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Background In 2018, CDC and the Vermont Department of Health investigated an outbreak of multidrug-resistant Shigella sonnei infections in a retirement community. Most Shigella infections are self-limited, but antibiotics are indicated for severe illness and sometimes to limit transmission. The Clinical and Laboratory Standards Institute has not yet established breakpoints for azithromycin, so laboratories cannot report resistance. Although breakpoints exist for ciprofloxacin, isolates with one fluoroquinolone resistance mechanism typically have minimum inhibitory concentrations within the susceptible range (≤ 0.25 µg/mL). Methods We reviewed charts for treatment outcomes of outbreak patients to evaluate clinical and microbiologic response. We defined clinical failure as ≥ 3 loose stools per day for ≥ 1 day after completion of antibiotics and microbiologic failure as a positive stool culture after completion of antibiotics. We used broth microdilution to perform antimicrobial susceptibility testing, and whole-genome sequencing to identify resistance mechanisms. Results Among the 24 patients with culture-confirmed Shigella infection, 4 were hospitalized and 2 died. All isolates were multidrug-resistant (Table 1) and harbored mechanisms for resistance to ampicillin, ceftriaxone, trimethoprim-sulfamethoxazole, azithromycin, and ciprofloxacin. Fifteen patients received one course of ciprofloxacin, 5 received multiple courses of antibiotics, and 4 received no antibiotics. Overall, 6 patients had treatment failure (Table 2); all 4 patients who received azithromycin had subsequent clinical failure and 2 also had microbiologic failure. Two patients had failure after ciprofloxacin (1 clinical, 1 microbiologic). Conclusion This outbreak of highly resistant shigellosis highlights the importance of comprehensive susceptibility testing and systematic outcome studies. Evidence of treatment failure after azithromycin suggests that an appropriate clinical breakpoint is needed to inform clinical decision-making. Ciprofloxacin treatment failures were observed in patients with a susceptible strain harboring a resistance mechanism, warranting further investigation. Disclosures All authors: No reported disclosures.

1603. Observation of Treatment Outcomes During an Outbreak of Multidrug-Resistant Shigella sonnei Infections in a Retirement Community—Vermont, 2018