Case Detection Outbreak Detection and Outbreak Characterization

Michael M. Wagner

RODS Laboratory, Center for Biomedical Informatics, University of Pittsburgh, Pittsburgh, Pennsylvania

Louise S. Gresham

San Diego County, Health and Human Services Agency and Graduate School of Public Health, Epidemiology and Biostatistics, San Diego State University, San Diego, California

Virginia Dato

Center for Public Health Practice, University of Pittsburgh, Graduate School of Public Health, Pittsburgh, Pennsylvania


In Chapter 2, we saw how some well-known outbreaks first came to the attention of investigators and how the investigators proceeded to elucidate the causative biological agent, source, route of transmission, and other characteristics of the outbreaks. In this chapter, we examine in detail how biosurveillance systems detect and characterize outbreaks. We introduce an important distinction between case detection and outbreak detection. We discuss outbreak characterization, which is the process by which investigators elucidate characteristics of an outbreak that are important for disease control (e.g., causative biological agent, source, and route of transmission). We break down and analyze biosurveillance in this manner so that it can be modeled more formally, which is a prerequisite to providing computer support. Table 3.1 summarizes the topics of this chapter and links them to examples in Chapter 2 and additional examples introduced in this chapter (marked by bold type). The table serves as both an outline and a summary of the chapter.


The objective of case detection is to notice the existence of a single individual with a disease. We say that this individual is a case of the disease. The importance of case detection is that detection of an outbreak typically depends on detection of individual cases (Figure 3.1).

Many entities are involved in case detection. People (e.g., physicians, veterinarians, nurse practitioners, infection control practitioners, medical examiners, and pathologists) and laboratories detect cases. Biosurveillance organizations detect cases through surveillance systems and screening programs. Increasingly, computers detect cases.

Some methods of case detection use case definitions, a written statement of findings both necessary and sufficient to classify a sick individual as having a disease or syndrome (Figure 3.2). More commonly, however, the determination of whether an individual has a disease (or syndrome) is left to the expert judgment of a clinician.

2.1. Case Detection by Clinicians

Case detection by clinicians (physician, veterinarian, nurse practitioner, pathologist) is a by-product of routine medical and veterinary care. It works as follows: A sick individual seeks medical attention or is brought to a clinician, who establishes a diagnosis. If the diagnosis is considered a notifiable disease1, the clinician reports it (for more information on notifiable diseases, see Chapter 5). If the sick individual is a person, the clinician reports the case to a state or local health department. If the sick individual is an animal, the clinician reports the case to a state department of agriculture (see Chapter 7). This mechanism of case detection played a role in the detection of every outbreak described in Chapter 2, with the exception of cryptosporidiosis.

A clinician establishes a diagnosis by collecting and interpreting diagnostic data, including symptoms, physical observations (e.g., rash or temperature), risk factors for disease (e.g., travel to a foreign country), pre-existing diseases in the individual (e.g., diabetes), results of microbiological tests, radiographic examinations, and autopsy findings. The interpretation of diagnostic data is a complex cognitive activity. The clinician first generates a differential diagnosis, which is a list of diseases that the patient could have given the information the clinician has thus far. The clinician then resolves the differential

1 A notifiable disease is a diagnosed condition for which a health statute requires reporting by physicians and laboratories. Health departments decide to make diseases notifiable based on their potential as threats to a community. We discuss notifiable diseases in Chapter 5.

Handbook of Biosurveillance ISBN 0-12-369378-0

Elsevier Inc. All rights reserved.

TABLE 3.1 Methods for Detection of Cases, Clusters, Outbreaks, and Methods for Characterization of Outbreaks




Case detection

Diagnosis by clinician*

SARS, mad cow disease, anthrax, meningitis, measles

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