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Can QMRA be used to Discount Pathogen Risk to Swimmers from Animal Fecal Contamination? Doheny Beach, CA Case Study
Citation:
SCHOEN, M. E. AND N. ASHBOLT. Can QMRA be used to Discount Pathogen Risk to Swimmers from Animal Fecal Contamination? Doheny Beach, CA Case Study. Presented at Society for Risk Analysis, Boston, MA, December 08 - 10, 2008.
Impact/Purpose:
To participate and present poster
Description:
Estimated health risks to swimmers from seagull and bather sources of fecal contamination at Doheny Beach, California were compared using quantitative microbial risk assessment (QMRA) with a view to aiding beach closure decisions. Surfzone pathogens from seagulls were thought to be less of a human health threat than human fecal contamination. To evaluate if the seagull fecal contribution can be discounted, possibly reversing a beach closure, a discounting process is proposed here that estimated the risk to swimmers of gastro-intestinal illness and compared the predicted risk to the 19 illnesses per 1000 benchmark. The discounting process was exemplified for Campylobacter jejuni using 1) the counts of seagulls and swimmer/shedders and assumed pathogen load distributions, or 2) a hypothetical source tracking tool that measures the proportion of total fecal contamination attributed to the two sources. A beta-Poisson dose-response model was utilized to calculate the probability of infection using Monte Carlo analysis of the uncertain input variables. The conditions under which the seagull fecal contribution could be discounted were specified for the observed range of surfzone enterococci (ENT) concentration over the 2007 recreational season. However, even using a perfect source tracking tool, seagull fecal contributions could not be discounted for ENT concentrations above the 108 CFU/100ml single sample limit, at the 95% confidence level, given the 19/1000 benchmark. The same conclusion was drawn using fecal load estimates from the number of gulls, and various ratios to swimmers. The proposed discounting process negates the belief that gull fecal-derived enterococci counts contribute less of a health threat to swimmers than human sources, given the current large uncertainty in human infectious campylobacters in gull feces, let alone their unknown environmental persistence. The discounting process suggested could be applied to additional locations and sources, using appropriate reference pathogens for locations with different sources of fecal contamination.