ment goal. Water quality standards have used either or both measures depending on the type of water use.

While coliform bacteria serve well as indicators of bacterial pathogens, they do not predict the inactivation or removal of enteric protozoa and viruses (Gerba and Rose, 1990; LeChevallier and Norton, 1993; LeChevallier et al., 1991a, 1991b; Rose et al., 1991). For instance, LeChevallier and Norton (1993) found that multiple linear regression models using coliforms and temperature could predict only 57 percent of the variation in Giardia cyst concentration, whereas no model using indicator bacteria could adequately predict Cryptosporidium oocyst levels.

Methods to detect viruses in water were first developed by Paul and Trask (1947) for measuring enteroviruses in untreated wastewater. Coin (1966) was one of the first to detect viruses in finished drinking water meeting the existing coliform standard. In the 1970s, improvements in collection filters and the use of antibodies led to the first isolation of rotavirus and hepatitis A virus from water. Today, standard methods for the detection of enteric viruses are based on the ability of viable enteric viruses to destroy monkey kidney cells grown in vitro; this cell-destroying ability is known as cytopathic effect or CPE (Benenson, 1995).

Methods for detecting microbial protozoa were first developed for Entamoeba in the 1940s. Starting in 1965, research focused on the detection of Giardia. In the 1980s, a standardized approach for Giardia detection was developed that used filtration for collection and antibodies labeled with a fluorescent isothiocyanate (FITC) for enhanced microscopic detection (Rose et al., 1988a, 1988b). This approach was applied to the detection of Cryptosporidium after its first recorded waterborne outbreak in the United States in 1985.

As more protozoan and viral waterborne outbreaks occurred in waters meeting existing water quality standards, the limitations of using indicator bacteria became apparent. In response to these health concerns, the Environmental Protection Agency (EPA) promulgated the Surface Water Treatment Rule for drinking water in 1989 (U.S. EPA, 1989a, 1989b). The rule established treatment-based performance goals of 99.99 percent reductions of viruses and 99.9 percent reduction of Giardia. The rule also emphasized the use of sand or multimedia filtration for the removal of Giardia and the use of improved disinfection methods for the control of both viruses and Giardia. The target reduction level was based on anticipated levels of pathogens in ambient surface waters, and the performance goals were derived from a desired annual risk of microbial disease of not greater than 1 in 10,000. An Enhanced Surface Water Treatment Rule (U.S. EPA, 1996) is under development; the enhanced rule will include an assessment of Cryptosporidium in source waters and its removal by treatment processes.

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