A Cruise Ship Norovirus Outbreak That Spread From Crew to Passengers, 2022–2023

CDC's Vessel Sanitation Program (VSP) monitors acute gastroenteritis — vomiting and diarrhea — on cruise ships heading to U.S. ports. A reportable case is three or more loose stools in 24 hours (or more than is normal for that person), or vomiting plus another symptom such as fever, diarrhea, bloody stool, muscle aches, abdominal cramps or headache. When 2% or more of passengers or crew are ill and a ship is due at a U.S. port within 15 days, a reporting system alerts VSP and an investigation begins.

The alert

In the first week of January 2023, VSP learned that more than 2% of passengers were ill on a ship that had completed three voyages in Europe and was within 15 days of a U.S. port, on its fourth voyage. Stool samples from sick travelers all tested positive for norovirus genotype II. VSP tracked cases as the ship sailed, and by mid-January 3.4% of passengers were ill.

CDC chart: gastroenteritis cases by voyage, November 2022–January 2023.

Tracing the outbreak

In mid-January, VSP's outbreak team boarded the ship to investigate how people were being exposed and how the virus was spreading, looking closely at crew members' work and social habits. Over five voyages, from November 2022 to January 2023, there were 410 cases: 356 passengers (87%) and 54 crew members (13%). Vomiting and diarrhea were the main symptoms.

• The index case was most likely a food and beverage crew member on a crew-only first voyage, who fell ill after boarding.
• Crew to crew: the virus then spread among crew members on the second voyage.
• Crew to passengers: passengers began falling ill on the third voyage, and cases continued on the fourth and fifth.

Sick crew members generally reported to the ship's medical staff promptly and were isolated until 48 hours after their symptoms ended. But about 70% of ill crew members worked in jobs that brought them into contact with passengers, in housekeeping and food and beverage service.

With the Agency for Toxic Substances and Disease Registry's Geospatial Research, Analysis, and Services Program, VSP built four-dimensional visual models of the ship. They showed where transmission continued and where people could be exposed, such as contaminated surfaces in sick people's cabins and frequently touched surfaces in public areas. VSP kept monitoring the ship on its fifth voyage until it left U.S. jurisdiction.

Lessons

The investigation shows how surveillance data combined with environmental and spatial analysis can speed up the response and pinpoint where norovirus is spreading. To stop illness from carrying over from one voyage to the next:

• crew and travelers should always practice good hand hygiene and sanitation;
• passengers and crew who feel ill should isolate immediately and report their symptoms to the ship's medical center; and
• cruise companies should train crew frequently on norovirus, especially newer crew members, such as those who have served fewer than three contracts.

Sources

• Carolyn A. Crisp, Keisha A. Jenkins, Ian Dunn, Andrew Kupper, Jona Johnson, Stefanie White, Erin D. Moritz and Luis O. Rodriguez, "Notes from the Field: Cruise Ship Norovirus Outbreak Associated with Person-to-Person Transmission — United States Jurisdiction, January 2023," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/72/wr/mm7230a5.htm