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2. Define and Find Cases

Developing a Case Definition

Example case definition of Cryptosporidium.

Health officials develop a case definition to identify which ill persons will be classified as part of the outbreak. Case definitions should be sensitive enough to include all cases, and specific enough to only identify the cases involved in the outbreak. Case definitions may include details about:

  • Features of the illness
  • The pathogen or toxin, if known
  • Certain symptoms typical for that pathogen or toxin
  • Time range for when the illnesses occurred
  • Geographic range, such as residency in a state or region
  • Other criteria, such as patterns of molecular characterization of pathogens

There might be several case definitions during an outbreak investigation, each with a different purpose. For example, one case definition might be for confirmed illnesses and another for probable illnesses. The number of illnesses that meet the case definition is called the case count. As the investigation proceeds with more information about the illnesses and outbreak, the case definition will be further refined to include compatible symptoms, laboratory confirmation, and onset dates. It is important to document the case definition development throughout the investigation. The Council of State and Territorial Epidemiologists (CSTE) has developed case definitions for many waterborne pathogens based on uniform criteria used to define a disease for public health surveillance that can be reviewed on the National Notifiable Disease Surveillance System (NNDSS) website. These case definitions may be helpful in determining if a case is confirmed, probable, or suspected in an outbreak.

Cryptosporidiosis (Cryptosporidium spp.)

Case Definition Example

Setting: Time: Onset between Jan 2012-Dec 2012 Geographic Range: Residents of State A or State B

Clinical Description:

A gastrointestinal illness characterized by diarrhea and one or more of the following: diarrhea duration of 72 hours or more, abdominal cramping, vomiting, or anorexia.

Laboratory Criteria for Diagnosis

Confirmed: Evidence of Cryptosporidium organisms or DNA in stool, intestinal fluid, tissue samples, biopsy specimens, or other biological sample by certain laboratory methods with a high positive predictive value (PPV), e.g.,

  • Direct fluorescent antibody [DFA] test,
  • Polymerase chain reaction [PCR],
  • Enzyme immunoassay [EIA], OR
  • Light microscopy of stained specimen.

Probable: The detection of Cryptosporidium antigen by a screening test method, such as immunochromatographic card/rapid card test; or a laboratory test of unknown method.

Case Classification

Probable: A case with supportive laboratory test results for Cryptosporidium spp. infection using a method listed in the probable laboratory criteria. When the diagnostic test method on a laboratory test result for cryptosporidiosis cannot be determined, the case can only be classified as probable, OR

A case that meets the clinical criteria and is epidemiologically linked to a confirmed case.

Confirmed: A case that is diagnosed with Cryptosporidium spp. infection based on laboratory testing using a method listed in the confirmed criteria.


Persons who have a diarrheal illness and are epidemiologically linked to a probable case because that individual was only diagnosed with cryptosporidiosis by an immunocard/rapid test/or unknown test method cannot be classified as probable cases. These epi-links can be considered suspect cases only.

Find Additional Cases

At the beginning of an outbreak investigation, it is important to identify as many potential cases as defined by the case definition as possible to guide determination of a common source or sources. The first illnesses that are recognized can be only a small part of the total outbreak. Identifying more persons who are ill is integral to understanding the magnitude, timing, severity, and possible sources of the outbreak for public health officials.

Using the case definition, investigators search for more illnesses related to the outbreak by:

  • Reviewing public health reportable condition surveillance reports
  • Reviewing state and national molecular typing databases
  • Asking local healthcare providers to order diagnostic testing of persons ill with the specific clinical profile, as well as to report cases as soon as they suspect the diagnosis
  • Reviewing emergency room records for similar illnesses
  • Surveying groups that may have been exposed
  • Asking health officials in surrounding jurisdictions to watch for illnesses that might be related
  • Alerting the affected community to the outbreak and asking any ill persons with similar exposures to notify the health department
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