Though many think of cruise ships at the top of the norovirus outbreak list, restaurant-related events are more common, with the culprits being a highly contagious pathogen that can spread easily when sick restaurant employees prepare food, according to a new analysis from the US Centers for Disease Control and Prevention (CDC).
According to a Vital Signs report released today, cruise ships account for only about 1% of all reported outbreaks, with foodborne transmission the primary mode in 23% of the events. More than half of those occurred at restaurants.
The CDC's analysis profiles the foodborne share of norovirus outbreaks and is based on data collected from 2009 through 2012 via the National Outbreak Reporting System (NORS), which contains electronic reports of acute gastroenteritis outbreaks transmitted through similar exposures. The CDC published the full findings of the study in an early online edition of Morbidity and Mortality Weekly Report (MMWR).
Of 4,318 norovirus outbreaks reported during the time frame, 1,008 (23%) involved foodborne transmission, a number that reflects 48% of the 2,089 foodborne illness outbreaks with a single suspected or confirmed cause during that time. Those factors make norovirus the leading cause of acute gastroenteritis and foodborne disease in the United States.
In total, the 1,008 food-related outbreaks sickened 21,065 people, according to the MMWR report.
At a media briefing today, CDC Director Tom Frieden, MD, MPH, said most of the foodborne transmission occurs in foodservice settings, with restaurants accounting for 64% of settings and catering or banquet facilities responsible for 17%.
Among outbreaks with contributing factors reported, 70% resulted from infected food workers who likely touched ready-to-eat foods with their bare hands, he said.
When CDC researchers looked at foods commonly linked to outbreaks, they found that of 324 with a specific food listed, 90% were contaminated during final preparation, such as making a sandwich with raw and already cooked ingredients. Leafy vegetables, fruits, and mollusks such as oysters stood out as the most common foods linked to the outbreaks.
'One tough bug'
Norovirus requires only a minuscule dose to be infectious, Frieden said. "The bottom line is that norovirus is one tough bug," he said.
Though norovirus outbreaks typically peak in the winter months, foodborne ones occur all year round, the CDC found. States varied widely in the number and rate of reported foodborne norovirus outbreaks, but Frieden said those patterns mainly reflect levels of outbreak reporting and how hard states are looking for outbreaks, not that the highest reporting states are more vulnerable.
The CDC's report today also came with key recommendations for the foodservice industry. "We do know now to stop the virus," Frieden said. Measures include making sure employees adhere to proper hand washing protocols, keeping sick staff members away from work at least 48 hours after symptoms resolve, and making sure certified kitchen managers supervise food preparation.
Aron Hall, DVM, MSPH, an epidemiologist with the CDC's Division of Viral Diseases, told reporters that having a certified kitchen manager is associated with lower levels of norovirus illnesses in foodservice settings.
He said though foodborne outbreaks make up only a relatively small percentage of the 20 million norovirus illnesses that occur in the United States each year, analyzing them—as scientists did for today's report—is helpful, because it reveals useful information about transmission exposure.
The disease kills 570 to 800 people a year, according to the study, and accounts for $777 million in annual healthcare costs.
Jun 3 CDC Vital Signs report
Jun 3 CDC MMWR report
Jun 3 CDC press release