Shipboard impact of a probable Norwalk virus outbreak from coastal Japan

Norwalk virus has been implicated in shipboard diarrheal disease outbreaks throughout Asia. A large outbreak of suspected Norwalk virus was investigat ed on a U.S. Naval aircraft carrier following the clinical recognition of 4 50 cases of gastroenteritis over a 2-week period (September 14-28, 1997) du ring coastal exercises. A random sampling of 44 cases from 450 personnel wh o sought medical attention was compared with 19 controls. Junior enlisted s ailors and marines comprised 97% of all cases. There was no evidence of shi pboard geographic clustering of cases. Furthermore, no single food type was associated with illness on the basis of comparative analysis (cases versus controls). Principal case signs and symptoms reported included watery stoo ls (89%), nausea (82%), and vomiting (77%). Anecdotal reports indicated > 5 0% of the cases received rehydration therapy. An absence of fever was also noted in 32% of the cases and only 5% had blood in their stools. The mean d uration of illness was 37 hr, with a range of 3-96 hr. Laboratory findings based on reverse trans cription-polymerase chain reaction and Southern hybr idization methods showed that 21 (72%) of 29 patients had evidence of the U K2 prototype of the Norwalk virus. A cross-sectional study of 131 crew memb ers from the ships population (n = 4,200) showed an attack rate of 44%. Att ack rate is a variant of an incident rate applied to a narrowly defined pop ulation observed for a limited period of rime, such as during an outbreak. The numerator is people who get sick and the denominator is people (populat ion) at risk. An extrapolation of these findings suggests as many as 1,806 sailors may have been affected during the outbreak, of which only 26% (of t he 57 outbreak related cases) where identified from sick call records. Ther e was no difference in the mean ages between outbreak and non-outbreak affe cted crewmen, or geographic clustering based on berthing or work spaces. Ou tbreak-related cases reported signs and symptoms of watery-stools (79%), na usea (65%), and vomiting (47%). The mean duration of illness was 28 hr, ran ging from 2 to 96 hr. Thirty-one percent of outbreak affected cases reporte d a sick call visit. Loss of work was reported by 39% of the outbreak affec ted population. This report documents the epidemic potential of Norwalk vir us and the associated impact on fleet operational readiness. Additionally, that this outbreak occurred against a background of 3 other consecutive gas troenteritis outbreaks onboard the same ship (March 1997, February/March 19 98, and June 1998), all sharing the same clinical and epidemiologic profile s, suggests possible shipboard persistence of Norwalk virus over time, desp ite periodic ship-wide disinfection efforts.