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Restaurants a major source of nasty norovirus infections, CDC study finds

Norovirus outbreaks on cruise ships may make the headlines, but you’re much more likely to catch the nasty bug in a local restaurant, according to a report released Tuesday by the U.S. Centers for Disease Control and Prevention (CDC).

In fact, cruise ships account for only 1 percent of all reported norovirus infections reported in the United States, while restaurants and catering services are responsible for 81 percent.

“Norovirus outbreaks from contaminated food in restaurants are far too common,” CDC Director Tom Frieden, M.D., said in a press statement released with the report. “All who prepare food, especially the food service industry, can do more to create a work environment that promotes food safety and ensures that workers adhere to food safety laws and regulations that are already in place.”

Common and highly contagious

An estimated 21 million Americans become infected with norovirus each year, making it the leading cause of acute gastroenteritis (diarrhea and vomiting) in the United States. The illness can lead to severe dehydration, a situation that is particularly problematic for young children, the elderly and people with other serious illnesses. Noroviruses are responsible for 70,000 hospitalizations each year in the United States and up to 800 deaths. The cost of treating these illnesses is about $777 million each year.

Most people mistakenly self-diagnose a norovirus illness as “a stomach bug” or “the flu.” It’s also sometimes referred to, incorrectly, as “food poisoning.” But the source of the illness is people not food — specifically the feces and vomit of infected individuals.

In most cases, the illness is passed person to person. You pick up the virus by touching a contaminated surface, and then you unknowingly transfer the virus to your mouth, eyes or nose. Swallowing as few as 18 norovirus particles can make you sick. That’s a very tiny — and unnoticeable (except under a microscope) — amount. As the CDC report points out, enough norovirus particles could fit on the head of a pin to infect more than 1,000 people.

To make matters worse, noroviruses are notoriously “environmentally stable.” They can remain infectious on surfaces for up to two weeks, and are resistant to common disinfectants, freezing temperatures and heat below 140 degrees F.

All that makes them, as the CDC report notes (with some understatement), “challenging to control.”

Analyzing four years of data

For the CDC’s current report, researchers analyzed data on norovirus outbreaks that had been reported between 2009 and 2012 through the National Outbreak Reporting System. These were outbreaks that involved two or more cases and that were linked to a common exposure source. (The vast majority of norovirus cases go unreported.)

Number and rate of reported foodborne norovirus outbreaks by state
Courtesy of the Center for Disease Control and Prevention
Number and rate of reported foodborne norovirus outbreaks (per 1 million
person-years*), by state — National Outbreak Reporting System, United States,
2009–2012 * Legend indicates rate ranges divided by quartile.

A total of 4,318 reported outbreaks were reported, resulting in 161,253 illnesses, 2,512 hospitalizations and 304 deaths. Most of those outbreaks (2,976, or 69 percent) were the result of person-to-person contact, but 1,008 (23 percent) were transmitted through infected food, mostly in restaurants (64 percent) or in catering or banquet settings (17 percent). (The rest of the reported cases were transmitted through water or an unknown source.)

Minnesota, by the way, tied California with the most reports of foodborne norovirus outbreaks during the four years of the study — 117 per 1 million person-years. CDC officials point out, however, that state differences in such numbers are more likely to reflect how well individual states identify and report outbreaks than in the actual number of outbreaks themselves. Minnesota has a very good reporting system.

For 520 of the national foodborne outbreaks, CDC officials were able to pinpoint a specific contributing factor. In 70 percent of those outbreaks, that factor was an infected food worker, and in more than half of those cases, the infected worker had been preparing food with his or her bare hands.

A further digging into the data revealed that in about one-third (324) of the foodborne outbreaks, one or more specific foods could be identified as the source. Some 75 percent of the contaminated foods were ones that are eaten raw, especially lettuce and other leafy vegetables, fruits and mollusks (such as oysters).

Infected workers need to stay home

CDC officials want restaurants, caterers and others in the food-service industry to take more stringent steps to reduce foodborne norovirus infections. They specifically recommend:

  • Making sure that food service workers practice proper hand washing and avoid touching ready-to-eat foods, such as raw fruits and vegetables, with their bare hands before serving them.
  • Certifying kitchen managers and training food service workers in food safety practices.
  • Requiring sick food workers to stay home [until 48 hours after symptoms have gone], and considering use of paid sick leave and on-call staffing, to support compliance.

That last recommendation may be particularly difficult to enforce. A 2013 study found that 1 in 5 restaurant workers had worked while ill with symptoms of vomiting or diarrhea for at least one shift during the previous year. Most said they worked through the illness because they feared losing their job or they were concerned about leaving their coworkers short-staffed.

You can read the CDC’s report in the June 3 issue of the agency’s Morbidity and Mortality Weekly Report. More information about noroviruses is also available on the CDC’s website.

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