Corbicula fluminea as an Environmental Assay for Cryptosporidium parvum Oocysts in Surface Waters

Cryptosporidium parvum, a protozoal parasite, can be transmitted to humans through water and has become a major health concern. The ability to detect the oocysts of this parasite has been hampered by techniques that are expensive, time-consuming and have low sensitivities. The goal of this research was to determine the applicability of using the resident filter-feeding bivalve, Corbicula fluminea, as a method of detecting C. parvum oocysts in surface waters. Oocysts were detected in the clam fecal bioassay for exposure concentrations of 10, 100, and 1,000 oocysts/L, with peak concentrations of oocysts in clam feces occuring just a few hours following exposure to oocysts. No oocysts were detected after 22 hours of depuration. There were significant differences in the number of oocysts excreated per hour per clam among exposure concentrations and among observations times within 100 and 1,000 oocysts/L exposure concentrations. Ninety percent of the total oocysts defecated by clams were defected within 6 hours post-exposure removal. Cooler ambient water temperature was a significant factor in decreasing defecation rates and thereby decreasing the sensitivity of the bioassay. Clam-passed oocysts remained fully infective and approximately 50% were viable as determined by excystation. The Corbicula assay had peak test sensitivities (probability of detecting one or more oocysts per assay) of 50%, 100%, and 100% for exposure concentrations of 10, 100, 1,000 oocysts/L respectively. This research has shown that the filter-feeder, Corbicula fluminea, could be used as an inexpensive, sensitive test for detecting C. parvum oocysts in surface waters.