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CDC 24/7: Saving Lives. Protecting People.CDC 24/7: Saving Lives. Protecting People. Quick-Learn Lesson Using an Epi Curve to Determine Most Likely Period of Exposure
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 Using an Epi Curve to Determine Most Likely Period of Exposure This Quick Learn lesson will take approximately 10 minutes to complete. When you are finished, you will be able to interpret an epidemic curve or “epi curve”, and determine the likely period of exposure that led to the outbreak. You can move through this lesson by using the NEXT and BACK icons below.
 What an Epi Curve Can Tell You An epi curve is a visual display of the onset of illness among cases associated with an outbreak. You can learn a lot about an outbreak from an epi curve, such as The outbreak's time trend, which is the distribution of cases over time
  										Outliers, that is,  cases that stand apart from the overall pattern
  										General sense of the illness magnitude 
  										Inferences about the outbreak's mode of spread
  										The most likely period of exposure In a 
                                        
                                        
                                        
                                        
                                        
                                        point source outbreak 
                                        
                                        
                                        
                                        
                                        
                                        
                                         of a known disease, you also can use the epi curve to identify the most likely time period of exposure that led to the outbreak. Knowing the period of exposure allows you to focus your search for the source of the outbreak. **Point Source Outbreak** Persons are exposed to the same source over
						  a brief time, such as through a single meal or at an event. The number of cases rises rapidly to a peak and falls  gradually. Most cases occur within one
						  incubation period.
 Incubation Period To determine the most likely period of exposure for an outbreak, you need to know the average incubation period for the disease and the range of incubation periods, which are the minimum and maximum reported incubation periods. The **incubation period** is the time from exposure to the causative agent until the first symptoms develop and is characteristic for each disease agent. Example: Shiga toxin-producing *E. coli* has an average incubation period of 3-4 days with a range from a minimum of 2 days to a maximum of 10 days.
 Determining the Period of Exposure The following steps will allow you to determine the most likely period of exposure for a 
                                        
                                        
                                        
                                        
                                        
                                        point source outbreak 
                                        
                                        
                                        
                                        
                                        
