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How to Investigate Unexplained Respiratory Disease Outbreaks (URDO)

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., occur within 2 weeks) and location (e.g., linked by institution, affiliation, exposure or small geographic area).

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

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

Which Outbreaks Should be Investigated?

When deciding how to respond to a respiratory disease outbreak, public health agencies 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 of the outbreak 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 for which identification of the causative agent or potential dual infections is needed, determined a priori
  • Outbreaks which 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 which have generated excessive public anxiety
  • Outbreaks which are either very large or rapidly progressing

Tools for Epidemiological Investigation of Outbreaks

  1. Start to collect information: Respiratory Outbreak Survey Learn more about Adobe Acrobat Reader (52K, 1 page)
  2. Compile available data
    1. Develop case definition Learn more about Adobe Acrobat Reader (62K, 2 pages)
    2. Complete line list Learn more about Adobe Acrobat Reader (132K, 2 pages)
    3. Generate an epicurve
    4. Collect and store available clinical and pathologic specimens
    5. List current control measures implemented to date, if any
  3. Develop public health response to outbreak. Consider:
    1. Number of cases and severity of disease
    2. Need and potential for interventions (e.g., cohorting, quarantine, vaccination, use of prophylaxis, elimination of a potential source of disease)
    3. Likelihood of natural versus intentional source of infection
    4. Level of public health, provider or community concern
  4. Recommended reporting and response
    1. Notify appropriate local and state public health officials
    2. Discuss with staff in your program (e.g., laboratory, epidemiology, environmental and veterinary and other personnel as appropriate)
    3. If indicated, notify CDC of the outbreak—Report an Emergency.

Data Collection Forms

Data Collection Forms
Note: This form is 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.

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