Norwalk thrives at sea -- and on land

Many Americans heard of the Norwalk virus only recently, after it sickened hundreds of cruise ship passengers. But the microbe is hardly a stranger.

Medical experts say the germ is usually the culprit behind those bouts of the “stomach bug” that sweep through homes, day-care centers, restaurants and other places people congregate. In fact, with the exception of the common cold, it’s the most frequent cause of illness in the United States, according to the federal Centers for Disease Control and Prevention in Atlanta.

“Just about everyone gets exposed to it in their lifetime,” says Dr. John Painter, a CDC epidemiologist. “Norwalk is the most common of infectious bugs, like salmonella or E. coli.”

First identified in 1971 after an outbreak in Norwalk, Ohio, the pesky germ strikes 23 million people a year, triggering stomach cramps, nausea, vomiting, diarrhea, headaches and low-grade fever. Transmitted through person-to-person contact or by ingesting contaminated food or water, the virus causes two-thirds of the known food-borne illnesses, said Dr. Marc-Alain Widdowson, a CDC epidemiologist, dwarfing the incidence of illnesses caused by salmonella or E. coli.

Scientists are just beginning to get a handle on the magnitude of this virus’ impact, however. Tests sensitive enough to detect the microbe were only recently developed, Widdowson said, “so we’re still in the process of determining its actual prevalence,” which could be even greater than current estimates.

The Norwalk virus is actually a group of similar viruses that are now called noroviruses. Like the common cold, which can be caused by more than 100 viruses, any strain of these noroviruses -- and many strains are circulating at any one time -- can trigger the same constellation of symptoms.

For most people, it’s just a terrible nuisance that can fell an entire family, leaving everyone reeling for a few days. The incubation period is 12 to 48 hours, and the symptoms persist from 12 hours to three days, though some cases linger longer.

For infants and the frail elderly, however, it can be deadly: The vomiting and diarrhea can cause severe dehydration, which in rare instances can be fatal. Each year, about 20,000 hospitalizations and 124 deaths are attributed to the Norwalk virus. “Usually, though, it lasts one to three days, and then people feel fine, if a little worn out,” Painter said.

People confined to close quarters -- on cruise ships, in nursing homes or camp grounds -- are especially susceptible when an outbreak occurs because the virus spreads so easily.

“It’s very contagious and just a few virus particles can make people ill,” Painter said. “And when one person is ill, he or she has the potential to contaminate many other people, who then infect others.” Suddenly, you have a major outbreak.

In about 14 million cases, people catch the bug through direct contact with someone who is infected, from touching or shaking hands, or being around them when they vomit. “The vomit can be aerosolized with multiple particles in the air,” said Michael P. Doyle, director of the Center for Food Safety at the University of Georgia. People can also unwittingly spread the virus for up to two weeks after they’ve recovered from the acute phase of their infection; the virus on their hands or in their stool continues to shed, though it gradually loses it potency.

In the other 9 million cases, people get sick from drinking water or eating food containing the virus, which can be in raw shellfish, such as oysters or clams, that have been harvested from seas contaminated with sewage, or in foods handled by someone infected with the virus -- fruits, salads, crab dishes, sandwiches or luncheon meats.

Since the virus can survive five days or longer on surfaces, it can also be picked up from carpeting, floors, walls or even toys touched by infected kids in day-care centers. And it’s hard to kill -- some cruise ships have been sanitized with strong disinfectants several times to no avail, while hospitals in Europe and Canada had to be shuttered until the virus ran its course. The CDC is investigating why this happens.

It’s tough to distinguish whether an upset stomach is caused by the Norwalk virus or other microbes carried in food, because the symptoms are similar. But Norwalk virus infections normally don’t last as long, and the symptoms are usually milder. “If people have high fevers, bloody stools and a number of individuals are hospitalized during an outbreak, it’s probably not caused by the Norwalk virus,” Painter said. These symptoms are more likely the result of a salmonella, E. coli or shigella infection.

There is no treatment for this illness other than staying in bed, drinking plenty of fluids to prevent dehydration and riding it out. If someone in your family gets sick, good hygiene can minimize the risk of further infections -- thoroughly wash towels, blankets and utensils they may have used, and keep your own hands clean. “Hand-washing may sound like an old-fashioned remedy, but it works,” Widdowson says.

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Norwalk virus outbreaks

From January 1996 to November 2000, the Centers for Disease Control and Prevention received reports of 348 outbreaks of Norwalk-like virus. Most were at restaurants.

*--* Setting Percentage Restaurants 39% Nursing homes and hospitals 29% Schools and day-care centers 12% Vacation settings, including cruise ships 10% Other 9% Note: Numbers do not equal 100 due to rounding

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Source: Centers for Disease Control and Prevention