What is Norwalk Virus?

This page was reviewed or revised on Thursday, August 13, 2009.

Norwalk and Norwalk-like viruses is implicated as the etiologic agent in about 1/3 of the non-bacterial outbreaks of gastroenteritis. Norwalk viruses are more commonly associated with outbreaks of gastroenteritis in nursing homes, homes for the aged and adult institutions. Enteric outbreaks in children's facilities like hospitals, daycares and hospital nurseries are more commonly caused by a Rotavirus. However both can be the causative agent in outbreaks of vomiting and diarrhea in both age groups. Both these viral conditions can occur year round. Incidence is higher in the fall and winter.

Norwalk induced gastroenteritis is usually a self-limiting, mild to moderate disease. Clinical symptoms include nausea, vomiting, diarrhea, abdominal pain, headache, malaise, low grade fever or a combination of these symptoms. A sudden onset of symptoms is not uncommon, with no previous feelings of being "unwell". Gastrointestinal symptoms usually last 24 to 48 hours. Man is the only known reservoir. Mode of transmission is not definitely known. The faecal-oral route is considered the most probable. Direct airborne transmission from patient to patient and transmission by fomites has been suggested to explain the rapid spread through hospitals and institutions. Food borne and waterborne outbreaks have also been identified. The organism is very contagious. Small numbers of the virus can cause illness. The more compromised the patient is, the more vulnerable.

The patient remains communicable during the acute stage of the disease and up to 48 hours after the diarrhea stops. Short term immunity lasting 14 days has been demonstrated but long term immunity is variable. Laboratory confirmation is done through direct Electron Microscopic examination of the patient's stool. IEM (Immune electron microscopy) can only be performed when the virus is detected in the stool specimens and when acute and convalescent sera are collected from patients. Isolation of the virus in stool is directly reflected on how quickly a specimen can be collected. The success of isolating the virus is increased if a specimen is collected as soon after the onset of symptoms as possible. The numbers of virus present are greatly decreased after each episode of diarrhea.

The disease is self-limiting and treatment is not usually required. Fluid and electrolyte replacement may be necessary in severe cases.

Interruption of the spread in an institution can prove difficult. Effective control measures should stress:

• ILL persons placed on enteric precautions
• ILL staff are to be excluded from work until they are symptom free for 48 hours
• rigorous attention to handwashing must be reinforced in both patient and staff populations
• regular disinfection of washrooms and all hand contact surfaces as needed
• encourage visitors to wash their hands before and after visiting the facility

Physicians and Institutions are reminded that epidemic diarrhea is reportable to the Health Unit under the Health Protection and Promotion Act.