Cyclospora oocysts were identified in the stools. Epidemiological investigations implicated the tap water in the physicians' dormitory and indicated that the storage tank may have been contaminated. Although this outbreak has been identified as a waterborne outbreak, a plausible scenario for the contamination of the water has not been developed.

Another set of emerging protozoan pathogens is microsporidia—a general term that describes a large group of primitive, obligate, intracellular protozoa. Most reported cases of microsporidial infections have occurred among persons infected with human immunodeficiency virus (HIV) (Bryan et al., 1991), and recent epidemiological studies suggest that one species, Enterocytozoon bieneusi, is an important cause of chronic diarrhea in patients with AIDS (Weber et al., 1992). The development of methods to detect H. pylori, Cyclospora cayetanensis oocysts, and microsporidia spores in water and wastewater and to evaluate the risks associated with waterborne exposure to these pathogens was recently identified as a priority research need by the EPA (U.S. EPA, 1996).

A final emerging waterborne pathogen is Isospora belli, a protozoan that has been associated with one documented waterborne disease outbreak in troops in Panama (Goodgame, 1996). The oocyst is large (20 to 30 µm) and, while it is endemic in Africa, Asia, and South America, it is extremely rare in the United States. For example, the organism infected 15 percent of AIDS patients in Haiti but only 0.2 percent of those in the United States.

Aquatic Bacterial Pathogens of Possible Concern for Potable Reuse Systems

Two types of aquatic microorganisms, aeromonads and cyanobacteria, may be of concern for potable reuse systems because their densities in water and/or their production of toxins could be influenced by wastewater nutrients. Indirect reuse systems that contain sufficient nutrients could create blooms of these organisms that may penetrate the treatment barriers and/or proliferate in the distribution system.

Aeromonads are commonly found in water and soil. Densities in water are related to fecal pollution and temperature, and aeromonads proliferate in domestic and industrial wastewater (Schubert, 1991). Some evidence suggests that Aeromonas may produce enterotoxins (Mascher et al., 1988), and several reports have suggested an association between gastroenteritis and Aeromonas in drinking water (Burke et al., 1984; Schubert, 1991). One study in Iowa concluded that three strains of Aeromonas were capable of causing diarrhea and that consumption of untreated water was a risk factor for Aeromonas infection (Moyer, 1987). A study in London found a correlation between Aeromonas isolates from water and iso-

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