BACKGROUND: The largest outbreak of shigellosis in Oregon state history occurred from July 2015 to June 2016. In November 2015, the outbreak strain spread among Portland’s homeless population, coinciding with onset of the wettest rainy season on record. Shigella transmission occurs through fecal-oral contact, and outbreaks tend to occur in overcrowded settings where personal hygiene is challenging. We hypothesized that substantial rainfall might have led to increased congregation and breakdown of hygienic conditions among homeless persons, facilitating Shigella transmission. We investigated whether rainfall was associated with shigellosis during this outbreak.
METHODS: A case was defined as a Shigella sonnei infection in an Oregonian with a pulsed-field gel electrophoresis pattern indistinguishable from the outbreak strain occurring 7/1/2015–6/30/2016. We interviewed patients and reviewed medical charts. Using Poisson regression, we analyzed the association between daily case count and cumulative rainfall during the period 8–14 days prior to each patient’s symptom onset. We stratified by homelessness status and controlled for temperature.
RESULTS: One hundred and five shigellosis cases were identified during the outbreak; 79 (75%) were among men; all occurred among persons aged ≥18 years. Forty-five (43%) infections were reported among homeless persons. Among homeless persons, the risk ratio (RR) for shigellosis was 1.46 (95% confidence interval [CI] = 1.28–1.68) for each additional inch of cumulative rainfall; the association was not significant for housed persons (RR = 1.05; 95% CI = 0.88–1.26).
CONCLUSIONS: Rainfall was associated with increased reports of shigellosis among homeless persons during this outbreak. Substantial rainfall might have caused behavior changes that facilitated Shigella transmission. During forecasted heavy rainstorms, public health departments might take proactive measures to mitigate the spread of infectious disease among homeless persons.