2. Module II Graphic Depiction of an Outbreak: Creating an Epidemic Curve
This Module was adapted from UNC Chapel Hill School of Public Health North Carolina Center for Public Health Preparedness training materials “I is for Investigation” and the FOCUS on Field Epidemiology training series.
There are two parts to this module. Part I focuses on how to draw an epidemic curve and Part II focuses on how to interpret an epidemic curve.

3. 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

4. Part I Creating an Epidemic Curve

5. 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
These are the basic steps to an outbreak investigation, as mentioned in the introduction module, which illustrate the context for drawing an epidemic curve. Creation of an epidemic curve fits in with steps two and three-as you conduct case finding and compile that information.
The following slides will detail what steps are involved.

6. 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
At this point during the outbreak investigation, data has been gathered and a line listing has been created. The next step is the creation of an epidemic curve (or epi curve) to illustrate the outbreak and help the investigators characterize the type of outbreak.
An epi curve graphically depicts the number of cases of illness by onset (or the frequency of cases over time). Information to be used in an epi curve can be obtained from the line listing or from case report forms.
An epi curve can be drawn at the beginning, middle or end of an outbreak. If it is drawn earlier in the outbreak, it should be updated to reflect the current cases- that way it can tell you where you are in the course of the outbreak (i.e. if there is an increase or decrease in cases).
It is useful in providing information on:
Pattern of spread
Size
Outliers
Time trend
Exposure and/or disease incubation period
These attributes will be explained further in Part II of the training.

7. Key Terms Exposure period Incubation period
Exposure period is the time period when exposure occurred.
Incubation period is the time between the moment the virus enters a person’s body and the appearance of symptoms.
In some instances, you will know the disease you are investigating and will be able to draw an epidemic curve using a known incubation period. For those outbreaks where you don’t initially know what disease you are working with, the epidemic curve will be able to give you clues about the incubation and exposure periods. More detail on what information an investigator can gain from an epidemic curve will follow in Part II of this training.

8. 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

9. Components of an Epi Curve
y-axis
This is an example of what an epi curve should look like.
There should not be space between the x-axis categories because an epi curve is a histogram.
Each axis should be labeled, and a descriptive title should be given to the graph.
A date or time frame is included in the title to clearly identify which outbreak the epi curve is referring to. This is useful when outbreaks of more common diseases occur more frequently (i.e. salmonella).
If you have information on the baseline number of cases prior to the epidemic, plotting the baseline number of cases may be useful in the outbreak investigation. This would illustrate the pre-epidemic period.
The incubation period for measles is 7-18 days (incubation period is the time between the moment the virus enters a person's body and the appearance of symptoms).
With measles, a person is contagious (capable of passing on the infection) from 3-5 days before symptoms appear to about four days after the rash shows up (the infectious period).
x-axis

10. 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
To create an epi curve:
Plot the time or date of illness on the x-axis.
Plot the number of cases of disease during the current outbreak y-axis.
If the disease causing the outbreak is known, use knowledge about the average incubation period (the time between the moment the virus enters a person's body and the appearance of symptoms) to determine the best time unit for the x-axis.
The unit used on the x-axis will depend on the incubation period for a specific disease. Use a time unit that is about one quarter of the incubation period length to start. For example, if the incubation period is 12 hours, you should start by using 3 hour time units on the x-axis, since one quarter of 12 hours is 3 hours.

11. 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
Day of illness is generally the best unit for the x-axis, as this will be appropriate for most diseases, except those where the incubation period is very short or very long. Remember, the incubation period is time between the moment the virus enters a person's body and the appearance of symptoms.
If incubation period is very short:
hours may be more appropriate to use as the unit.
Example of a short incubation period: botulism (12-36hours)
If the incubation period is very long:
weeks or months might be more appropriate to use.
Example of a long incubation period: Hep B (60-90 days);
Facilitator:
Ask audience for other examples of diseases with short or long incubation period.
Technical Tips:
Choice of time unit for x-axis depends upon the average incubation period
Begin with a unit approximately one quarter the length of the incubation period
If the incubation period is not known, graph several epi curves with different time units
Disease specific incubation periods can be found in your state health department’s surveillance guide, the American Academy of Pediatrics “Red Book,” as well as

