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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 Mode of Spread
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 This Quick Learn lesson will take approximately 10 minutes to complete. When you are finished, you will be able to determine the outbreak's likely mode of spread by analyzing an epidemic curve, or “epi curve.” You can move through this lesson by using the NEXT and BACK icons below.
 X and Y Axes An epidemic curve, or “epi curve,” is a visual display of the onset of illness among cases associated with an outbreak. The epi curve is represented by a graph with two axes that intersect at right angles. The horizontal **x-axis** is the date or time of illness onset among cases. The vertical **y-axis** is the number of cases. Each axis is divided into equally spaced intervals, although the intervals for the two axes may differ.
 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 Time trend of the outbreak, that is, the distribution of cases over time 
  										“Outliers,” or cases that stand apart from the overall pattern 
  										General sense of the outbreak's magnitude 
  										Inferences about the outbreak's pattern of spread 
  										Most likely time of exposure
 What an Epi Curve Can Tell You, continued The **magnitude** of an outbreak can be assessed easily with a glance of the epi curve.  Are there many cases or just a few? The **time trend**, or the distribution of cases over time, will give an indication of where the outbreak is in its course.  Are cases still rising or has the outbreak already peaked?  Does it appear that the outbreak is over?  How long has it been since the last case occurred? **Outliers** are cases that stand apart from the other cases.  Outliers include the index case, which might be the source of the outbreak, and cases that occur well after other cases, which might indicate secondary spread of the illness. **Index case**: the first reported case in an outbreak
 **Secondary spread**: the spread of disease to others from cases exposed through the original source of infection; others can include household members, roommates, and health-care providers by airborne, vehicleborne, and vectorborne direct and indirect transmission
 Magnitude, Time Trend, and Outliers Below is the epi curve from an outbreak of hepatitis A.  If today's date is August 17, what can you conclude about the outbreak? Select a link below to learn more. Note: The incubation period for hepatitis A is 25-30 days. Magnitude 
  										Time trend
  										Outliers **Hepatitis A Cases by Date of Onset in Port Yourtown, Washington, June - August 2010**  **Magnitude** Based on currently reported cases, the outbreak involves 17 total cases. **Hepatitis A cases by date of onset in Port Yourtown, Washington, June - August 2010**
 **Time Trend** The first case of hepatitis A occurred during the third week of June, almost 3 weeks before the other cases.  The majority of cases began the week of July 14 and peaked the week of July 21.  The last case onset of symptoms appeared during the week of August 10. **Hepatitis A Cases by Date of Onset in Port Yourtown, Washington, June - August 2010**
 **Outliers** The first case arouses suspicion because it occurs about one incubation period before the other cases. This could be the source of the outbreak. **Hepatitis A Cases by Date of Onset in Port Yourtown, Washington, June - August 2010**
 Mode of Spread: Point Source An epi curve can also be used to make inferences about inferences about an outbreak's most likely mode of spread, suggesting how a disease is transmitted. Transmission occurs in the following ways: Point source
  										Continuous common source
  										Person-to-person spread (propagation) In a **point source outbreak**, persons are exposed over a brief time to the same source, such as a single meal or an event. The number of cases rises rapidly to a peak and falls gradually. The majority of cases occur within one incubation period of the disease. **Cryptospordiosis Cases Associated with a Child
			  Care Center by Date of Onset in Port Yourtown, Washington, June 1998**
 Mode of Spread: Continuous Common Source In a **continuous common source** outbreak, persons are exposed to the same source but exposure is prolonged over a period of days, weeks, or longer. The epi curve rises gradually and might plateau. **Salmonellosis Cases Exposed to Contaminated Salami by Date of Onset, United States, December 2009 – January 2010**
 Mode of Spread: Propagated Outbreak In a **propagated outbreak**, there is no common source because the outbreak spreads from person-to-person. The graph will assume the classic epi curve shape of progressively taller peaks, each being one incubation period apart. **Measles Cases by Date of Onset in Aberdeen, South Dakota, October 15, 1970 – January 16, 1971**
 Analyzing the Mode of Spread Of course, the shape of an epi curve rarely fits any of these descriptions exactly. For propagated outbreaks, the shape might show overlapping waves of cases that obscure subsequent peaks, and peaks might diminish more slowly over time. You can, however, get a general sense about the mode of spread of an outbreak from its epi curve.
 Your Turn: Exercise 1 The average incubation period for *Salmonella* is 12 to 36 hours and has a range of 6 hours to 10 days. What do you think is the most likely mode of spread? Point Source Continuous Common Source Propagated (person-to-person) **Exercise 1: Correct** This epi curve is an example of point source outbreak.  The epi curve rises quickly to a peak around August 24-25 before slowly tapering off. ***Salmonella Enteritidis* Gastroenteritis Cases by Date of Onset in Maryland, August 2008**
 **Exercise 1: Incorrect** This epi curve is an example of point source outbreak.  The epi curve rises quickly to a peak around August 24-25 before slowly tapering off. ***Salmonella Enteritidis* Gastroenteritis Cases by Date of Onset in Maryland, August 2008**
 **Exercise 1: Incorrect** This epi curve is an example of point source outbreak.  The epi curve rises quickly to a peak around August 24-25 before slowly tapering off. ***Salmonella Enteritidis* Gastroenteritis Cases by Date of Onset in Maryland, August 2008**
 ***Salmonella Enteritidis* Gastroenteritis Cases by Date of Onset in Maryland, August 2008**
 Your Turn: Exercise 2 Let’s look at the epi curve for another outbreak of salmonellosis. Remember, the average incubation period for *Salmonella* is 12 to 36
			  hours and has a range of 6 hours to 10 days. What do you think is the most likely mode of spread? Select the most likely pattern of spread. Point Source Continuous Common Source Propagated (person-to-person) **Exercise 2: Incorrect** This outbreak is a continuous common source outbreak, in which exposure to contaminated peanut butter continued over many months. Cases gradually rise and plateau from mid-October to mid-December. **Onset of Illness Among Cases of *Salmonella Typhimurium* Infection Associated with Peanut Butter, United States, 2008-2009.**
 **Exercise 2: Correct** This outbreak is a continuous common source outbreak, in which exposure to contaminated peanut butter continued over many months. Cases gradually rise and plateau from mid-October to mid-December. **Onset of Illness Among Cases of *Salmonella Typhimurium* Infection Associated with Peanut Butter, United States, 2008-2009.**
 **Exercise 2: Incorrect** This outbreak is a continuous common source outbreak, in which exposure to contaminated peanut butter continued over many months. Cases gradually rise and plateau from mid-October to mid-December. **Onset of Illness Among Cases of *Salmonella Typhimurium* Infection Associated with Peanut Butter, United States, 2008-2009.**
 **Onset of Illness among Cases of *Salmonella Typhimurium* Infection Associated with Peanut Butter, United States, 2008-2009.**
 Summary Congratulations!
			    Now you should be able to analyze an epi curve and determine an outbreak’s mode of spread. To view other Quick Learns, 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.
                           
                  Introduction to Epidemiology
 CDC Learning Connection LogoFor more Quick-Learn lessons please visit CDC Learning Connection
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