Pandemic noroviruses have a faster rate of evolution than non-pandemic strains, which could explain why they are better adapted to cause worldwide outbreaks of viral gastroenteritis, according to research published free in PLoS Pathogens this week.
Norovirus is an RNA virus that is responsible for the majority of viral gastroenteritis outbreaks worldwide. Norovirus infection—dubbed ‘winter vomiting disease’—is notoriously associated with cruiseships and can cause havoc in hospitals. Symptoms of norovirus infection include diarrhoea and projectile vomiting, which can spread viral particles easily from person-to-person, on contaminated surfaces or in contaminated food and water. Moreover, the virus is incredibly contagious—only 10 or so viral particles are needed to cause infection—and able to survive for several days in a contaminated area.
Despite the fact that over 40 genotypes of norovirus circulate within a population at the same time, only one, known as genogroup II genotype 4 (GII.4), causes winter vomiting disease pandemics. 62% of worldwide norovirus outbreaks are caused by GII.4 and very little is known about why this particular norovirus genotype causes mass disease outbreaks.
Bull et al. investigated how quickly different norovirus genotypes replicated and mutated, and how this could contribute to the ‘fitness’ of the virus during infection. The researchers used in vitro RNA dependent RNA polymerase assays and bioinformatics data to measure the rates of replication, mutation and evolution for the GII.4 pandemic norovirus compared with rates for the less frequently detected non-pandemic norovirus genotypes (recombinant GII.b/G.III, GII.3 and GII.7), and hepatitis C virus as a control. They found that GII.4 strains of norovirus had much higher rates of mutation, replication and evolution than the other norovirus strains tested. Evolution rates were measured within the viral capsid (the outer protein coat of the virus) and GII.4 strains had more mutations that made changes to the capsid’s amino acid sequence than the other noroviruses.
Bull and colleagues argue that the rapid mutations seen in the GII.4 norovirus make it similar to influenza, in which “an increase in antigenic drift has been associated with increased outbreaks.” The research will help scientists better understand how norovirus causes winter vomiting disease pandemics and could prove useful during development of a vaccine or treatment for norovirus. So just remember this when you’re quarantined in your cruise cabin and upchucking into your toilet—the norovirus has mutated fast to make it ‘fit’ to infect you.
Bull, R., Eden, J., Rawlinson, W., & White, P. (2010). Rapid Evolution of Pandemic Noroviruses of the GII.4 Lineage PLoS Pathogens, 6 (3) DOI: 10.1371/journal.ppat.1000831