if any, phenotypic divergence but differs genetically and as seen in this example shows no recombination occurring between the two genotypes.

The ability of Cryptosporidium to survive outside the host suggests that evolution favors strains that can generate and input many oocysts into the environment without seriously harming the host. Oocysts were found in three species of rodents in Poland (Bajer et al., 2002). Interestingly, there were significant differences between the Cryptosporidium species identified in rodents over time, though fewer older animals carried infection and there were marked seasonal differences. Rickard et al. (1999) tracked both Cryptosporidium and Giardia species in populations of white-tailed deer in the southern United States. These researchers found higher infection rates associated with Cryptosporidium than Giardia. As for rodents, the probability of protozoan infection decreased with increasing age of the deer.

A few studies have tried to determine primary sources of Cryptosporidium and Giardia cysts and oocycts in watersheds. Studies comparing agricultural and wildlife sources have shown that the lowest prevalence of Giardia and Cryptosporidium was found in wildlife (Heitman et al., 2002), while the highest concentrations were found in cattle feces. Given the potential runoff from agricultural sources into watersheds and waterways, this observation is significant. In a 17-month survey (Bodley-Tickell et al., 2002) of Cryptosporidium oocysts in surface waters draining a livestock operation, the parasites were found to be present year-round, with maximum concentrations and highest frequency of occurrence during autumn and winter. Cryptosporidium was also found in an isolated pond (no livestock), indicating that wild animals alone could transport or import oocysts to surface waters. Waterfowl can also disseminate infectious C. parvum and Giardia in the environment (Graczyk et al., 1998). Although this finding is not surprising, the widespread levels of contamination may be, since seven out of nine sites showed occurrence of C. parvum and all nine sites were positive for Giardia in the feces of migrating Canada geese. Given the size of the migrating waterfowl populations in the United States, this observation is of concern.

Cryptosporidium oocysts transported from river waters into estuarine and marine systems have been shown to be taken up and sequestered by filter-feeding invertebrates. Lowery et al. (2001) showed that the marine filter-feeding mussel (Mytilus edulis) collected from the shores of Belfast Lough in Northern Ireland had Genotype 1 oocysts of C. parvum. Thus, these filter-feeding invertebrates serve as environmental reservoirs of this protozoan. While the natural rate of release from these Cryptosporidium reservoirs is not known (because Mytilus edulis is consumed by humans), this aspect of the ecology of C. parvum may have direct public health ramifications.

Terrestrial insects have been implicated as both control agents and vectors of dissemination of Cryptosporidium oocysts (Dumoulin et al., 2000; Follet-Dumoulin et al., 2001; Mathison and Ditrich, 1999). Mathison and Ditrich (1999) demonstrated that many oocysts can pass safely through the mouth parts and