                                        
                                         based on the epi curve. Identify the peak of the outbreak, which is the time period then the largest number of cases occurred.
  												Count back from the peak, the average incubation period for disease. Note that date. 
  												Identify the earliest case in the outbreak and count back the minimum incubation period.  Note that date.
  												Identify the last case in the outbreak and count back the maximum incubation period.  Note that date. The range of dates identified in Step 2-4 represent the most likely period of exposure and ideally should fall within a few days of each other. If the three dates are widely separated, this indicates that the incubation period has a wide range or the outbreak is not a point source outbreak. **Point Source Outbreak** Persons are exposed to the same source over
						  a brief time, such as through a single meal or at an event. The number of cases rises rapidly to a peak and falls  gradually. Most cases occur within one
						  incubation period.
 Determining the Period of Exposure Let's look  at an example and walk through the steps to determine the most likely period of exposure.  Below is the epi curve from an outbreak of Shiga toxin-producing *E. coli* (STEC).  The outbreak appears to be due to a point source. **Cases of Shiga Toxin-producing *E. coli* by Date of Onset, Port Yourtown, December 2011.** Epi curve depicting Cases of Shiga toxin-producing E coli by date of onset, Port Yourtown, December 2011
 Step 1: Identify the Peak Identify the peak of the outbreak.  In the outbreak below, the peak occurs on December 10, which is the date with the highest number of cases. **Shiga Toxin-producing *E. coli* Cases by Date of Onset, Port Yourtown, December 2011** Epi curve depicting Cases of Shiga toxin-producing E coli by date of onset, Port Yourtown, December 2011. Epi curve identifies the peak of onset on the December 10th
 Step 2: Count Back the Average Incubation Period The average incubation period for Shiga toxin-producing E. coli (STEC) is 4 days. Now, count back 4 days from the peak.  That date is December 6. **Shiga Toxin-producing *E. coli* Cases by Date of Onset, Port Yourtown, December 2011** Epi curve depicting Cases of Shiga toxin-producing E coli by date of onset, Port Yourtown, December 2011. Epi curve identifies the peak of onset on the December 10th and the average incubation from December 6th to December 10th
 Step 3: Count Back the Minimum Incubation Period Identify the earliest case in the outbreak and count back the minimum incubation period, which is two days. That date is December 7. **Shiga Toxin-producing *E. coli* Cases by Date of Onset, Port Yourtown, December 2011** Epi curve depicting Cases of Shiga toxin-producing E coli by date of onset, Port Yourtown, December 2011. Epi curve identifies the peak of onset on the December 10th, the average incubation from December 6th to 10th, and the minimum incubation period from December 7th to 9th.
 Step 4: Count Back the Maximum Incubation Period Identify the last case and count back the maximum incubation period. In this case, the last case is December 13, and the incubation period is 10 days.  You should count back to December 3. **Shiga Toxin-producing *E. coli* Cases by Date of Onset, Port Yourtown, December 2011** Epi curve depicting Cases of Shiga toxin-producing E coli by date of onset, Port Yourtown, December 2011. Epi curve identifies the peak of onset on the December 10th, the average incubation from December 6-10th, the minimum incubation period from December 7th to 9th, and the maximum incubation period from December 3rd to December 13th.
 Result: Most Likely Period of Exposure After analyzing the epi curve, we determined the first case occurred on December 9, the peak of the outbreak on December 10, and the last case occurred on December 13.  With the described methods, we have determined the Average incubation period from peak — December 6
				  						Minimum incubation period from the first case — December 7
				  						Maximum incubation period from the last case — December 3 Using these range of dates, we have can determine the most likely period of exposure for this outbreak is December 3-7. Investigators will focus on this time period when searching for the exposure that resulted in this outbreak. **Shiga Toxin-producing *E. coli* Cases by Date of Onset, Port Yourtown, December 2011** Epi curve depicting Cases of Shiga toxin-producing E coli by date of onset, Port Yourtown, December 2011. Epi curve identifies the peak of onset on the December 10th, the average incubation from December 6-10th, the minimum incubation period from December 7th to 9th, and the maximum incubation period from December 3rd to December 13th. The most likely period of exposure, December 3rd  to the 7th has been highlighted.
 Your Turn: Exercise 1 Notice in the example we just completed the interval on the x-axis 1 day. Some epi curves will use intervals of multiple days, weeks, or even months. The interval depends on the time frame that cases occurred. Always be sure to look at the interval so that you interpret the epi curve appropriately. Now, it’s your turn. Walk through the steps to determine the most likely period of exposure for a point source outbreak of Hepatitis A in Alabama. The incubation range for Hepatitis A is 15-50 days and has an average incubation of 25-30 days.  Use an average incubation period of 28 days. Because the cases are spread over multiple months, the x-axis is divided into 4-day intervals, starting on the day noted. **Hepatitis A Cases By Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006.
 Exercise 1: Determine the Peak What is the peak for the epi curve below? Select the correct answer. 4-day period beginning 10/28 4-day period beginning 10/20 4-day period beginning 11/1 **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. There are 3 cases during October 20th-24th, 9 cases during October 24-28, 15 cases during October 28-November 1,  10 cases during November 1-5, 9 cases during November 5-9, 3 cases November 9-13, and 1 case during the November 13-17 interval. **Exercise 1: Correct** The peak, which occurred during the 4-day interval beginning 10/28, is the highest point of the epi curve. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak, October 28-November 1 is highlight.
 **Exercise 1: Incorrect** The peak, which occurred during the 4-day interval beginning 10/28, is the highest point of the epi curve. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak, October 28-November 1 is highlight.
 **Exercise 1: Incorrect** The peak, which occurred during the 4-day interval beginning 10/28, is the highest point of the epi curve. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak, October 28-November 1 is highlight.
 Exercise 2: Count Back the Average Incubation Period Count back the average incubation period from the peak. Select the appropriate area of the graph. The average incubation period for hepatitis A is 28 days. Select the correct answer below. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak was identified as October 28-November 1. 4-day period beginning 9/22 4-day period beginning 9/30 4-day period beginning 10/12 **Exercise 2: Incorrect** You should have counted 28 days from the peak during 10/28. That date falls within the 4-day period beginning 9/30. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval and the average incubation period is highlighted as from September 30-October 28th.
