What are noroviruses?
Noroviruses are a group of viruses. They're the leading cause of gastroenteritis, which causes diarrhea and vomiting. They affect the whole age range from young children to the elderly, and, in the US, they cause about 20 million cases annually.
How many different strains of norovirus are there and how many are known to affect humans?
There are five genogroups of norovirus, and two of those primarily affect humans. Those genogroups can be further broken down into so-called genotypes. In total, there are about 30 known genotypes that can cause illness in humans. Noroviruses are constantly evolving, and every 2 to 4 years a new strain emerges.
What makes the norovirus so contagious?
They have a very low infectious dose, as low as eighteen to a thousand virus particles, and that contrasts to the incredible amount that are shed in the stools of infected persons, greater than one billion virus particles per gram of stool.
They're also transmitted through multiple infection routes:
- direct contact between people
- food-borne transmission
- water-borne disease
- environmental spread
These viruses are very stable in the environment and hardy against disinfection.
Why are there currently no vaccines or treatments for norovirus?
We're just now defining the burden of disease of norovirus, which groups are affected, and, therefore, the need for norovirus vaccines.
But there are also technical challenges, including that it is not possible to grow norovirus in cell culture, and we don't have a complete understanding of norovirus immunity, so it's been challenging to develop a vaccine or specific treatment.
Why can’t norovirus be controlled just by improved water and sanitation?
There are multiple transmission routes for norovirus. Water, sanitation, and hygiene are important, particularly in ensuring a safe food supply and water sources
But noroviruses, because they're highly infectious, are also transmitted directly between people and through the environment, including frequently-touched surfaces.
So, improving water and sanitation can reduce food-borne disease and water-borne disease, but there's still a substantial amount that comes through direct person-to-person transmission.
How important is the development of a vaccine for norovirus?
Because control of norovirus is so difficult, and norovirus is so infectious, prevention is the key, and vaccines could be an important tool for prevention.
We now know that noroviruses are the number one cause of pediatric gastroenteritis hospitalizations in the US. They cause 70,000 hospitalizations across the age range, so a vaccine would be a very welcome addition to our prevention tools.
What will be the main challenges in the development of a vaccine for norovirus?
As I mentioned, you can't grow norovirus in cell culture, so that hampers a lot of the development. We don't have a complete understanding of natural immunity to norovirus, so that, also, is a challenge.
A vaccine will need to protect against the diverse range of strains that are out there, and norovirus is constantly evolving, so the vaccines will have to be designed in order to protect against the most common strains causing disease.
Will it be necessary to develop different vaccines for each strain of norovirus?
There are two main genogroups that cause disease in humans, so a vaccine will probably at least need to be bivalent, and will need to contain a component of genogroup one and genogroup two.
But noroviruses are constantly evolving, so it's possible that a vaccine would need to be regularly updated. Really, we don't know yet, and we'll only know what is needed in a vaccine once vaccines are developed and trialled in humans.
Please can you outline your recent research on noroviruses?
We recently conducted a review of all the published papers on norovirus, and we included all eligible papers globally that looked at the fraction of diarrheal disease cases that were caused by norovirus.
From this, we estimated the fraction of gastroenteritis was due to norovirus, and, based on 175 papers from around the globe, we estimate that norovirus is the cause of around a fifth of all gastroenteritis cases.
Were you surprised by this figure?
It's an increase from previous estimates, but there has also been a huge increase in the amount of information that's out there, a sign of increasing interest in norovirus. A similar review that was conducted about five years ago found only 30 studies; we found 175.
Diagnostics have improved tremendously in the last decade and they've become more widely-available, so there's much better information now on the role of norovirus as the cause of diarrheal disease.
So there's a lot more information out there on norovirus, but there's still a lot to be learned, particularly regarding the burden of disease in poorer countries.
Were there any other important findings?
We found norovirus is an important cause of diarrheal disease both in poorer countries as well as developed regions, highlighting that it's a public health concern globally.
Does that differ from other types of viruses?
It's similar for other viruses like rotavirus, which also has a global burden of disease and affects nearly all countries; but it does differ from certain bacteria and parasites, which are much bigger problems in developing countries, and are controlled, to an extent, by improvements in water, sanitation, and hygiene.
What prevention tools should be adopted to prevent norovirus infections?
Hand hygiene may help preventnorovirus. Disinfecting areas when someone has become ill, and, for example, has vomited and contaminated an area around them, is an important way to stop onward transmission.
Good food safety practices in the kitchen are also important. Infected food handlers can transmit norovirus, so food handlers should use best practices, and should not work while ill, and can still shed virus for after they recover.
What are CDC’s plans for the future with regards to noroviruses?
We will continue our work measuring the burden of disease, which can help identify which groups might be appropriate for vaccination. We'll continue monitoring norovirus strains and the evolution of the virus, and we'll also be keeping a close eye on vaccine development, as well as other interventions that could be used to control norovirus.
Where can readers find more information?
- Global prevalence of norovirus in cases of gastroenteritis: a systematic review and meta-analysis, Sharia M Ahmed MPH, Aron J Hall DVM, Anne E Robinson BA, Linda Verhoef PhD, Prasanna Premkumar PhD, Umesh D Parashar MBBS, Marion Koopmans PhD, Dr Benjamin A Lopman PhD, The Lancet Infectious Diseases - 27 June 2014, DOI: 10.1016/S1473-3099(14)70767-4, http://www.thelancet.com/journals/laninf/article/PIIS1473-3099%2814%2970767-4/abstract
About Dr Benjamin Lopman
Dr. Ben Lopman is an epidemiologist with the Viral Gastroenteritis Team, Division of Viral Diseases at the US Centers for Disease Control and Prevention (CDC). He is an Adjunct Assistant Professor at the Emory University Rollins School of Public Health.
Dr. Lopman's research is directed at understanding the burden of disease, epidemiology and transmission of viral gastroenteritis (mainly norovirus and rotavirus) as well as developing effective methods for their control. He has authored over 100 peer-reviewed publications in addition to invited editorials, book chapters, and conference presentations.