Norovirus

The family Caliciviridae consists of two broad genera, Norwalk-like viruses and Sapporo-like human Caliciviruses. The ‘Norwalk like viruses’ are known as Noroviruses. They were first identified as the cause of gastroenteritis in a community outbreak in Norwalk, Ohio, in 1968. The majority of the viruses causing diarrhea affects, infants and children but Norovirus can cause disease across all age groups. They are also known to cause sporadic infections and hospital acquired outbreaks of gastroenteritis. The disease has a typical seasonal pattern and primarily occurs in winter, therefore it is known as ‘winter vomiting disease’ or ‘stomach flu’.

Structure

The Noroviruses are small round structured Viruses (SRVs) having characteristic surface morphology in the form of 32 cups or ‘calices’ formed by 90 dimers arranged in T=3 symmetry. Noroviruses are non-enveloped viruses with a single stranded RNA forming the nucleic acid core. There are more than 12 antigenic strains identified in various outbreaks which have occurred throughout the world.

Source of Infection

  • The source of infection is contaminated food and water. But outbreaks caused by person to person transmission have also been reported. Shellfish and salads are often contaminated with Norovirus. Foods other than shellfish can become contaminated through infected food handlers. 
  • Self inoculation can take place by touching contaminated surfaces.
  • One can become infected while caring for a person actually suffering from disease.  Persons such as healthcare workers or close family members of patients with gastroenteritis are particularly at risk.
  • It is important to note that the virus spreads very rapidly among closed, crowded groups such as nurseries for children, day care centers and nursing homes. Poor sanitation is critical causative agent for large outbreaks of Norovirus illness.
  • Norovirus has been identified as the cause of almost 90% of the outbreaks of nonbacterial gastrointestinal illness, especially those on cruise ships.

Symptoms of illness

The infectious dose is 10 to 100 viral particles. Mild illness with fever (< 100° F) begins approximately 12 to 72 hours after the infection is acquired from contaminated food and water. Subsequently there is diarrhea, nausea, vomiting and abdominal pain. Frequently, the illness is very mild and the patient does not require medical help due to its self limiting nature. Complications occur due to dehydration caused by the diarrhea and vomiting. Case fatalities are very rare.

Diagnosis

The diagnosis can be made via electron microscopy of the stool samples from affected patients. Research laboratories in the UK, USA and Japan have developed assays for the detection of antigens and antibodies from stool specimens. Reverse transcriptase PCRs are also available for the identification small quantities of virus. This is especially useful in epidemiological investigations to identify people who are asymptomatic carriers of the virus and shed the virus in their stool thereby acting as a source of infection. Molecular studies are also useful in identifying the point of infection, especially when it is contaminated food or water.

Control of food borne outbreaks

The Public Health Laboratory Service Working Party on Viral Gastroenteritis, UK, has proposed the following guidelines for the control of food-born outbreaks:

  • All staff members who develop symptoms of diarrhea and / or vomiting must be excluded from work for up to 48 hours after the symptoms have subsided. This is because detectable numbers of viruses are shed up to the duration of symptoms.
  • The kitchen area or other areas in the vicinity which may be contaminated either by the food or the vomitus of patients must be cleaned thoroughly and disinfected with 10,000 ppm of hypochlorite.
  • All uncooked and uneaten food must be destroyed.
  • Hand washing and other hygiene practices must be reinforced.
  • Foods such as shell fish or salads which are at high risk of contamination must be excluded from the kitchen menu.
  • The traffic to and from the kitchen should be reduced to a minimum. The kitchen should not be used as a shortcut passage to other areas especially during an outbreak.

Management of outbreaks in a healthcare setting

  • A high degree of diligence is extremely important, especially in crowded settings where early measures will help in preventing the spread of an outbreak. 
  • Both bacterial and virological investigations must be instigated at the same time. 
  • Whenever possible, the affected patients must be isolated and dedicated staff should be provided for their care, espeially hospital acquired outbreak is anticipated.
  • In healthcare settings, all staff and patients of the affected ward must be screened as asymptomatic infection is quite common with Noroviruses.
  • Appropriate hand washing practices are of utmost importance. Use of 70% alcohol can easily destroy the virus. 
  • Group activities and visits to children must be discouraged as measures to prevent the spread of infection.

Treatment

Norovirus induced gastroenteritis is a self limiting disease which subsides after 1 to 4 days. No specific treatment is therefore required for Norovirus infections. Symptomatic treatment such as rehydration and correction of electrolyte imbalance by administration of oral or parenteral fluids is the only important measure required. The search for a Norovirus vaccine is still on. Many researchers are working on different antigenic types of the virus which will offer protective immunity to high risk individuals. An intranasal and oral vaccine containing Norovirus virus-like particles (VLPs) is under trial in mice models. Recombinant virus-like particles have induced specific humoral and cellular immunity in BALB/c mice. VLPs, when administered via the intranasal route, have induced good serum antibody response as well as fecal IgA in the gastrointestinal tract. The genetic diversity of the virus is an important consideration for the development of a suitable vaccine.

Disinfectants that can be used for Noroviruses

The CDC, Atlanta, recommends the use of chlorine bleach in a concentration of 1000 ppm to be applied on nonporous hard surfaces. Phenol based disinfectants can also be used but the concentration used for destroying Noroviruses is 2 to 4 times greater than that recommended by the manufacturer. Since Noroviruses are non-enveloped viruses, Quaternary ammonium compounds should be used for surface disinfection during outbreaks. Heat disinfection (pasteurization to 60 º C) can be used for instruments which cannot be treated with chemical disinfectants.