12. Activity: Creating an Epi Curve
We’ve introduced a number of concepts and terms, so now we’re going to use a fictitious scenario to practice applying these concepts.
Over the next minutes we will review the scenario, and then review a sample line listing.
Facilitator:
Please choose one method (pen and paper or Microsoft Excel) to walk through with participants. This will be dependent upon the facility (i.e. if participants have access to a computer or laptop). If choosing to create an epidemic curve using Microsoft Excel, please note the instructions below.
During the Excel portion, it is very important that all participants complete each step before moving on to the next step. Even if you are very comfortable with Excel, please follow along with the slides, going step by step.
There are two reasons for this – first, there are any number of ways to arrive at similar results, and these steps may be different than the steps you would take. If you take a different route, and run into trouble, it will be harder for me to help you sort it out!
And second, I want to be certain that all participants are confident about the steps, especially others who may be less familiar with Excel.

13. 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 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.
If you participated in the Line Listing Module, this scenario should look familiar.
Facilitator:
Have trainees create their own epi curve-either using pen and paper or Excel.

14. Line listing of 10 cases Patient # Age Sex Onset Date
Severe Abdominal Pain
No. Times Diarrhea
Stool Testing 1 17 M
Dec 8 Y 3
Not Done 2 16 F
Dec 6 N
Negative
Dec 10
E. coli 0157 4 5
Dec 5 8 6
Dec 7 7
E. Coli 0157
Dec 9 9 10
Facilitator:
Ask trainees components needed to create an epi curve.
These components include:
Descriptive title
X-axis (date of symptom onset, unit of time is generally the average incubation period)
Y-axis (number of cases on that particular day of symptom onset)

15. 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
Remember, epi curves are bar graphs so you need to have an x- and y-axis and there should be no space between the bars.

16. 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
Excel is a simple way to create an epi curve.
In the next few slides we will walk you through how to do this
First, I will walk through the steps, then you will have the opportunity to create one on your own. Remember to not go ahead to the next step, wait for the group.
Facilitator:
Included in the training package, there is an pre-populated Excel template that could be adapted for this portion of the training.

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

18. 2. Total cases for each date
Facilitator:
The auto sum function in Excel is useful for this task. By highlighting the cells you wish to sum and then clicking the ‘auto sum’ button on the task bar (Σ), Excel automatically sums those cells.

19. 3. Highlight data, click on ‘chart wizard’ and select ‘column’ as the chart type
Facilitator:
Click on chart wizard
Select “column type” as the chart type
Click “next”

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

21. 5. Add descriptive title and label axes clearly
Facilitator:
Point to:
Graph title
X-axis title
Y-axis title
When done entering the necessary information, click ‘finish.’

22. 6. Change ‘Gap Width’ to ‘0’
By changing the gap width to ‘0,’ it eliminates the spaces between the bars; making the graph look more like a histogram.
Double click on the bars in the graph to access ‘format data series’ box.
NOTE: Double click on bars in graph to access ’format data series’ box

23. 7. Adjust axis units (if needed)
For example, chart wizard may select inappropriate units for the x- or y-axis (i.e. you can’t have ½ a case).
Double click on an axis, for dialogue box to open.

24. Completed Epi Curve Facilitator:
Please point out all the components needed:
Descriptive title
X-axis (date of symptom onset)
Y-axis (number of cases)

25. A ‘real life’ example
This is the epidemic curve for the 2009 Salmonella Typhimurium outbreak associated with peanut butter.
Source:

26. Source: http://www.cdc.gov/salmonella/typhimurium/epi_curve.html
This is an epidemic curve for the same 2009 peanut butter outbreak, but by week of illness onset, rather than by day. Note the difference in the shape of the two epi curves.
In the next section, we are going to discuss how to interpret the curve.
Facilitator:
Next section is more complex, determine who your audience is before continuing.
Source:

27. Part II Interpreting an Epidemic Curve
Now that we’ve created an epi curve – we can learn more about how to describe the curve, and so the outbreak. We’ll be reviewing common patterns and additional information that can be gained from epi curves.

28. 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
As mentioned in Part I, an epi curve graphically depicts number of cases of illness by date of onset (frequency of cases over time).
By illustrating the outbreak, it organizes the information visually and it can show where you are in the course of the outbreak.
It is also useful in providing information on:
Size of the outbreak
Time trend of the outbreak
Person or place information
Period of exposure (time period when exposure occurred)
Incubation period (the time between the moment the virus enters a person’s body and the appearance of symptoms)
Information to be used in an epi curve can be obtained from the line listing or from case report forms.

