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Embarking on an Outbreak Investigation

Learn the importance of verifying case reports, methods to determine outbreak necessity, and diagnosis confirmation techniques. Understand how to decide whether to launch an investigation and create effective case definitions for proper evaluation. Explore examples and factors influencing investigation decisions.

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Embarking on an Outbreak Investigation

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  1. Embarking on an Outbreak Investigation

  2. Goals • The goals of this presentation are to discuss: • The importance of verifying case reports • Methods to determine if an outbreak investigation is necessary • Creating and using case definitions

  3. Verify the Diagnosis • Before launching a full investigation, verify: • Signs • Symptoms • Test results • Ways to reduce diagnosis error • Confirm appropriate lab tests were performed • Confirm symptoms were reported accurately • For rare conditions, educate clinicians

  4. Diagnosis in an Outbreak • Patients may present with a known/highly suspected agent • Verify with standard lab test • Not every case needs to be lab-confirmed • Example: Listeriosis

  5. Diagnosis in an Outbreak • Patient may present with an unknown agent but with characteristic symptoms • Identify probable agent based on: • Signs and symptoms • Age of patients, season, incubation period • Lab results • Example: gastrointestinal illness • No outbreak exists if cases result from different agents

  6. Diagnosis in an Outbreak • If cases have a common link or are the same illness, you can investigate without knowing the agent • If cases do not appear to be related or share a common exposure, you may not want to proceed with an investigation

  7. To Investigate or not to Investigate • Consider the following factors when deciding whether or not to investigate an outbreak • It could be “true” outbreak with common cause • It could be unrelated cases of the same disease • Severity of illness • Transmissibility • Local politics • Public concern • Available resources

  8. To Investigate or not to Investigate • Key deciding factor is often if there are unusually high numbers of cases • “Unusually high”=more cases than expected • This depends on the disease: • Multiple cases of respiratory illness in grade school during winter may be usual • Single case of botulism or anthrax is more than expected

  9. To Investigate or not to Investigate • How do you determine if you have more cases than expected? • For notifiable diseases • Cases are reported to health department • Compare number of current reports with previous weeks • Compare number of current reports with same time period or season in previous years

  10. To Investigate or not to Investigate • How do you determine if you have more cases than expected? • For non-notifiable conditions: • Check hospital discharge records, mortality data, cancer registries, birth defect registries or other available records • Use data from neighboring areas • Call local health care providers • Call community members

  11. Case Definitions • A case definition • Allows a simple, uniform way to identify cases • “Standardizes” the investigation • Is unique to outbreak but is based on objective criteria

  12. Case Definitions • Always includes: Person, Place and Time • Person: relevant information about personal characteristics • Place: information about where the exposure is thought to have occurred • Time: dates during which exposure was thought to have occurred

  13. Case Definitions • Can emphasize sensitivity or specificity in case definition • Usually emphasize sensitivity early in investigation • Can narrow case definition as more information is obtained

  14. Case Definition • Example: Listeriosisoutbreak • Person: mother of a stillborn or premature infant infected with Listeria or a pregnant woman/mother with febrile illness • Place: lived in Winston-Salem, North Carolina • Time: October 24, 2000-January 1, 2001

  15. Case Definition • Example: Salmonellosis outbreak • Person: Has culture-confirmed Salmonella enteriditis, is a North Carolina resident • Place: North Carolina • Time: July 1, 2001-September 1, 2001

  16. Case Definition • Categories of cases • Confirmed • Symptoms characteristic of the agent • Lab test • Epidemiologic link • Probable • Symptoms confirmed • No lab or epidemiologic link • Suspected • Symptoms reported but not confirmed • No lab or epidemiologic link

  17. Conclusion • Deciding whether to conduct an outbreak investigation requires an balance of disease reporting, correct diagnosis, background research, and good judgment.

  18. References • Centers for Disease Control. Outbreak of Listeriosis Associated with Homemade Mexican-Style Cheese – North Carolina, October 2000 – January 2001. MMWR July 6, 2001; 50 (26):560-2. (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5026a3.htm) • Dombrowski, Julie. Hepatitis A Among Men who have Sex with Men. 2002 (http://www.epi.state.nc.us/epi/gcdc/pdf/HepatitisA.pdf) • Dicker RC, et al. Investigating an Outbreak. In: Principles of Epidemiology: An Introduction to Applied Epidemiology and Biostatistics. Centers for Disease Control and Prevention, 1992: 347-350. (http://www.phppo.cdc.gov/PHTN//catalog/pdf-file/Epi_Course.pdf)

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