gastrointestinal tracts of dung beetles; however, the majority are destroyed. Predation by coprophagous (dung eating) insects can be important in both reducing oocysts and spreading the oocysts that are not destroyed throughout the environment. In contrast, free-living ciliated protozoa, such as Paramecium caudatum, are capable of consuming up to 170 oocysts per hour (Stott et al., 2001). Thus, predation, under certain conditions, may affect the density of oocysts.

Although Cryptosporidium infects mammals, it is unable to infect other vertebrate groups such as fish, amphibians, and reptiles (Graczyk et al., 1996). This inability greatly limits the spread and dissemination of oocysts in surface waters. However, depending on the characteristics of the watershed, runoff can carry oocysts shed by infected mammals in major pulses. Once in the waterway, oocysts can be maintained until consumed either by humans or by other mammals.

Most studies of waterborne protozoa of public health concern have focused on the occurrence of the enteric protozoa Cryptosporidium and Giardia. Although these two enteric protozoan parasites are not related taxonomically, they are closely related from an epidemiological, regulatory, and public health point of view. In addition, methods for their simultaneous detection in water have led to much information on their occurrence, more so than for any of the other protozoan parasites. In surveys from the early 1990s, it was reported that for 66 surface water treatment plants in 14 states and 1 Canadian province, 81 percent of the raw water samples had Giardia cysts and 87 percent tested positive for Cryptosporidium (LeChevallier et al., 1991a,b). Further reviews of the literature have found that most waters contain some level of cysts and oocysts ranging from a high in treated wastewaters (10⁴ per 100 liters) to a low in pristine waters (0.1 per 100 liters) (Rose, 1997; Rose et al., 2001a; Slifko et al., 2000). Similar levels have been found throughout the United States and in Europe (Ong et al., 1996; Smith and Rose, 1998). As noted previously, domestic animals, cattle in particular, and sewage discharges have been identified as some of the primary sources of Cryptosporidium and Giardia in water.

The cysts and oocysts of Giardia and Cryptosporidium may also be transported from irrigation waters to row crops, some of which may be eaten raw. Thurston-Enriquez et al. (2002), using both molecular and immunofluorescent techniques, were able to detect oocysts of Cryptosporidium in 36 percent and cysts of Giardia in 60 percent of all irrigation water samples. They also found that 28 percent of samples had Microsporidia. Average concentrations for Giardia and Cryptosporidium varied from 559 cysts and 229 oocysts per 100 liters in samples from Central America to 25 cysts and <19 oocysts per 100 liters in the United States, respectively. These researchers demonstrated that agricultural irrigation waters may be a significant vector for the transmission of waterborne protozoa and corresponding diseases.