One concern about potable reuse of reclaimed water is the potential health risk from little-known or unknown pathogens. In more than half of all reported outbreaks of waterborne disease, no etiologic agent was ever determined. Some outbreaks that were thoroughly investigated suggest the existence of unrecognized pathogens. For example, "Brainerd diarrhea," first described in an outbreak in Brainerd, Minnesota, in 1983 (Osterholm et al., 1986), is characterized by chronic diarrhea lasting an average of 12 to 18 months. Similar symptoms were noted in several subsequent outbreaks in seven other states where the disease etiology was associated with poor-quality or untreated drinking water (Parsonnet et al., 1989). Intense microbiological analyses failed to identify any etiologic agent for this syndrome.
"Emerging" infectious diseases have been defined as those whose incidence in humans has increased within the past two decades or threatens to increase in the near future (Institute of Medicine, 1992). Some infectious agents, such as Cryptosporidium, were first described in the past 10 to 20 years but have more recently emerged as major causes of waterborne disease. Drinking water from potable reuse systems may pose a risk of exposure to emerging enteric pathogens because raw wastewater contains many enteric pathogens, the removal of which by treatment processes can only be inferred by other measures of microbial quality. The occurrence and health significance of many of these agents in finished drinking water are currently unknown.
Table 3-4 summarizes information on a number of recently recognized enteric pathogens known to have waterborne transmission or to have the potential for waterborne transmission via fecal-contaminated water. The table includes the sizes of these organisms (when known), since this may be relevant to their removal by specific water and wastewater treatment processes. (Several emerging enteric waterborne pathogens that are important outside the United States e.g., hepatitis E virus, group B rotavirus, and Vibrio cholerae O139 are not discussed here because these infections have not been transmitted within the United States.)
Norwalk virus and related human caliciviruses are considered emerging pathogens because new diagnostic techniques have recently identified their roles as major waterborne and foodborne pathogens. A number of other viruses are known or putative enteric pathogens. However, little or no evidence exists regarding waterborne transmission of these organisms. Methods to detect them in water and wastewater have not been developed, and little or no information exists about their survival or transmission in water. For instance, astroviruses are recently recognized