Weather can influence both the transport and the dissemination of these protozoa (Fayer, 2000; Fayer et al., 2000; Rose et al., 2001b). A study conducted in the United States demonstrated similar results with both wildlife and dairy farms contributing to Cryptosporidium oocysts in the watershed and implicated cold seasons as high-risk periods for oocyst contamination of surface waters (Jellison et al., 2002). High concentrations of oocysts during winter and autumn may be indicative of reduced predation. Alternatively, cold temperatures may actually preserve oocysts and thus provide sources during warmer weather. Skerrett and Holland (2000), in a temporal survey at five sites near Dublin, Ireland, showed that the maximum number of oocysts was found after a period of heavy rainfall associated with increased runoff. Knowledge of how changes in rainfall, snow-melt, and runoff affect the transport of protozoan parasites is critical.

Another important aspect of Cryptosporidium and Giardia (oo)cyst ecology that may affect their distribution and survival is their sedimentation velocity. These velocities are much too low to cause significant sedimentation in surface waters or reservoirs (Medema et al., 1998). However, both cysts and oocysts attach readily to organic particles, which greatly increase their sedimentation velocities. Attachment to particles will affect not only the deposition onto sediments but also the potential for consumption. Depending on the size of the particles, ciliated protozoa may not be able to graze the (oo)cysts. Although infection of fish, amphibians, and reptiles has not been shown, bottom-feeding fish may transport the (oo)cysts to new locations.

Parasite interactions with their hosts and the environment are diverse, complex, and continually evolving. These interactions involve a considerable amount of genetic exchange and selection, as well as host-dependent phenotypic expression. For many parasites, the host immune response is a major factor influencing pathogen-host interaction and its effects on the health of the host. In addition, parasites and their hosts coevolve and show a variety of population-based ecological interactions influencing population density, geographic location, and other factors related to the parasite and host environment.

Factors such as temperature, source of shedding (i.e., from hosts), rainfall, and predation can affect the survivability of these parasites in the aquatic environment. However, other aspects of the ecology of cysts and oocysts are not well understood. For example, why do low temperatures seem to promote survival, and why are the highest concentrations often found in autumn and winter months? Is there more than a proximate answer to this question? Does this observation confer any selective advantage on the parasites and increase their chance of infecting new hosts? In other words, are certain genetic variants better able to adapt to and survive the changing environment than others? If so, these strains would be expected to increase over time. Are there density-independent factors that