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Foodborne norovirus outbreaks

Noroviruses are a major cause of viral gastroenteritis and have been detected with increasing prevalence in recent years. Noroviruses are genetically diverse and the ones, which have been detected in humans are currently divided into 5 genogroups (G) based on their capsid and/or polymerase sequences. Only GI, II, and to a much lesser extent IV are clinically important for man. Clinical symptoms including nausea, vomiting, diarrhea, and fatigue start suddenly after an incubation period of 1−3 days and normally resolve within few days. Infants, elderly people and particularly immunocompromized persons can excrete noroviruses for several months to more than a year.

Laboratory diagnosis relies mainly on PCR detection of noroviral RNA in stool samples. Antigen assays for rapid testing are also available but less sensitive and specific. They may be used in outbreaks, if testing of many persons is necessary. Several factors contribute to the ability of noroviruses to cause widespread outbreaks: high amounts of virus excreted during infection, high stability of virions in the environment, low infectious dose (10–100 viral particles). Foodborne transmission has been well-documented but may be difficult to prove in an individual outbreak and thus is often misinterpreted. The main reason for detection failure is the limited sensitivity of PCR assays if the amount of input sample material is low compared to the amount of food ingested.

End of September 2012 an extended gastroenteritis outbreak affected East Germany with 11.000 cases many of which received lunch from the same provider. Not all gastroenteritis cases were necessarily due to noroviruses; however they were the major cause. Various GI and II strains were detected in stool samples from sick patients. This fitted to a foodborne outbreak starting from highly contaminated food. The dominance of non-pandemic strains could also help to explain the lack of widespread transmission within households. Frozen strawberries delivered by a large-scale caterer were identified as the likely source both epidemiologically and by detecting noroviral RNA. These strawberries had been distributed to all affected settings, but different manifestation rates were observed due to variable handling of the frozen berries before serving.

The outbreak demonstrates the ability of noroviruses to spread quickly as a foodborne pathogen and the importance of a quick coordinated response of virologists with local, state, and federal agencies in outbreak control.

Dieter Hoffmann, München