Outbreak of waterborne cryptosporidiosis associated with low oocyst concentrations.

Water treatment plants in the United Kingdom at significant risk of cryptosporidiosis and using conventional filtration methods have been required to install 24-h monitoring systems since April 2000. No major waterborne outbreaks have been described since 2001. Small outbreaks have been associated with water. This paper describes such an outbreak. Data from a local multi-agency surveillance system was used to describe the outbreak, including mapping cases against water supply zones. A case-control study investigated hypotheses raised. All cases were genotype 1. Early cases were in the supply zone of a surface water-treatment plant that had met treatment standards. Later cases included residents in a different supply zone that temporarily received water from the implicated plant. Cases reported more consumption of domestic mains water than controls. Descriptive and analytical epidemiology thus supported drinking water as a source of cryptosporidiosis from a plant meeting regulatory requirements. The evidence for setting drinking-water standards needs review.