Descriptive epidemiology – Outbreak Toolkit

Overview

Descriptive epidemiology describes the outbreak in terms of person, place and time. “Person” refers to socio-demographic characteristics of cases and includes variables such as age, ethnicity, sex/gender, occupation, and socioeconomic status. “Place” refers to spatial relationships that are important in describing the occurrence of illnesses and may include variables that describe clustering, rural-urban status, city, province/territory, or country. “Time” refers to the examination of when and over what time period the illnesses occur and may describe a point source epidemic, secular trends, or temporal clustering. Descriptive epidemiology forms one of the main parts of an epidemiological summary.

The goals of descriptive epidemiology in enteric outbreak investigations are:

To assess trends in health and disease: illnesses are monitored in order to identify emerging problems (e.g., potential outbreaks). Comparisons can be made among population groups (e.g., different age groups, or sexes), geographic areas, and time periods.
To identify problems and generate hypotheses (e.g., if illnesses are occurring in a specific demographic or geographic area, this could suggest initial hypotheses for the source of an outbreak). Hypotheses can then be tested using analytic methods, such as a case-control study.

Descriptive analyses

Person

People’s socio-demographic characteristics and behaviors can increase or decrease their risk for developing an illness. For example, the elderly and very young are often at elevated risk for bacterial and viral infections. Average (median), minimum, and maximum ages of cases, as well as proportions of cases according to sex and other relevant variables, should be part of any descriptive analysis. Cross-tabulations among these variables may be calculated to identify inter-relationships. Analyses of case demographics may provide insight into the source of an outbreak, for example, if the majority of cases are female, young or old, or from a specific ethnic group or religious community.

Place

Pathogens do not necessarily respect or follow political borders. An examination of the spatial associations of cases can play a key role in determining the source of the outbreak. For example, the distribution of cases amongst provinces could be a reflection of the availability of the contaminated food product (distributed in Provinces A and B, but not C). Maps may be a useful tool in describing these spatial associations.

Time

Time is important in characterizing illness to assess if incidence rates or case numbers have increased or decreased over time and if there is seasonal variation. In outbreaks, the relationship between time and the number of illnesses is graphically displayed in an epidemic curve. The nature of the outbreak can often be deduced by the appearance of the epidemic curve and may reveal whether an exposure is attributable to a point-source, a continuing common source or is intermittent. The incubation period of a particular pathogen–the time between infection and symptom onset–in any given outbreak is another important aspect of time and also will affect the shape of an epidemic curve.

Examples

Tools

Toolkit line list and data dictionary

This Microsoft Excel-based tool is designed to be used as a template for foodborne outbreak investigation line lists. Once data has been entered, common descriptive statistics are automatically calculated. A data dictionary describing each data field in the line list is available in the final tab.

Toolkit epi curve exercise

This exercise shows how to make an epidemic curve in Microsoft Excel, where each case is represented by a single box using this data set.

Toolkit pivot tables exercise

This exercise uses an outbreak line list to create PivotTables in Microsoft Excel and use them to extract information for descriptive epidemiological summaries and create epidemic curves.

Toolkit epidemiological summary template

This Microsoft Word document provides a suggested template for the content and layout of an epidemiological summary.

Toolkit outbreak investigation report template

This Microsoft Word document provides a suggested template for the content and layout of an outbreak investigation report or final epidemiological summary.