Quantitative microbial risk assessment of acute gastrointestinal illness attributable to freshwater recreation in Ontario.

Objectives: The burden of disease associated with acute gastrointestinal illness (AGI) in Canada is estimated to be ~ 20 million cases/year. One known risk factor for developing AGI is recreation in freshwater bodies such as lakes. The proportion of cases attributable to freshwater recreation in Canada, however, is currently unknown. The study objective was to estimate the risk of developing AGI from exposure to Giardia, Cryptosporidium, Campylobacter, Escherichia coli O157:H7, norovirus, and Salmonella during freshwater recreation in Ontario, Canada.

Methods: A quantitative microbial risk assessment (QMRA) was conducted to estimate the number of AGI cases per 1000 recreational events associated with freshwater recreation. QMRA utilizes four steps: hazard identification, exposure assessment, dose-response modelling, and risk characterization. A probabilistic model was developed using the following inputs accounting for uncertainty and variability: published data on pathogen prevalence and concentration in freshwaters in Ontario (hazard identification), recreator water ingestion volumes (exposure), pathogen-specific dose-response models, and ratios between numbers of infections and symptomatic disease cases to estimate illness risks (risk characterization).

Results: The mean estimated AGI risk associated with recreation ranged from 0.8 to 36.7 cases per 1000 swimmers (5th-95th probability interval: 0-226.3 cases/1000) which is in line with previous studies conducted in Lake Ontario, as well as prior QMRAs of freshwater recreation. Upper range predicted values exceeded the Health Canada guideline of less than 20 cases per 1000 recreators.

Conclusion: This study shows that QMRA can be used to estimate disease risk in the absence of large-scale epidemiological studies. The results demonstrate a range of risk that is in line with exposure to pristine (low risk estimates) and more contaminated waters (high risk estimates) and capture the potential risk to vulnerable populations.