We describe a 2016 community-wide recreational water-associated cryptosporidiosis outbreak investigation and response in Maricopa County, Arizona. Persons with a laboratory-confirmed illness were interviewed using a standardized questionnaire that assessed exposures 2 weeks before symptom onset. A convenience sample of managers and operators of chlorine-treated public aquatic facilities was surveyed regarding permanent supplemental treatment systems for Cryptosporidium. Among 437 cases identified (median age 12, range <1–75 years), 260 persons were interviewed. Public-treated recreational water was the most frequently reported exposure (177, 68%) of interviewed persons; almost 1 in 5 (43, 17%) swam when diarrhea was ongoing.
After the 2016 outbreak, managers of some facilities expressed intentions to install supplementary water treatment systems, and by May 2017, at least one large facility installed an ultraviolet light system. Strategies to prevent additional illness included community messaging, education, and targeted remediation of affected facilities on the basis of interviews. Challenges to remediation during a cryptosporidiosis outbreak in a large jurisdiction with primarily outdoor pools underscore the importance of promoting healthy swimming practices that help prevent contamination from occurring.