Shigellosis is a reportable disease in Illinois. We reviewed surveillance data from the Illinois National Electronic Disease Surveillance System (I-NEDSS) to identify all laboratory-confirmed shigellosis cases among Chicago residents [7]. Because the first case of ESBL S. sonnei initially identified in the cluster had a reported illness onset of 8 August 2022, we reviewed all shigellosis cases reported in I-NEDSS starting the month before this date; we followed up cases to the end of the month when the last case was identified. For cluster cases among Chicago residents, we defined cases as laboratory-confirmed ESBL S. sonnei among Chicago residents during 1 July to 31 October 2022. ESBL S. sonnei isolates were identified by (1) intermediate or resistant AST result to third-generation cephalosporins using automated broth microdilution method on the VITEK2 instrument (Biomerieux) followed by ESBL confirmation with disk diffusion method or (2) detection of the blaCTX-M gene through WGS [8]. To identify cases among patients residing outside of Chicago, we used the national System for Enteric Disease Response, Investigation, and Coordination (SEDRIC). We defined travel-associated cases as shigellosis cases reported in SEDRIC during 1 July to 31 October 2022, with isolates within 0–9 cgMLST allele differences of the initial 3-case cluster or with genomes containing a blaCTX-M gene, and among patients who traveled to Chicago during the 7 days before symptom onset.

We obtained basic demographic and clinical information from the surveillance systems for cluster cases. In addition, we attempted to interview patients using a standardized supplemental survey conducted by telephone. The supplemental survey collected additional information on clinical characteristics, potential exposures, travel, sexual behaviors, patient outcomes, and treatment.

Among 84 Chicago residents with laboratory-confirmed S. sonnei during 1 July to 31 October 2022, there were 45 (54%) isolates with resistance information available, including 5 (6%) with AST results only, 18 (21%) with WGS results only, and 22 (26%) with both. Five isolates were considered ESBL. Three of the 5 had indistinguishable AST profiles. In addition, 4 travel-associated cases were identified through SEDRIC. All 9 isolates were sequenced.

Among the 9 ESBL S. sonnei cases, all patients were male, with a median age of 33 years (interquartile range = 31–38 years). All patients were white; 1 of 9 patients was Hispanic or Latino. We interviewed all patients with the supplemental survey. No common food borne exposures were identified. Eight patients reported ever having sexual contact with men, including 5 of 9 patients who reported sexual contact with a man within a week before symptom onset. Six patients reported attending the same festival for lesbian, gay, bisexual, transgender, queer and other (LGBTQ+) community members 4–7 August 2022, presenting exposure opportunities before symptom onset, including 3 out-of-state travelers who attended the event. Of the 5 patients who reported recent sexual contact with men, 4 reported oral-penile contact (4 insertive, 3 receptive), 3 reported anal sex (3 insertive, 2 receptive), and 3 reported oral-anal contact. One patient had a prior Shigella infection in the past year and was coinfected with mpox and non-ESBL Shigella flexneri at the time of S. sonnei diagnosis. Only 3 patients had heard about Shigella before their most recent infection.

We had complete symptom information for 7 of 9 ESBL Shigella patients who completed our supplemental survey. Among these 7 patients, all reported diarrhea, 5 reported abdominal cramps, and 4 self-reported fever at the time of their gastrointestinal symptoms. Among all 9 patients, 3 sought medical care in the emergency department, and 2 were hospitalized for dehydration. No patients were immunocompromised. Antibiotic treatment information was available for 5 patients; all had isolates that were resistant to the recommended treatment antibiotics ampicillin, azithromycin, ceftriaxone, ciprofloxacin, and trimethoprim-sulfamethoxazole but susceptible to tetracycline (not a recommended treatment agent). Four of the 5 patients who were treated with antibiotics received an antibiotic to which their isolates were resistant, and 4 patients received tetracycline or doxycycline. One patient received cefixime, despite reduced susceptibility to ceftazidime. No patients reported receiving antibiotics the month before treatment, and there was no reported clinical treatment failure, although we have incomplete follow up for all patients’ outcomes.

This cluster is a local example of S. sonnei becoming more antibiotic resistant. Overall, there was resistance to multiple classes of antibiotics, including cephalosporins, and 1 resistance gene (qnr-B19) was located on a plasmid, indicating potential for transmission to other Enterobacterales. Plasmid-mediated ESBL Shigella remains a concern. A plasmid-mediated gene (bla_-CTX-M-27) resistant to all first-line antibiotics was found in a similar global cluster first identified in the United Kingdom, predominantly circulating among MSM in early 2022 [9].

MSM are at increased risk of infection from drug-resistant forms of Shigella, including ESBL Shigella [6]. Eight of 9 ESBL cases in our cluster were MSM; however, we were only able to confirm sexual contact within 1 week before symptom onset for 5 of 8 MSM (56%) patients in the cluster. Given that no common food sources were found, direct transmission through sexual contact is still likely in this cluster, and the LGBTQ+ festival might have led to opportunities for transmission, be it sexual or food borne. While sexual contact is likely the route of transmission for many people in our cluster, we also observed an unrelated isolate among a patient without reported epidemiologic risk factors for sexual transmission, suggesting that resistance is growing among isolates that might have been transmitted through other mechanisms.

Clinicians should perform AST on Shigella isolates from patients at higher epidemiologic or clinical risk of shigellosis, including MSM, persons experiencing homelessness, international travelers, and immunocompromised individuals [10]. If AST is not possible due to limitations of the clinical laboratory, close monitoring for signs of treatment failure may be warranted, particularly for patients at high risk for severe illness. During this cluster investigation, AST was conducted for only 27% of all S. sonnei isolates among patients in Chicago. AST data are needed to identify drug-resistant shigellosis and ensure appropriate treatments for all at risk groups, including MSM, the immunocompromised and patients with severe shigellosis.

Timely diagnosis of shigellosis and a detailed history of potential exposures and social history is important. Patients experiencing fever or bloody diarrhea should be evaluated for enteropathogens for which antimicrobial agents may confer clinical benefit, including Shigella. In an integrated approach, health care providers should provide shigellosis prevention education along with other sexual health prevention measures, such as vaccination for mpox, viral hepatitis, and meningitis, and human immunodeficiency virus (HIV) preexposure prophylaxis (PrEP) at screenings for sexually transmitted diseases, particularly in treatment settings for patients that face greater risks of sexually transmitted disease, such as HIV PrEP providers. Prior to large events that could present opportunities for new or increased numbers of sexual contacts, it is important to include shigellosis prevention education in activities promoting sexual health. In the event of an outbreak, WGS should be performed for public health intervention.