European investigation into recent norovirus outbreaks on cruise
The European investigation into norovirus infection outbreaks
on cruise ships, carried out by DIVINE- NET, the European network for the
prevention of emerging (food-borne) enteric viral infections (http://www.eufoodborneviruses.co.uk/DIVINEVENT/DIVIndex.asp
is continuing. By 5 July 2006, a total of 35 outbreaks of gastrointestinal
infection had been reported on 13 cruise ships travelling around Europe. Norovirus
(NoV) has been confirmed as the aetiological agent for outbreaks on 9 of the
13 ships. A total of 1088 cases have been reported, of which at least 642
(59%) are known to have been in passengers and 64 (6%) in crew members. The
most recent suspected outbreak was reported on 5 July.
The investigation has focused on two main hypotheses:
1. the introduction of NoV on the ships from a common source (food or water
2. the circulation of NoV on cruise ships as a reflection of illness in
the community, possibly increased due to circulation of new variant strains.
Of the 13 ships involved, nine have reported consecutive outbreak episodes,
ranging from two to five. Samples to confirm the diagnosis have not been
collected consistently in consecutive outbreaks, but the data suggests persistence
of NoV on ships. Alternatively, new passengers may reintroduce the virus,
precipitating new outbreaks.
The virological investigation has shown that the confirmed outbreaks are
caused by two new variant GGII4 NoV strains. An open access strain matching
system has been established to allow harmonised sequence comparisons worldwide
for commonly used genome regions (www.rivm.nl/bnwww,
follow the links to ‘Foodborne viruses in Europe’ and ‘quick typing database’).
Requests for strain comparison can also be sent to email@example.com.
A survey is ongoing among DIVINE-NET members to provide background levels
of outbreak reporting in the community. A retrospective cohort study and
a detailed review of hygiene protocols are planned on the ship where the
most recent outbreak was reported. In addition, guidelines on NoV control
in different settings are being reviewed.
Based on the available information, and pending the completion of the analysis
of data, there is no indication that a common source event introduced NoV
on these ships. The most likely hypothesis is that cruise ship outbreaks
reflect increased level of illness in the community.