Analysis of norovirus outbreaks reveals the need for timely and extended microbiological testing

Norovirus outbreaks in hospitals remain a substantial threat despite many recommendations for prevention published recently.

To analyse the factors contributing to onset of a norovirus outbreak in hospitals in order to identify new prevention options.

Data from 71 norovirus outbreaks occurring in five German hospitals between 2002 and 2012 were analysed focusing on the start conditions: the weekday of outbreak, the time span between the first symptomatic cases and the outbreak onset date, the timing of a positive norovirus test result in an outbreak, and presence of concomitant Clostridium difficile infections.

In 68 (96%) outbreaks index cases were identifiable. In 30 of 44 (68%) outbreaks the index case patient acquired norovirus infection in hospital. In 20% of all outbreaks, the index case was a staff member. Nine outbreaks were caused by not isolating contact patients during the incubation time after their exposure to a symptomatic case. Case numbers in norovirus outbreaks were lower when the norovirus test results were available before the outbreak onset (P = 0.028). In 30 of 46 (64%) norovirus outbreaks, C. difficile toxin tests were positive in up to ten patients. Co-infection or subsequent infection with norovirus and C. difficile in single patients occurred in nine (20%) outbreaks.

Future prevention strategies should focus not only on patients but also on staff. Constant surveillance for new cases of diarrhoea and vomiting and timely adherence to contact precautions for all exposed persons is crucial in outbreak control, as is the need for extended microbiological testing.