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Norovirus top source of reported foodborne outbreaks, says CDC

By Joe Whitworth+

09-Jun-2014
Last updated the 09-Jun-2014 at 10:31 GMT

Fruits and vegetables were among the most frequently named food categories
Fruits and vegetables were among the most frequently named food categories

Noroviruses are the leading cause of reported foodborne disease outbreaks, according to the Centers for Disease Control and Prevention (CDC).

It is most often associated with food contamination in restaurants during preparation by infected workers, said the agency in its Morbidity and Mortality Weekly Report (MMWR).

During 2009–2012, 1,008 foodborne norovirus outbreaks were reported through the National Outbreak Reporting System (NORS), constituting 48% of all outbreaks with a single known cause.

Contaminated food

Of 324 outbreaks with an implicated food, most resulted from food contaminated during preparation (92%) and consumed raw (75%).

Specific categories were implicated in only 67 outbreaks; the most frequently named were vegetable row crops (e.g., leafy vegetables) (30%), fruits (21%), and mollusks (19%).

CDC analyzed 2009–2012 data on suspected and confirmed outbreaks reported by state, local, and territorial health departments to characterize the epidemiology.

Foodborne norovirus outbreaks were reported by 43 states, with the number per state ranging from one to 117.

Foodborne outbreaks more often affected men (44%) and persons aged <75 years and the reported case-hospitalization and case-fatality ratios in foodborne outbreaks (1% and 0.01%) were lower than those in nonfoodborne cases.

However, a greater proportion of cases among foodborne outbreaks resulted in emergency department visits than among nonfoodborne outbreaks.

Common setting

Restaurants were the most common setting (64%) of food preparation reported in outbreaks and food workers were implicated as the source in 70% of 520 cases.

“Steps to curtail contamination of ready-to-eat foods by food workers include adherence to appropriate recommendations for hand washing and avoiding bare-hand contact with ready-to-eat foods (e.g., through use of gloves or utensils),” said the CDC.

“Compliance with policies to prevent ill staff members from working until ≥48 hours after symptom resolution and supervision by a certified kitchen manager, as recommended by the Food and Drug Administration Food Code.”

CDC cited an observational study of workers in restaurants which found proper hand washing in only 27% of activities for which it is recommended and even less frequently (16%) when gloves were used.

One in five food workers in restaurants report having worked while ill with vomiting or diarrhea for at least one shift in the previous year, found the study.

Scott DeFife, executive vice president for policy and government affairs at the National Restaurant Association said it agreed with many of CDC’s suggestions as to what can be done but not all.

"However, we believe that some of the language is misleading and that norovirus is not common in the industry, as the report claims.

"The report shows that the overwhelming majority of norovirus outbreaks during the studied time period were non-foodborne. Restaurants serve 130 million meals each day, and while any instance is serious, there are very few instances of norovirus contamination.”

Norovirus diagnostics generally rely on molecular methods but data collected through investigations provide insights that can help guide prevention efforts, said the agency.

The public health burden exacted by noroviruses is substantial. Although candidate norovirus vaccines are in development and show promise, behavioral interventions focused on food workers continue to be primary means to prevent foodborne norovirus disease, added the report. 

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