title: Disease transmission and passenger behaviors during a high morbidity Norovirus outbreak on a cruise ship, January 2009.
abstract: BACKGROUND
  Norovirus continues to pose a significant burden on cruise ships, causing an average of 27 confirmed outbreaks annually over the past 5 years. In January 2009, the report of a suspected norovirus outbreak among passengers on a cruise ship prompted an investigation.
  
  
  METHODS
  A retrospective cohort study among passengers was conducted on board the ship. Questionnaires about health care-seeking behaviors, hygiene practices, and possible norovirus exposures were placed in every cabin. Stool samples from several ill passengers were tested for norovirus by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR) and confirmed by sequence analysis.
  
  
  RESULTS
  Of 1842 passengers, 1532 (83.2%) returned questionnaires, and 236 (15.4% of participants) met the case definition. Of these, 95 (40%) did not report to the infirmary. Case passengers were significantly more likely to have an ill cabin mate (relative risk [RR] = 3.0; P < .01) and to have witnessed vomiting during boarding (RR = 2.8; P = .01). Over 90% of all passengers reported increased hand hygiene practices following the outbreak; 38% of ill passengers and 11% of well passengers decreased participation in public activities. Of 14 samples tested, 12 were positive for norovirus by RT-qPCR; 5 of these were confirmed by sequence analysis and typed as GII.4 Minerva.
  
  
  CONCLUSIONS
  Person-to-person transmission, including an incident of public vomiting during boarding, likely contributed to this high morbidity outbreak. Infirmary surveillance detected only 60% of acute gastroenteritis (AGE) cases involved in this outbreak. Adjustments to outbreak reporting thresholds may be needed to account for incomplete voluntary AGE reporting and to more rapidly implement control measures.
authors.affiliation: National Center for Immunization, and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, USA. mwikswo@cdc.gov
authors.forename: Mary E
authors.initials: ME
authors.lastname: Wikswo
authors.forename: Jennifer
authors.initials: J
authors.lastname: Cortes
authors.forename: Aron J
authors.initials: AJ
authors.lastname: Hall
authors.forename: George
authors.initials: G
authors.lastname: Vaughan
authors.forename: Christopher
authors.initials: C
authors.lastname: Howard
authors.forename: Nicole
authors.initials: N
authors.lastname: Gregoricus
authors.forename: Elaine H
authors.initials: EH
authors.lastname: Cramer
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D017250:Caliciviridae Infections / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004196:Disease Outbreaks; D005260:Female; D005759:Gastroenteritis / Q000453:epidemiology / Q000821:virology; D006801:Humans; D008297:Male; D009017:Morbidity; D029322:Norovirus / Q000235:genetics / Q000302:isolation & purification; D012189:Retrospective Studies; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012621:Seasons; D012767:Ships; D014195:Travel; D055815:Young Adult