Quantitative Microbial Risk Assessment of Swimming in Sewage Impacted Waters Using CrAssphage and Pepper Mild Mottle Virus in a Customizable Model

Fecal indicator bacteria currently employed for microbial water quality management are poor representatives of viruses. Viral water quality indicators have recently been proposed based on the human gut bacteriophage crAssphage and the food virus pepper mild mottle virus (PMMoV) due to their high abundance in sewage and association to human waste. Here, we develop a model relating crAssphage and PMMoV abundance to the risk of swimmer illness in a recreational water contaminated with fresh, untreated domestic sewage. This model, entitled QMRAswim, is available via a Web-based user interface and is generalizable to any indicator or pathogen. The majority of predicted illnesses from exposure to untreated domestic sewage-contaminated water were attributable to viruses, primarily norovirus. The mean crAssphage and PMMoV concentrations correlating with 30 illnesses per 1000 bathers were 4648 GC/100 mL and 5054 GC/100 mL, respectively, approximately 50 times their standard detection limit. This study reaffirms the importance of monitoring viral water quality to adequately protect public health, suggests the high potential of both crAssphage and PMMoV for this application, and establishes a basis to relate viral indicator abundance with probability of illness due to viral pathogens.