AWWA WQTC69507

In an ongoing study, treated water from conventional surface water filtration plants across a broad geographic area was sampled multiple times for the presence of infectious oocysts. Large volume samples (up to 1,000 L) were analyzed using a modification of U.S. Environmental Protection Agency (USEPA) Method 1623 followed by in vitro cell culture. A comparison of the three most commonly used cell culture-based infectivity methods for Cryptosporidium determined that the HCT-8 cell culture followed by immunofluorescence microscopy was the most appropriate method for the study. Desirable characteristics of an infectivity method for finished water include: distinguishing infectious from non-infectious oocysts; eliminating or minimizing false positives and false negatives; robust enough to support infection despite environmental contaminants that are isolated along with the oocysts; and, allow for molecular analysis of positive samples to determine the species or genotypes responsible for infection. The study is ongoing but 201,000 L of water have been analyzed so far, with no positives yet detected. Positive controls and routine matrix spikes indicate that the method is working, so the lack of positives is not due to false-negative results. The eventual goal is to analyze 280,000 L. Assuming a single infectious cluster arises from one oocyst, if a single sample is positive, the annual risk will be calculated as 0.05 - 1.3 infections per 10,000 individuals, depending on the values for water consumption and risk of infection from a single oocyst selected for model input. Includes 37 references, tables, figures.