6.1 Outbreak criteria

Defining an outbreak assists those responsible for managing the outbreak to decide whether an outbreak may be occurring, and to report it as early as possible to a PHU and other responsible authorities. The PHU will need to verify that the report does constitute an outbreak, which should be based on more cases occurring than would be expected in a defined time period. If a decision cannot be made as to whether the occurrence of cases represents an outbreak at the time the report or notification is received, additional information will need to be collected. The clustering of cases by time, person and place may signal the possibility of an outbreak.

A case definition is a set of criteria for determining who should be classified as a case. The outbreak management team forms a case definition once an outbreak has been declared. The case definition includes four components:
  1. well-defined clinical symptoms (with or without laboratory confirmation)
  2. information relating to time (timing of onset of symptoms)
  3. persons affected
  4. the place or location where the outbreak has or is occurring.
The case definition should be established early in the outbreak and may be revised or updated during the course of the investigation. Case definitions are important to allow early identification of cases and implementation of control measures and limiting the risk of transmission.

The definitions of what constitutes an outbreak and an outbreak-related case may differ for norovirus spread from person-toperson as opposed to that due to contaminated food or water. The definitions are only intended to be used as a guide and will vary according to the cause of the outbreak

6.2 Definition of a clinical case

Outbreaks can be defined initially on clinical diagnosis and subsequently confirmed by laboratory diagnosis once an outbreak has been declared and faecal specimens have been collected for testing. An outbreak is decided based on the initial clinical symptoms of cases, which allows for early detection and therefore a rapid public health response. Key indicators of a norovirus outbreak may be the sudden onset of vomiting or diarrhoea amongst a group of people and a rapidly rising attack rate.

In the early 1980s, Kaplan developed clinical and epidemiological criteria for norovirus outbreaks, as diagnosis of norovirus in the laboratory was very insensitive. Kaplan proposed that outbreaks of gastroenteritis could be identified as norovirus if they met the four criteria listed in Box 1 [87].

Box 1: Kaplan’s criteria for a clinical case definition for suspecting that an outbreak is due to norovirus
  • vomiting in >50% of affected persons
  • mean (or median) incubation period of 24–48 hours
  • mean (or median) duration of illness of 12–60 hours
  • no bacterial pathogen isolated in stool specimens
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A recent re-evaluation of Kaplan’s criteria confirmed a high specificity (99%) and moderate sensitivity (68%) in distinguishing confirmed outbreaks due to bacteria from those caused by norovirus. This recent calculation is in line with the original estimations [87]. Kaplan’s criteria can be used early in an outbreak to define cases. However, it is necessary to obtain faecal specimens for laboratory testing to exclude bacterial aetiology and to confirm the diagnosis of norovirus. In particular, it can be difficult to estimate the incubation period of illness where person-to-person transmission is suspected.

6.3 Definition of an outbreak

A small percentage of people have gastrointestinal problems for other reasons, particularly in ACFs, making it difficult to determine if the level of illness initially reported in a ‘suspected outbreak’ is above the normal ‘background level’. The outbreak definitions are intended to be sensitive for identifying gastrointestinal outbreaks. They should be used as a guide and a PHU will need to decide if a report truly represents an outbreak based on information collected. Generally, the mode of transmission is considered unknown at the commencement of an outbreak investigation and a general definition should be used.

6.3.1 Gastroenteritis outbreak caused by person-to-person transmission

In general, a practical definition for an outbreak suspected to be caused by person-to-person spread is ‘two or more associated cases of diarrhoea and/or vomiting in a 24 hour period’ (excluding cases which have a known cause, e.g. bowel disease, alcohol, or pregnancy).

Alternatively, an outbreak can be defined as ‘two or more cases of diarrhoea and/or vomiting in a defined time frame in a setting that is prone to outbreaks of norovirus, such as hospitals or ACFs.’

A study of norovirus outbreaks in health care settings in the UK used a broad clinical outbreak definition and considered cases to be part of the same outbreak if they occurred within seven days of each other [88].

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6.3.2 Gastroenteritis outbreak caused by foodborne or waterborne transmission

Foodborne or waterborne outbreaks may be defined as ‘two or more associated cases of diarrhoea and/or vomiting caused by the consumption of common source of food or water within a specified time frame’. Often it is difficult to identify if contaminated food or water are the vehicle of infection until epidemiological or microbiological investigations have been conducted.

6.4 Definition of a case

What constitutes a case is usually defined once an outbreak of gastroenteritis has been declared. Separate definitions may be used for suspected and confirmed cases.

6.4.1 Suspected case of norovirus

A person from the population at risk (e.g. institution, community group, restaurant patron) with clinical symptoms from a defined time period, characterised by:
  1. three or more loose stools or bowel movements in a 24 hour period that are different from normal AND/OR
  2. two or more episodes of vomiting in a 24 hour period.
People reporting these symptoms who have a known alternative cause for their illness, such as bowel disease, excessive alcohol intake or pregnancy, should not be considered suspected cases.

For the purpose of control in the early part of an outbreak, suspected cases are regarded as potentially infectious until proven otherwise (i.e. alternative pathogen demonstrated to have caused illness) or until 48 hours has elapsed after resolution of symptoms.

6.4.2 Confirmed case of norovirus

Confirmed cases of norovirus infection must meet the suspected case definition above along with a positive laboratory test fromone of the following definitive diagnostic tests:
  1. detection of human norovirus by antigen detection
  2. detection of human norovirus by Nucleic Acid Assays (NAAs)
  3. visualisation of norovirus by EM.