Norwalk and other Norwalk-like viruses cause the same types of symptoms in humans regardless of the differences in genogroups.

Infection can occur with as little as 10 to 100 viral particles. Once infected, incubation takes 24 hours, however incubation has been shown to be dose dependent ranging from 18-48 hours.

Epidemiology

Norwalk-like viruses are now classified as the leading cause of non bacterial outbreaks of gastroenteritis. They have increased in prevalence within the last few years, however it has been hypothesized that the documented rise in prevalence may be due to improvements in biotechnology. Outbreaks in the winter suggest that the virus may be seasonal. Another important feature of Norwalk-like virus outbreaks is that outbreaks affect people of all ages while Sapporo-like virus affects mostly children. Outbreaks often occur because of contaminated water supplies or food. Contaminated drinking water, commercial ice cubes, water on board cruise ships, and pools for recreational swimming have all led to outbreaks. Shellfish have been the cause of several outbreaks because they are grown in contaminated in waters. Other outbreaks have occurred through food handlers often at military or vacation camps.

Norwalk-like viruses are all faecal-orally transmitted but vomit from patients also contains virus and is a potential source of transmission. Infection with the virus causes a sudden onset of projectile and severe vomiting and diarrhoea accompanied by low grade fever. Mild abdominal cramps, malaise, headaches, and cramps can also occur. Hospitalization is rare and severe dehydration happens occasionally. Death is extremely rare and no evidence has proven a Norwalk-like virus to be the sole cause of death. Immunity to Norwalk-like viruses can wane and is rarely lifelong.

Norwalk-like viruses infects and replicates in the epithelial mucosal cells. Virus causes the villi to flatten and the microvilli to shorten.

To date there is no vaccine available due to waning immunity. Prevention of outbreaks consists of monitoring the water supply and growing shellfish in uncontaminated waters. Also keeping food handlers and healthcare workers who have been infected from working until at least two days after symptoms have ceased. Hospitals, kitchens, and rest rooms should be cleaned with hot detergent and disinfected with chlorinated solution to minimize transmission.