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Investigate an Outbreak

Defining an Outbreak of Unexplained Respiratory Illness

For the purposes of this website, an outbreak or cluster of respiratory illness is defined as disease in excess of what is expected for a given time (e.g., within 2 weeks), within a specific location (e.g., linked by institution, affiliation, exposure, small geographic area), and for a target population (e.g., students, long-term care residents).

As additional information about a particular cluster of unexplained respiratory disease is obtained, the outbreak definition should be adjusted to incorporate the predominant clinical presentations (e.g., pneumonia, pharyngitis), severity of illness (e.g., hospitalizations, deaths), demographic profiles, illness onset dates of the cases (see Line List Template), and, if available, a comparison to the historical or baseline rates for similar disease in the community.

In certain situations a single case of unexplained respiratory disease may need to be evaluated as a possible outbreak because of the potential need for immediate public health intervention (e.g., suspect pulmonary anthrax, plague, SARS, MERS, hantavirus pulmonary syndrome).

Which Outbreaks Should be Investigated?

When deciding how to respond to a respiratory disease outbreak, public health officials must take into consideration many factors, such as the availability of resources and competing agency priorities. While each agency needs to determine the level of public health response appropriate for each outbreak, several characteristics of respiratory outbreaks typically warrant further investigation and an urgent response. The characteristics below should not be viewed as a comprehensive or definitive list, but serve as a general guide to determine which outbreaks merit further investigation.

  • Outbreaks of unknown etiology
  • Outbreaks associated with severe disease manifestations, such as need for hospitalization or death
  • Outbreaks that may be useful to answer epidemiologic, laboratory, or infection control questions
  • Outbreaks of possible vaccine-preventable diseases
  • Outbreaks associated with institutional settings or with a likely (controllable) environmental source
  • Clusters of respiratory infection potentially caused by a bioterrorism agent
  • Outbreaks among a vulnerable population
  • Outbreaks that have generated excessive public anxiety
  • Outbreaks that are either very large or rapidly progressing

See Request CDC Assistance for guidance on when and how to reach out to CDC for help investigating an unexplained respiratory disease outbreak.

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Components for Epidemiological Investigation of Outbreaks

Below are some of the common components of an epidemiological outbreak investigation. The components below should not be viewed as a comprehensive or definitive list. Additional steps may be needed depending on the circumstances in any given outbreak.

  • Start to collect information
    Begin collecting available epidemiologic, clinical, and laboratory information to help assess the situation. See Respiratory Outbreak Survey for an example on the types of information you might want to gather. The following tools will help guide the next steps:
  • Develop public health response to outbreak
    Based on the information gathered, below are additional steps that may be appropriate to gauge the scope of the outbreak and develop an appropriate response.
    • Determine the number of cases and severity of disease
    • Assess the need and potential for interventions (e.g., cohorting, quarantine, vaccination, use of prophylaxis, elimination of a potential source of disease)
    • Consider the likelihood of natural versus intentional source of infection
    • Evaluate the level of public health, clinician, or community concern
  • Report and respond
    Coordinate the public health response by
    • Discussing with staff in your program (e.g., laboratory, epidemiology, environmental, veterinary, other personnel as appropriate)
    • Notifying appropriate state public health officials
    • If indicated, notifying CDC of the outbreak by e-mailing URDO (M-F; 8 am–4 pm EST) or calling the emergency, after-hours hotline at 770-488-7100.
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Data Collection Forms

Data Collection Forms
Note: These forms are designed to assist investigators in collecting information relevant to identifying etiologies of respiratory outbreaks. These are not for reporting an outbreak of disease to the CDC. For reporting assistance, please refer to your State Health Department.

Background

Outbreaks of acute respiratory illness are common and can occur in a variety of settings ranging from communities to institutions, such as nursing homes or military barracks. Specific therapies, infection control practices, and other preventive measures may be necessary to control outbreaks. Consequently, investigation into the clinical and epidemiologic features and the etiology of outbreaks is particularly important to public health. Severe unexplained respiratory outbreaks may be of particular importance to rule out avian flu or new health threats.

The predominant clinical respiratory syndromes addressed by this website include outbreaks of

  • Prolonged paroxysmal cough
  • Bronchitis
  • Pneumonia
  • Influenza-like illness
  • Acute respiratory distress syndrome or rapidly progressive pneumonia

Despite the continual development of new diagnostic techniques, the etiology of many acute respiratory illnesses may not be determined due to various circumstances, including lack of staff or funding, or diagnostic tests. Investigation of these unexplained respiratory outbreaks may identify new infectious agents that require unique strategies for control and prevention (e.g., hantavirus pulmonary syndrome, SARS); however, outbreaks investigations for previously identified etiologies are equally important. This process can be enhanced through coordinated efforts that

  1. Alert clinicians to identify and report cases and clusters of unexplained infectious diseases
  2. Prepare health departments to investigate these clusters
  3. Guide laboratory scientists to develop and apply both traditional and novel diagnostic approaches to define the organism-specific etiology of the outbreak
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