Human-Associated Fecal Quantitative Polymerase Chain Reaction Measurements and Simulated Risk of Gastrointestinal Illness in Recreational Waters Contaminated with Raw Sewage

We used quantitative microbial risk assessment to simulate the risk of gastrointestinal (GI) illness associated with swimming in waters containing different concentrations of human-associated fecal markers from raw sewage, HF183 and HumM2. The volume/volume ratio of raw sewage to ambient water was determined by comparing marker concentrations in recreational water to concentrations in raw sewage from 54 geographic locations across the United States. Concentrations of reference GI pathogens in raw sewage, volumes ingested by swimmers, dose–response functions, and fractions of infected that become ill were adopted from previous studies. Simulated GI risk increased with concentration of the human quantitative polymerase chain reaction markers in recreational waters. A benchmark illness rate of 30 GI illnesses per 1000 swimmers occurred at median concentrations of 4200 copies of HF183 and 2800 copies of HumM2 per 100 mL of recreational water. This study establishes a risk-based approach for interpreting concentrations of human fecal markers in ambient waters.