Shellfish have been used as bio-indicators of fecal pollution in freshwater and marine ecosystems. Filter feeding shellfish have been shown to bioconcentrate pathogens, including parasites, bacteria, and viruses from surrounding waters. In laboratory experiments, shellfish exposed to high doses of Cryptosporidium have maintained detectable levels of the parasite for up to three weeks post-exposure, and shellfish-passed oocysts have been proven viable with mouse bioassays. Cryptosporidium has been detected in wild shellfish, but depuration dynamics after exposure to environmentally realistic doses of Cryptosporidium are unknown.
A tank exposure experiment was conducted using Cryptosporidium parvum oocysts and freshwater clams (Corbicula fluminea). Two batches of oocysts, from different California dairy farms, were used for a set of three exposed tanks and one unexposed tank at 10° C, and a set at 20° C. Doses of C. parvum consistent with high environmental loading rates (100, 1000, and 10000 parasites per liter of freshwater) were circulated in aerated freshwater tubs containing clams for six hours. Clams were then moved to clean tubs, and maintained for three weeks with clams removed at three, six, and nine hours and one, three, seven, 14, and 21 days post-exposure. Ten percent clam mortality was observed, spread amongst treatment and control groups.
Immunomagnetic Separation (Dynal) and Direct Fluorescent Antibody (Merifluor) testing was used to enumerate C. parvum oocysts in clam tissues, with oocysts being detected in clams kept at 10° C and 20° C, and at all exposure doses. Because oocysts were detectable for days to weeks after exposure to environmentally plausible doses of C. parvum oocysts, at realistic ecosystem temperatures for California, these findings suggest that clams can be useful bioindicators of fecal pollution. The dose of Cryptosporidium to which the clams are exposed may affect how long post-exposure the oocysts can be detected. Due to the within-clam-batch variability observed, multiple clams should be sampled in environmental monitoring programs. Because shellfish concentrate protozoa and other pathogens from the surrounding waters, marine mammals or humans eating raw shellfish may be exposed to significant doses of these pathogens.