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Fundamentals of Epidemiology and Outbreak Investigations

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					Fundamentals of Epidemiology and Outbreak Investigations

Version 2 February 2006
Source: CDC Excite

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Fundamentals of Epidemiology and Outbreak Investigations
Developed by • Connecticut Partnership for Public Health Workforce Development
• Yale University School of Public Health

Adapted for distribution by
• Yale Center for Public Health Preparedness

Source: CDC Excite

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Fundamentals of Epidemiology and Outbreak Investigations
Course content source: US Department of Health and Human Services, Centers for Disease Control and Prevention http://www.cdc.gov/excite/index.htm

Source: CDC Excite

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Fundamentals of Epidemiology and Outbreak Investigations
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Funded by: Connecticut Department of Public Health Centers for Disease Control and Prevention Public Health Preparedness and Response for Bioterrorism Cooperative Agreement Centers for Disease Control and Prevention Center for Public Health Preparedness Cooperative Agreement New England Alliance for Public Health Workforce Development Health Resources and Services Administration Grant No. D20HP00003
Source: CDC Excite

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Training Schedule
Welcome and Housekeeping Items Pre-Test Purpose and Learning Objectives Lecture • Epidemiology Fundamentals • Outbreak Investigations 5. Group Exercise 6. Post-Test 1. 2. 3. 4.

Source: CDC Excite

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Purpose of Training
To provide a quick ―refresher‖ on • Basic epidemiology • Steps in outbreak investigations

Source: CDC Excite

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Learning Objectives
At the end of today‘s session, you will be able to:

 Describe how the sanitarian can work with the epidemiologist and other team members in outbreak investigations  List the reasons for and steps in an outbreak investigation  Describe and use some of the basic tools of field epidemiology

Source: CDC Excite

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Role of Non-Epidemiologists in Outbreak Investigations
1. Assure outbreak comes to attention ASAP of those with experience and authority to investigate.
– – Report outbreaks to the State DPH. Joint investigations by the LHD and DPH.

2. Provide assistance, as needed, depending on expertise.
– – Sanitarians can help to develop questionnaires, conduct environmental investigation. Surge capacity needed to: answer phones/ public inquiry; conduct interviews for analytic studies

Source: CDC Excite

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Roles in Foodborne Outbreak Investigations

Source: CDC Excite

Graphic developed by Terry Rabatsky-Ehr, Regional Epidemiologist, CT DPH

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Fundamentals of Epidemiology and Outbreak Investigations
What is Epidemiology? Introduction to the Categories of Epidemiology and Types of Epidemiological Studies III. Overview of the Disease Transmission Cycle IV. Outbreak Investigations: Steps and Tools But first: Consider the following three incidents and ask “What does public health do now?” I. II.

Source: CDC Excite

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Incident #1
• While reviewing surveillance data, a nurse noted three cases in a single month of hepatitis B • Three patients did not seem to have the usual risk factors • All three received injections at the same health care facility • Is this a coincidence? • Did these three cases occur by chance? • Is there a link?
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Source: CDC Excite

Incident #2
• Nurse reported two cases of severe respiratory illness, one of which had been fatal • Both people had attended the annual American Legion Convention • Between July 26 and August 2, 18 conventioneers had died • Led to the discovery of the gramnegative pathogen, Legionnella pneumophila
Source: CDC Excite

Fluorescent microscopy of Legionella pneumophilia

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Incident # 3
• Physician reports three patients with a marked increase in their white blood cell count and severe muscle pain • All had taken oral preparations of L-tryptophan • New syndrome identified: EMS • Problem traced to a contaminant introduced in the production process at a single facility

Source: CDC Excite

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Summary
• What do these three incidents have in common?

– They were unexpected.
– They demanded a response. – The investigators had to go out into the field to solve the problem. • They illustrate some of the key reasons for needing applied, or field, epidemiology

Source: CDC Excite

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I. What is Epidemiology?
―The study of the distribution and determinants of health-related states in specified populations, and the application of this study to control health problems."

The following key words will be defined - Study - Distribution - Determinants - Health-related states - Population

Source: CDC Excite

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Examination of Key Words
• Study: Epidemiology is a quantitative discipline based on statistics and research methodologies. – It is the basic science of public health. • Distribution: Epidemiology characterizes disease based on person, place and time. • Determinants: Epidemiology looks for causes or factors associated with an increased risk of disease.

