Removal of Escherichia coli, Enterococcus fecalis, coliphage MS2, poliovirus, and hepatitis A virus from oysters (Crassostrea virginica) and hard shell clams (Mercinaria mercinaria) by depuration.

Filter-feeding bivalve mollusks (shellfish) can bioaccumulate pathogenic microorganisms in up to 1000-fold higher levels than overlying waters, and therefore disease risks are associated with consuming raw or partially cooked shellfish. Many of these shellfish-borne diseases are due to enteric bacteria and viruses associated with fecal contamination. To control shellfish-borne diseases, guidelines for shellfish harvest waters and shellfish meat have been devised, which include cleansing of contaminated shellfish by depuration in controlled systems, heat pasteurization, or relay to clean waters. This study examines the depuration of oysters (Crassostrea virginica) and hard shell clams (Mercinaria mercinaria) in a flow-through depuration system under variable temperature (12 °C, 18 °C, and 25 °C), salinity (8 ppt, 18 ppt, and 28 ppt), turbidity (<1NTU, 10NTU, and 20NTU), pH (pH 7 and pH 8), and algae conditions (0 cells/mL and 50,000 cells/mL), with constant dissolved oxygen (5-7 mg/L). Oysters and hard shell clams were artificially contaminated with enteric microorganisms: Escherichia coli, Enterococcus faecalis, coliphage MS2, Poliovirus type-1 and Hepatitis A virus HM-175 (HAV), then depurated in 5-day trials with daily sampling. In oysters, optimizing environmental parameters of water temperature improved E. coli, MS2, poliovirus and HAV depuration, and optimized salinity improved E. coli, E. faecalis, and MS2 depuration rates. In hard shell clams, salinity improved E. coli and E. faecalis depuration rates. Adjusting turbidity, pH or algae did not improve microorganism depuration in either oysters or hard shell clams, with the exception of turbidity on E. faecalis in hard shell clams. Microorganism depuration rates in oysters from greatest to least were: MS2>E. coli>E. faecalis>poliovirus>HAV, and in clams depuration rates from greatest to least were: E. coli>E. faecalis>HAV>MS2>poliovirus. Because E. coli and E. faecalis were removed at faster rates than HAV and poliovirus, these fecal bacteria appear to be poor process indicators of the virological quality of depurated oysters and hard shell clams.