Epidemiologic Investigation
We performed a retrospective cohort study on passengers from four July 2004 cruises to determine the burden of gastrointestinal illness among passengers and risk factors for illness. The case definition for this part of the study was a person with acute onset of three or more episodes of watery diarrhea in a 24-hour period while on board the ship during the summer of 2004. We administered a questionnaire by telephone or in person to all the passengers that we contacted, recording demographic information, characteristics of the illness, and information about the food consumed on board the ship, and we calculated attack rates and risk factors for illness.
In collaboration with the staff of the Municipality of Anchorage Department of Health and Human Services, we performed active surveillance by communicating with the public and with health care providers through the media in an effort to identify additional persons in whom gastroenteritis had developed as a result of consuming Alaskan oysters. The case definition for this portion of the investigation was a person with acute onset of three or more episodes of watery diarrhea in a 24-hour period that started within two days after the consumption of raw oysters collected from Alaskan waters during 2004.