What is already known on this topic?
“From 1990 to 2004, the reported rates of diarrheal disease (three or more loose stools or a greater than normal amount in a 24-hour period) on cruise ships decreased 2.4%, from 29.2 cases per 100,000 travel days to 28.5 cases. In 2001, the Vessel Sanitation Program and the cruise industry expanded the diarrheal illness case definition to include acute gastroenteritis (diarrhea, or vomiting that is associated with loose stools, bloody stools, abdominal cramps, headache, muscle aches, or fever). The most common causative agent has been norovirus.
What is added by this report?
From 2008 to 2014, the rate of acute gastroenteritis on cruise ships decreased among passengers from 27.2 cases per 100,000 travel days in 2008 to 22.3 in 2014, while the rate among crew members was essentially unchanged. The rate among both passengers and crew members was higher in 2012 compared with the preceding and following years, likely because of the emergence of a new norovirus strain. Among 73,599,005 passengers on cruise ships during 2008–2014, a total of 129,678 (0.18%) cases of acute gastroenteritis were reported during outbreak and nonoutbreak voyages; among 28,281,361 crew members, 43,132 (0.15%) cases were reported. Only a small proportion of those cases were part of a norovirus outbreak.
Approximately 73.5 million passengers sailed on voyages that required a VSP report during 2008–2014. During that period, 172,810 passengers and crew members met Vessel Sanitation Program (VSP) case definition for acute gastroenteritis, accounting for 0.18% of passengers and 0.15% of crew members (outbreak and nonoutbreak illnesses combined). Among cruise ship outbreaks with clinical specimens tested, 92% were caused by norovirus, with enterotoxigenic E. coli the second most common etiologic agent. Noroviruses are highly transmissible, can spread easily, especially in environments where persons live in close quarters such as long-term care facilities or dormitories, and can remain infectious on environmental surfaces for long periods of time. Good hand hygiene is vital to preventing outbreaks of acute gastroenteritis, including on cruise ships. This is best accomplished by washing hands with soap and water because it allows for the mechanical removal of the virus from the hands. Alcohol-based hand sanitizer use alone has shown limited efficacy, but can be used in conjunction with handwashing with soap and water.
The enterotoxigenic E. coli outbreaks all occurred outside the United States on ships sailing back to the United States after visiting Central or South America. Overall, 14,911 passenger and crew acute gastroenteritis cases were associated with norovirus outbreaks during 2008–2014; these accounted for only 0.01% of the estimated 140 million norovirus cases in the United States during that period. Monthly rates of acute gastroenteritis on cruise ships were higher during October–April for all years of the study period, with the highest rates of illness occurring during 2012, when a novel strain of norovirus was identified (GII.4 Sydney). The overall seasonality and higher levels of illness in 2012 were similar to non-cruise ship U.S. acute gastroenteritis outbreak data, which showed higher rates of illness during November–April each year, and more norovirus illnesses during years when a novel strain of the virus was identified.
Rates of acute gastroenteritis and the number of acute gastroenteritis outbreaks were consistently lower among crew members than passengers, likely for multiple reasons. First, there are strict reporting and isolation requirements for crew members who experience acute gastroenteritis. Crew members who do not report symptoms of diarrhea or vomiting face discipline, which can include employment termination, because the risk they pose either through food handling or passenger interactions can lead to the spread of acute gastroenteritis. Additionally, because good hand hygiene is known to limit the spread of norovirus, hand washing stations are required at the entrances to all crew eating areas, and crew members are monitored in their use. Hand washing stations also are required to be located in food handling areas such that “no employee must walk more than 8 meters (26 feet) to reach a station”. As an additional food safety measure, crew members are not allowed bare hand contact with ready to eat foods. Finally, cruise lines have worked diligently to remove reporting barriers for passengers and crew members and encourage immediate reporting of any diarrhea or vomiting for medical assessment, treatment, and monitoring.
The findings in this report are subject to at least four limitations. First, voyages that did not include a foreign port or did not exceed VSP’s alert or outbreak thresholds were not required to have a VSP report submitted, so the results might not reflect all voyages that occurred in U.S. waters. Second, VSP reports are only required when sailing from a foreign port to a U.S. port or when the cumulative incidence of acute gastroenteritis in either passenger or crew populations exceeds VSP’s alert or outbreak thresholds; ships sailing between U.S. ports with a cumulative incidence lower than VSP’s thresholds are not required to submit a report. Therefore, these data might not reflect the final case count at the end of the voyage. Third, case counts reported by ships and included in this study include only those persons who had symptoms while on the cruise ship and reported their symptoms to a crew member. The number of persons who experienced symptoms of acute gastroenteritis but did not report them is not known; thus, total case counts are likely underreported. Finally, in 2011, VSP included the phrase “or what is above normal for the individual” to the definition of diarrhea to align with federal regulation and the World Health Organization definition of diarrhea. This addition might have affected the number of persons who met the case definition because the definition now requires an assessment of “normal” rather than simply noting a frequency of ≥3 episodes in 24 hours.
The number and severity of cruise ship outbreaks of acute gastroenteritis varied during the study period, but were lower than rates reported during 2001–2004. Collaborative efforts with the cruise industry have allowed VSP to provide more rapid support to cruise lines and ships experiencing higher than expected levels of acute gastroenteritis. Fewer and less severe outbreaks are likely the result of earlier detection of acute gastroenteritis, along with cruise industry efforts to identify and control outbreaks by developing and implementing required Outbreak Prevention and Response Plans, using processes and chemical disinfectants known to be effective against a norovirus surrogate and proactively seeking strategies to limit acute gastroenteritis spread, using the most currently available evidence.”
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