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Monitoring of 82 surface water treatment plants revealed that 1.4% of 1,690 100‐L finished water samples tested positive for infectious Cryptosporidium using the cell culture‐polymerase chain reaction). Infectious oocysts were detected in finished water samples from 22 water treatment plants (26.8%). Genotype analysis identified 23 isolates as Cryptosporidium parvum and one isolate as Cryptosporidium hominis. All isolates subgenotyped were shown to be genetically distinct environmental isolates. Analysis of the water quality and treatment plant characteristics showed no differences between the positive and negative sites. More than 70% of the positive samples occurred in filtered water samples of <0.1 ntu, and 20% of the positive samples were in water of <0.05 ntu. There was no association among Cryptosporidium occurrence and source water type, microbial indicators, or plant operation and treatment. It was concluded that given sufficient testing, nearly all conventional treatment plants would be at risk for passing infectious oocysts. Based on this study, the overall risk of Cryptosporidium infection for conventionally treated drinking water was 52 infections/10,000 people/year, with an 80% credible range of 9–119 infections/10,000 people/year. It was also concluded based on these studies that conventional treatment requires an additional treatment barrier, such as ultraviolet light disinfection, to meet the US Environmental Protection Agency risk goals.

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