Gary S. Logsdon

Large community outbreak of cryptosporidiosis due to contamination of a filtered public water supply.

Between January 12 and February 7, 1987, an outbreak of gastroenteritis affected an estimated 13,000 people in a county of 64,900 residents in western Georgia. Cryptosporidium oocysts were identified in the stools of 58 of 147 patients with gastroenteritis (39 percent) tested during the outbreak. Studies for bacterial, viral, and other parasitic pathogens failed to implicate any other agent. In a random telephone survey, 299 of 489 household members exposed to the public water supply (61 percent) reported gastrointestinal illness, as compared with 64 of 322 (20 percent) who were not exposed (relative risk, 3.1; 95 percent confidence interval, 2.4 to 3.9). The prevalence of IgG to cryptosporidium was significantly higher among exposed respondents to the survey who had become ill than among nonresident controls. Cryptosporidium oocysts were identified in samples of treated public water with use of a monoclonal-antibody test. Although the sand-filtered and chlorinated water system met all regulatory-agency quality standards, sub-optimal flocculation and filtration probably allowed the parasite to pass into the drinking-water supply. Low-level cryptosporidium infection in cattle in the watershed and a sewage overflow were considered as possible contributors to the contamination of the surface-water supply. We conclude that current standards for the treatment of public water supplies may not prevent the contamination of drinking water by cryptosporidium, with consequent outbreaks of cryptosporidiosis.

Microbiology and Drinking Water Filtration

Water filtration research has been undertaken for a variety of reasons. Studies have been performed to develop information for filtration theories and for design of filtration plants to remove suspended matter such as clays, algae, suspended matter in general, and asbestos fibers from water. Filtration studies related to removal of microorganisms have generally been motivated by the need to learn about the removal of pathogens or indicator organisms, or both. Reducing the risk of waterborne disease has been a goal of microbiologically related filtration research for nearly 100 years. This chapter briefly reviews that research and then discusses the results of recent investigations.

Filtration as a Barrier to Passage of Cysts in Drinking Water

Numerous outbreaks of waterborne giardiasis have occurred in the United States since 1973. Generally, the causes have been inadequate disinfection or inadequate filtration (perhaps none), or sometimes both. Even though research on water filtration for Giardia cyst removal is expected to continue in the future, sufficient information on water filtration for cyst removal is available now to provide some guidance for engineers who design filtration plants and for operators who run them.