What is Norovirus?
Noroviruses are positive-sense single-stranded RNA viruses belonging to the genus Norovirus within the family Caliciviridae. They are the leading cause of acute gastroenteritis (diarrhea and vomiting), or the “stomach flu”, worldwide.
Genetically, this virus is classified into at least 6 genogroups and nearly 40 genotypes based on viral protein 1 (VP1) sequences. One genotype in specific, known as genogroup II genotype 4 (GII.4), has been the major circulating genotype in the community and health-care settings since 1996 with a periodic emergence of novel GII.4 variants every 2 to 4 years.
The virus is highly contagious and a common cause of epidemics. In the USA, long-term care facilities and other healthcare facilities are the most commonly reported setting of norovirus outbreaks, followed by restaurants and catered events and schools and day-care facilities. Studies from Finland, Germany, England and Wales and Australia demonstrated similar patterns in terms of settings with hospitals, nursing homes, and other inpatient care facilities being the site for the most norovirus outbreaks. Other places affected by norovirus are military bases and cruise ships.
Who is at risk?
Norovirus is the most common cause of diarrheal cases across for all ages. It is a leading cause of diarrheal death in children under the age of 5 years, and a common cause of diarrheal death over 5 years of age. In developed countries, the elderly are at higher risk.
In low- and middle-income countries, norovirus is the first and second most common cause of diarrheal disease in the first and second year of life, respectively. In high- and middle-income countries with successful rotavirus vaccination programs, norovirus is the most common cause of pediatric gastroenteritis
Norovirus infections are associated with severe clinical manifestations and hospitalization. It is characterized by sudden onset of:
- Diarrhea (may or may not be present)
- Abdominal Cramping
- Low-grade fever
Vomiting is a cardinal symptom of norovirus. When combined with a low-grade fever, myalgia and malaise it is commonly referred to as the “stomach flu”. Diarrhea is a key symptom, however, it is not always present. These symptoms are usually self-limited lasting 2–3 days in most cases, but, the infected individual’s age affects the symptom profile. Children, for example, sometimes have a longer duration of diarrhea, while the elderly generally have less vomiting and commonly report anorexia, headache, and abdominal pain. Noroviruses are also more likely to cause severe disease, and sometimes even death, in the elderly. Elevated hospitalization and mortality rates in nursing homes during norovirus outbreak periods challenging the idea that norovirus is a mild, self-limiting illness. Of the estimated 800 annual norovirus deaths each year in the USA, 90% are persons aged 65 years or greater, while low-income countries have severe outcomes with more than 25% of deaths in children younger than 5 years being attributable to acute gastroenteritis.
Why is it so contagious?
Globally, norovirus activity peaks in the colder winter months. The virus is transmitted either directly from person-to-person, via the fecal-oral route or aerosolized vomitus. It can also be transmitted indirectly through contaminated food, water, and environmental surfaces. Noroviruses spread easily because it only requires a few viral particles to cause an infection. With each gram of stool from an infected individual containing greater than a thousand RNA copies at peak, a single infected individual has the potential to infect several hundred or even thousands of others.
The virus can also enter the body by swimming contaminated water or in water in which an infected individual is or has been swimming. It is commonly associated with eating vegetables, like lettuce or salad greens, containing contaminated water. Clams and oysters are also a source of infection because they efficiently concentrate the virus when filter-feeding in contaminated water.
Compounding this copious viral shedding is the potential for pre-, post-, and asymptomatic shedding as well as the environmental stability, resistance to disinfectants and thermal stability of the virus. These characteristics further underscore the public health challenges posed by noroviruses and the major reason why they cause outbreaks.
In most cases, oral fluid replacement with isotonic liquids should suffice in replacing body fluid losses. In severe cases, however, hospitalization with intravenous therapy may be required. Analgesics and antiemetic’s can be used to decrease pain, fever, and vomiting. The infection is normally self-limited with a full recovery in just a few days. Specific antiviral therapy is not available.
Given the morbidity, mortality and economic costs of norovirus, vaccines against this pathogen have been developed and are currently being tested. The primary control measures for outbreaks are the prevention of contamination of water and food supplies, and proper hygiene. Infected individuals should be isolated and because of the resistance of noroviruses to freezing, heating, and standard cleaning solutions, decontamination procedures should use EPA recommended disinfectants.
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