FAQ: Norwalk and Noroviruses
Responses verified by Allison McGeer, MD
Mount Sinai Hospital
» 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
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
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
» How are Noroviruses
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
» What are the Infection
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
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
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.
» Where can I get more
information about Norwalk Virus?
This website has been made possible through an unrestricted educational grant from
Pfizer Canada Inc.