Norwalk Virus ‐‐ Off and Running

H ospitals across Canada have been struggling with a series of outbreaks of viral gastroenteritis caused by the Norwalk virus (NV).Nursing homes, schools and hospitals in Alberta, British Columbia, New Brunswick and Ontario have been most affected, with additional outbreaks reported from Manitoba and Quebec (1-4).By the end of December 2002, in Edmonton's Capital Health Region, there were 42 outbreaks directly affecting more than 1000 individuals.Thirty outbreaks of viral gastroenteritis have been reported to Public Health in the Calgary Health Region since the beginning of October 2002, with 18 of these in December alone (2).In British Columbia from January to June 2002, more than 30 gastrointestinal illness outbreaks were found by reverse transcriptase polymerase chain reaction (RT-PCR) testing to be caused by Norwalk-like virus (NLV) (3).These outbreaks, involving both patients and staff, have occurred in various closed and semiclosed populations, including long term care facilities, assisted living sites, schools and acute care facilities.In most of the outbreaks where stool samples have been obtained for testing, NLV has been identified as the causative agent.This kind of outbreak activity usually signals a high amount of illness in the community and suggests that considerable under-reporting is occurring, with a resultant underestimation of the numbers of affected people.Many acute care hospitals across southern Ontario were forced to curtail visiting hours or temporarily close wards and emergency rooms in recent weeks to try to slow the spread of the virus (4).In addition, persistent outbreaks of gastrointestinal illness due to NLV among passengers and crew of cruise ships have been increasingly reported over the past few years.During the autumn of 2002, at least one large cruise ship was withdrawn from active service to provide an opportunity for a thorough environmental cleaning.What do we know about the NLV?What are the factors associated with its spread?The recent series of outbreaks provides an opportunity to review some of the epidemiology and features of NLV infections.In October 1968, an outbreak of acute gastroenteritis among students and teachers in an elementary school in Norwalk, Ohio was investigated by the Centers for Disease Control and Prevention (5).During a two-day period, 50% of the students and teachers (116 of 232) developed a gastrointestinal illness with a secondary attack rate of 32% among contacts of primary cases.The illness was characterized by nausea and vomiting in more than 90% of cases and diarrhea in 38%

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