Norovirus Steals the Crown from Rotavirus

Published - Written by Sushrut Jangi

A decade ago, if a young child with profuse diarrhea and vomiting was brought into the pediatrician’s office, the doctor would think immediately: rotavirus. No longer, says Daniel Payne, author of a new study in NEJM this week. According to their results, norovirus has displaced rotavirus as the predominant cause of acute infectious gastroenteritis in children.

Both rotavirus and norovirus have circulated in the United States for a long time. A hundred years ago, the acute GI syndrome caused by these viruses was called the “Winter Vomiting Disease.” Even now, we see viral gastroenteritis cases peak in the winter, although both viruses are thought to circulate within the United States throughout every month of the year.

But recently, the epidemiology of these viruses has begun to change. With the introduction of the rotavirus vaccines in 2006 and in 2008, cases of rotavirus are on the decline, not only in the United States, but throughout the world. Yet every year, countless adults and children suffer violent bouts of gastoenteritis. As rotavirus has begun to decline, has norovirus begun to bloom?

Prior to this week’s report – we didn’t know. “Norovirus is difficult and costly to diagnose in a clinical setting,” Payne explains. Generally, a clinician has no major reason to identify the viral culprit of acute gastroenteritis; whether rotavirus or adenovirus, children are treated with rehydration therapy. Because doctors rarely test for norovirus, we haven’t known how frequently its presence underlies pediatric diarrhea. Understanding this epidemiology is critical if we are to replicate the success of rotavirus vaccination for norovirus.

In the study, Payne and colleagues examined American children with acute gastroenteritis to determine how many of these cases were caused by norovirus. They found that nearly a quarter of sick children tested positive for the virus; the number climbed to 33% in the winter months.   Rotavirus was found in only 10% of cases – suggesting for the first time that norovirus is now the predominant culprit of acute gastroenteritis in the US pediatric population. “Primarily, this is probably because of the rotavirus vaccine,” says Payne.

So might it be possible to create a norovirus vaccine too?

Payne thinks we will face challenges in such an undertaking. “The geno groups of norovirus seem to change every two to three years. When a new major variant appears, humans often don’t have immunity, and they become newly suspectible with symptomatic disease.” It’s a problem similar to the variability of the influenza virus – thus a new flu shot is offered every year.

While the new study has implications for vaccine development and implementation, the findings also provide information for the clinician. Given that norovirus is now recognized as the most common cause of acute pediatric gastroenteritis, doctors may have increasing suspicion that a child has norovirus, rather than a less contagious culprit, on their hands. “Norovirus is incredibly infectious.  It’s been documented that 18 viral particles can cause a full blown infection,” says Payne. Furthermore, norovirus sheds for weeks after a child has recovered from a gastroenteritis episode, raising the chance of infection considerably. Although viral diarrheas in children have other causes – adenovirus, astrovirus, and sapovirus, for example – the high infectivity of norovirus coupled with its ascendancy over rotavirus suggests sanitation and hygeine are of utmost importance to contain the disease, at least until a vaccination campaign is successfully in place.

While this study answers a lot of questions about the epidemiology of norovirus, other mysteries abound.  Some people, Payne tells me with rising excitement, allow norovirus to transit through their GI tract without any symptoms. These hosts may have a mutation in one of the antigens carried on the ABO blood group, an observation which may hold promise for alternative therapeutic avenues.

The winter season is now almost over and the peak of norovirus will momentarily ebb – but Payne isn’t planning on slowing down.  He’s already off to another research meeting to plan the next move.

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