Outbreak identification

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Outbreak identification is the first step in an outbreak investigation. Clusters of illnesses may occur in the same location over the same timeframe, but they do not necessarily arise from a common source. Defining and understanding the expected number of illnesses (i.e., the baseline), is key to detecting clusters and outbreaks. Clusters and outbreaks may also be identified through a variety of sources including surveillance networks, public health partners, the public, and front-line health care providers. There are various ways to define clusters and outbreaks and the terminology used will depend on the organization and the specific situation (see Table 1 below).

Preliminary follow-up activities such as collecting information from cases, analyzing exposure information to identify commonalities, and liaising with public health and food safety partners may help determine if an outbreak truly exists and/or if further action is warranted. In many situations, once the information is assessed further, investigative partners may conclude that there is no public health action required.

Table 1: Cluster and outbreak definition comparisons

Organization (Document) Cluster Outbreak
Dictionary of Epidemiology, 5th edition (Porta, 2008) Aggregations of relatively uncommon events or diseases in space and/or time in amounts that are believed or perceived to be greater than could be expected by chance. Outbreak: An epidemic limited to localized increase in the incidence of a disease.
Epidemic: the occurrence in a community or region of cases of an illness, specific health-related behaviour, or other health-related events clearly in excess of normal expectancy (…relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year).
Dictionary of Public Health (Last, 2007) A collection of events such as new cases of an uncommon or rare disease that occur so closely together in space and/or time as to arouse suspicion that this is not a chance occurrence but has a cause that should be investigated. A small localized cluster of cases of a condition, usually an infectious disease. The word is sometimes a euphemism used to downplay the seriousness of an epidemic.
Guidelines for Foodborne Disease Outbreak Response (CIFOR, 2014) An unusual aggregation of cases grouped in time or space. The term is commonly used in pathogen-specific surveillance, when multiple infections caused by similar microbial strains are identified by a public health laboratory. The purpose of identifying clusters is to trigger further investigations to determine whether they might represent an outbreak. The number of cases needed to form a cluster cannot be absolutely defined; cluster definition can vary by type of agent, novelty of the subtype, season, and resources available for further investigation. Two or more cases of a similar illness shown by an investigation to result from a common exposure, such as ingestion of a common food. An outbreak is a cluster with a clear association between cases, with or without a recognized common source or known disease agent. Single cases of certain rare and serious conditions, such as gastrointestinal anthrax, botulism, or cholera, elicit an outbreak-like response.
Methods in Field Epidemiology (MacDonald, 2012) A geographical or temporal collection of cases that seem to be greater than the expected number for the given place and/or time. An increase – often sudden – in the observed number of cases of a disease or health problem compared with the expected number for a given place or among a specific group of people over a particular period of time.
Foodborne Disease Outbreaks: Guidelines for Investigation and Control (WHO, 2008) Epidemiologists may use “cluster”, “outbreak”, and “epidemic” interchangeably. Typically, “cluster” is used to describe a group of cases linked by time or place, but with no identified common food or other source. In the context of foodborne disease, “outbreak” refers to two or more cases resulting from ingestion of a common food.

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Observed vs. expected

Outbreak identification relies on the ability to detect higher than expected rates of illness. Therefore, it is important to be able to define the expected number of illnesses in a particular region over a particular period of time – this is also known as the baseline level of illness. The best way to define the baseline would be to look at data from previous years to determine how much illness you would expect to see at a particular time each year. Baseline illnesses are usually made up of sporadic illnesses, meaning that they are not linked to a common source. Many illnesses see a seasonal distribution, meaning the number of illnesses is higher in some months than others, and this pattern repeats itself from year to year.

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Information sources

There are several sources of information you can use to help identify outbreaks.

Surveillance systems routinely track and store data on the number of illnesses in a defined region. Increases in illnesses can be detected by comparing current data to historical data in the system, which acts as a baseline. For enteric illnesses, surveillance systems are usually laboratory-based, meaning they only include cases that have been confirmed to be infected with a pathogen using laboratory methods. This will be an underestimate of the true amount of disease in the region, since it will exclude milder cases that were not sick enough to see a doctor and have a sample taken.

Public health partners such as neighbouring regions, provincial/territorial/federal investigators and food safety partners may send notifications of clusters of illness or outbreaks that they are actively investigating. These notifications may suggest potential exposures to monitor, such as specific food items, animal contact, or restaurant or tourist attraction visits.

The public can report their illnesses and suspected exposures directly to public health, either as part of routine public health follow-up or as a complaint. This can be particularly helpful in identifying restaurant clusters. If the individual reports that dining partners also fell ill, a quick review of what common food items were eaten may help identify the source of illness. In certain situations, ill individuals may decide take their story to the media, therefore monitoring news reports and social media may be another useful way to identify potential outbreaks.

Health care providers such as family, emergency room, and long term care home nurses and physicians may communicate directly with public health if they notice an unusual increase in illnesses. Front-line health care providers can detect these increases faster than traditional surveillance systems, thus allowing for timelier follow-up.

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Moving from monitoring to action

After it has been determined that the reported number of cases is above baseline, the investigation can begin. This will include collecting information from cases, analyzing exposure information to identify commonalities, and liaising with public health and food safety partners to share information and coordinate action. In many situations, once the information is assessed further, investigative partners may conclude that there is no public health action required.

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Toolkit sources of public health intelligence

  • This tool provides a list of online resources that may be used for outbreak identification and notification purposes, as well as journals that publish enteric outbreak investigation findings.

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Council to Improve Foodborne Outbreak Response (CIFOR). 2014. Guidelines for Foodborne Disease Outbreak Response (2nd edition). Council of State and Territorial Epidemiologists, Atlanta, GA. Available at: https://cifor.us/clearinghouse/cifor-guidelines-for-foodborne-disease-outbreak-response

Porta, M (ed.). 2008. A Dictionary of Epidemiology, 5th Edition. Oxford University Press, Oxford, England.

Last, J.M. (ed.). 2007. A Dictionary of Public Health. Oxford University Press, Oxford, England.

MacDonald, P.D.M. 2012. Methods in Field Epidemiology. Jones & Bartlett Learning, Burlington, MA.

World Health Organization. 2008. Foodborne Disease Outbreaks: Guidelines for Investigation and Control. WHO Press, Geneva, Switzerland. Available at: https://apps.who.int/iris/handle/10665/43771 

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