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Background During May 2004, the Vessel Sanitation Program (VSP) investigated an outbreak of norovirus gastroenteritis on board a cruise ship sailing in Alaska waters. The objectives were to identify a common food item source and explore behavioral risk factors for person-to-person transmission among passengers.

Methods A case was defined as three or more episodes of loose stools within 24 hours or two or fewer episodes of loose stools accompanied by one or more episodes of vomiting. Vomitus and stool samples from affected passengers were tested for norovirus by reverse transcriptase–polymerase chain reaction. Environmental health officers performed an environmental investigation following VSP protocol. Questionnaires about food items consumed and behavioral risk factors were placed in cabin mailboxes (n = 2,018). A case-control study design using multivariable logistic regression tested associations between risk factors and disease.

Results A total of 359 passengers (24.1% of respondents) met the case definition. Four of seven clinical specimens tested positive for norovirus. No significant deficiencies in environmental health practices were identified, and no meal servings were associated with disease. Having a cabin mate sick with diarrhea or vomiting [odds ratio (OR): 3.40; 95% confidence interval (CI) = 1.80–6.44] and using a specific women’s toilet that was contaminated with vomit (OR: 5.13; 95% CI = 1.40–18.78) were associated with disease. Washing hands before meals was protective (OR: 0.25; 95% CI = 0.12–0.54) against disease.

Conclusions Widespread person-to-person norovirus outbreaks can occur on board cruise ships, even with appropriate environmental health practices. Programs to prevent and control norovirus outbreaks on board cruise ships should involve strategies that disrupt person-to-person spread and emphasize hand washing.