Shigellosis outbreak associated with contaminated well water in a rural elementary school

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, x2 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