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Vibrio parahaemolyticus (Vp) is found naturally in coastal saltwater. In the United States, Vp causes an estimated 35,000 domestically acquired foodborne infections annually (1), of which most are attributable to consumption of raw or undercooked shellfish. Illness typically consists of mild to moderate gastroenteritis, although severe infection can occur. Demographic, clinical, and exposure information (including traceback information on implicated seafood) for all laboratory-confirmed illnesses are reported by state health departments to CDC through the Cholera and Other Vibrio Surveillance system. Vp isolates are distinguished by serotyping (> 90 serotypes have been described) and by pulsed-field gel electrophoresis (PFGE).

Vp serotypes O4: K12 and O4: K (unknown) comprise the Pacific Northwest (PNW) strain and, within the United States, had not been associated with shellfish outside the Pacific Northwest before 2012. During May–July 2012, Vp of the PNW strain associated with shellfish from Oyster Bay Harbor in New York caused an outbreak of 28 illnesses in nine states. Simultaneously, Vp of the PNW strain caused an outbreak of illnesses on a cruise ship docked on the Atlantic Coast of Spain; illness was associated with cooked seafood cooled with ice made from untreated local seawater. All Vp isolates from ill persons in the US and Spanish outbreaks that were further subtyped were indistinguishable by PFGE (2).