Studies of health risks associated with recreational water exposure require investigators to make choices about water quality indicator averaging techniques, exposure definitions, follow-up periods, and model specifications; however, investigators seldom describe the impact of these choices on reported results. Our objectives are to report illness risk from swimming at a marine beach affected by nonpoint sources of urban runoff, measure associations between fecal indicator bacteria levels and subsequent illness among swimmers, and investigate the sensitivity of results to a range of exposure and outcome definitions.
In 2009, we enrolled 5674 people in a prospective cohort at Malibu Beach, a coastal marine beach in California, and measured daily health symptoms 10–19 days later. Concurrent water quality samples were analyzed for indicator bacteria using culture and molecular methods. We compared illness risk between nonswimmers and swimmers, and among swimmers exposed to various levels of fecal indicator bacteria.
Diarrhea was more common among swimmers than nonswimmers (adjusted odds ratio = 1.88 [95% confidence interval = 1.09–3.24]) within 3 days of the beach visit. Water quality was generally good (fecal indicator bacteria levels exceeded water quality guidelines for only 7% of study samples). Fecal indicator bacteria levels were not consistently associated with swimmer illness. Sensitivity analyses demonstrated that overall inference was not substantially affected by the choice of exposure and outcome definitions.
This study suggests that the 3 days following a beach visit may be the most relevant period for health outcome measurement in recreational water studies. Under the water quality conditions observed in this study, fecal indicator bacteria levels were not associated with swimmer illness.
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From the aSchool of Public Health, University of California, Berkeley, CA; bSouthern California Coastal Water Research Project, Costa Mesa, CA; cKaiser Permanente Division of Research, Oakland, CA; dUnited States Environmental Protection Agency, National Environmental Health Effects Research Laboratory, Chapel Hill, NC; eSurvey Research Center, University of California, Berkeley, CA; fOrange County Sanitation District, Orange County, CA; and gInstitute of Environment and Sustainability, University of California, Los Angeles, CA.
The study was funded by the California State Water Resources Control Board, the US Environmental Protection Agency (EPA), and the Los Angeles County Flood Control District. With the exception of the scientific contribution of T.J.W. of the EPA, the funders played no role in the collection, analysis or interpretation of the data, nor in the writing of this report and the decision to submit the article for publication. The views expressed in this article are those of the authors and do not necessarily reflect the views or policies of the US Environmental Protection Agency. The authors have no competing interests.
Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (www.epidem.com). This content is not peer-reviewed or copy-edited; it is the sole responsibility of the author.
Correspondence: John M. Colford, Jr, Department of Epidemiology, School of Public Health, University of California, Berkeley, 113A Haviland Hall, MC #7358, Berkeley, CA 94720–7358. E-mail: firstname.lastname@example.org.
Received November 21, 2012
Accepted June 7, 2013