2Module 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.
3Goal To enable users to create and interpret an epidemic curve Learning ObjectivesDefine an epidemic curveExplain the utility of epidemic curvesDescribe methods to create epidemic curves
5Basic Steps to an Outbreak Investigation Verify the diagnosis and confirm the outbreakDefine a case and conduct case findingTabulate and orient data: time, place, personTake immediate control measuresFormulate and test hypothesisPlan and execute additional studiesImplement and evaluate control measuresCommunicate findingsThese 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.
6Epidemic Curves Defined A graphic depiction of the progression of an outbreak over timeCan provide information about:Size of the outbreakTime trend of the outbreakPerson or place informationPeriod of exposureIncubation periodAt 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 spreadSizeOutliersTime trendExposure and/or disease incubation periodThese attributes will be explained further in Part II of the training.
7Key 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.
8What does an epi curve look like? Epi curves are bar graphs (histograms)No space between x-axis categoriesEach axis is clearly labeledA descriptive title is included
9Components of an Epi Curve y-axisThis 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
10Drawing an Epi Curve Refer to line listing data Plot the date a person became ill (date of illness onset) on the x-axisPlot the number of persons who became ill (cases of disease) on each date reported on the y-axisTo 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.
11Choosing the best unit of time for the x-axis Day of illness onset is bestHour of onset appropriate for very short incubation periodWeek or month of onset appropriate for very long incubation periodDay 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 periodBegin with a unit approximately one quarter the length of the incubation periodIf the incubation period is not known, graph several epi curves with different time unitsDisease specific incubation periods can be found in your state health department’s surveillance guide, the American Academy of Pediatrics “Red Book,” as well as
12Activity: 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.
13Outbreak ScenarioIn 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.
14Line listing of 10 cases Patient # Age Sex Onset Date Severe Abdominal PainNo. Times DiarrheaStool Testing117MDec 8Y3Not Done216FDec 6NNegativeDec 10E. coli 015745Dec 586Dec 77E. Coli 0157Dec 9910Facilitator:Ask trainees components needed to create an epi curve.These components include:Descriptive titleX-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)
15Drawing an Epi Curve using Pen and Paper Draw the x and y axesDivide each axis into the appropriate measure (unit of time for the x-axis and count for the y-axis)LabelGraph each case for the selected period of timeTitleRemember, epi curves are bar graphs so you need to have an x- and y-axis and there should be no space between the bars.
16Using Excel to Create Epi Curves To create an epi curve in Microsoft Excel:Highlight data to be included in chartClick the “Chart wizard” on the tool barChoose “Column” as the chart typeClick “Next” twice and specify the chart optionsClick “Next”Click “Finish”Change the “Gap width” to “0” to get the bars to touchExcel is a simple way to create an epi curve.In the next few slides we will walk you through how to do thisFirst, 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.
182. 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.
193. Highlight data, click on ‘chart wizard’ and select ‘column’ as the chart type Facilitator:Click on chart wizardSelect “column type” as the chart typeClick “next”
215. Add descriptive title and label axes clearly Facilitator:Point to:Graph titleX-axis titleY-axis titleWhen done entering the necessary information, click ‘finish.’
226. 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
237. 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.
24Completed Epi Curve Facilitator: Please point out all the components needed:Descriptive titleX-axis (date of symptom onset)Y-axis (number of cases)
25A ‘real life’ exampleThis is the epidemic curve for the 2009 Salmonella Typhimurium outbreak associated with peanut butter.Source:
26Source: 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:
27Part 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.
28Epidemic CurveA picture of the number of cases on the dates of illness onsetProvides outbreak information including:Pattern of spreadSizeOutliersTime trendPeriod of exposureDisease incubation periodAs 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 outbreakTime trend of the outbreakPerson or place informationPeriod 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.
29Basic Steps to an Outbreak Investigation Verify the diagnosis and confirm the outbreakDefine a case and conduct case findingTabulate and orient data: time, place, personTake immediate control measuresFormulate and test hypothesisPlan and execute additional studiesImplement and evaluate control measuresCommunicate findingsThese 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.
30Review components required for an epi curve: X-axis: date of symptom onsetY-axis: number of casesDescriptive title
31Outbreak Pattern of Spread The overall shape of the epi curve can reveal the type of outbreak3 types of epi curves:Common sourcePoint sourcePropagatedThe 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 sourcepoint sourcepropagatedThese will be reviewed on the following slides.
32Point Source Outbreak Characteristics: Brief period of exposure All cases in one incubation periodTypically a sharp upward slope and a gradual downward slopeIn 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.
33Facilitator: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).
34Common Source Outbreak Two types of exposure:ContinuousIntermittentIn a common source outbreak, people can be either intermittently or continuously be exposed to a harmful source.
35Continuous Common Source Outbreak Characteristics:Long period of exposureGradual increase in casesThen a plateau in number of casesA 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.
36Example 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.
37Intermittent Common Source Outbreak Characteristics:Brief, sporadic exposure periodIrregular peaks reflect timing and extent of exposureIntermittent exposure epi curves will have irregular peaks that suggest a brief and sporadic exposure.
38Example 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.
39Propagated Outbreak Characteristics: Spreads from person to person Longer lasting than common source outbreaksMultiple waves possibleProgressively taller peaksA 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.
40Example 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.
41Additional Information from Epi Curves Size of the outbreakTime trend of the outbreakPerson or place informationPeriod of exposureIncubation periodThe 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 caseCreating 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.
42Outbreak OutliersVery first and last cases on curves that may not appear to be related to the outbreakMay represent:Baseline level of illnessOutbreak sourceA case exposed earlier or later than othersAn unrelated caseA case with a long incubation periodOutbreak 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.
44What 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.
46ReferencesLast, 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.