The degree of V. parahaemolyticus contamination in raw shellfish is also known to relate to the water temperatures. Therefore, it is more likely to detect V. parahaemolyticus in oysters harvested in the spring and the summer than in the winter. Although the density of V. parahaemolyticus in contaminated oysters is usually lower than 103 cfu/g at harvest (Kaysner and DePaola, 2000), it could exceed
103 cfu/g in oysters harvested from warmer seawater (DePaola et al., 2000) and the organism can multiply rapidly in oysters upon exposure of elevated temperatures.
Studies have shown that populations of V. parahaemoly- ticus in unrefrigerated oysters could increase rapidly to
50–790 folds of its original level within 24 h of harvest if oysters were exposed to 26 1C (Gooch et al., 2002). A survey of 370 lots of oysters sampled from restaurants, oyster bars, retail and wholesale seafood markets through- out the US between June 1998 and July 1999 found a seasonal distribution of V. parahaemolyticus in market oysters with high densities (some exceeded 1000 MPN/g) being detected in the summer months (Cook et al., 2002).