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Mount Sinai Hospital is a University of Toronto patient care, teaching, and research centre.
Mount Sinai Hospital is a University of Toronto patient care, teaching, and research centre.

Frequently Asked Questions

» What is Norwalk Virus?

Norwalk virus is 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.

The Norovirus group were first identified as the cause of a primary school outbreak of vomiting/diarrhea in Norwalk, Ohio during the early 1970's. The group consists of related viruses that share similar symptoms and epidemiology. This virus group causes viral gastroenteritis predominately during the winter in temperate climates.

The Noroviruses are small, round, structured, non-enveloped viruses which are 27 to 32 nm in diameter. Their genome consists of a single 7.5 kb positive strand of RNA. The protein coat is composed of a 60 kDa structural protein. The virus is resistant to pH levels between 5 to 10, acid, ether, and is capable of surviving temperatures of 60C for 30 minutes.

» Where are Noroviruses Found?

Noroviruses are only capable of infecting humans. Outbreaks of Norwalk virus frequently occur in institutional settings such as schools and nursing homes, where infection is passed person to person. They also occur in group settings such as banquet halls, cruise ships, dormitories, and campgrounds, where outbreak sources are usually contaminated food and/or water. Potential sources of contaminated water include municipal water supplies, recreational lakes, swimming pools, and water stored on cruise ships. Potential sources of contaminated food include fruits, salads, and shellfish such as clams and oysters, which are contaminated by water at source. Food that has been handled by infected individuals may also become contaminated and cause outbreaks.

» What are the Symptoms of Norwalk and Noroviruses?

Norwalk virus causes viral gastroenteritis, which is also known as acute nonbacterial gastroenteritis, food poisoning, food infection, stomach flu, and winter vomiting disease. A Norwalk virus infection results in illness 24 to 48 hours after exposure, and symptoms last from 12-48 hours. Illness is characterized by the abrupt onset of vomiting and/or non-bloody diarrhea; abdominal cramps are common. 25-50% of patients report headache, nausea, malaise, myalgias and low-grade fever. It is important to note that there are other organisms such as rotaviruses which also cause viral gastroenteritis. Unlike rotaviruses, Norwalk virus infection is more frequent in older children and adults than in infants and toddlers.

» How are Noroviruses Transmitted?

Norwalk and Noroviruses are transmitted primarily by the fecal-oral route. Having direct contact with vomit or feces from an infected person and then transferring the virus to the mouth from the hands primarily spreads the viruses. Secondary person-to-person transmission through sharing food, water, and/or utensils, or contact with the contaminated environment is also possible. Touching a contaminated door knob and/or stair railing and then touching your mouth is a possible method of transmission. In some outbreaks, there is evidence that the virus may be spread by the airborne route. These viruses are very contagious and only a few are needed to cause an illness.

The disease is transmitted through ingestion of contaminated foods and drinks, or by contact with an infected individual. Shellfish from waters exposed to raw sewage will concentrate virus particles from the water since they are filter feeders. Contaminated water can also introduce the virus to beverages, ice, eggs, fruits, and salad ingredients. Cooked foods may also be contaminated though handling by an infected individual. The virus can also be transmitted through close contact with an infected individual and/or touching contaminated surfaces.

» What are the Infection Control Measures?

Prevention of outbreaks in institutions depends on good food handling practice, and compliance with hand washing on the part of staff. Staff who display symptoms of nausea, diarrhea or vomiting must be excluded from work, and should not return to work until 48 hours after these symptoms subside. Hospitals and nursing homes also maintain surveillance for clusters of gastroenteritis, so that more stringent precautions may be taken if there is evidence of transmission within the facility.

Healthcare professionals who take care of patients with symptoms of Norwalk virus should wear a gown and gloves. Special attention to handwashing (with soap) must be practiced by everyone. Patient movement through the hospital should be minimized.

» What are the Personal Preventative Measures?

Immunity to Norwalk and Noroviruses is strain-specific and temporary, so everyone should take precautions to prevent the spread of Norwalk virus to others.

Frequent hand washing with soap and running water is important, especially after visits to the washroom and before handling food. Prompt disinfection of contaminated surfaces and soiled clothing will also help prevent the spread of the disease. Regular household bleach based cleaners are effective.

Thorough cooking destroys Norwalk virus, so make sure the food you eat is cooked completely. Remember that the virus can survive for 30 minutes at 60°C. If you suspect that the water supply is polluted while travelling, choose to drink pasteurized milk or bottled beverages without ice.

» What Treatments are Available?

There is currently no vaccine available for prevention of Norwalk virus. Since Norwalk virus is a virus, antibiotics are ineffective against it.

Serious illness is rare, and is usually associated with dehydration and/or electrolyte imbalance in elderly, disabled, or immunocompromised individuals. Maintaining adequate fluid intake to replace losses from vomiting or diarrhea is important. Drink plenty of fluids, such as water and juices. The use of oral rehydration solutions (ORS) may be recommended by physicians. These are available in pharmacies without a prescription.

» Who is at Risk of Norwalk Virus Infection?

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

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