Norovirus

"The cruise ship is just like lots of other contexts in which we know disease transmission can take place," said Megan Murray, an assistant professor in the Department of Epidemiology and a clinician at Massachusetts General Hospital. Murray aided an investigation by Holland America in November when passengers on one of its ships fell ill.

Highly contagious but non life-threatening, noroviruses have sickened passengers on several cruise ships recently. The term "noroviruses," generally called Norwalk virus in the media, is the designation approved by the International Committee on Taxonomy of Viruses for the pathogens thought to cause sudden-onset viral gastroenteritis.

Noroviruses are transmitted person to person or through contaminated food or water. The pathogens are usually passed among people living or working in close quarters. Prisons, schools, hospitals, nursing homes, military barracks, and naval ships have all been settings for norovirus outbreaks in the past. Communities of poor people in crowded living situations are also at risk of infection.

"Whereas usually these conditions are associated with poverty, in the case of cruise ships, they are associated with affluence, and they have received attention," said Murray.

Part of the attention may have resulted from the fact that cruise ships are required by the Centers for Disease Control and Prevention to report gastroenteritis cases when patients exceed a specific number, she said.

While the cruise ship illnesses captivated American media, the sicknesses pale in comparison to the toll of other infectious diseases. People infected with noroviruses may experience vomiting, diarrhea, nausea, and stomach cramps, but the symptoms usually dissipate after a few days. So far, several hundred cruise line passengers and crew members have fallen ill.

By comparison, tuberculosis claims approximately two million people each year, despite the fact that the illness is curable.

Noroviruses are "not on the same level as other infectious disease threats such as tuberculosis, malaria, and HIV," said Murray. "Those are the ones that kill lots of people."

An unknown number of people fall ill from noroviruses each year. There is no systematic surveillance of gastroenteritis in the community at large. However, while not proving an increase, the Program for Monitoring Emerging Diseases of the International Society for Infectious Diseases lists far more reports of norovirus outbreaks on its web site in 2002 than in 2001.

"Are there higher rates this year and if so, why?" said Murray. The possibly changing infection rates may be a product of a disease transmission dynamic or of a more contagious strain of noroviruses that emerged last year. The answers are unknown right now, she said.

Murray expects more cases of gastroenteritis to pop up on cruise ships in the future.

"The virus is in the community at large, and people will continue to board ships," she said.