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BACKGROUND: Cryptosporidium, a chlorine-tolerant parasite, causes gastrointestinal illness affecting approximately 748,000 U.S. residents annually and has caused massive, communitywide waterborne outbreaks. Laboratory-based surveillance detected a cryptosporidiosis outbreak in Baker City, Oregon in July 2013, anecdotally linked to municipal drinking water. We investigated to confirm the outbreak source, assess attack rate, and evaluate detection timeliness.
METHODS: We tested raw and treated city water and assessed potential environmental sources of contamination. Trained interviewers administered a standardized survey to 380 randomly sampled households. To determine attack rate, survey cases were defined as Baker City residents with onset after July 1 of acute diarrhea lasting ≥3 days. Data were weighted to reflect the sampling design. We compared survey case symptom onset with report dates of laboratory-confirmed cases detected by passive surveillance to determine outbreak detection timeliness.
RESULTS: Cattle grazing encircles the watershed; cattle feces were observed within watershed barriers. City water tested positive for Cryptosporidium; one watershed sample had 913 oocysts. The attack rate was 28.3% (95% confidence interval: 22.1%–33.6%), indicating 2,780 residents became ill. The first survey case onset was July 1; first laboratory-confirmed case was reported July 29. A 3-week boil water advisory began July 31; the implicated reservoir was taken offline and no subsequent cases occurred. Cryptosporidium parvum subtype IIaA15G2R1, a common parasite of U.S. dairy calves, was found in stool specimens from nine patients.
CONCLUSIONS: This community-wide outbreak highlights the importance of regulations requiring source water protection and treatment to control Cryptosporidium. Watershed contamination by cattle likely caused this outbreak, which was undetected by traditional surveillance for four weeks. Prevention and control of cryptosporidiosis outbreaks requires improvements in drinking water treatment and enhanced surveillance.