Norovirus
Approximately 179 million cases of acute gastroenteritis (AGE) occur in the United States each year. No etiology is determined in about 40% of cases. In the remaining 60% of cases where an etiology is determined, almost 90% are caused by norovirus. Other reported microorganisms, in order of decreasing prevalence, include Shigella, Salmonella, Shiga toxin producing Escherichia coli and rotavirus.
Most norovirus outbreaks occur in winter months, between October and March. In contrast, outbreaks by the other organisms occur in the spring or summer months. Outbreaks often occur in nursing homes, other long-term care facilities, childcare centers, cruise ships, hospitals and schools. Most cases of norovirus involve adults >49 years of age. Approximately 70% of norovirus cases occur in females and 30% in males. This female predominance is most likely due to the fact that 70% of residents living in nursing homes and other long-term care facilities are female.
Sources of infection are highly variable & include contaminated drinking water, swimming pool water, ice cubes, or almost any food substance including cooked meat, salads, and fresh fruit. Undercooked contaminated shellfish is likely a major source of transmission. The initial infection generally results from a common contaminated source, with secondary infections resulting from person to person transmission. Norovirus has a low infectious dose and a very high attack rate.
The incubation period of norovirus is generally 18 to 72 hours, with symptoms including malaise, myalgia, and abdominal cramps. Vomiting and diarrhea are the most commonly reported symptoms. Vomiting is more commonly associated with norovirus infections than the other microorganisms. Vomiting is more common in children, while watery, nonbloody diarrhea is more common in adults. Symptoms may persist for 48 to 72 hours. Treatment is supportive, and aimed primarily at preventing dehydration, as no specific antiviral therapy is available.
In the United States, Norovirus causes 70,000 hospitalizations and 800 deaths per year. Individuals over the age of 65 years are most likely to be hospitalized.
Although NV is becoming more prevalent, neonates and very young children are more likely infected by rotavirus or adenovirus, particularly during the winter months. Rotavirus is transmitted primarily by the fecal-oral route, generally without an identifiable common source. Regional outbreaks usually persist for 4-5 months. Susceptibility to rotavirus is thought to be lifelong, with the most severe disease occurring during an infant’s primary infection. Adenovirus is another prominent pathogen in infantile diarrhea, and symptoms may persist for one to two weeks after infection.
Most Shigella outbreaks involve children under the age of 10 years attending childcare centers. Shigella is more often associated with fever and bloody stools than norovirus. Person to person transmission is common during Shigella outbreaks. Adults over the age of 65 years are much more likely to be hospitalized than children. Antibiotics are frequently prescribed for Shigella infections, but multidrug resistance is common, limiting antibiotic treatment options.
Prevention and control of person-to-person gastroenteritis outbreaks depend primarily on proper hand hygiene and isolation of ill persons.
