Are Noroviruses Emerging?

(previously called " Norwalk-like viruses ") were discovered as the first viruses definitively associated with acute gastroenteritis. During the next 2 decades, researchers were unable to develop simple methods to detect these common viruses or to find the etiolog-ic agents of nonbacterial gastroenteritis outbreaks and hospitalizations. Indeed, of >2,500 foodborne outbreaks reported to the Centers for Disease Control and Prevention from 1993 to 1997, <1% were attributed to noroviruses, and 68% were of " unknown etiology " (1). As a result, noroviruses were out of sight and mind and thus relegated to a minor role as agents of gastroenteritis at a time when high-profile outbreaks of Salmonella Enteritidis (2) and Escherichia coli (3) had focused attention and budgets on preventing foodborne bacterial illnesses. The development of reverse transcription–polymerase chain reaction in the early 1990s provided the breakthrough needed to facilitate diagnosis of norovirus infection. Today, noroviruses are recognized as the most common cause of infectious gastroenteritis among persons of all ages (4). They are responsible for <50% of all food-borne gastroenteritis outbreaks in the United States (5) and are a major contributor to illness in nursing homes (6) and hospitals (7). Noroviruses have been detected in 35% of persons with sporadic gastroenteritis of known cause (8) and in 14% of all children <3 years old hospitalized for gastroenteritis (9). Norovirus infection has put apparently healthy people in intensive care (10) and has been associated with chronic diarrhea among transplant patients (11). In addition, we now know of a myriad of strains of norovirus, which have been classified into 5 genogroups, distinguishable from sapoviruses, a separate genus of human caliciviruses, but also in the Caliciviridae family. This diversity represents a dramatic increase from the single calicivirus strain discovered >30 years ago. Moreover, the known host range of noroviruses has expanded: the virus is found in mice (12), cows (13), and pigs (14), and antibodies to bovine strains have been found in humans, which has stimulated speculation about zoonotic transmission (15). However, a fundamental question remains—is the increased detection of norovirus the result of better application of improved diagnostics or does evidence exist that norovirus disease is an emergent problem? Recent reports have established that norovirus strains can periodically emerge either globally or nationally, displace other strains, and increase disease incidence (16,17). In winter 2002, a new virus variant was attributed to a well-publicized surge of norovirus outbreaks on cruise ships and …