Question

1. List two to three qualities of the norovirus that make it so infectious.
2. Only approximately 70% of norovirus outbreaks meet the Kaplan criteria, and therefore it is often used in concert with other diagnostic methods. Why then are the Kaplan criteria used?
3. Give two to three reasons why diagnostic tests like RT-qPCR are typically only performed during an outbreak and not to diagnose individual cases of norovirus.
4. If only the residents and none of the staff had fallen ill, what would be your first suspicion of the cause of illness?
5. What treatment could be prescribed for individuals suffering from a norovirus infection?
6. Though the exact mechanism of a norovirus infection is not known, give a generalized account of how the norovirus could cause its signature symptoms, vomiting and diarrhea.

Answer 1

Norovirus is the diverse group with a single-stranded positive-sense RNA genome. They are non enveloped viruses belonging to the family of Calciviridae. The genus Norovirus has one species called “Norwalk virus”. They have five different genogroups that are GI, GII, GIII, GIV, GV. Geno groups GI and GII are ones which infect humans. The most variable region in the virus is the viral capsid P2 domain. It contains carbohydrate receptor binding region and antigen presenting sites.

The virus is very contagious and can spread by:

1. Eating or drinking food contaminated with the virus
2. Touching the surface or objects that are contaminated with the virus and then eating with the contaminated hands.
3. Sharing utensils with the ones which are already infected with a virus

Infected individuals shed large numbers (in billions) of norovirus particles; tiny drops of vomit would spread the infectious particles in air and land, on various objects nearby thus contaminating the surroundings badly.

A very small quantity of virus is enough to make a person sick. 10-100 viral particles are sufficient to cause infection. They are easily transmitted by oral-fecal route, or by direct person to person transmission. Vomiting and diarrhea create aerosols with highly contagious viral particles in billions which can enter in the body by the oral route. They can survive for long periods on different surfaces to cause infection and make the environment a source of infection.

Kaplan criterion to determine if the cause of the outbreak is norovirus:

1. Mean or median illness of 12 to 60 hrs
2. A mean or median incubation period of 24 -48 hrs
3. >50% of people suffering from vomiting
4. Absence of any bacterial agent

Norovirus outbreak is most likely when all four criteria are met. But these criteria are not met in 30 % of a norovirus outbreak. If the criteria are not met it does not mean the outbreak is not the result of norovirus hence the Kaplan criterion are used along with other laboratory diagnostic methods.