Source: CDC Excite

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Examination of Key Words
• Health-related states: Epidemiology applies to the whole spectrum of health-related events – Chronic disease, environmental problems, behavioral problems, injuries, and infectious disease • Populations: Epidemiology deals with groups of people rather than with individual patients

Source: CDC Excite

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Role of Epidemiology
Epidemiology is more than just an analytical tool for studying diseases and their determinants. Epidemiologic data: 1) Steers public health decision making 2) Aids in developing and evaluating interventions to control and prevent health problems Use of epidemiological data to steer decision making to control and prevent health problems Is the primary function of applied, or field, epidemiology
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Source: CDC Excite

Comparison Between Healthcare and Public Health Practice
Healthcare
1. Focus = one person 2. Data from medical history

Public Health
1. Focus = population 2. Data from surveillance or descriptive studies

3. Make differential diagnosis
4. Diagnostic studies 5. Medical treatment

3. Generate hypothesis
4. Analytical studies

5. Community intervention
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Source: CDC Excite

What Do Epidemiologists Do?
– Count cases of disease or injury – Define the affected population – Compute rates of disease or injury in that population – Compare rates with those found in other populations – Make inferences regarding patterns of disease – Determine whether a problem exists

Source: CDC Excite

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II. Types of Epidemiological Studies
A. Experimental: Epidemiologist controls exposure of subjects to an intervention and observes the outcome B. Observational: Epidemiologist observes exposure and outcome without controlling either • Descriptive: Epidemiologist collects information to characterize and summarize health event • Analytical: Epidemiologist compares groups to identify risk factors

Source: CDC Excite

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Types of Epidemiology
• Descriptive epidemiology is concerned with – Person (who) – Place (where) – Time (when) • Analytical epidemiology uses the information gathered during the descriptive process to answer – How – Why

Source: CDC Excite

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Descriptive Studies
Deaths Associated with Tractor Injuries by Month of Death, Georgia 1971-1981

• What might this data mean?
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Source: CDC Excite

Descriptive Studies
Deaths Associated with Tractor Injuries by Time of Day

• What might this data mean?
Source: CDC Excite

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Descriptive Studies
Deaths Associated with Tractor Injuries by Place

Source: CDC Excite

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Descriptive Studies
Deaths Associated with Tractor Injuries by Age

• What might this data mean?
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Source: CDC Excite

Analytic Studies 1 – Cross-sectional study
• • Basically a survey that takes a ‗snapshot‘ of the population. Steps – Define the population to be studied – Collect information from members of population on disease and exposure – Examine the relationship between disease and exposure

Source: CDC Excite

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Analytic Studies 2 – Cohort study
• • Prospective study - follows populations of people over time Steps – Select populations that were and were not exposed to a hypothesized risk factor – Follow populations for occurrence of disease – Compare disease occurrence in those with and without exposure at beginning

Source: CDC Excite

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Analytic Studies 3 – Case-Control Study
• • Retrospective study – Epidemiologists work backward from the case Steps – Select subjects with disease (case) and without disease (control) – Collect information on hypothesized risk factors – Compare risk factors in cases and controls

Source: CDC Excite

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Limit of Analytical Studies
• Analytic studies enable us to quantify (mathematically calculate) the relationship between an exposure and a health outcome • However, a mathematical relationship between an effect and health outcome is not enough to establish causation.

Source: CDC Excite

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Cause-and-Effect Relationship
In general, five criteria must be met to establish a cause-and-effect relationship 1. Strength of Association: clear mathematical relationship 2. Consistency: observation must be repeatable 3. Temporality: cause must precede effect 4. Plausibility: must make sense biologically 5. Biological Gradient: must be doseresponse relationship
Source: CDC Excite

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III. Disease Transmission
Key terms defined – Epidemic is the occurrence of more cases than would normally be expected in a specific place or group over a given period of time – Outbreak is basically the same thing – Cluster is a group of cases that may or may not represent a greater than expected rate – Endemic is a persistent level of occurrence of a disease – Pandemic is a very widespread, often global epidemic
Source: CDC Excite

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Disease Transmission
• For an outbreak, or epidemic, to occur, the basic elements of disease causation and an adequate chain of transmission must be present. • Disease occurs when an outside agent capable of causing the disease meets a host that is vulnerable to the agent in an environment that allows the agent and host to interact. • Then, given a chain of transmission from one host to another and a suitable mode of spread, an outbreak can develop.

