METHODS: A case was a S. sonnei infection matching the outbreak strain by PFGE in an Oregonian since July 2015. We collected data from case reports, structured case interviews, and medical records. We used Fisher exact test to compare frequencies before and after November 1, 2015.
RESULTS: During July 1, 2015–March 4, 2016, a total of 92 cases were identified, all in adults. All had case reports; we interviewed 57 (62%) patients and reviewed 73 (79%) medical records. Thirty-six (39%) were MSM and 40 (43%) were homeless; 3 of these were both and 19 were neither. Before the shift, patients were more likely to be MSM (82% vs. 26%, P<0.01); 3 (14%) were homeless, 1 of whom was MSM. After the shift, most patients were homeless (53% vs 14%, P<0.01); most MSM were housed (16, 89%).
CONCLUSIONS: Shigella outbreaks among MSM are well documented, but rarely reported among homeless populations, even though homeless persons experience inadequate access to hygiene and sanitation, overcrowded living arrangements, and potentially contaminated water and food. In this outbreak, Shigella circulating among an MSM population shifted into a homeless population, which has important implications for outbreak management. Outbreak investigation and public health response should be tailored to the epidemiology of an evolving outbreak.