Rates of notified cryptosporidiosis and quality of drinking water supplies in Aotearoa, New Zealand

An ecological study was conducted in New Zealand to examine the relationship between incidence of human cryptosporidiosis and indicators of community drinking-water supply quality, using routinely collected data from June 1996 to August 1998. A total of 790 cases of cryptosporidiosis lived in communities with a registered drinking-water supply. Within each category of the grading system there was a wide range of rates of cryptosporidiosis. The lowest mean rates of notified cryptosporidiosis were observed in communities served by drinking water supplies with completely satisfactory public health gradings, and the highest rates in communities served by ungraded drinking-water supplies. Mean rates of notified cryptosporidiosis in drinking-water zones that complied with New Zealand drinking-water standards were lower than rates in zones that did not comply with the standards. Current methods of assessing the quality of New Zealand drinking-water supplies reflect in broad terms the risk to human health from Cryptosporidium parvum. Data limitations and uncontrolled confounding mean that the excess risk due to poor quality of the drinking water supply cannot be quantified, but improvements in drinking water quality are likely to be associated with health benefits at the population level. Occurrence of a high proportion of cases in drinking water supplies with a “satisfactory” public health grading highlights the importance of non-waterborne modes of transmission in human cryptosporidiosis.