Infected staff, raw items typify foodborne norovirus outbreaks

Sep 18, 2012 (CIDRAP News) – The United States has an average of one foodborne norovirus outbreak every day, typically involving an infected food worker who handles raw or ready-to-eat items such as leafy vegetables, according to an analysis of 8 years' worth of data.

The study, published yesterday in Emerging Infectious Diseases, shows that the nation had an average of 365 foodborne norovirus outbreaks each year from 2001 through 2008, resulting in more than 10,000 illnesses and 1 death annually. Infected food handlers were implicated as the source of contamination in more than half of the outbreaks in which contributing factors were listed.

In the report, researchers from the Centers for Disease Control and Prevention (CDC), the University of Illinois, and North Carolina State University note that noroviruses are the leading cause of foodborne outbreaks in the United States. They analyzed outbreaks that were reported to the CDC's Foodborne Disease Outbreak Surveillance System over the 8 years.

A total of 9,206 outbreaks were reported, including 6,355 with a known cause. Noroviruses were the confirmed or suspected cause in 2,922 (46%) of those outbreaks, the report says. Lab confirmation was achieved in 58% of the 2,922.

Foodborne norovirus outbreaks were reported in every state except Vermont, and the rates of outbreaks varied widely from state to state, which probably reflects differences in reporting practices and varying degrees of underreporting, the researchers say.

Outbreaks were slightly more common in the winter than in other seasons, as 33% of them occurred in December, January, and February.

The number of reported illnesses associated with the norovirus outbreaks averaged 10,324 per year, resulting in an estimated 1,247 healthcare visits, 156 hospitalizations, and 1 death, the authors determined. Eighty percent of the illnesses involved adults (age 20 or older), and 56% involved women. Children under age 5 had a significantly lower rate of illness than all other age-groups.

At least one specific food was implicated in 44% (1,298) of the norovirus outbreaks. For 364 outbreaks associated with a single food, the foods most often involved were leafy vegetables (33%), fruits or nuts (16%), and mollusks (13%). And for 191 outbreaks with available information, the researchers concluded that the likely point of contamination was preparation or service in 85% and production or processing in 15%.

Factors that may have contributed to contamination were listed in 886 (68%) of the outbreaks with implicated foods. Contributing factors included food handler contact with a ready-to-eat food, 82%; eating a contaminated raw food, 13%; cross-contamination during preparation, 12%; and inadequate cooking or heating, 3%. A food handler was specifically implicated as the source of contamination in 53% (473) of the outbreaks.

"Food handler contact with raw and ready-to-eat foods was identified as the most common scenario resulting in foodborne norovirus outbreaks, underscoring the need to better understand and control endemic norovirus disease as a means of foodborne disease prevention," the report states.

The authors say steps to correct the problem include adherence to recommendations for hand washing and use of gloves; compliance with policies to prevent ill staff from working; and the presence of a certified kitchen manager, as recommended by the Food Code of the US Food and Drug Administration.

The researchers note that no specific food could be implicated in 56% of the outbreaks, which reflects challenges such as multiple transmission pathways and time lags in citizen reporting but may also reflect a relatively low emphasis on investigating norovirus outbreaks.

"Misperceptions may exist that foodborne norovirus outbreaks result only from local contamination events and afford little opportunity for further prevention and/or control," and inadequate public health resources and staffing compound the problem, the report says.

On the science side, the authors say needed steps include further development and validation of analytic methods for detecting norovirus in foods, more data on how long sick food handlers remain infectious, and clarification of the role of asymptomatic food handlers in norovirus transmission.

Hall AJ, Eisenbart VG, Etingue AL, et al. Epidemiology of foodborne norovirus outbreaks, United States, 2001-2008. Emerg Infect Dis 2012 October;18:10 (Early online publication) [Full text]

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