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I is for Investigation

I is for Investigation. Outbreak Investigation Methods from Mystery to Mastery. Session I. Recognizing an Outbreak. Session Overview. Overview of outbreak investigation Identifying a potential outbreak Verifying the diagnosis and confirming the outbreak Defining and finding cases

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I is for Investigation

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  1. I is for Investigation Outbreak Investigation Methods from Mystery to Mastery

  2. Session I Recognizing an Outbreak

  3. Session Overview • Overview of outbreak investigation • Identifying a potential outbreak • Verifying the diagnosis and confirming the outbreak • Defining and finding cases • Orientingdata by person, place, and time

  4. Learning Objectives • Identify steps of an outbreak investigation • Develop a case definition • Identify a process for case finding in an outbreak • Apply methods used to orient data by person, place, and time • Create and interpret epidemic curves

  5. Overview of Outbreak Investigation

  6. What is an Outbreak? The occurrence of more cases of a disease than expected for a particular place and time Number of cases Expected number of cases Days

  7. Basic Steps of an Outbreak Investigation • Verify the diagnosis and confirm the outbreak • Define a case and conduct case finding • Tabulate and orient data: time, place, person • Take immediate control measures • Formulate and test hypothesis • Plan and execute studies • Implement and evaluate control measures • Communicate findings

  8. Exceptions to the Rule Basic steps provide a model for systematic outbreak investigations • No two outbreaks are alike! • Steps of an outbreak could… • occur in a different order • occur simultaneously • be repeated

  9. Identifying a Potential Outbreak

  10. Outbreak Information Sources • Laboratory-confirmed reports of notifiable diseases • Clinician reports of notifiable disease and unusual increases in disease • Concerned parent/citizen reports to health department • Media

  11. Gathering Information from Case Reports Collect: • As much information as possible • Negative as well as positive information • Food history • 3 days (72 hrs) to 5 days, if unknown agent • Within incubation period, if known agent • Exposure sources such as water, people, animals

  12. Disease Surveillance: Case Report What questions would you ask an ill person? WHO: age, sex, occupation, any others ill WHAT: physical condition, symptoms, medication, and medical care sought WHEN: when did the affected become ill WHERE: city/school, address, telephone number of ill person(s) WHY/HOW: suspected cause of illness, risk factors, modes of transmission, notes on people who did not become ill

  13. Disease Surveillance:What Next? • File the report and stop? • Investigate further?

  14. Deciding to Investigate • Ideally, all reports of possible outbreaks should be investigated to: • Prevent other persons from becoming ill • Identify potentially problematic practices • Add to the knowledge of infectious diseases

  15. Why Investigate? • Surveillance detects increases in cases of disease • Characterize the problem • Prevention and control • Research and answer scientific questions • Train epidemiologists • Political / legal concerns

  16. Maybe You Should Investigate... • If illness is severe (life-threatening) • If there are confirmed clusters or large numbers of a similar illness • If foodborne illness is from a food handler • If illness is associated with commercially-distributed food • If there is outside pressure to investigate (media, politicians)

  17. Maybe You Shouldn’t Investigate... • If affected persons might not have the same illness • If affected persons are not able to provide adequate information for investigation • If the diagnosis and/or clinical symptoms are not consistent with the related exposures • If there are repeated complaints made by the same individual(s) for which prior investigations revealed no significant findings

  18. Verifying the Diagnosis and Confirming the Outbreak

  19. Verify the Diagnosis Evaluate: • Predominant signs and symptoms • Incubation period • Duration of symptoms • Suspected food • Suspected toxin, virus, or bacteria • Laboratory testing of stool, blood, or vomitus

  20. Identify the Pathogen • Ensure all suspect patients have the same pathogen • Identify the potential incubation period for hypothesis generation • Can proceed while waiting for laboratory diagnosis

  21. Verify the Diagnosis Potential reasons for negative laboratory results: • Illness could be due to an organism that wasn’t tested for • Mishandling of specimen resulting in death of the pathogen (during storage, transport, processing, or culture) • Specimens collected too late in the illness

  22. Defining and Finding Cases

  23. Case Definition A standard set of criteria for deciding whether an individual should be classified as having the disease of interest, including: • Clinical criteria (signs, symptoms, and laboratory tests) • Person, place, and time criteria

  24. Case Definition • Can be modified as more data are obtained • Should not include a possible cause of the outbreak

  25. Case Finding • Contact local care providers • Contact schools, large businesses • Contact state health department / neighboring health departments • Ask case-patients if they know of others who are ill Additional sources may be appropriate, depending on the outbreak.

