Shigellosis Outbreak Associated with Contaminated Well Water in a Rural Elementary School: Sichuan Province, China, June 7–16, 2009

OBJECTIVES: We investigated a shigellosis outbreak in an elementary school to identify the source of infection, mode of transmission and risk factors for illness. METHODS: In a case-control investigation, we compared the source of drinking water, consumption of untreated well water and suspected food items, and hygienic habits between case-students and randomly selected asymptomatic control-students, frequency-matched by class on a 1∶1 ratio. RESULTS: 18% of the 533 students and no teachers developed Shigella. 52%(44/85) of case-students and 17% (12/71) of control-students drank untreated well water (OR = 2.3, 95% CI = 1.1–5.8); 47% (n = 40/85) of case-students and 14% (10/71) of control-students drank untreated water from Well A (OR = 3.7, 95% CI = 1.3–11). The odds ratio increased with the amount of untreated Well A water consumed (p = 0.035, χ(2) test for trend). Rectal swabs from 5 of 6 case-students and water from Well A yielded Shigella flexneri 2b. CONCLUSIONS: This shigellosis outbreak was caused by drinking untreated water from a well polluted by Shigella flexneri 2b.