Source: CDC Excite

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Key Terms Defined: Agent
• Agent is the entity necessary to cause disease in a susceptible host – An agent can be biological, physical, chemical, or nutritional • Agents have important characteristics – Infectivity: capacity to cause infection in host – Pathogenicity: capacity to cause disease in host – Virulence: severity of disease that agent causes

Source: CDC Excite

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Key Terms Defined: Host
• Host is the person that may be acted upon by the agent • Status of the host is classified as – susceptible to the agent – immune to the agent – infected by the agent • Host‘s response to exposure can show – no effect – manifest subclinical disease – atypical symptoms – straightforward illness – severe illness
Source: CDC Excite

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Key Terms Defined: Environment
• Environment = conditions or influences that are not part of either the agent or the host, but that influence their interaction • Factors can include – physical – climatologic – biologic – social – economic conditions

Source: CDC Excite

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Disease Transmission
• Agent, host, and environment alone are not sufficient to cause an epidemic • An adequate chain of transmission must be present • A chain of transmission requires the following elements: – a source of the agent – a portal of exit from the source – a mode of transmission – and a portal of entry into the susceptible person

Source: CDC Excite

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Transmission Elements Defined
• Source of infection may be a human, an animal or the environment • Portal of exit is a pathway by which the agent can leave the source • Mode of transmission is the means of carrying agent to the host • Portal of entry is the pathway that gives the agent access to tissue where it can multiply or act

Source: CDC Excite

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Modes of transmission
• Direct – Direct contact – Droplet nuclei • Indirect – Airborne – Vehicleborne – Vectorborne • Mechanical • Biologic

Source: CDC Excite

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Field Epidemiology
• Field epidemiology is practice or application of epidemiology to control and prevent health problems – Act when problem is acute and unexpected and when quick action is required – High levels of community concern – Involvement of the press – Political pressure

Source: CDC Excite

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Field Epidemiology
• Field investigations are action oriented, with the main goal being to solve a pressing public health problem • There is a need to institute the controls necessary to safeguard health as soon as possible • Challenges to investigators: – limited control over the situation – little time for planning a study – limited date sources and laboratory samples • Must do best science possible under the circumstances
Source: CDC Excite

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IV. Outbreak Investigations: Steps and Tools

Source: CDC Excite

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Challenges in Outbreak Investigations
• Cause, source may not be known • Large numbers of people may be affected • Residents fear more illness • Possible hostility or defensiveness if an individual, product or company is accused of being the source. • Epidemiological team must remain calm, professional and scientifically objective
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Source: CDC Excite

Uncovering Outbreaks
• Call from a doctor, healthcare provider, citizen • Routine analysis of public health surveillance data – Data on health which are systematically collected, analyzed, interpreted, and disseminated. – Data analysis shows an increase over normal background level
Source: CDC Excite

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Why Investigate Outbreaks?
1. Control and prevention

2. Severity and risk to others
3. Research opportunities 4. Training opportunities

5. Program considerations
6. Public, political, or legal concerns

Source: CDC Excite

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Control and Prevention
• Control the outbreak at hand and prevent future outbreaks • Assess its extent and the characteristics of the population at risk • Design measures to prevent additional cases • Investigate further to identify its source and use that information to develop measures that will prevent future outbreaks. • Strike a balance between instituting control measures and conducting further investigation.
Source: CDC Excite

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Severity and Risk to Others
• It is urgent to investigate an outbreak when the disease is severe and could affect more people

Source: CDC Excite

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Research Opportunities
• Each outbreak offers a unique opportunity to study the natural history of the disease in question— including the agent, mode of transmission, and incubation period. • Epidemiologists can learn more about the impact of control measures and the usefulness of new epidemiological and laboratory techniques.

