Defining and Finding Cases

Often, the first illnesses that are recognized are only a small part of the total outbreak. Finding more persons who are ill is important to help public health officials understand the size, timing, severity, and possible sources of the outbreak.

Health officials develop a case definition to spell out which ill persons will be included as part of 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 DNA fingerprint (if the pathogen is tracked by PulseNet)

There might be several case definitions for 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.

Using the case definition, investigators search for more illnesses related to the outbreak. They do this by:

• Reviewing regular surveillance reports
• Reviewing laboratory reports to PulseNet
• Asking local clinical and laboratory professionals to report cases of the particular illness more quickly, 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 areas to watch for illnesses that might be related

Investigators watch the progression of an outbreak by keeping track of who becomes ill, when they become ill, and where they live. To help keep track of the number of illnesses over time, investigators use a graph called an epidemic curve or epi curve. The epi curve shows the number of illnesses over time. The pattern of the epi curve can help investigators decide if ill persons were exposed to the same source of illness over a short period or if the exposure to the source was over a longer time. Investigators use maps to mark where ill persons live so they can easily see whether and how the outbreak is spreading over an area or community.