Researchers tested oysters from seven sites used for commercial harvesting in the Chesapeake Bay area. Oysters from all the sites contained Cryptosporidium species both from cows and people. This finding shows that the water at these sites contained human and animal feces during a period when the oysters were filtering. The risk of contamination is probably higher after a heavy rain, but some risk is present year-round.
Oysters feed by filtering water through their gills.
When water is contaminated by run-off from pastures or sewage, oysters can keep the parasite in their gills and spread illness. Infection with Cryptosporidium m can cause prolonged diarrhea. The infection is especially dangerous for persons with weakened immune systems. However, heating to temperatures above 162 F kills the parasite, so the authors urge that oysters be cooked before they're eaten.