29. 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
These are the basic steps to an outbreak investigation which illustrate the context for drawing an epidemic curve. A epidemic curve fits in with steps two and three-as you conduct case finding and compile that information.
The following slides will detail what steps are involved.

30. Review components required for an epi curve:
X-axis: date of symptom onset
Y-axis: number of cases
Descriptive title

31. 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
The shape of an epi curve can tell us about the type of outbreak.
There are 3 general types of outbreaks that can be determined by the shape of the curve-
common source
point source
propagated
These will be reviewed on the following slides.

32. Point Source Outbreak Characteristics: Brief period of exposure
All cases in one incubation period
Typically a sharp upward slope and a gradual downward slope
In a point source outbreak there is usually a sharp upward slope, followed by a gradual downward slope.
The exposure period is brief, and all cases will occur within one incubation period, since all the cases were exposed at approximately the same time.

33. Facilitator:
Note how the number of cases seems to rise quickly and cluster after an initial case a few days prior.
Examples of a point source outbreak:
A Hepatitis A outbreak due to an infected food handler at a catered event (Hepatitis A incubation period: days (average=30 days)).
Salmonella outbreak among attendees at a church luncheon (salmonella incubation period 1-3 days).

34. Common Source Outbreak
Two types of exposure:
Continuous
Intermittent
In a common source outbreak, people can be either intermittently or continuously be exposed to a harmful source.

35. Continuous Common Source Outbreak
Characteristics:
Long period of exposure
Gradual increase in cases
Then a plateau in number of cases
A continuous exposure will cause cases to rise gradually, due to the often long exposure period.
Exposure to source is prolonged over an extended period of time and may occur over more than one incubation period.
Down slope may be sharp, if the common source is removed OR gradual, if the outbreak is allowed to exhaust itself.

36. Example of a continuous common source outbreak:
A cholera outbreak from a contaminated water source.
(cholera incubation 1-3 days)
Facilitator:
Note how the curve rises gradually and has cases throughout the time frame examined.

37. Intermittent Common Source Outbreak
Characteristics:
Brief, sporadic exposure period
Irregular peaks reflect timing and extent of exposure
Intermittent exposure epi curves will have irregular peaks that suggest a brief and sporadic exposure.

38. Example of a intermittent common source outbreak:
Patrons of a restaurant where a cook (infected with Hepatitis A) works only 2 days/week. Hepatitis A incubation period is (average=30 days)
Facilitator:
Note how the curve has several gaps in cases on the time axis (x-axis), along with several peaks in cases.

39. Propagated Outbreak Characteristics: Spreads from person to person
Longer lasting than common source outbreaks
Multiple waves possible
Progressively taller peaks
A propagated outbreak is spread from person to person, and can last longer than common source outbreaks. There may be multiple waves of progressively taller peaks. The peaks are generally one incubation period apart from each other.

40. Example of a propagated common source outbreak:
Influenza outbreak. Influenza is spread through respiratory droplets from person to person. As more people are exposed and become ill, they in turn can infect more people, increasing the number of people who are sick in each wave. Think of a sick child who attends the local elementary school. He will first infect his own classmates, next those in the same grade, and eventually the illness will spread through the entire elementary school.
Facilitator:
Please note the clusters of a cases, with multiple waves of cases and taller peaks as time progresses.

41. Additional Information from Epi Curves
Size of the outbreak
Time trend of the outbreak
Person or place information
Period of exposure
Incubation period
The size (or magnitude) of the outbreak and trends in time can be determined from the curve, by considering the:
-Date of illness onset for the first case
-Date when the outbreak peaked
-Date of illness onset for the last case
Creating several sub-samples according to specific criteria, such as age and gender, can tell us further information about the outbreak. An example would be a GI outbreak at a school that affected only females under the age of 5.

42. 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
Outbreak outliers are cases at the beginning or end of an outbreak that don’t appear to be related to the other cases.
They may actually tell us about the baseline level of illness, the outbreak source, cases exposed earlier or later than the rest, unrelated cases, or may have a long incubation period.

43. Additional Information Gained from an Epi Curve

44. What type of epidemic curve does the following graph illustrate?
Facilitator:
We’ve had a chance to talk at length about the different types of epi curves. Can anyone identify what type of epidemic curve this is?
The curve shown here shows a sharp upstroke which is followed by a decline, and all of the cases occurred within one incubation period. These characteristics are consistent with a "point source" epidemic.

45. Questions?

46. 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–