Noroviruses are the most common causes of nonbacterial gastroenteritis in the United States. Noroviruses (also known as Norwalk-like viruses) are members of the calicivirus family and are small nonenveloped RNA viruses. They are estimated to cause 23 million cases of acute gastroenteritis in the United States each year and are responsible for 81%-90% of all acute gastroenteritis outbreaks. These outbreaks occur in a variety of settings including nursing homes, retirement centers, hospitals, restaurants, schools, daycare centers, and cruise ships. Sporadic cases are also common.

Noroviruses are extremely infectious, with as few as 10 infectious particles causing disease. Transmission can occur by person-to-person contact, fecal-oral spread, food/water contamination, and the inhalation of aerosolized vomit. Infectious virus can survive on fomites for very long periods of time. The most common mode of transmission is person-to-person contact, although the highest attack rates are seen with food-borne transmission. Outbreaks are most common during the winter months, but transmission can occur year-round. All age groups are at risk of infection, although the disease is generally more severe in the very young and very old. Breastfed infants are at lower risk of infection because secreted blood group antigens in breast milk can bind certain norovirus strains.

Symptomatic disease occurs in up to 68% of infected individuals. Symptoms appear within 12-48 hours of infection. Disease is most severe in the first 24 hours. Vomiting is common – seen in 69%-75% of symptomatic individuals – and is present early, usually resolving within the first 24-48 hours of disease. Diarrhea is present in 79%-87% of individuals and may be prolonged, lasting 5 days or more in healthy individuals. Abdominal pain or cramping is seen in 50%-96% of patients. Low-grade fever, nausea, headache, and malaise are also frequently seen. Diarrhea is non-bloody and often watery. Fecal leukocytes can be detected. Laboratory studies can show a mild leukocytosis with elevated neutrophil counts and bandemia. A mild hyperbilirubinemia can also be present.

Most cases of norovirus gastroenteritis are mild and self-limited. Death in the United States due to norovirus gastroenteritis is rare and is usually due to dehydration or electrolyte imbalance. Disease manifestations can be more pronounced in the very young and the elderly.

Immunocompromised patient considerations: Immunocompromised individuals and those with underlying chronic diseases can also have more severe disease. Norovirus infections are more severe in hospitalized patients. These groups often require more aggressive rehydration and electrolyte repletion. Individuals who have undergone organ transplant can have very prolonged symptoms lasting weeks to months.