Norwalk virus gastroenteritis aboard a cruise ship: an outbreak on five consecutive cruises.

An explosive outbreak of gastroenteritis caused by a parvovirus-like (PVL) agent (Norwalk agent) affected 521 (64%) cruise ship passengers in 1977. The illness was characterized as mild-to-moderate and lasted one to two days. Principal symptoms experienced by ill passengers were nausea, vomiting, and diarrhea; fever was reported for approximately 25% of the patients. The outbreak was compatible with a common-source exposure, but no such exposure was identified. On the next four cruises, passengers experienced a similar gastrointestinal illness, but the outbreaks were less explosive, affected fewer persons, and symptoms appeared milder, on each subsequent cruise. No common source was identified, and person-to-person transmission may have occurred. The etiology of these outbreaks would have remained unknown were it not for the recent development of a specific radioimmunoassay (RIA) for the serologic identification of Norwalk virus and the measurement of antibody titers to the virus. A fourfold or greater rise in serum antibody titers to Norwalk antigen in serum from seven of eight ill passengers and the identification of Norwalk antigen in stool specimens from two ill passengers on the first cruise demonstrated that Norwalk virus caused this outbreak. The symptoms experienced by passengers on the four subsequent cruises suggest that a Norwalk agent probably also caused these outbreaks. In addition, a stool specimen from an ill passenger on cruise 3 contained Norwalk antigen.