Norovirus Outbreak 2012
In autumn 2012, many cases of diarrhoea and vomiting occurred as a result of norovirus infections in children and adolescents who had eaten meals in schools and kindergartens. The norovirus outbreak affected the federal states of Berlin, Brandenburg, Saxony, Saxony-Anhalt and Thuringia. According to the Robert Koch Institute, the outbreak accounted for almost 11,000 cases, thus making it the biggest foodborne disease outbreak known in Germany to date.
Cause of the outbreak: frozen strawberries
According to the results of an epidemiological and microbiological outbreak investigation conducted jointly by national and regional governments, imported deep-frozen strawberries of a particular batch are regarded as the cause of the norovirus outbreak. Noroviruses were also detected on strawberries taken from originally sealed packagings.
The affected batch of frozen strawberries was delivered to various commercial kitchens and did not enter the retail chain. In the interest of precautionary consumer protection, the batch was taken from the market and the buyers notified accordingly. Frozen strawberries from this batch were processed into compote or soft white cheese dishes in commercial kitchens before being handed out to catering participants in community institutions. The frozen strawberries were prepared in different ways: it was reported from several kitchens associated with a disease outbreak that the frozen strawberries had been defrosted and sugared. Individual kitchens stated that they had only stirred the strawberries into boiling water or boiled them briefly. Kitchens not associated with an outbreak almost exclusively served the strawberries after boiling them. The BfR does not know what core temperatures were reached during the heating processes.
Berries can come in contact with noroviruses at various points during the production process, such as improper watering or fertilisation. Persons infected with noroviruses can also transfer the virus to the berries during harvest or packaging. Deep-frozen berries can also come in contact with noroviruses via contaminated water added during the freezing process.
Resistance of noroviruses
Little is generally known about the tenacity (resistance) of human noroviruses because the infectiousness of these viruses cannot be measured due to a lack of cell culture systems. Overall, however, it can be derived from the available data that noroviruses tolerate a low pH value and that they lose infectiousness in the temperature range above 70 °C depending on the selected holding time. Smooth and even heating of strawberry compote to over 90 °C appears to be suitable to completely inactivate the virus. It has to be assumed, on the other hand, that stirring large quantities of deep-frozen strawberries into boiling water or uneven heating when briefly bringing a large quantity of compote to the boil cannot safely inactivate all of the noroviruses on the strawberries. The BfR has published an assessment of the tenacity of noroviruses in strawberry compote (PDF file,75.11 KB) in the internet in the form of an opinion.