The norovirus outbreak on board the Star Princess cruise liner this spring was a passenger’s worst nightmare. The ship departed from San Francisco on April 29 en route to Hawaii. By the time it returned two weeks later, 135 passengers and 16 crew members contracted norovirus. 1

According to the Centers for Disease Control and Prevention (CDC),, eight other cruise ships with American ports of call also had norovirus outbreaks this year, sickening anywhere from 5 to 10% of the passengers onboard.2 (Other outbreaks also occurred on vessels from other countries.)3

Although norovirus outbreaks on cruise ships always attract a lot of media attention, it’s important to keep in mind that they’re still relatively rare occurrences. “They can be large outbreaks, but they represent a tiny fraction of the total number of outbreaks reported to the CDC,” says Benjamin Lopman, PhD, an epidemiologist with the Centers for Disease Control and Prevention. CDC statistics estimate that about 1% of all norovirus outbreaks happen on cruise liners.4 

Certain Factors Help Norovirus Spread Quickly on Cruise Ships

The speed at which the virus spreads on cruise ships can still be alarming. The unique environments of ocean liners make them ideal settings for rapid transmission of noroviruses, said Amesh Adalja, MD, an infectious disease physician with Center for Health Security at the University of Pittsburgh Medical Center.

“They’re full of people that come in from all over the world,” Adalja said. “Everyone’s in very close quarters, interacting with each other – going to restaurants, bars, standing on deck. And, if someone gets sick, they can’t necessarily be taken off the ship.”

 And once the virus is onboard, many factors can confound attempts to control its spread. As few as 18 norovirus particles can cause infection,5 and noroviruses are also resistant to many common disinfectants. They even have the ability to live on surfaces for an extended period of time, according to the CDC.6  Plus, some cruise passengers who become ill are reluctant to report it to the ship’s staffers, Adalja said, for fear of being confined to their cabins.

 Health care professionals may see more patients asking about precautionary measures they can take before they board a cruise liner. Preventing norovirus is best accomplished by the same measures taken to control the spread of other contagious diseases.

 “It’s just the usual hygiene and sanitary practices – things like washing hands with soap and water before and after dining and using the restroom,” Lopman said.

 Cruise passengers can also help control norovirus by contacting the ship’s physician promptly if they become ill and steering clear of others who appear to be sick because the virus can spread through the air.

“If someone’s vomiting over the side of the deck, go the other way,” Adalja said. “Don’t get yourself exposed to that vomit aerosol. Go report it. That will help the ship’s physician have some idea of what’s going on, epidemiologically, on their cruise ship.”

Cases of Norovirus May Increase

 In the coming years, doctors may see an uptick in the amount of norovirus cases they see – both on cruise ships and in other settings. Since the mid-1990s, most cases of norovirus have been caused by the GII.4 strain. But research published in the July 2015 edition of the journal Eurosurveillance found an increase in the amount of cases of the novel norovirus strain GII.17. This strain has been circulating in humans for at least 37 years but had been limited to sporadic outbreaks until 2004, when samples from Korea began showing more widespread circulation. Last winter GII.17 was blamed for 24 of 29 outbreaks in China, and today it is the dominant strain circulating in some areas of China, Taiwan, and Hong Kong.

Although there’s currently no evidence that GII.17 is more virulent than GII.4, the study’s authors concluded that public health officials “need to be prepared in case of a potential increase of norovirus activity” due to GII.17. Though GII.4 is still very contagious because of its ability to mutate and bypass many people’s immune systems, GII.17 is less common, which means it could have an even greater chance of causing gastroenteritis symptoms in people who have been exposed to it.7 

 “[Fewer] people would be immune to this strain than ones previously circulating,” said Adalja.

References

1. CDC, Investigation Update on the Star Princess, http://www.cdc.gov/nceh/vsp/surv/outbreak/2015/may14_star_princess.htm.

2.   Centers for Disease Control, Noroviruses Summary Document, http://www.cdc.gov/nceh/vsp/cruiselines/norovirus_summary_doc.htm.

3.   CDC, Cruise Ship Outbreak Updates, http://www.cdc.gov/nceh/vsp/surv/gilist.htm#2015.

4.   Daily Mail, Passengers struck down with norovirus on board Fred Olsen ship tell of days stuck in their cabins as vomiting bug wrecked Scandinavian trip, http://www.dailymail.co.uk/news/article-3132557/Passengers-struck-norovirus-board-Fred-Olsen-ship.html.

5.   Centers for Disease Control, Norovirus is the leading cause of disease outbreaks from contaminated food in the U.S., http://www.cdc.gov/media/releases/2014/p0603-norovirus.html.

6.   CDC, Noroviruses in Healthcare Facilities Fact Sheet, http://www.cdc.gov/hai/pdfs/norovirus/229110-ANoroCaseFactSheet508.pdf.

7.   Centers for Disease Control, Norovirus Trends and Outbreaks, http://www.cdc.gov/norovirus/trends-outbreaks.html

8.   EMERGENCE OF A NOVEL GII.17 NOROVIRUS – END OF THE GII.4 ERA? Eurosurveillance, Volume 20, Issue 26, 02 July 2015. Available at: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=21178.