The Evolution of Norovirus, the “Gastric Flu”

Perspectives L inguistically speaking, the predominant viral cause of gastroenteritis has been evolving. Once evocatively called winter vomiting disease, the pathogen's name has changed alongside improved scientific understanding. First called Norwalk virus (or Norwalk-like virus) in reference to the Ohio town where specimens from a school outbreak enabled the seminal work that first characterised the virus, it was later dubbed the Small Round Structured Virus following visualisation by electron microscopy. The International Committee on Taxonomy of Viruses later settled on the present name of " norovirus " , classifying it as a member of the Caliciviridae family based on both morphology and phylogeny. A new study published in this issue of PLoS Medicine suggests that the colloquial " gastric flu " may have been the most apt term of all [1]. When media reports call " norovirus " just " a fancy word for gastric flu " , they allude to similar seasonality and the lack of effective therapeutics for influenza and norovirus, but the analogy may run deeper [2]. There are parallels with respect to influenza and norovirus evolution and human immunity. In the new study, Ralph Baric and colleagues present compelling data to demonstrate that norovirus evolution is driven by immune selection pressure. The domain of the exposed viral capsid protein that binds carbohydrates in the human gut evolves in the face of herd immunity. Histoblood group antigens (HBGAs), a heterogeneous group of related carbohydrates on mucosal surfaces, provide a " docking station " for noroviruses, and there is a large variety of such HBGAs. These sites are similar, but distinct, so as the capsid mutates and subtly changes its shape, it can still find new binding sites on the mucosal surface of the gut. The virus survives despite the build-up of immunity in the population because there is room in antigenic space for the virus to evolve and remain infectious. As with influenza, the predominant circulating norovirus may also change its antigenicity frequently, giving it the potential to cause pandemics and necessitate regular reformulation of vaccine. Noroviruses are the most commonly detected pathogen both in sporadic cases and outbreaks of gastroenteritis. They are particularly problematic in environments where groups congregate and infection can be rapidly transmitted through both faecal and vomitus routes. Outbreaks affect health care facilities worldwide, and may cause massive disruption to providing care, substantial economic loss, and, according to some reports, mortality in vulnerable …