BACKGROUND: Multidrug resistant Shigella outbreaks among men who have sex with men (MSM) have increased. In July 2016, the California Department of Public Health (CDPH) notified Los Angeles County of 7 Shigella flexnericases, caused by serotype 7 (provisional 88-893), an uncommon strain. We investigated to find additional cases, determine risk factors and molecular relatedness of isolates, provide clinical guidance, and prevent additional cases.
METHODS: Locally nontypable S. flexneri isolates were serotyped at CDPH’s microbial disease laboratory. Cases were defined as persons with confirmed S. flexneriserotype 7 in Southern California with onset during March–October 2016. Patients were interviewed about risk factors and medical records abstracted. Six isolates were tested for antimicrobial susceptibilities by broth microdilution. Pulse-field gel electrophoresis and whole genome sequencing (WGS) were conducted. Kruskal-Wallis and Fisher exact tests were used to compare isolate subclusters.
RESULTS: Of 34 patients identified, 13 required hospitalization; 1 died. All were male (median age 36 years); 97% self-identified as MSM; 80% (24/30) with known HIV status were positive, of whom 54% were not taking antiretrovirals consistently. Of the clinically relevant antimicrobials, all isolates were non-susceptible to azithromycin and trimethoprim-sulfamethoxazole, and 83% to amoxicillin. WGS of 28 isolates revealed all were ST-245 sequence type; 27 were clustered with 2-45 single-nucleotide polymorphisms (SNP) difference. Three subclusters were identified and differed by isolate date (P <0.001) and local health jurisdiction (P<0.04).
CONCLUSIONS: This outbreak, caused by an uncommon Shigella strain, provides important data on genetic relatedness of Shigella isolates in a cluster with high selective pressure. Investigators may want to consider molecular testing to identify clusters and help guide investigation of risk factors and transmission. Treatment guidance to clinicians should be determined by antimicrobial susceptibility testing.