Surveillance of Norovirus Infections

Noroviruses are the most frequent cause of acute gastroenteritis outbreaks in humans worldwide and second only to rotavirus in causing sporadic gastroenteric disease.  Approximately 50% of all foodborne outbreaks are attributable to norovirus and, approximately one in fourteen Americans will become ill with norovirus gastroenteritis every year.  These highly infectious viruses usually cause self-limited disease in persons of all ages but the young, elderly and sick may suffer more severe consequences of illness such as dehydration requiring medical attention. In closed settings such as nursing homes, cruise ships, schools, and the military, norovirus often is transmitted person-to-person resulting in high attack rates and large outbreaks.  Food may become contaminated with norovirus at the source (i.e., irrigation, shellfish) or at point of service (e.g. infected foodhandler).  Recent advances in detection and typing methods for norovirus such as TaqMan-based realtime RT-PCR assays and genotyping allows for more standardized surveillance (e.g., through electronic network such as CaliciNet) of outbreaks and sporadic cases. Worldwide, the majority of norovirus outbreaks are caused by the GII.4 genotype, which was first recognized in the mid-1990’s and has been shown to be associated with several pandemics with increased outbreak activity including the winter of 2006-2007. Phylogenetic analyses suggest an epochal evolution for GII.4 viruses over the last twenty years with periods of stasis followed by rapid evolution of novel epidemic strains. Recent research shows that host genetics, such as secretor status, plays an important role in susceptibility of individuals to norovirus infection and that certain histoblood group antigens (HBGAs) on their mucosal surfaces are more susceptible to infection. These HBGA molecules act as attachment factors required for infection by noroviruses but their polymorphism contributes to restriction of the transmission of any given strain. Although no specific therapies, such as antivirals or a vaccine are yet available, recently published vaccine studies using intranasal delivery of recombinant virus-like-particles demonstrated that these vaccines are safe and immunogenic. To reduce the NoV disease burden in certain target groups such as the elderly, healthcare workers and the military, an effective norovirus vaccine could play a pivotal role to control NoV disease although several significant challenges, such as which strains to include in a future vaccine, remain to be solved. Therefore, surveillance systems such as CaliciNet will be critical to monitor how the virus is changing over time. This seminar will cover the most current knowledge on norovirus surveillance in the U.S.