This massive outbreak of watery diarrhea was caused by cryptosporidium oocysts that passed through the filtration system of one of the city's water-treatment plants. Water-quality standards and the testing of patients for cryptosporidium were not adequate to detect this outbreak.
Monitoring for indigenous spores of aerobic spore‐forming bacteria proves a viable method of assessing treatment plant performance.
Spores of aerobic spore‐forming bacteria were used as microbial surrogates for evaluating drinking water treatment plant performance. A method for assaying for the microbial surrogate was developed. Various water sources were surveyed, and data were collected from coagulation and chlorine inactivation studies. Evaluations for spore removals and turbidity and particle removals were compared for pilot‐scale and full‐scale water treatment plants.
Trials have provided conflicting estimates of the risk of gastrointestinal illness attributable to tap water. To estimate this risk in an Iowa community with a well-run water utility with microbiologically challenged source water, the authors of this 2000-2002 study randomly assigned blinded volunteers to use externally identical devices (active device: 227 households with 646 persons; sham device: 229 households with 650 persons) for 6 months (cycle A). Each group then switched to the opposite device for 6 months (cycle B). The active device contained a 1-microm absolute ceramic filter and used ultraviolet light. Episodes of "highly credible gastrointestinal illness," a published measure of diarrhea, nausea, vomiting, and abdominal cramps, were recorded. Water usage was recorded with personal diaries and an electronic totalizer. The numbers of episodes in cycle A among the active and sham device groups were 707 and 672, respectively; in cycle B, the numbers of episodes were 516 and 476, respectively. In a log-linear generalized estimating equations model using intention-to-treat analysis, the relative rate of highly credible gastrointestinal illness (sham vs. active) for the entire trial was 0.98 (95% confidence interval: 0.86, 1.10). No reduction in gastrointestinal illness was detected after in-home use of a device designed to be highly effective in removing microorganisms from water.
Members of the USEPA team that advised Milwaukee officials during the outbreak describe their findings.
In March and April 1993, the largest US waterborne disease outbreak occurred in Milwaukee, Wis., causing illness in more than 400,000 people. The etiological agent responsible for most cases was Cryptosporidium. Milwaukee was served by two water treatment plants that used raw water from Lake Michigan. Before the outbreak, severe spring storms caused the lake's turbidity and bacterial counts to rise dramatically. During the outbreak, effluent produced by one plant had a turbidity approaching 2.5 ntu, a high reading that indicated an increase in particulates passing through the plant. The increase may have also meant an increase in passage of Cryptosporidium oocysts. This article discusses the investigation by the Water Supply and Water Resources Division (formerly the Drinking Water Research Division) of the US Environmental Protection Agency into what may have happened to allow the oocysts to pass through the treatment plant.
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