 **Exercise 2: Correct** You should have counted 28 days from the peak during 10/28. That date falls within the 4-day period beginning 9/30. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval and the average incubation period is highlighted as from September 30-October 28th.
 **Exercise 2: Incorrect** You should have counted 28 days from the peak during 10/28. That date falls within the 4-day period beginning 9/30. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval and the average incubation period is highlighted as from September 30-October 28th.
 Exercise 3: Count Back the Minimum Incubation Period Identify the earliest case in the outbreak and count back the minimum incubation period.  What is the approximate date? Select the correct answer below. Note: The range in incubation periods for hepatitis A is 15-50 days. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval and the average incubation period is highlighted as from September 30-October 28th. 4-day period beginning 10/12 4-day period beginning 10/4 4-day period beginning 10/16 **Exercise 3: Incorrect** The first identified case occurred during the 4-day period beginning on 10/20. By counting back the minimum incubation period of 15 days, we can determine the minimum incubation period from the peak occurred during the 4-day period beginning 10/4. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, and the minimum incubation period is depicted from the October 4th-October 20th interval.
 **Exercise 3: Correct** The first identified case occurred during the 4-day period beginning on 10/20. By counting back the minimum incubation period of 15 days, we can determine the minimum incubation period from the peak occurred during the 4-day period beginning 10/4. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, and the minimum incubation period is depicted from the October 4th-October 20th interval.
 **Exercise 3: Incorrect** The first identified case occurred during the 4-day period beginning on 10/20. By counting back the minimum incubation period of 15 days, we can determine the minimum incubation period from the peak occurred during the 4-day period beginning 10/4. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, and the minimum incubation period is depicted from the October 4th-October 20th interval.
 Exercise 4: Count Back the Maximum Incubation Period Identify the last case in the outbreak and count back the maximum incubation period.  What is this date? Select the correct answer below. Note: The range in incubation periods for hepatitis A is 15-50 days **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, and the minimum incubation period is depicted from the October 4th-October 20th interval. 4-day period beginning 10/4 4-day period beginning  9/26 4-day period beginning 9/22 4-day period beginning  10/16 **Exercise 4: Incorrect** The last identified case was during the 4-day period beginning 11/13. By counting back the maximum incubation period of 50 days, the identified date falls within the 4-day period beginning 9/22. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval.
 **Exercise 4: Incorrect** The last identified case was during the 4-day period beginning 11/13. By counting back the maximum incubation period of 50 days, the identified date falls within the 4-day period beginning 9/22. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval.
 **Exercise 4: Correct** The last identified case was during the 4-day period beginning 11/13. By counting back the maximum incubation period of 50 days, the identified date falls within the 4-day period beginning 9/22. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval.
 **Exercise 4: Incorrect** The last identified case was during the 4-day period beginning 11/13. By counting back the maximum incubation period of 50 days, the identified date falls within the 4-day period beginning 9/22. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval.
 Exercise 5: Determine the Most likely Period of Exposure You have now determined the following dates and time periods: Peak of the outbreak occurred — 10/28 
				  						Average incubation period from peak — 4-day period beginning 9/30
				  						Minimum incubation period from the first case — 4-day period beginning 10/4
				  						Maximum incubation period from the last case — 4-day period beginning 9/22 Based on the information above, what is the most likely period of exposure? Select the correct answer. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval. 10/6 – 10/26 9/24 – 9/31 9/24 – 10/6 9/22 – 10/7 **Exercise 5: Incorrect** The most likely period of exposure is 9/22 – 10/7. The minimum incubation period from the first case occurred during the 4-day period beginning on 10/4 and ending on 10/7. The maximum incubation period from the last case occurred during the 4-day period beginning on 9/22. The average incubation period from the peak occurred during the 4-day period beginning on 9/30, which falls between the minimum and maximum periods. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval. The most likely period of exposure highlights the September 22 to October 4th intervals which includes October 7.
 **Exercise 5: Incorrect** The most likely period of exposure is 9/22 – 10/7. The minimum incubation period from the first case occurred during the 4-day period beginning on 10/4 and ending on 10/7. The maximum incubation period from the last case occurred during the 4-day period beginning on 9/22. The average incubation period from the peak occurred during the 4-day period beginning on 9/30, which falls between the minimum and maximum periods. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval. The most likely period of exposure highlights the September 22 to October 4th intervals which includes October 7.
 **Exercise 5: Incorrect** The most likely period of exposure is 9/22 – 10/7. The minimum incubation period from the first case occurred during the 4-day period beginning on 10/4 and ending on 10/7. The maximum incubation period from the last case occurred during the 4-day period beginning on 9/22. The average incubation period from the peak occurred during the 4-day period beginning on 9/30, which falls between the minimum and maximum periods. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval. The most likely period of exposure highlights the September 22 to October 4th intervals which includes October 7.
 **Exercise 5: Correct** The most likely period of exposure is 9/22 – 10/7. The minimum incubation period from the first case occurred during the 4-day period beginning on 10/4 and ending on 10/7. The maximum incubation period from the last case occurred during the 4-day period beginning on 9/22. The average incubation period from the peak occurred during the 4-day period beginning on 9/30, which falls between the minimum and maximum periods. **Hepatitis A Cases by Date of Onset in Colbert Country, Alabama September-November 2006** Epi Curve depicting Hepatitis A cases by date of onset in Colbert Country, Alabama September-November 2006 with 4 day intervals. The peak has been identified as October 28 interval, the average incubation period is highlighted as from September 30-October 28th, the minimum incubation period is depicted from the October 4th-October 20th interval, and the maximum incubation period is shown from  September 22nd to the November 13th interval. The most likely period of exposure highlights the September 22 to October 4th intervals which includes October 7.
 Summary **Congratulations!** You have analyzed an epi curve to determine the likely period of exposure that led to the outbreak. Depiction of basic epi curve and similar epi curve with period of exposure identified. To view other Quick Learn Lessons, visit CDC Learning Connection.
 Resources on Basic Epidemiology Basic Epidemiology
                  					CDC TRAIN
                  					E is for Epidemiology
                           
                 					Principles of Epidemiology in Public Health Practice, 3rd Ed.
                           
                  					What is Epidemiology? (00:43)
 CDC Learning Connection LogoFor more Quick-Learn lessons please visit CDC Learning Connection
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