Source: CDC Excite

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Training Opportunities
• Investigative skills improve with practice and experience. • Pairing a seasoned epidemiologist with an epidemiologist-intraining has value

Source: CDC Excite

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Program Considerations
• Investigating an outbreak of a disease targeted by public health authorities for prevention and control may highlight: – Populations at risk that have been overlooked – Failures in the program‘s intervention strategy – Changes in the agent causing the disease – Events beyond the scope of the program

Source: CDC Excite

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Public, Political, or Legal Considerations
• Public, political, or legal concerns sometimes override scientific concerns in the decision to conduct an investigation. • Many health departments have learned that it is essential to be "responsibly responsive" to public concerns

Source: CDC Excite

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10 Steps in an Outbreak Investigation
1 2 3 4 5 Prepare for field work Establish the existence of an outbreak Verify the diagnosis Define and identify cases Describe and orient the data in terms of time, place, and person Develop hypotheses Evaluate hypotheses Refine hypotheses and carry out additional studies Implementing control and prevention measures Communicate findings
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6 7 8 9 10

Source: CDC Excite

Step 1. Prepare for Field Work
• Research the disease and gather supplies and equipment • Make necessary administrative and personal arrangements for such things as travel, etc. • Consultations with all parties to determine their role in the investigation and who local contacts are
Source: CDC Excite

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Step 2. Establish Existence of an Outbreak
• Is a suspected outbreak a real outbreak? – Some are true outbreaks with common cause – Some are unrelated cases of the same disease – Others are unrelated cases of similar, but unrelated, diseases • To determine if an outbreak exists (i.e., whether the observed number of cases exceeds the expected number), first the expected number of cases for the area in the given time frame must be determined.

Source: CDC Excite

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Step 2. Establish Existence of an Outbreak
• Compare current cases with previous occurrences – Check health department records – Consult local data sources – Make estimates from neighboring states or national data

Source: CDC Excite

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Step 2. Establish Existence of an Outbreak
• If current number of reported cases exceeds expected number, further investigation is needed • Many factors affect changes in total number of cases reported
– – – – Change in reporting procedures or case definition Increased local interest or public awareness Improved diagnostic procedures Seasonal population changes

Source: CDC Excite

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Step 3. Verify the Diagnosis
• Epidemiologists identify as accurately as possible the nature of the disease. – First, to ensure the problem has been properly diagnosed and that it really is what it is reported to be. – Second, for outbreaks involving infectious or toxic-chemical agents, to be certain that the increase in diagnosed cases is not the result of a mistake in the laboratory.

Source: CDC Excite

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Step 3. Verify the Diagnosis
• Review clinical findings and laboratory results for people affected • Verify laboratory findings • Prepare for any specialized laboratory work • Visit and interview several people who became ill

Source: CDC Excite

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Step 4. Define and Identify Cases
• Epidemiologists establish a case definition: a standard set of criteria for deciding whether a person should be classified as having the disease or condition under study. Usually includes components: 1. Clinical information about the disease 2. Characteristics about the people who are affected 3. Information about the location or place 4. A specification of time during which the outbreak occurred.
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•

Source: CDC Excite

Step 4. Define and Identify Cases
• Criteria are be based on objective measures that are consistently applied without bias to all people included in the investigation • Case definitions are broad enough to include most actual cases while avoiding ―falsepositive‖ cases (when the case definition is met, but the person does not have the disease).
Source: CDC Excite

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Step 4. Define and Identify Cases
Investigators often classify cases as one of the following: • Confirmed: usually has laboratory verification • Probable: usually has clinical features without lab verification • Possible: usually has fewer of typical clinical features

Source: CDC Excite

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Step 4. Define and Identify Cases
• When an outbreak is first recognized the epidemiologist must ―cast the net wide‖ to identify cases • Possible sources of cases – Health care facilities – Public alert – Survey of population, particularly if outbreak occurs in restricted population – Case patient referral

Source: CDC Excite

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Step 4. Define and Identify Cases
The following information is collected: 1. Identifying information 2. Demographic information • Details to characterize population at risk 3. Clinical information • Allows the creation of a epidemic curve and a description of the spectrum of illness 4. Risk factor information • Helps to tailor investigation to the specific disease in question
Source: CDC Excite

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Case Report Form
• This information is entered on a case report form. • CT DPH has a number of different case report forms, depending on the nature of the outbreak
Sample report form from the DPH Food Protection Program

Source: CDC Excite

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Line Listing
Next, selected critical items are abstracted into a table called a ―line listing‖ – Each column represents an important variable, such as age and sex – Each row represents a different case, by number • This simple format allows the investigator to scan key information on every case and update it easily

Source: CDC Excite

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Example of a Line Listing

Source: CDC Excite

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Step 5. Describe and Orient the Data
• Characterize the outbreak by time, place, and person (descriptive epidemiology)

• Benefits
– Allows you to become familiar with the data, especially what is and is not reliable – Provides a comprehensive description of the outbreak – Allows you to develop a causal hypothesis based on what is known about the disease

Source: CDC Excite

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Characterizing By Time: Epidemic Curve
Epidemic curve or “epi curve” = a graph of the number of cases by their date of onset.

