one of the most common causes of acute gastroenteritis (18% of global cases), and presents as sudden onset vomiting and diarrhoea
it is highly contagious and is more common in winter
outbreaks occur in residential care facilities, hospitals, schools and child care centres
Virology
non-enveloped RNA Caliciviridae virus
noroviruses can genetically be classified into five different genogroups (GI, GII, GIII, GIV, and GV), which can be further divided into different genetic clusters or genotypes.
Groups 1, 2, 3 and 4 last shared a common ancestor in 867 AD
it can survive for months, maybe even years in contaminated still water
it can survive for weeks on most surfaces
it is destroyed when food is heated above 75deg C
Human genetic factors
a non-functional fucosyltransferase FUT2 provides high protection from the most common norovirus strain, GII.4.
homozygous carriers of any nonsense mutation in the FUT2 gene are called non-secretors, as no ABH-antigen is produced
20% of Caucasians are non-secretors due to G428A and C571T nonsense mutations in FUT2 and therefore have strong protection from GII.4
Incubation period
24-48hrs
Spread
only need 5-20 virus particles to cause an infection!
when a patron at a restaurant vomited on the floor which was quickly cleaned up, the attack rate was 90% at her table, 70% at the adjacent table and even 25% at the most distant table!
direct contact (or indirect contact via fomites) of vomitus or stool (eg. shaking hands, sharing drinks)
airborne particles when people vomit (eg. after toilet flush or vomit onto floor)
eating oysters and other shellfish which have lived in water contaminated with norovirus
eating food that has been handled by infected persons eg. salads
Clinical features
sudden onset vomiting and diarrhoea
may also have nausea, fever, abdominal cramps, headache, myalgias
recovery usually occurs within 1-3 days but globally causes 200,000 deaths pa mainly in the developing world, and mainly in the very young, the elderly or the immunocompromised
patient remains infectious from onset to at least 48hrs after resolution although some can remain up to 2 weeks after recovery