Another Reason Not to Eat Your Oysters Raw Cryptosporidium, a parasite that causes diarrhea and can be fatal for persons with weak immune systems, has been found in oysters harvested from the Chesapeake Bay area.
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. 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 humans. 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.