Cruise ship norovirus outbreak highlights how infections spread

Norovirus is the leading cause of acute gastroenteritis in the United States and is estimated to cause nearly 21 million cases annually. It is highly transmissible through person-to-person contact and contaminated food, water, and environmental surfaces. The results of an investigation of a 2009 outbreak on a cruise ship shed light on how the infections can spread and the steps both passengers and crew can take to prevent them. The findings are published in a new study in Clinical Infectious Diseases and available online.

Questionnaires about when people did or did not seek medical care, hygiene practices, and possible norovirus exposure were placed in every cabin after the outbreak began. The ship had 1,842 passengers on board, and 83 percent returned the questionnaires. Of the 15 percent of respondents who met the case definition for acute gastroenteritis, only 60 percent had sought medical care on the ship. Infected passengers were significantly more likely to have an ill cabin mate and to have resided or dined on the deck level where a vomiting incident had occurred during boarding. The most common symptom reported was diarrhea, followed by vomiting. Stool samples from several ill passengers tested positive for norovirus.

Less than 1 percent of the crew reported illness, and their low attack rate may have been due to the few crew members who had direct contact with passengers. This included separate sleeping and dining areas and alternate passages for boarding and exiting the ship. Another factor may have been an acquired short-term immunity from previous cruise ship outbreaks.

"Cruise line personnel should discourage ill passengers from boarding their ships," according to study author Mary Wikswo, MPH, of the Centers for Disease Control and Prevention. "Once on board, passengers and crew who become ill should report to the ship's medical center as soon as possible. These quick actions are crucial in preventing the introduction and spread of norovirus on cruise ships and allow ship personnel to take immediate steps to prevent the spread of illness."