• Simple visual display of outbreak‘s magnitude and time trend. • Shows course of epidemic • May enable estimation of probable time period of exposure • May enable inferences to be drawn about the epidemic pattern

Source: CDC Excite

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Characterizing By Time: Epidemic Curve
• • • How to draw an epidemic curve Know the time of onset for each person Number of cases is plotted on y-axis Time is plotted on the x-axis – The unit of time is based on incubation period and length of time over which cases are distributed. Select a unit that is one-fourth to onethird as long as the incubation period. Show the pre- and post-epidemic period to illustrate the activity during those periods

•

Source: CDC Excite

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Example 1: Epidemic Curve
• Insert EPI Curve

Source: CDC Excite

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Example 2: Epidemic Curve

Source: CDC Excite

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Epidemic Curve: Interpreting the Shape
• Point source epidemic • Shape – a steep up slope, a peak and a gradual down-slope • Interpretation - people are exposed to the same source over a relatively brief period • Continuous common source epidemic • Shape - curve will have a plateau instead of a peak • Interpretation - people are exposed to the same source over an extended period • Propagated epidemic • Shape - a series of progressively taller peaks • Interpretation - person-to-person spread
Source: CDC Excite

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Example 1: Epidemic Curve
• Insert EPI Curve

Source: CDC Excite

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Example 2: Epidemic Curve

Source: CDC Excite

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Characterizing By Time: Epidemic Curve
Outliers = cases that stand apart • Early case may represent: – a background (unrelated) case – source of epidemic (index case) – an early exposure • Late cases may be: – unrelated – have long incubation periods – indicate later exposure – secondary cases
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Source: CDC Excite

Example 2: Epidemic Curve

Source: CDC Excite

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Characterizing By Place: Spot Map
• Assessment of an outbreak by place provides information on the geographic extent of a problem • A spot map of cases in a community may show clusters or patterns that reflect water supplies, wind currents, or proximity to a restaurant or grocery store.
John Snow and Broad Street Pump map
Source: CDC Excite

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Characterizing By Place: Spot Map
• If the size of the overall population varies between the areas under comparison, a spot map, because it shows numbers of cases, can be misleading. This is a weakness of spot maps. Discuss the data in the spot map on the right. What are some possible interpretations?

•

Dead crow sightings, 2000 (CT DPH Mosquito Management Program 2000 Annual Report)
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Source: CDC Excite

Source: CDC Excite

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Characterizing By Place
• To compensate, maps can show the proportion of people affected in each area • This also represents the rate of disease or, in the investigation of an outbreak, the "attack rate"

Connecticut Epidemiologist, July 2001

Source: CDC Excite

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Characterizing By Person
• Determine the populations at risk by characterizing the outbreak by person • Define populations by – Personal characteristics (Examples: age, race, sex, or medical status) – Exposures (Examples: occupation, leisure activities, use of medications, tobacco, drugs) • Age and sex are usually assessed first, because they are often the characteristics most strongly related to exposure and to the risk of disease.
Source: CDC Excite

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Step 5. Describe and Orient the Data in Terms of Time, Place, Person
Summarize • After characterizing an outbreak by time, place, and person (descriptive epidemiology), epidemiologists need to summarize what they know to see whether their initial hypotheses are on track. • New hypotheses may need to be developed to explain the outbreak.

Source: CDC Excite

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Step 6. Develop Hypotheses
• Hypotheses may be based on – Interviews with affected people – Consultation with health officials in community – Descriptive epidemiology - person, place and time It should incorporate the known characteristics of the agent It should be testable.

• •

Source: CDC Excite

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Step 7. Evaluate Hypotheses
Two approaches 1) Compare hypotheses with the established facts. This method is used when the evidence is so strong that the hypothesis does not need to be tested 1) Use analytic epidemiology to test hypotheses by using a comparison group to quantify relationships between various exposures and the disease.

Source: CDC Excite

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Step 7. Evaluate Hypotheses Analytic Studies
There are two types of analytic studies – Cohort Studies = compare groups of people who have been exposed to suspected risk factors with groups who have not been exposed. – Case-Control Studies = compare people with a disease (case-patients) with a group of people without the disease (controls).

