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An epidemiology study published in the Lancet Infectious Diseases looks at the spread of sexually transmissible drug-resistant shigellosis.
Dr Vincenzo Torraca, Lecturer in Infectious Disease, King’s College London, said:
Is this good quality research? Are the conclusions backed up by solid data?
“Yes I endorse the research quality and the conclusions.
How does this work fit with the existing evidence? Does it challenge it – and if so does it provide sufficient strong evidence to do so?
“The work provides additional evidence of existing trends (see for example this https://www.gov.uk/government/news/increase-in-drug-resistant-shigella-cases-among-gay-and-bisexual-men).It does not challenge previous trends but provides a quantification of the impact of MSM-transmitted Shigella sonnei.
Have the authors accounted for confounders? If not, do the confounders fatally undermine this study?
“There are potentially confounding factors in this study. The main one is that cases in men in the age 16-60 brackets with no recent travel history were presumptively assigned to the MSM category. These cases could still be non sexually associated and acquired from the community (i.e. by ingestion of contaminated food/drinking or swimming in contaminated waters). However, the study makes the most of the available datasets and indicates that additional studies should be performed.
Are there important limitations to be aware of? Are there any flaws or weaknesses in this work that journalists need to know about?
“As discussed above. The limitations is that both the MSM and the travel associated cases are presumptive. This is a good proxy but it’s not 100% accurate. I don’t think however this should be reported. This work is still very solid due to the large sample size. In the future however additional studies should be conducted where the recent sexual history of the patients is recorded and taken into account to validate findings and interpretations.
What are the implications in the real world? Is there any overspeculation?
“Implication for real world is to inform the MSM community that Shigella is an emerging sexually transmitted pathogen. Differently from other more notorious STIs, Shigella is not exclusively transmitted sexually. It can also be transmitted by ingesting contaminated food and water. This pathogen therefore is a threat to patients, their sexual partners and also the surrounding community. An additional threat of Shigella is antibiotic resistance, which makes it difficult to treat it with several mainstream antibiotics. Transmission can be however prevented by abstaining from behaviours that lead to high risk of transmission to others. The CDC for example recommends maintaining high standards of hand hygiene, avoiding having unprotected sex or engaging with other behaviours at high risk of foecal contamination, avoid preparing foods for others, and avoid swimming until fully recovered. https://foodsafety.uw.edu/sites/foodsafety.uw.edu/files/documents/shigellosis/Shigella_Do-you-have-diarrhea_Adults_CDC_EN.pdf.”
*Amended comment* Dr Mathew Beale, Senior Staff Scientist and molecular and computational microbiologist, Wellcome Sanger Institute, said:
This is an impressive and comprehensive analysis of transmission of Shigella sonnei in England. The dataset is large and well sampled, and the analyses represent cutting edge tools for addressing questions around transmission and spread. The authors show that sexually transmitted Shigella transmits more rapidly and spreads more broadly than non-sexually transmitted Shigella. This has been suggested by previous work, but here the authors formally and comprehensively quantify this. While there are some key limitations, the study is very strong, and given the sample size and comprehensiveness of the dataset, it is likely that the broad findings of the study hold true.”
“These limitations are acknowledged by the study’s authors. Due to the lack of more granular data, the authors assume that most men infected with Shigella without recent travel history to a high-risk area come from the GBMSM community, and that most women with Shigella without recent travel history to a high-risk area do not. This is an assumption made with care and best use of the available data, and has been used previously for similar analyses. However, non-sexually transmitted shigellosis (non-pMSM) is expected to affect both men and women roughly equally and was estimated to count for 36% of cases. The assumption that men age 16-60 without travel history acquired their shigellosis sexually whilst women of the same demographic did not is inconsistent with this, since an approximately equal proportion of men should have acquired their Shigella non-sexually. The study shows 25% of the non-pMSM group were male, largely drawn from individuals younger than 16 or older than 60. It therefore seems likely that a large number of men classified in the pMSM (sexual transmission) group acquired their Shigella through non-sexual means, meaning the estimates of pMSM likely represent an overestimate GBMSM.”
“Additionally, the authors also use the location of the laboratory where Shigella samples were tested as a proxy for geographical location of the patient, but this is also problematic, since samples taken for testing are often referred to testing laboratories far from a patient’s home. It is also worth noting that many individuals involved in GBMSM sexual transmission networks do not acquire their infection close to their home (or to the hospital laboratory), but travel across the country (e.g. to clubs in metropolitan areas such as London), and only present as sick to their GP after returning home.”
“In general, although the study does indeed show that sexually transmitted Shigella spreads faster and that this transmission has at times been associated with increasing drug resistance, the manuscript does not show that sexually transmitted Shigella is currently ‘growing drug resistant much quicker’ than non-sexually transmitted Shigella as the press release suggests, in particular because the azithromycin resistance noted actually declined after 2018.”
Comment from our colleagues at the Spanish SMC:
Dr Alicia Calvo-Villamañán, Postdoctoral researcher at the Gulbenkian Institute for Molecular Medicine in Lisbon (Portugal):
“The study is well done, with a good selection of data and the usual limitations of this type of study. For example, in cases where not all necessary patient data are available, the authors assume that if it is not noted that the person has traveled to a high-risk country, it means the person has not been in a high-risk country in the past 28 days; or, if gender is not specified, the person is female (which automatically places the sample in the group of non-men who have sex with men).
“This study highlights the need to adopt antibiotic treatment guidelines in our healthcare systems that take into account the complex dynamics of antimicrobial resistance. The term “bystander resistance” refers to situations in which an antibiotic is used to treat an infection caused by a specific bacterium, but the treatment has the side effect of selecting for resistance to that antibiotic in a different bacterium. In this case, treating a patient with a gonorrhea infection using azithromycin could promote the selection of azithromycin resistance in Shigella if the patient were also infected with this second bacterium. When the United Kingdom’s national treatment guidelines stopped recommending the use of this antibiotic to treat gonorrhea infections, the emergence of azithromycin resistance in Shigella slowed down.
“The spread of antimicrobial resistance is a highly complex phenomenon due to several factors. First, bacteria live in highly complex communities that foster intense interaction among them, as is the case in the human microbiome. Second, different bacteria exhibit varying susceptibility to the same antibiotic. This means that the dose adequate to eliminate one bacterial species may be insufficient to eliminate another, which can promote the emergence of antibiotic resistance in the latter. Third, human communities are also highly complex, and a wide variety of individual behaviors coexist within them. All these factors make it difficult to establish appropriate national guidelines for antibiotic use. As our knowledge of antimicrobial resistance grows, national guidelines must be updated to reflect these new advances and our increasingly precise understanding of this phenomenon.”
She does not declare any conflict of interest.
‘The spread of sexually transmissible drug-resistant shigellosis in England: a genomic epidemiology study’ by Julia E Marshall et al. was published in the Lancet Infectious Diseases at 23:30 UK time on Wednesday 8 July 2026.
DOI: 10.1016/S1473-3099(26)00227-6
Declared interests
Dr Vincenzo Torraca: “I have co-authored papers with the authors of this study.”
Dr Mathew Beale: “No conflicts of interest to declare.”
This Roundup was accompanied by an SMC Briefing.