The Nasty Bug That Can Take Out an Entire Cruise Ship

Health and medicine explained.
Jan. 30 2014 11:22 PM

Why Norovirus Is Such a Menace

The amazing little pathogen does so much more than cause cruise-ship disease.

The Carnival Liberty Cruise ship docked at Port Everglades in November 2006 in Fort Lauderdale, Florida.
The Carnival Liberty cruise ship docks in Fort Lauderdale, Fla., in November 2006, after hundreds of the ship's passengers were stricken with norovirus symptoms.

Photo by Joe Raedle/Getty Images

Passengers on Royal Caribbean’s Explorer of the Seas came home two days early this week after more than 600 people fell ill with suspected norovirus, also known as Norwalk disease. Norovirus loves a good cruise ship, with its tight quarters full of people from all over the world. There have been nearly 200 confirmed norovirus outbreaks on cruise ships in the past 20 years, plus many other suspected maritime norovirus outbreaks that couldn’t be definitively linked to the virus. But it isn’t an exclusively sea-going pathogen. It accounts for more foodborne illness in the United States than E. coli and salmonella combined. It lives on doorknobs, handrails, and even soft surfaces like couches and carpets. Norovirus is all around you. And it is sickening more people than ever. It is a wondrous pathogen that should fascinate, disgust, and frighten you in equal measure.

Most viruses are encased by a lipid envelope, which has a couple of vulnerabilities. First, it dries out when exposed to the elements, which is one of the reasons HIV, for example, dies almost immediately outside of a host. In addition, alcohol-based sanitizers easily penetrate a lipid envelope and destroy the virus. Norovirus has a protein shell with no such weaknesses. It can live in the open for weeks and possibly months, and it is resistant to hand sanitizer and soap, unless you scrub the heck out of it. When a cruise ship suffers an outbreak, the Centers for Disease Control and Prevention advises that virtually every inch of the vessel and everything on it be drenched in a 5-percent bleach solution. Since schools, concert halls, movie theaters, and private homes rarely undergo such a thorough scrub-down, it’s disturbing to imagine how often we encounter norovirus. (If you decide to bleach your house after your child falls ill, it’s better to go with a 10-percent solution, since bleach degrades rather quickly in a bucket.)

The situation is all the more worrying when you consider norovirus’ second neat trick—it takes an incredibly small number of viral particles to make you sick. Most pathogens, such as influenza, need to invade you with an army of thousands to cause symptoms. That’s one of the reasons we’re generally healthy in a world teeming with viruses. Mathematical modeling suggests that as few as 10 norovirus particles can make an adult sick. The average norovirus virion is around 35 nanometers across—one-third the size of most viruses—so the volume of an infectious dose is uniquely small. It can easily find its way into your mouth through your hands or a whiff of infected air.

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You can contract norovirus in the most subtle and disgusting ways imaginable. In 1999, after one gentleman vomited in a concert hall and nearby bathroom in Wales, more than 300 people inhaled enough airborne norovirus to become ill. Many of the victims were school children who came on a field trip the following day. In another case, several members of a college football team from North Carolina came down with norovirus and managed to infect members of the opposing team from Florida through contact with their uniforms, which were contaminated with particles of feces and vomit.

Norovirus is also eerily persistent. People with colds, for example, are typically not contagious beyond a week after symptoms commence. By contrast, laboratory experiments on human volunteers suggest that people infected with norovirus continue to shed virions for up to three weeks, long after the vomiting and diarrhea have passed.

Why would researchers use human volunteers to study such a horrible bug? Researchers have thus far been unable to culture norovirus in a dish. Last year, microbiologist Christiane Wobus and her colleagues at the University of Michigan were able to grow norovirus in immunocompromised mice, and some progress has been made infecting pigs whose guts had been rid of other microbes, but those are baby steps toward understanding the virus. Until we can grow this stuff in a laboratory, there’s little hope of developing a vaccine or an effective treatment. (Wobus calls in vitro culturing “the holy grail” of norovirus research.) Most of what we currently know comes from deliberately infecting volunteers who submit themselves to three days of nausea, vomiting, and diarrhea for the good of science and around a thousand bucks.

Incidence of norovirus has increased in the past couple of decades, and researchers have proposed an intriguing explanation: It may be learning to infect more people.

Your blood type—A, B, AB, or O—is defined by certain sugars on your blood cells. Similarly, people differ in the types of sugars on the cells that line the intestine. Certain strains of norovirus bind specifically to certain sugars—but the strains that have been traveling the world recently bind to a wider range of sugars. That seems to mean the strains can sicken a wider range of victims.

Humans aren’t the only hosts for norovirus. When filter-feeding shellfish ingest norovirus particles, the virus can bind to sugars in their systems and accumulate without making them sick. A recent study found norovirus contamination in 9 percent of oysters on the French market. In some cases, epidemiologists have traced human norovirus outbreaks to specific storms, as heavy rainfall caused sewage overflow and contamination of the oysters’ water supply.

How freaked out should you be? Moderately. Norovirus can kill, but usually indirectly; most deaths are among children or the elderly who become dehydrated, particularly in the developing world. The average adult will experience three days of miserable gastrointestinal distress. If it happens to you, avoid concert halls, football games, and any other social interaction, bleach everything you touch—and drink constantly.

Brian Palmer writes about science, medicine, and the environment for Slate and the Natural Resources Defense Council. Email him at explainerbrian@gmail.com. Follow him on Twitter.

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