbourne Hospital, wrote that there were several key takeaways from the study.

First, outbreaks of sexually transmissible pathogens continue to pose a serious threat to global public health, including the emergence of new pathogens.

Second, antimicrobial resistance is a pressing concern with several sexually transmitted pathogens. They said that antimicrobial stewardship in the management of sexually transmitted infections and the development and use of new antimicrobials are urgently needed to reduce resistance.

Third, public health responses that quickly identify and contain outbreaks of sexually transmitted infections remain critical.

“Although the COVID-19 pandemic led to restrictions on social mobility that might have curtailed the spread of sexually transmissible infections, the lifting of restrictions and resumption of international travel mean we are likely to see a rebound in the local and global transmission of these infections,” they wrote.

“The study by Charles and colleagues, coupled with the recent spread of monkeypox through sexual contact, are timely reminders of the need for highly accessible sexual health services and effective public health responses.”


CDC. Antibiotic resistance and Shigella infections. https://www.cdc.gov/shigella/treatment/antibiotic-resistance-general.html. Accessed on July 20, 2022.

Charles H, et al. Lancet Infect Dis. 2022;doi:10.1016/S1473-3099(22)00370-X.

Chen MY, at al. Lancet Infect Dis. 2022;doi:10.1016/S1473-3099(22)00361-9.

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Highly resistant Shigella sonnei outbreak detected in UK

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