Comparison of two extraction methods for viruses in food and application in a norovirus gastroenteritis outbreak

Noroviruses are important causes of gastroenteritis; however, due to a lack of sensitive detection methods, the distinct role of contaminated food in norovirus outbreaks remains unclear. Two published virus extraction procedures combined with real-time RT-PCR for the detection of norovirus from food inoculated experimentally were compared. The elution-precipitation method was most efficient in all food matrices tested showing detection limits of 20 RT-PCRU for lettuce and ham, and 200 RT-PCRU for raspberries. The average recovery rates were 23%, 7% and 24% for lettuce, raspberries and ham, respectively. The ultrafiltration method yielded detection limits of 200 RT-PCRU for lettuce and ham, and 2000 RT-PCRU for raspberries; recovery rates were 9%, 7%, 3%, respectively. Subsequently, food items implicated in a norovirus outbreak were examined by the elution-precipitation method. Virus recovery rates determined by using MS2 phage ranged from 1 to 20% depending on the food matrix. However, norovirus could not be detected in the food items examined. This negative result may be explained by a low virus titer and heterogeneous virus distribution, or by random selection of food samples that contained no norovirus. Both, proper sampling and virus extraction from foods may be improved further to identify vehicles of infection.