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Background: During March and April 2010, after 8 months without rain, 254 Hepatitis A (HA) cases a 10-fold increase relative to the past 3 years, were reported from two townships of Qianxi County.We investigated this epidemic to identify risk factors, mode of transmission and recommend control and prevent measures.Methods: We defined suspected case as onset since January 2, 2010 of≥2 of the following: fever, malaise, anorexia, nausea, abdominal discomfort and jaundice in a resident of either township.A confirmed case was a suspected case with HAV-IgM (+).We searched for cases in the hospitals and villages.We compared exposures of 75 case-patients to those of 150 control-persons matched by age and village.Results: We identified 308 HA cases (attack rate [AR]=2.0/1000) from February 15 to April 19, 2010, including 54 suspected and 254 confirmed cases.The AR was 9.3/1000 in 8 neighboring villages where one pipeline from a lake supplied water compared to 0.42/1000 in other villages (risk ratio, [RR]=22, 95% confidence interval [CI]=16-31).Among case-villagers 80% drank unboiled water piped from the lake compared to 44% of control-villagers (odds ratio, OR=14; 95% CI=4.6-43), and 24% of case-villagers and 46% of control-villagers washed hands ≥3 times/day (OR=0.22; 95%CI=0.09-0.54).Multivariable logistic regression showed both drinking unboiled, piped lake-water and washing hands infrequently were independently associated with HA.Lake water had > 1600 MPN total coliforms/100mL.Two weeks after water chlorination and mass vaccination with HA vaccine, no new cases were reported.Conclusions: Drinking contaminated unboiled water was responsible for the outbreak.As a long term solution, chlorinated water was piped in from an existing water-treatment plant.