M.L. Speck

Occurrence of Enteric Bacteria and Viruses in Oysters

Enteric bacteria and virus levels were determined in oysters from paired stations that were opened or closed for commercial shellfishing on the basis of total coliform levels in the water. Six pairs of stations were sampled quarterly over a 1-year period. Enteric viruses were found in 3 of 24 50-g oyster samples from closed areas and in none of 23 samples from open areas. Salmonella was found in 2 of 47 samples of 40 g each, one from an open and the other from a closed area. Although enteric pathogens of fecal origin were found only in oysters that exceeded the recommended market limit of 230 fecal coliforms per 100 g of meat, the fecal coliform levels in some virus-positive samples were much lower than those in Salmonella -positive samples. Vibrio parahemolyticus levels were similar in oysters from both open and closed beds, indicating no particular association with fecal pollution. However, there was a marked seasonal variation in V. parahemolyticus levels. Total but not fecal coliform levels in oysters from open beds correlated with the occurrence of rainfall 1 or 2 days before sample collection. Neither total nor fecal coliform levels in oysters from closed beds correlated with rainfall. These findings suggest that fecal coliforms levels in oysters are less influenced by rainfall than are total coliforms, and therefore may be a more specific indicator of recent fecal pollution.