Noroviruses
Hardly a day goes by without another outbreak of Norovirus infection being reported in the news. That’s particularly true at this time of year, and it’s no accident that the consequences of Norovirus infection used to be known as “winter vomiting disease”, although infections occur year round. And yet, in spite of how common they are, these viruses are not household names in the same way as influenza and polio, so MicrobiologyBytes comes to the rescue!
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Historically, noroviruses have been named after the places where outbreaks occurred. By far the most famous Norovirus is Norwalk virus, named after an outbreak of acute gastroenteritis in children at an elementary school in Norwalk, Ohio, in November 1968. These viruses remained poorly understood however, since for the most part, they do not grow in the laboratory. The taxonomy of these viruses became confused and they were previously grouped on the basis of their appearance in the electron microscope, and this led to the name “Small Round-Structured Viruses”. By adding labelled antibodies and performing immune electron microscopy (IEM), it became possible to be a bit more specific which virus was being studied. However, it was not until the genomes of these viruses were cloned and sequenced in the 1990s that the situation became clearer. Based on this molecular information, Norwalk virus and its troublesome little friends became a new genus, Norovirus, in the Calicivirus family, with which they share similarities.
Noroviruses are the most commonly identified cause of infectious gastroenteritis in Western Europe. They account for at least 10% of all gastroenteritis outbreaks in Europe, and an estimated 23 million cases in the USA each year (Lopman et al. Two Epidemiologic Patterns of Norovirus Outbreaks: Surveillance in England and Wales, 1992-2000. Emerging Infectious Diseases 9: January 2003). Noroviruses are responsible for more than 85% of all non-bacterial outbreaks of gastroenteritis (Hutson AM, et al. Norovirus disease: changing epidemiology and host susceptibility factors. Trends in Microbiol. 2004 12: 279-287). Only the common cold is reported more frequently than viral gastroenteritis as a cause of illness in the USA.
The symptoms of Norovirus illness usually include nausea, vomiting, diarrhoea, and stomach cramps. Sometimes patients may also have a low-grade fever, chills, headaches and muscle aches. The illness usually begins suddenly, and the infected person feels very sick. In most healthy people the illness is self-limiting with symptoms lasting for about a few days days. In general, children experience more vomiting than adults. There are usually no long-term health effects related to this illness. However, people who are are unable to drink enough liquids to replace the liquids they lost because of vomiting and diarrhoea can become dehydrated and may need special medical attention. This can be a particular problem in tropical climates, but also in the very young, the elderly, and persons with weakened immune systems.
Norovirus particles are excreted in large quantities in the stool or vomit of infected people. This means that the virus can spread rapidly in closed environments such as hospitals, nursing homes, schools and cruise ships. Hospital ward closures due to Norovirus outbreaks are a significant problem. Infection occurs when people eat food or drink liquids that are contaminated with the virus. Fruits, vegetables and shellfish contaminated with sewage are a common way for an outbreak to start, after which the virus can spread directly when people touch surfaces or objects contaminated with the virus, then place their fingers in their mouth. Norovirus outbreaks are fought with hygiene measures such as thorough and frequent handwashing, disinfection of contaminated surfaces and objects using a bleach-based household cleaner, washing clothing or linen that may be contaminated with virus after an episode of illness with hot water and detergent.
People infected with Norovirus are contagious from the moment they begin feeling ill (as soon as 12 hours after infection) to at least three days after recovery. Some people may be contagious for as long as two weeks after recovery, allowing “silent” spread of the virus. Currently, there is no antiviral medication that works against Norovirus and there is no vaccine to prevent infection. There is some evidence that blood types B and AB may confer partial protection against symptomatic Norovirus infections (Hutson AM, et al. Norwalk virus infection and disease is associated with ABO histo-blood group type. J Infect Dis 2002 185: 1335-1337).
It’s still not clear how many serotypes of Norovirus there are, or how protective infection with one virus is against another. However it is known that immunity is not permanent and reinfections do occur. One thing is sure, Noroviruses are not going away anytime soon, so unless we find out more about them, we might come to a sticky end!
- Caliciviruses
- Wikipedia: Noroviruses
- Estes MK, et al. Noroviruses everywhere: has something changed? Curr Opin Infect Dis. 2006 19: 467-74.
Tags: Biology, Health, Medicine, Microbiology, norovirus, Science, Virology, virus
[...] started January off with noroviruses and ancient plague, then relaxed a bit by playing with Lego and brewing [...]
[...] Historically, noroviruses have been named after the places where outbreaks occurred. By far the most famous Norovirus is Norwalk virus, named after an outbreak of acute gastroenteritis in children at an elementary school in Norwalk, Ohio, in November 1968. These viruses remained poorly understood however, since for the most part, they do not grow in the laboratory. The taxonomy of these viruses became confused and they were previously grouped on the basis of their appearance in the electron microscope, and this led to the name “Small Round-Structured Viruses”. By adding labelled antibodies and performing immune electron microscopy (IEM), it became possible to be a bit more specific which virus was being studied. However, it was not until the genomes of these viruses were cloned and sequenced in the 1990s that the situation became clearer. Based on this molecular information, Norwalk virus and its troublesome little friends became a new genus, Norovirus, in the Calicivirus family, with which they share similarities. Continue reading [...]
I wish that people in the US would stop referring to Norovirus infections as “the flu.” There is actually a widespread misconception over here that a flu shot will prevent viral gastroenteritis. In fact, I had an extended argument on my blog in October, when I was ill for a week with a particularly vindictive form of viral gastroenteritis; readers kept scolding me for not having gotten a flu shot. Even in my 104-degree-fever-and-continual-running-to-the-bathroom extremis, I did my best to educate the public about what is, and isn’t, the flu. *sigh*
How long does immunity typically last? We had a bout last summer: will immunity have worn off by now?
Hi
Thank you for posting such a clear description of this very common but largely misunderstood virus. As an Emergency Planning Officer for a UK local health authority, I was involved in setting up an Incident Control Team (ICT) on several occasions in recent years to address such outbreaks. I remember that it was first described to me as a “Norwalk like” virus. Perhaps a year later, it became SRSV, “Small Round-Structured Viruses” and only latterly was it described as a Norovirus.
Establishing exactly what one is dealing with at the earliest possible stage is essential in determining an appropriate response.
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