14% of norovirus outbreaks from food

To reduce the disease burden, it is essential to understand how it spreads and transmission via contaminated foods is one mode of transmission, said the researchers.

This will give insight into the effectiveness of measures such as market recalls and communication of cooking recommendations for mussels and frozen berries.

The European Food Safety Authority (EFSA) panel on Biological Hazards (BIOHAZ) said last year that outbreaks associated with norovirus in frozen raspberries and strawberries are an emerging public health risk​.

Analysis methods​

Researchers used norovirus transmission and genotyping information from international outbreak surveillance systems (Noronet, CaliciNet, EpiSurv) and a review of peer-reviewed literature.

The FBVE/Noronet database included 5,583 outbreaks from 22 countries during 1999–2012.

The CaliciNet database included information about 3,094 outbreaks in the US during 2009–2012.

The ESR-EpiSurv database included 818 outbreaks from New Zealand during 2008–2012 and the systematic review gave information on 966 outbreaks.

Applying profiles per surveillance database, the proportion of outbreaks attributed to foodborne transmission varied slightly (12% for FBVE/Noronet, 13% for ESR-EpiSurv, and 16% for CaliciNet), the aggregated global estimate was 13.7%.

Researchers said the proportion of norovirus outbreaks attributed to foodborne transmission is comparable in different parts of the world.

With one in seven norovirus outbreaks being attributable to food, the transmission route represents a major target for intervention, particularly given the possibility of preventing primary and secondary cases if contaminated foods are recalled, they said.

Primary and secondary transmission​

After primary introduction of the virus through food, secondary person-to-person and environmental transmission can make it hard to trace the disease back to contaminated food.

Another complexity is foodborne transmission can follow different routes – it can be contaminated during production or during handling by an infected food handler.

If the primary or secondary mode of transmission was reported as foodborne, the outbreak was considered as foodborne.

If person-to-person transmission was said to be one of the possible routes, it was considered as person-to-person.

Norovirus can be transmitted from person to person directly or indirectly through contaminated food, water, or environments.

The results of the study, which was coordinated by the National Institute for Public Health and the Environment (RIVM) in the Netherlands, were published in the Emerging Infectious Diseases journal​. 

It was commissioned and paid for in part by the Foodborne Disease Burden Epidemiology Reference Group (FERG) of the World Health Organization (WHO), the New Zealand Ministry of Health, and by the Government of the Netherlands on behalf of FERG.