July to September 1998, an outbreak of O3:K6 V.parahaemolyticus infections associated with consumption of oysters and clams harvested from Long Island Sound occurred among residents of Connecticut, New Jersey, and New York. Laboratory testing of 12 V.parahaemolyticus clinical isolates, including the 8 traced to Oyster Bay, identified the O3:K6 serotype, which had not previously been detected in coastal waters of the
United States. It is possible that this strain was introduced to U.S. coastal waters by ballast water discharged from ships
which had traveled to Asia (3, 4). In this study, we collected 139 isolates of O3:K6 V. parahaemolyticus strains recently isolated in Taiwan. Some of these strains were isolated from travelers originating in several other Asian countries. The clonal relationship of these strains to the O3:K6 strains isolated in other Asian countries was analyzed by the pulsed-field gel electrophoresis (PFGE) method (25). Biological characteristics, such as high toxin productivity or ability to survive in the natural environment, that are unique to the pandemic strains might provide further insight into the mechanism of the emergence and spread of these strains. Resistance to antibiotics and to environmental stresses, such as low- or high-temperature inactivation or a mild acid or lowsalinity treatment, may enhance the survival rates and spread of these O3:K6 strains in the host and the environment. The recently isolated O3:K6 strains were also compared to O3:K6 strains isolated before 1996 and to non-O3:K6 strains for the
presence and expression of the tdh gene encoding TDH and for susceptibilities to antibiotics and various environmental stresses.