Cruise Ship Norovirus Outbreak Highlights How Infections Spread

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Norovirus is the leading cause of acute gastroenteritis in the United States. There are an estimated 21 million cases annually. The virus poses a significant burden on cruise ships, causing an average of 27 confirmed outbreaks annually over the past 5 years.

In January 2009, the report of a suspected norovirus outbreak among passengers on a cruise ship prompted an investigation. The results of the investigation highlights how the infections can spread and the steps both passengers and crew can take to prevent them.

The findings of the study have been published online in Clinical Infectious Diseases.

Mary Wikswo, MPH, of the Centers for Disease Control and Prevention, and colleagues conducted a retrospective cohort study among passengers on board the ship. Questionnaires about health care–seeking behaviors, hygiene practices, and possible norovirus exposures were placed in every cabin after the outbreak began.

The ship had 1,842 passengers on board, and 83% returned the questionnaires. Of the 15% of respondents who met the case definition for acute gastroenteritis, only 60% had sought medical care on the ship.

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Acute gastroenteritis is defined by inflammation of the stomach and small and large intestines. The main symptoms of viral gastroenteritis are watery diarrhea and vomiting. The affected person may also have headache, fever, and abdominal cramps. In general, the symptoms begin 1 to 2 days following infection with a virus that causes gastroenteritis and may last for 1 to 10 days.

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% 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.

Norovirus is highly transmissible through person-to-person contact and contaminated food, water, and environmental surfaces.

"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."

Source
Mary E. Wikswo, Jennifer Cortes, Aron J. Hall, George Vaughan, Christopher Howard, Nicole Gregoricus, and Elaine H. Cramer; Disease Transmission and Passenger Behaviors during a High Morbidity Norovirus Outbreak on a Cruise Ship, January 2009; Clin Infect Dis. cir144 first published online March 22, 2011 doi:10.1093/cid/cir144

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