Norwalk and Noroviruses
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The Bug

What is it?

The Norwalk virus was first identified as the cause of a primary school outbreak of vomiting/diarrhea in Norwalk, Ohio in the early 1970’s. Similar viruses were responsible for subsequent outbreaks in Montgomery County and Hawaii. These morphologically similar viruses were named after their geographic origins and are collectively referred to as small round structured viruses (SRSV) or more recently as Noroviruses.

Early attempts to replicate noroviruses in cell culture failed. To date, noroviruses are still considered non-cultivatable in any tissue culture or animal system. Early characterization of noroviruses was based on surface morphology observed from electron microscopy of fecal samples obtained from infected individuals.

The genome of Norwalk virus has been sequenced from a library of cDNA obtained from stool samples of human volunteers. The genome consists of a single positive ~7.6 kb strand of RNA which is polyadenylated at its 3’ terminus. The genome was found it contain 3 open reading frames. The first open reading frame (ORF1) encodes a polyprotein precursor to an RNA-dependent RNA polymerase. The second open reading frame (ORF2) encodes a structural capsid protein. The function of the third open reading frame (ORF3) is unknown, but is suspected to encode a minor structural protein of ~23kD.

In 1999, a research group using X-ray crystallography to characterize the protein coat of the Norwalk virus which is 27 to 32 nm in diameter. The capsid of Norwalk virus is constructed using 180 copies of a ~60 kD capsid protein, organized into 90 dimers. The capsid protein contains two principal domains linked by a flexible hinge. The S domain near the NH2–terminus is involved in forming of the icosahedral shell, and the P domain at the COOH-terminus forms the protrusion emanating from the shell. Noroviruses display antigenic and genetic diversity, which is likely the result of a variable region within the P domain of the Norwalk capsid protein.



Where is it found?

Outbreaks of Noroviruses frequently occur in institutional settings such as schools and nursing homes, where infection is passed person to person. In these cases, the virus is introduced through common-source exposure (e.g. contaminated food or drink) and spread through person-to-person transmission. Crowded quarters and reduced levels of hygiene may be a factor in the spread of Noroviruses within these institutions.

Norovirus outbreaks also occur in restaurants, banquet halls, cruise ships, dormitories, and campgrounds, where contaminated food and/or water is consumed. Contamination may occur at the food source, such as when oysters or fruits are treated with contaminated water and harvested. The virus is capable of surviving temperatures of 60°C for 30 minutes, therefore thorough cooking of food and boiling of water is recommended. Food that has been handled by infected individuals after cooking can also become contaminated and cause outbreaks. Frequent hand washing with soap is essential, especially after using the washroom and before handling food and drink.

One of the most common causes of Norovirus outbreaks is consumption of raw shellfish (oysters and clams) that has been contaminated with the virus. Shellfish from waters exposed to raw sewage will concentrate virus particles from the water since they are filter feeders. Other potential sources of contaminated food include fruits, sandwiches, eggs, salads, bakery products, and semi-liquid foods (e.g. cake frosting, sauces, salad dressings). Potential sources of contaminated water include municipal water supplies, recreational lakes, swimming pools, and water stored on cruise ships. Contaminated water will introduce the virus to beverages, ice, fruits, and salad ingredients.


How is it transmitted?

The low infectious dose of Noroviruses along with its high concentration in human stool create a situation where only a few virus particles are needed to start a large outbreak. Asymptomatic shedding has also been reported creating a situation where individuals such as food handlers could be spreading the virus without displaying any symptoms of illness. The virus particles are also environmentally stable, able to withstand freezing, heating up to 60C, chlorine levels <= 10 ppm, stomach acid, and pH levels between 5 and 10.

Norwalk and Noroviruses are very contagious and transmitted primarily through the fecal-oral route. The majority of outbreaks are caused by introduction of contaminated food. Consumption of raw oysters was responsible for several outbreaks, including one in Australia in August 1996, one in Louisiana in December 1996, one in Florida in January 1995, and two in northwestern Florida and Louisiana in November 1993. Outbreaks caused by infected water sources are caused by human waste contamination of water supplies.

Noroviruses can also be spread by secondary person-to-person transmission through sharing food, water, and/or utensils. The virus can be transmitted by close contact with an infected individual and/or touching contaminated surfaces. This method of transmission extends the duration of an outbreak, as virus particles infect new individuals after the initial exposure event. In environments such as cruise ships, enclosed living quarters and shared resources promote person-to-person transmission. Arrival of new susceptible passengers every few weeks will allow an outbreak to propagate over several successive cruises unless major corrective actions are taken.

In some outbreaks, there is evidence that Noroviruses are spread by the airborne route. Virus particles contained in vomit are released in aerosol form and are capable of contaminating surfaces over a large radius.


What diseases does it cause?

Norwalk and Noroviruses are a common cause of vomiting and diarreal illness each winter and has often been referred to as "stomach flu" or "Winter Vomiting Disease". Norwalk virus infections have been linked to outbreaks of vomiting and/or diarrhea in institutions such as child-care centres and long term care facilities as well as on cruise ships, camps, schools and households.


Who is at risk of infection?

The virus does not discriminate. The symptoms of Norwalk virus infection may be more severe in older individuals, young children and those with underlying medical conditions who may become dehydrated due to the vomiting and diarrhea.



This website has been made possible through an unrestricted educational grant from Pfizer Canada Inc.
Copyright 1999-2007 Department of Microbiology, Mount Sinai Hospital, Toronto, Canada. All rights reserved.