Recent Norovirus Gastroenteritis Activity

The South Eastern Sydney Illawarra Public Health Unit has recently received increased notifications of outbreaks of gastroenteritis, which are frequently caused by Norovirus. Noroviruses are RNA viruses that belong to the family Caliciviridae, and are a very common cause of gastroenteritis in both adults and children. During January, February and March, the Public Health Unit was notified of outbreaks of gastroenteritis at a childcare centre, a school, a hospital, a nursing home and groups of cases on board three cruise ships.

You may already be familiar with norovirus under one of its previous names:

• Norwalk virus (after an outbreak in Norwalk, Ohio in 1968)
• Norwalk-like virus (NLV)
• Calicivirus
• Small round structured virus (SRSV)

There are at least four genotypes which are further divided into a number of genetic clusters.

Epidemiology

The mechanisms of infection include:

• direct person-to-person spread
• aerosolisation of virus particles from vomitus
• consumption of contaminated food or water

Norovirus is highly infectious. An inoculum of only ten virus particles may be enough to infect a person.

Clinical Features

The incubation period typically is 24-48 hours but can extend outside this range (12-72 hours). The clinical features are typical for a viral gastroenteritis:

• Nausea
• Vomiting
• Abdominal cramps
•  Watery diarrhoea
• Systemic features such as fever, headache and myalgia can also occur

Do not be discouraged from a diagnosis of norovirus gastroenteritis if all of these features are not present. It is quite common for a person to have a norovirus infection with only nausea.

The natural history tends to be one of complete recovery within 24-60 hours. However dehydration, amongst the very young and the elderly, may necessitate hospitalisation. Please be aware that individuals can continue to shed virus particles in their stool. Therefore it is important to emphasise to patients to continue good hygienic practices even after recovery. One study (Rockx et al) demonstrated that 26% of individuals continue to shed norovirus up to three weeks after the onset of illness.

Diagnosis is generally undertaken in specialised laboratories using either an enzyme immunoassay ora a reverse transcriptase polymerase chain reaction (RT-PCR) on stool specimens.
 

Treatment 

There is no antimicrobial treatment for norovirus infections. There is no long-term immunity to noroviruses due to the heterogeneity of norovirus strains and the lack of a vaccine.
 

What to do if you suspect a norovirus gastroenteritis outbreak

Outbreaks of gastroenteritis must be notified, irrespective of the pathogen. Contact your local Public Health Unit if you have a suspected or confirmed norovirus outbreak. 
Wherever possible, please collect stool specimens from sick individuals. 
We cannot stress enough the importance of this!

Further Information

NSW Health Viral Gastroenteritis Fact Sheet
NSW Health Multicultural Health Viral Gastroenteritis Fact Sheet
NSW Health Foodborne Illness Fact Sheet
Gastroenteritis in Children - Fact Sheet
CDC Norovirus Fact Sheet

References 

Rockx B, De Wit M, Vennema H, Vinje J, De Bruin E, Van Duynhoven Y, Koopmans M. Natural history of human calicivirus infection: a prospective cohort study. Clinical Infectious Diseases, 1 August 2002; 35(3): 246-53
.