Risk-Based Threshold of Gull-Associated Fecal Marker Concentrations for Recreational Water

A sensitive and specific marker of gull fecal contamination, Catellicoccus (CAT), has been used to conduct microbial source tracking in surface waters throughout the world, yet there are no guidelines for interpreting measured concentrations. Here, we use quantitative microbial risk assessment to evaluate CAT concentrations within a risk-based framework and develop a threshold at which the U.S. Environmental Protection Agency illness benchmark (similar to 3 illnesses/100 swimmers) is exceeded. We modeled illness risk from exposure to different concentrations of CAT in bathing waters using a Monte Carlo approach that considered densities of CAT and infectious zoonotic pathogens Salmonella and Campylobacter in gull feces, volume of water ingested during bathing, and dose-response relationships. We measured CAT densities in 37 fresh gull fecal droppings from six California beaches. log(10) densities ranged from 4.6 to 9.8 log(10) copies CAT/g of wet feces. When the level of CAT exceeds 4 x 10(6) copies/100 mL of water, the median predicted illness exceeds 3 illnesses/100 swimmers.