In general, raw oysters are most likely to be contaminated in the water before harvesting. Regardless, Public Health Environmental Health Investigators conduct onsite visits for each report of illness associated with raw oyster consumption. This helps control risk factors that may be present that could contribute to the spread of norovirus or Vibrio parahaemolyticus. Risk factors for each illness vary, but may include: oysters from unapproved sources, improper refrigeration of oysters (temperatures must be at or below 41°F), handling of raw foods in such a way to minimize the risk of cross-contamination, ill food workers, and/or improper handwashing practices.

Public Health routinely reports the illnesses to Washington State Department of Health (DOH) Shellfish Program, which is responsible for tracking the harvest locations of the oysters implicated in these illnesses. Multiple illnesses tracked to a common growing area may result in the closure of implicated harvest locations or other enforcement actions. For more information, see the DOH Shellfish Program website.