  26. Orienting Data by Person and Place

  27. Descriptive Epidemiology • Comprehensively describes the outbreak • Person • Place • Time • Line listings • Graphs • Bar graphs • Histograms • Measures of central tendency

  28. Line Listing

  29. Bar Graph (Person)

  30. Histogram (Person)

  31. Measures of Central TendencyMean (Average) • Equals the sum of all values divided by the number of values. • Example: • Cases: 7,10, 8, 5, 5, 37, 9 years old • Mean = (7+10+8+5+5+37+9)/7 • Mean = 11.6 years of age

  32. Measures of Central TendencyMedian (50th percentile) • The value that falls in the middle position when the measurements are ordered from smallest to largest • Example: • Ages: 7,10, 8, 5, 5, 37, 9 • Ages sorted: 5, 5, 7, 8, 9,10, 37 • Median age = 8

  33. Calculate a Median Value • If the number of measurements is odd: • Median = value with rank (n+1) / 2 • n = the number of values • Example: • 5, 5, 7, 8, 9,10, 37 • n = 7 • (n+1) / 2 = (7+1) / 2 = 4 • The 4th value = 8

  34. Calculate a Median Value • If the number of measurements is even: • Median=average of the two values with: • rank of n / 2 and rank of (n / 2) + 1 • Example • 5, 5, 7, 8, 9, 10, 12, 37 • n = 8 • (8 / 2) = 4, so “8” is the first value • (8 / 2) + 1 =5, so “9” is the second value • (8 + 9) / 2 = 8.5 • The median value = 8.5

  35. Descriptive Epidemiology: Place Spot map • Shows where cases live, work, spend time • If population size varies between locations being compared, use location-specific attack rates instead of number of cases

  36. Descriptive Epidemiology: Place Source: http://www.phppo.cdc.gov/PHTN/catalog/pdf-file/LESSON4.pdf

  37. Orienting Data by Time Epidemic Curves

  38. Descriptive Epidemiology: Time

  39. Descriptive Epidemiology: Time • An epidemic curve (epi curve) is a graphical depiction of the number of cases of illness by the date of illness onset • Can provide information on the outbreak’s: • Pattern of spread • Magnitude • Outliers • Time trend • Exposure and / or disease incubation period

  40. Epi Curve: Pattern of Spread The overall shape of the epi curve can reveal the type of outbreak (the pattern of spread) • Common source • Intermittent • Continuous • Point source • Propagated

  41. Common Source Outbreak • People are exposed to a common harmful source • Period of exposure may be • brief (point source) • long (continuous) or • intermittent

  42. Epi Curve: Common Source Outbreak with Point Source Exposure Pattern of Spread

  43. Epi Curve: Common Source Outbreak with Continuous Exposure Pattern of Spread

  44. Epi Curve: Common Source Outbreak with Intermittent Exposure Pattern of Spread

  45. Epi Curve: Propagated Outbreak Pattern of Spread

  46. Epidemic Curves: Magnitude of the Outbreak Magnitude

  47. Epi Curves Provide Information about the Time Trend of an Outbreak • Date of illness onset for the first case • Date when the outbreak peaked • Date of illness onset for the last case Number of cases Days

  48. Clues from the Epi Curve • Incubation period • The time from the moment of exposure to an infectious agent until signs and symptoms of the disease appear • Period of exposure • Point source outbreak • Timeframe during which the exposure likely occurred

  49. Using Epi Curves to Estimate the Incubation Period • Use when timing of exposure is known and agent is unknown • Estimated incubation period is between • Time of suspected exposure • Time of peak of epi curve

  50. Using Epi Curves to EstimatePeriod of Exposure • Use when incubation period for the disease is known • Period of exposure is between • Peak of epi curve counting back the average incubation period • Earliest case, counting back the minimum incubation period

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