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
Part I
Creating an Epidemic Curve
Basic Steps to an Outbreak
Investigation
1. Verify the diagnosis and confirm the
outbreak
2. Define a case and conduct case finding
3. Tabulate and orient data: time, place,
person
4. Take immediate control measures
5. Formulate and test hypothesis
6. Plan and execute additional studies
7. Implement and evaluate control measures
8. Communicate findings
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
Key Terms
• Exposure period
• Incubation period
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
x-axis
10
/2
8/
10 2 00
7
/2
9/
10 2 00
7
/3
0/
10 2 00
7
/3
1/
2
11 007
/1
/2
11 0 07
/2
/2
11 0 07
/3
/2
11 0 07
/4
/2
11 0 07
/5
/2
11 0 07
/6
/2
11 0 07
/7
/2
11 0 07
/8
/2
11 0 07
/9
/2
00
7
Number of Cases
Components of an Epi Curve
y-axis
Measles Outbreak 2007
8
7
6
5
4
3
2
1
0
Date of Onset
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
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
Activity:
Creating an Epi Curve
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 27. 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.
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
1
Negative
3
16
M
Dec 10
Y
2
E. coli 0157
4
17
F
Dec 8
Y
3
Not Done
5
16
F
Dec 5
Y
8
E. coli 0157
6
16
M
Dec 7
Y
3
Negative
7
17
M
Dec 7
Y
3
E. Coli 0157
8
17
F
Dec 9
Y
3
E. Coli 0157
9
16
F
Dec 7
N
1
Negative
10
17
F
Dec 6
Y
3
Not Done
Drawing an Epi Curve using
Pen and Paper
1. Draw the x and y axes
2. Divide each axis into the appropriate
measure (unit of time for the x-axis and
count for the y-axis)
3. Label
4. Graph each case for the selected period
of time
5. Title
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
1. Enter data in Excel and sort by
date
2. Total cases for each date
3. Highlight data, click on ‘chart
wizard’ and select ‘column’ as the
chart type
4. Confirm data selected is correct,
click next
5. Add descriptive title and label
axes clearly
6. Change ‘Gap Width’ to ‘0’
NOTE:
Double
click on bars in graph to access ’format data series’ box
7. Adjust axis units (if needed)
Completed Epi Curve
Number of Cases
E.Coli Outbreak December 2003
4
3
2
1
0
09
09
09
09
09
09
09
09
09
09
0
0
0
0
0
0
0
0
0
0
2
2
2
2
2
2
2
/2
/2
/2
3/
4/
5/
6/
7/
8/
9/
0
1
2
/
/
/
/
/
/
/
/1
/1
/1
12
12
12
12
12
12
12
12
12
12
Symptom Onset
A ‘real life’ example
Source: http://www.cdc.gov/salmonella/typhimurium/epi_curve.html
Source: http://www.cdc.gov/salmonella/typhimurium/epi_curve.html
Part II
Interpreting an Epidemic Curve
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
Basic Steps to an Outbreak
Investigation
1. Verify the diagnosis and confirm the
outbreak
2. Define a case and conduct case finding
3. Tabulate and orient data: time, place,
person
4. Take immediate control measures
5. Formulate and test hypothesis
6. Plan and execute additional studies
7. Implement and evaluate control measures
8. Communicate findings
Sample Epidemic Curve of Illness Onset
Number of Cases
4
3
2
1
0
12/3
12/4
12/5
12/6
12/7
12/8
12/9
Date of Onset
12/10
12/11
12/12
12/13
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
Point Source Outbreak
Characteristics:
• Brief period of exposure
• All cases in one incubation period
• Typically a sharp upward slope and a
gradual downward slope
Point Source Outbreak
Number of Cases
14
12
10
8
6
4
2
0
1
3
5
7
9
11 13
15 17 19
Day of Onset
21 23
25 27 29
Common Source Outbreak
Two types of exposure:
• Continuous
• Intermittent
Continuous Common Source
Outbreak
Characteristics:
• Long period of exposure
• Gradual increase in cases
• Then a plateau in number of cases
Continuous Common Source Outbreak
Number of Cases
10
8
6
4
2
0
1
2 3
4 5
6
7 8
9 10 11 12 13 14 15 16 17 18 19 20
Day of Onset
Intermittent Common Source
Outbreak
Characteristics:
• Brief, sporadic exposure period
• Irregular peaks reflect timing and extent
of exposure
Number of Cases
Intermittent Common Source Outbreak
8
7
6
5
4
3
2
1
0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
Day of Onset
Propagated Outbreak
Characteristics:
• Spreads from person to person
• Longer lasting than common source
outbreaks
• Multiple waves possible
• Progressively taller peaks
Propagated Common Source Outbreak
Number of Cases
14
12
10
8
6
4
2
0
1
5
9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77
Day of Onset
Additional Information
from Epi Curves
•
•
•
•
•
Size of the outbreak
Time trend of the outbreak
Person or place information
Period of exposure
Incubation period
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
Additional Information Gained
from an Epi Curve
What type of epidemic curve does
the following graph illustrate?
Number of Cases
Hepatitis A Outbreak
4
3
2
1
0
04 /04 /04 /04 /04 /04 /04 /04 /04 /04 /04 /04 /04 /04 /04
/
10 /12 /14 /16 /18 /20 /22 /24 /26 /28 3/1 3/3 3/5 3/7 3/9
/
2
2
2
2
2
2
2
2 2
2
Date of Onset
Questions?
References
1.
2.
3.
4.
5.
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