Between 24 April and 9 June 2006, four cruise ships each reported one on-board outbreak of gastroenteritis, and three ships each reported two outbreaks to the appropriate health authorities. Infection was confirmed in two ships after samples tested positive for norovirus; other samples are still being tested. Based on the current information, it appears that the seven ships are owned by five different companies. Most of the ships have been operating in the Baltic sea.

DIVINE-NET has sent a request for information to its focal points in the 13 European countries that participate in the network, providing the travel routes of the involved ships and requesting additional information. By 14 June, there was no indication of a common factor linking these outbreaks. The European Commission issued an alert to European Union member states via the European Early Warning and Surveillance System on 15 June.

Norovirus outbreaks are a well-known problem on cruise ships and ship operating companies have guidelines for sanitation procedures. Despite implementing thorough disinfection measures, outbreaks tend to continue on consecutive cruises [2]. Because norovirus is highly contagious and can be spread via several different routes, including person-to-person, food, water and environmental contamination, the ships provide an excellent environment for the virus to spread rapidly [3]. The outbreak investigations are usually performed by local health authorities in collaboration with ship operating companies. However, it is not always clear who should be in charge of investigations and to what extent the local authorities can collect confidential information, especially if the ships are in international waters. As the ships visit several cities in various countries, the actions taken in one country may not be readily informed to the authorities in other countries.

Although the norovirus outbreaks occur regularly on cruise ships, the seven events in a relatively short time period raise questions on possible common links between the outbreaks and underline a need for international guidelines on outbreak investigations on cruise ships.

Information reported to DIVINE-NET on 13 June is presented below. Information is not complete, but gives an idea of the extent of the outbreaks.

Ship A, The Netherlands
• Route: Zutphen (Netherlands) -Antwerp (Belgium)
• Cases: On 26 April 2006, an outbreak of gastroenteritis (15 cases) was reported on a cruise ship chartered by? a Japanese tour organisation. The index patient became ill on 24 April. The ship left for Antwerp, Belgium on 24 April.
• Samples: Swab samples were taken from door handles, toilets, and a reception counter and found to be positive for norovirus. The sequences were determined: GGII.4.
A stool sample was collected from one of the patients and tested negative for norovirus.
• Control measures taken: At the time of reporting some hygiene measures had been taken. The ship was thoroughly cleaned before the arrival of a new group of passengers. No new cases have been reported.

Ship B, The Netherlands
•Route: Kiel (Germany), Nijmegen (Netherlands), Vienna (Austria)
•Cases: On 22 May a general practitioner reported several cases of norovirus infection to the Municipal Health Service in Nijmegen (eastern Netherlands, on the Rhine and close to the border with Germany). No further investigation was performed at that time.
•On 9 June new cases were reported from the ship B. Ship B was at that time sailing towards Vienna.
•Samples: During the second outbreak, patient samples were collected in Germany and sent to RIVM in Bilthoven for norovirus testing. No further information available at this time.

Ship C, docked at Harwich (England, UK)
• Route: Harwich (UK), Bergen (Norway) , Flam (Norway), Gudangan(Norway), Rosendal (Norway), Harwich.
• Cases: On 29 May, an outbreak of gastroenteritis was reported, thought to be caused by norovirus. About 70 passengers and 15 crew members were infected. Most of the passengers on ship C are from the United Kingdom. It was reported that two passengers were ill when the ship departed from Bergen to Flam.
• On 9 June, a second outbreak was reported, affecting 28 people in a new group of passengers. Bergen in Norway was visited during this trip.
• Samples: During the first outbreak, stool samples from passengers were examined to look for norovirus at a laboratory in Trondheim, Norway. Due to miscommunication, no patient or environmental samples were taken during the second outbreak.
• Food supply: Both trips included an excursion to a fish market [where?], where passengers ate various fish and shellfish. Therefore this market is a potential source of infection.

Ship D, Nijmegen (The Netherlands)
• Route: Kiel (Germany) - Nijmegen (Netherlands) – Vienna (Austria)
• Cases: On 30 May, a cruise ship reported 61 cases of gastroenteritis in a group of American passengers. The index case had illness onset on 20 May, and the last reported case had illness onset on 30 May. There was a peak of 15 new cases on 28 May. MHS Nijmegen and the VWA (Dutch Food Safety Authority) visited the ship for further investigation.
• During ship D’s previous trip, it was reported that a small number of gastroenteritis cases occurred. According to the ship’s captain, some of the passengers mentioned having had the same symptoms before boarding the ship. However, this was not confirmed by a medical doctor.
• Both ship B and ship D are owned by the same company. However, none of the crew members had worked on both ships.
• Samples: Six patient samples were collected. Stool samples from two patients tested positive for norovirus; testing of samples from a further four patients is still in progress. Environmental swab samples tested positive, and sequences from swabs, patients and other cruise ships will be compared.
• Food supply: ship B and D were both supplied with food in Kiel, by a company that may supply other cruise ships. Fresh food was purchased locally. It is unclear whether high risk foods such as shell fish and soft fruit were included in this local purchase.

Ship E, docked at Southampton, (England, UK)
• Route: Vigo (Spain), Southampton (UK)
• Illness reported 30 May
• Arrival from Vigo, Spain. It is not known whether there were any stops between Vigo and Southampton.
• Dealt with by Southampton and Spain.
• No further information currently available

Ship F, docked at Dundee (Scotland, UK)
• Route: Tallinn (Estonia), Copenhagen (Denmark), Stockholm (Sweden), Helsinki (Finland), St Petersburg (Russia)
• Cases: on 29 May a first outbreak was reported in Dundee.
• On 9 June, a second outbreak was reported in Hull
• In one of the outbreaks, 70 passengers and 15 crew members were affected.. It is not known whether passengers brought the virus onto the ship or whether the outbreak is linked to its sister ship, ship C, which is owned by the same company. Ship F did not visit Bergen.
• No further information currently available

Ship G, docked at Leith (Scotland, UK)
• On 12 June an outbreak was reported with 116 cases.
• This ship is owned by a different company.
• No further information is currently available.

Acknowledgments
J Harris (DIVINE-NET)