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Module II Graphic Depiction of an Outbreak: Creating an Epidemic Curve

Module II Graphic Depiction of an Outbreak: Creating an Epidemic Curve. Goal To enable users to create and interpret an epidemic curve. Learning Objectives Define an epidemic curve Explain the utility of epidemic curves Describe methods to create epidemic curves.

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Module II Graphic Depiction of an Outbreak: Creating an Epidemic Curve

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  1. Module IIGraphic Depiction of an Outbreak: Creating an Epidemic Curve

  2. Goal To enable users to create and interpret an epidemic curve Learning Objectives • Define an epidemic curve • Explain the utility of epidemic curves • Describe methods to create epidemic curves

  3. Part ICreating an Epidemic Curve

  4. Basic Steps to an OutbreakInvestigation • 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 additional studies • Implement and evaluate control measures • Communicate findings

  5. Epidemic Curves Defined • A graphic depiction of the progression of an outbreak over time • Can provide information about: • Size of the outbreak • Time trend of the outbreak • Person or place information • Period of exposure • Incubation period

  6. Key Terms • Exposure period • Incubation period

  7. What does an epi curve look like? • Epi curves are bar graphs (histograms) • No space between x-axis categories • Each axis is clearly labeled • A descriptive title is included

  8. Components of an Epi Curve y-axis x-axis

  9. Drawing an Epi Curve • Refer to line listing data • Plot the date a person became ill (date of illness onset) on the x-axis • Plot the number of persons who became ill (cases of disease) on each date reported on the y-axis

  10. Choosing the best unit of time for the x-axis • Day of illness onset is best • Hour of onset appropriate for very short incubation period • Week or month of onset appropriate for very long incubation period

  11. Activity:Creating an Epi Curve

  12. Outbreak Scenario In December 2003, an outbreak of E. coli 0157 occurred among tenth-grade students from City High School. The students traveled between December 2-7. Although the students were broken down into smaller groups, the itineraries were similar for each group. Teachers and other adult chaperones accompanied the students, but no adult reported illness. In addition, no illness was reported among students who did not go on the field trip, and no cases of E. coli 0157 were reported in the community that week. Symptoms of gastroenteritis include severe abdominal pain and/or diarrhea and the average incubation period is 3-4 days.

  13. Line listing of 10 cases

  14. Drawing an Epi Curve using Pen and Paper • Draw the x and y axes • Divide each axis into the appropriate measure (unit of time for the x-axis and count for the y-axis) • Label • Graph each case for the selected period of time • Title

  15. Using Excel to Create Epi Curves To create an epi curve in Microsoft Excel: • Highlight data to be included in chart • Click the “Chart wizard” on the tool bar • Choose “Column” as the chart type • Click “Next” twice and specify the chart options • Click “Next” • Click “Finish” • Change the “Gap width” to “0” to get the bars to touch

  16. 1. Enter data in Excel and sort by date

  17. 2. Total cases for each date

  18. 3. Highlight data, click on ‘chart wizard’ and select ‘column’ as the chart type

  19. 4. Confirm data selected is correct, click next

  20. 5. Add descriptive title and label axes clearly

  21. 6. Change ‘Gap Width’ to ‘0’ NOTE: Double click on bars in graph to access ’format data series’ box

  22. 7. Adjust axis units (if needed)

  23. Completed Epi Curve

  24. A ‘real life’ example Source: http://www.cdc.gov/salmonella/typhimurium/epi_curve.html

  25. Source: http://www.cdc.gov/salmonella/typhimurium/epi_curve.html

  26. Part IIInterpreting an Epidemic Curve

  27. Epidemic Curve • A picture of the number of cases on the dates of illness onset • Provides outbreak information including: • Pattern of spread • Size • Outliers • Time trend • Period of exposure • Disease incubation period

  28. Basic Steps to 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 additional studies • Implement and evaluate control measures • Communicate findings

  29. Outbreak Pattern of Spread The overall shape of the epi curve can reveal the type of outbreak 3 types of epi curves: • Common source • Point source • Propagated

  30. Point Source Outbreak Characteristics: • Brief period of exposure • All cases in one incubation period • Typically a sharp upward slope and a gradual downward slope

  31. Common Source Outbreak Two types of exposure: • Continuous • Intermittent

  32. Continuous Common Source Outbreak Characteristics: • Long period of exposure • Gradual increase in cases • Then a plateau in number of cases

  33. Intermittent Common Source Outbreak Characteristics: • Brief, sporadic exposure period • Irregular peaks reflect timing and extent of exposure

  34. Propagated Outbreak Characteristics: • Spreads from person to person • Longer lasting than common source outbreaks • Multiple waves possible • Progressively taller peaks

  35. Additional Information from Epi Curves • Size of the outbreak • Time trend of the outbreak • Person or place information • Period of exposure • Incubation period

  36. Outbreak Outliers • Very first and last cases on curves that may not appear to be related to the outbreak • May represent: • Baseline level of illness • Outbreak source • A case exposed earlier or later than others • An unrelated case • A case with a long incubation period

  37. Additional Information Gained from an Epi Curve

  38. What type of epidemic curve does the following graph illustrate?

  39. Questions?

  40. References • Last, JM. A Dictionary of Epidemiology. Oxford Univ Press, 2001. • Nelson, KE and Williams CM. Infectious Disease Epidemiology Theory, and Practice. Jones and Bartlett, 2nd edition, 2007. • “Focus Series: Epidemic Curves Ahead”. UNC Chapel Hill School of Public Health North Carolina Center for Public Health Preparedness training materials. • “I is for Investigation, Session I: Recognizing an Outbreak.” UNC Chapel Hill School of Public Health North Carolina Center for Public Health Preparedness. • CDC Investigation Update: Outbreak of Salmonella Typhimurium Infections, 2008–2009. http://www.cdc.gov/salmonella/typhimurium/epi_curve.html

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