Spatial analysis of Acanthamoeba keratitis and water samples in a case-control study

Purpose: An outbreak of Acanthamoeba keratitis (AK), a rare infection with significant morbidity caused by a ubiquitous waterborne organism, has persisted despite the recall of a contact lens solution strongly associated with disease. Environmental Protection Agency regulations reducing allowable disinfection byproducts in potable water were implemented concomitantly; a microbial shift may be an unintended consequence. The relationship between AK and potable water was evaluated in Chicago to understand the contribution. Methods: 65 AK cases diagnosed 6/03 – 6/08 and 175 controls matched on contact lens use, age, and service date were recruited from the UIC Cornea Service. Water samples collected 7/06 – 7/08 from subjects' homes were analyzed for Acanthamoeba microbiologic presence. Geographic information systems (GIS) and SaTScan software were used in spatial analysis. Among subjects with Lake Michigan water, conditional logistic regression was used to estimate the association between AK and the distance between water treatment and end usage, and also positive water samples and the distance from water treatment. Results: Acanthamoeba were identified in 17 of 64 (26.6%) case and 30 of 165 (18.2%) control homes with Lake Michigan water. Spatial analysis identified an area of elevated AK risk (OR: 3.62; p = 0.02) that was relatively distant from the water treatment facility. AK and Acanthamoeba-positive water sample risk increased significantly (OR: 2.38; 95%CI: 1.46 – 3.88 and OR: 2.63; 95%CI: 1.61 – 4.29, respectively) with each 10-mile increase between water treatment and end usage. Conclusion: Results support hypotheses that the domestic water supply is contributing to the persisting AK outbreak.