An Epidemic of Presumed Acanthamoeba Keratitis That Followed Regional Flooding: Results of a Case-Control Investigation

OBJECTIVES To investigate an outbreak of presumed Acanthamoeba keratitis (AK), to identify risk factors associated with its development, and to characterize the changing epidemiology of AK. METHODS We performed a pairwise-matched case-control study involving 31 patients who were diagnosed as having AK between July 1993 and December 1994. Risk factors were identified using conditional logistic regression analysis. To investigate the impact of regional flooding, we stratified counties within Iowa by whether their water facilities were affected and then calculated population-based estimates of the incidence of AK. RESULTS During the study, 43 presumed incident cases of AK were diagnosed; 31 were included in the case-control study. Cases were diagnosed based on the clinical presentation of keratitis, positive tandem scanning confocal microscopy examination results, and confirmatory cytopathologic findings. There were no positive culture specimens. On average, cases had symptoms for 8 weeks before diagnosis, most notably photophobia (94%), red eyes (94%), and pain (80%). Contact lens use (odds ratio [OR]=44.16; P=.02) and fishing (OR=22.62; P =.04) were independent predictors of the development of AK. The presence of a humidifier in the home (OR=0.08; P=.03) and having household water that originated from a private well instead of the municipal water supply (OR=0.12; P=.08) were protective. Twenty-nine of 30 cases resided in counties in which the water supplies were affected by flooding as determined by the Department of Natural Resources, Des Moines, Iowa. The incidence of AK in these counties was more than 10 times higher than that in the unaffected counties (relative risk=10.83, 95% confidence interval, 1.48-79.49; P