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.

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

Part I Creating an Epidemic Curve

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.

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.

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.

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

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

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.

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 www.CDC.gov.

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 15-20 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.

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. 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.

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)

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.

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.

1. Enter data in Excel and sort by date

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.

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”

4. Confirm data selected is correct, click next

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.’

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

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.

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

A ‘real life’ example This is the epidemic curve for the 2009 Salmonella Typhimurium outbreak associated with peanut butter. Source: http://www.cdc.gov/salmonella/typhimurium/epi_curve.html

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

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.

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.

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.

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

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.

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.

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: 15-50 days (average=30 days)). Salmonella outbreak among attendees at a church luncheon (salmonella incubation period 1-3 days).

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.

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.

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.

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.

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 15-50 (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.

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.

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.

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.

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.

Additional Information Gained from an Epi Curve

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.

Questions?

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