Loose Stools Sink Ships

Why do so many cruise ship passengers get diarrhea?

Download the MP3 audio version of this story here, or sign up for The Explainer’s free daily podcast on iTunes.

A gastrointestinal virus hit the world’s biggest cruise ship last week, afflicting almost 400 passengers with diarrhea and other symptoms. A similar bug afflicted almost twice as many people on a cruise in mid-November. What makes cruise ships so germy?

The close quarters. The virus responsible for the two most recent outbreaks starts out as a food or water contaminant but spreads rapidly from person to person. An epidemiological study of a similar epidemic from a cruise in 1989 revealed that passengers who shared toilets were twice as likely to get sick as those who had private baths, and patients who vomited in their cabins were much more likely to have infected their cabinmates.

To make matters worse, cruise ships often make stops in countries with questionable food and water safety. Once an outbreak starts, crew members might serve as reservoirs for the virus, and ships take on new passengers every week or two, which makes it easier for a single outbreak to extend across consecutive cruises. Taking on new passengers can also increase the chances of bringing someone onboard who’s already sick. Strict government surveillance and reporting requirements that have been in place since the 1970s make sure that even the smallest outbreaks get noticed.

The virus responsible for both of the recent outbreaks is in a category of noroviruses that includes the Norwalk virus. (Researchers discovered the little, round virus after an epidemic at an elementary school in Norwalk, Ohio, in 1968.) Noroviruses have been identified as the cause of cruise ship illnesses going back to at least 1977, but there’s been a sharp rise in outbreaks in the past five years or so. This may be due to improved surveillance or to the recent appearance of new viral strains.

Noroviruses are by no means limited to sea vessels. In fact, the CDC estimates that 23 million Americans get one every year. A study of similar outbreaks in the second half of the 1990s revealed that 39 percent occurred in restaurants, 29 percent in nursing homes or hospitals, and 12 percent in schools or day care centers. Only one in 10 took place in a “vacation setting,” which includes cruise ships.

The virus will spread in any situation in which lots of people are in close contact. Norovirus outbreaks have been documented at sporting events, for example. Both teams in a college football game got sick in 1998. According to a report in the New England Journal of Medicine, some players from North Carolina “were playing in uniforms soiled with vomitus and feces. Both fecal-oral transmission and aerosol transmission of vomitus probably occurred, given the intense physical contact and use of bare hands that are characteristic of the game of football.”

Not all cruise ship outbreaks are viral or contagious. Legionnaires’ disease results from a bacterium that has on some occasions been found in cruise ship hot tubs and spas. It won’t spread from one passenger to another, but anyone who inhales the steam from an infected spa can get sick. (On a Japanese cruise ship, the organism was found in the porous “Maifanshi” stones of the spa filters.)

Got a question about today’s news? Ask the Explainer.

Explainer thanks Dave Forney of the Vessel Sanitation Program.