Some people may associate cruise ships with norovirus outbreaks (and there happened to be one on the Crown Princess last week), but they are actually an uncommon setting for acute gastroenteritis (AGE) outbreaks. The CDC’s Vessel Sanitation Program (VSP), a group that works with the cruise industry and routinely inspects the ships and assists during outbreak investigations, recently published an article in the Morbidity and Mortality Weekly Report about the occurrence of these outbreaks on cruise ships, and there have been some noticeable decreases in the number of cases over the years.
You can find the article here.
The group looked at data from 2008-2014, both from reports submitted by passenger ships, and laboratory data on samples submitted during outbreaks. By law, passenger vessels are required to report the number of people with symptoms of diarrheal disease to the VSP 24-36 hours before arriving in the U.S. from a foreign port, even if that number is zero, or if over 2% of the people onboard are sick, or if it reaches an outbreak level (over 3%). For the 2008-2014 interval, there were 32,084 reports submitted to the VSP, and 29,107 of these voyages were analyzed for this article because they were 3-21 days in length and had 100 or more passengers.
There are a plethora of neat findings in the article, but here are a few of the highlights:
- Of the 29,107 voyages that were analyzed, 133 of them (0.5%) experienced an outbreak of AGE. Acute gastroenteritis can be caused by a variety of bacterial and viral agents, but of the 95 outbreaks that had specimens tested, norovirus was the cause 97% of the time.
- When a specific norovirus genotype was identified, it was usually a GII.4 virus (73.8% of the 80 specimens that were genotyped).
- Of the 73,599,005 people in the U.S. who took a cruise between 2008 and 2014, there were 129,678 cases of acute gastroenteritis reported (0.18% of passengers), and only a small number of these cases (14,911 among passengers and crew) were part of a norovirus outbreak.
- Compared to the estimated 140 million cases of norovirus that happened in the U.S. between 2008 and 2014, the number of cruise-related cases seems even smaller, at about 0.01% of the nation’s total!
- In 2008, the rate of acute gastroenteritis among passengers was 27.2 cases per 100,000 travel days, and that decreased to 22.3 in 2014. Back in 1990, this number was 29.2 cases per 100,000, and in 2004 it was 28.5. There was not always a statistically significant difference between the years, but there has been an overall downward trend.
- There was a spike in the number of cases in 2012, among both passengers and crew, which was likely due to the emergence of a new norovirus strain, GII.4 Sydney, that year (and it is common to see this happen when a new norovirus starts going around).
Reasons given for the decrease in the number of cases included the collaboration between the VSP and the cruise industry, leading to earlier detection of outbreaks; the cruise industry’s development and implementation of prevention and control plans for AGE outbreaks; the selection of disinfectants; and the use of strategies to prevent the spread of diseases onboard.
The article goes on to recommend that travelers should practice good hand hygiene, and immediately report their symptoms to the ship’s medical center if they experience vomiting or diarrhea on a ship for assessment, treatment, and monitoring, as ways to further reduce the occurrence of acute gastroenteritis on cruise ships.