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

Abstract

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 (∼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. Log10 densities ranged from 4.6 to 9.8 log10 copies CAT/g of wet feces. When the level of CAT exceeds 4 × 106 copies/100 mL of water, the ...