5.1 Available diagnostic tests

Diagnostic testing available for norovirus includes EM, RT-PCR and ELISA tests. Tests performed will depend on what is available in laboratories in each state and territory.

RT-PCR is regarded as the preferred diagnostic method. It is commonly used and a result can be provided within 24 hours. However, RT-PCR testing requires skilled technicians and specialised facilities.

EM is not widely used in laboratories in Australia, as sensitivity is limited because the virus can be present in low numbers and may be missed by microscopy. Despite this, EM can be a useful diagnostic tool where specimens are negative by RT-PCR and antigen detection.

ELISA kits allow for rapid detection of the virus in faecal specimens, although they are less sensitive than RT-PCR. However, their ability to detect a range of genotypes, availability of results within 24 hours, ease of use and cost effectiveness make them useful as an initial tool for confirming norovirus as the aetiology in outbreak settings [23, 84].

Serology is not diagnostically useful because antibody presence does not always correlate with protection from re-infection [18, 85].

5.1.1 Determining the strain

Genotyping using polymerase and capsid sequences is the universal method used for determining the strain of norovirus. The Health Protection Agency of the United Kingdom (UK) manages a norovirus database for polymerase and capsid gene sequences and epidemiological data (see the United Kingdom’s Health Protection Agency website: <http://www.hpa.org.uk> and search for ‘Norovirus Molecular Epidemiology Database’). The database may be searched by entering ‘polymerase gene sequences’ and it is planned to add a search facility for ‘capsid sequences’ [85].

Because PCR testing varies in sensitivity by genogroup, there is a need for national collaboration to identify the optimal primers, protocols and reference reagents to better define the epidemiology of norovirus in Australia.

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5.2 Types of specimens to be collected

Submitting clinical specimens is an important part of an outbreak investigation because knowing the causative pathogen can help to determine control measures. Specimens should be collected as soon as possible after symptoms begin, preferably during the symptomatic phase of the illness. However, positive results from RT-PCR tests have been detected in asymptomatic individuals and in some cases 10 days after onset of illness.

5.2.1 Faeces

Faeces are the most suitable specimens to collect, as norovirus can be detected in faeces by the three diagnostic tests commonly used. Faeces should also be tested for bacterial, parasitic and other viral pathogens that cause gastrointestinal illness to exclude these pathogens as the causative agent. However, where patients’ illnesses are consistent clinically with norovirus and the outbreak has occurred in a setting commonly affected by norovirus, norovirus tests should be conducted first. If detected there is no need to conduct further testing of faecal specimens for other pathogens.

5.2.2 Vomitus

Norovirus can be detected in vomitus, but this should only be collected after consultation with nominated laboratories. The yield of virus is better from faeces than vomitus, making it preferable to obtain faecal specimens. If testing by RT-PCR or by antigen detection, faeces are the preferred specimens.

5.2.3 Food, environmental and water

Detection of norovirus in food samples is technically difficult, expensive and is not routinely performed in laboratories. There has been some success testing molluscs by two methods: macerating the shellfish flesh or by depurating the shellfish in water and then concentrating the water for examination [44, 86]. If there is strong epidemiological evidence linking a suspected food to the outbreak it may be possible to test food samples. Testing of foods is usually only warranted where the implicated food is widely distributed and may be the cause of geographically dispersed outbreaks. Given the complexity of testing food, it is inappropriate to test foods contaminated by food handlers that have caused localised point source outbreaks.

If PHUs or other agencies involved in an outbreak investigation decide that it is important to confirm the presence of viruses in food, they will need to discuss with laboratories in their jurisdiction about the collection of appropriate food samples. Similarly, where investigators suspect waterborne or environmental contamination, it is important to discuss collection of environmental and water samples with the laboratory. In general, it is best to ensure that duplicate samples are collected and at least one sample sent to a laboratory that is skilled at testing these technically difficult samples for noroviruses.