CDC: Infected Food Handlers Cause Half of Norovirus Outbreaks

Emma Hitt, PhD

September 12, 2012

September 12, 2012 — Leafy vegetables, fruits/nuts, and mollusks are the most common sources of foodborne norovirus outbreaks, and infected food handlers are thought to be the source of contamination in at least half of reported outbreaks, according to new data from the Centers for Disease Control and Prevention (CDC).

Data from 2922 reported cases collected between 2001 and 2008 by the Foodborne Disease Outbreak Surveillance System was analyzed by Aron J. Hall, DVM, MSPh, an epidemiologist with the CDC, and colleagues. The findings were published online September 12 and in the October issue of Emerging Infectious Diseases.

According to the researchers, an estimated 5.5 million foodborne illnesses each year in the United States are attributed to norovirus, which represents the leading cause of foodborne disease outbreaks. This virus can be transmitted in several ways: the fecal–oral route, direct person-to-person contact, ingestion of aerosolized vomitus, and indirect contact through contaminated surfaces, food, or water. Norovirus is known for its ability to spread rapidly and efficiently.

The goal of the analysis was to identify areas for potential intervention in the contamination process. Data collected included outbreak and patient characteristics, place of food preparation, and probable food vehicle. Of the states, 49 and the District of Columbia reported foodborne norovirus outbreaks; there was considerable variation in reporting rates between states.

Of norovirus outbreaks involving foods prepared in commercial settings, 62% occurred in restaurants or delicatessens, 11% via a catering service, and 4% at a grocery store. A food handler was identified as the contamination source in 53% of the outbreaks and as a potential source in up to 82%, the researchers report.

According to the researchers, "[t]hese proportions likely reflect lack of positive evidence and therefore may be underestimated because of disincentives for reporting illness and asymptomatic infections among food handlers."

A single food source was implicated in 364 foodborne norovirus outbreaks. The most common food to be implicated was leafy vegetables at 33%, followed by fruits and nuts at 16% and mollusks at 13%. All commodities except sprouts were implicated in at least 1 outbreak.

"Steps to curtail contamination of ready-to-eat foods by food handlers in these settings include adherence to appropriate recommendations for hand washing and use of gloves; compliance with policies to prevent ill staff from working; and presence of a certified kitchen manager, as recommended by the Food Code of the US Food and Drug Administration," Dr. Hall and colleagues conclude.

They add that there is an "average of 1 foodborne norovirus outbreak reported every day," and that their study "highlights the frequency of norovirus contamination of raw and other ready-to-eat foods."

This study was supported in part by an Agriculture and Food Research Initiative Competitive grant from the US Department of Agriculture, National Institute of Food and Agriculture.

Emerg Infect Dis. Published online September 12, 2012.