Cryptosporidium risk from swimming pool exposures.

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Infection risk estimates from swimming in treated recreational water venues are lacking and needed to prioritize public health interventions in swimming pools. Quantitative infection risk estimates among different age groups are needed to identify vulnerable populations. High risk populations can be targeted during public health interventions, like education campaigns and pool operation improvements. This study estimated per-swim and annual Cryptosporidium infection risks in adults (>18) and children (≤18) using new experimental data collected in the U.S. on swimmer behavior. Risks were estimated using oocyst concentration data from the literature, and data collected in this study on pool water ingestion, swim duration and pool use frequency. A sensitivity analysis identified the most influential model variables on infection probability. The average estimated risk of Cryptosporidium infection was 2.6×10-4 infections/swim event. The per-swim risk estimate in the present study differed from others because behavior data (ingestion rates, swim duration, and visit frequency) were collected in different countries and varied from U.S. estimates. We found swimmer behaviors influence infection risk. This is the first study to report annual risk of Cryptosporidium infection among swimmers by age group. Using U.S. exposure data, annual risk was estimated at 2.9×10-2 infections/year for children and 2.2×10-2 infections/year for adults. Annual risk for all swimmers was estimated at 2.5×10-2 infections/year from swimming in treated recreational water venues. Due to increased ingestion and swim duration, child swimmers had the highest annual risk estimate. Cryptosporidium concentration is the most influential variable on infection probability. Results suggest the need for standardized pool water quality monitoring for Cryptosporidium, education, development of interventions to reduce ingestion, consideration of behaviors unique to swimming populations in future risk assessments and improvement of oocyst removal from pool water. Child swimmers were the most vulnerable sub-population, and should be targeted in healthy swimming education campaigns.