Source: CDC Excite

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Study Type: Cohort Studies
• Best for analyzing an outbreak in a small welldefined population – Example: gastroenteritis among people who attended a wedding • Ask each attendee the same set of questions about potential exposures • In cohort studies, an attack rate can be calculated for people who ate a particular item (were exposed) and an attack rate for those who did not eat that item (were not exposed).

Source: CDC Excite

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Attack Rates
For the exposed group, the attack rate equals the number of people who ate item and became ill divided by (’) the total number of people who ate that item.
Total # of people who ate the item and became ill
Total # of people who ate that item

Exposed Group =

Source: CDC Excite

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Attack Rates
For the not exposed group, the attack rate equals the number of people who did not eat item but still became ill divided by (’) the total number of people who did not eat that item.

Total # of people who did not eat the item but still became ill Not Exposed Group = Total # of people who did not eat that item

Source: CDC Excite

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Relative Risk
• To identify source of outbreak, look for – High attack rate among those exposed and – Low attack rate among those not exposed and In addition – Most of the people who became ill should have consumed the item • Calculate the relative risk: mathematical association between exposure and illness for each food and beverage

Source: CDC Excite

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Relative Risk
Relative risk is calculated by dividing (’) the attack rate for people who were exposed to the item by the attack rate for those who were not exposed.

Attack rate for people who were exposed to the item Relative Risk = Attack rate for those who were not exposed

Source: CDC Excite

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Attack Rate Table 1
Number of people who ate specified item Food Baked Ham Mashed potatoes Spinach Cabbage Salad Milk Ice Cream (Van) Fruit salad Ice Cream (Choc) Ill 29 23 26 18 2 43 4 25 Well 17 14 17 10 2 11 2 22 Total 46 37 43 28 4 54 6 47 Attack Rate % Number of people who did not eat specified item Ill 17 23 20 28 44 3 42 20 Well 12 14 12 19 27 18 27 7 Total 29 37 32 47 71 21 69 27 Attack Relative Rate % Risk

Source: CDC Excite

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Example: Attack Rates
• • • • • • • •
Food Baked Ham Mashed potatoes Spinach Cabbage salad Milk Ice Cream (Van) Ice Cream (Choc) Fruit salad Exposed Group 29/46 = 63% 23/37 = 62% 26/43 = 60% 18/28 = 64% 2/4 = 50% 43/54 = 80% 25/47 = 53% 4/6 = 67%

Source: CDC Excite

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Example: Attack Rates
• • • • • • • •
Food Baked Ham Mashed potatoes Spinach Cabbage salad Milk Ice Cream (Van) Ice Cream (Choc) Fruit salad Not Exposed Group 17/29 = 59% 23/37 = 62% 20/32 = 62% 28/47 = 60% 44/71 = 62% 3/21 = 14% 20/27 = 74% 42/69 = 61%

Source: CDC Excite

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Attack Rate Table 2
Number of people who ate specified item Food Baked Ham Mashed potatoes Spinach Cabbage Salad Milk Ice Cream (Van) Fruit salad Ice Cream (Choc) Ill 29 23 26 18 2 43 4 25 Well 17 14 17 10 2 11 2 22 Total 46 37 43 28 4 54 6 47 Attack Rate % 63 62 60 64 50 80 67 53 Number of people who did not eat specified item Ill 17 23 20 28 44 3 42 20 Well 12 14 12 19 27 18 27 7 Total 29 37 32 47 71 21 69 27 Attack Relative Rate % Risk 59 62 62 60 62 14 61 74

Source: CDC Excite

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Attack Rate Table 3
Number of people who ate specified item Food Baked Ham Mashed potatoes Spinach Cabbage Salad Milk Ice Cream (Van) Fruit salad Ice Cream (Choc) Ill 29 23 26 18 2 43 4 25 Well 17 14 17 10 2 11 2 22 Total 46 37 43 28 4 54 6 47 Attack Rate % 63 62 60 64 50 80 67 53 Number of people who did not eat specified item Ill 17 23 20 28 44 3 42 20 Well 12 14 12 19 27 18 27 7 Total 29 37 32 47 71 21 69 27 Attack Relative Rate % Risk 59 62 62 60 62 14 61 74 1.07 1.00 0.97 1.07 0.81 5.71 1.10 0.72

Source: CDC Excite

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Study Type: Case Control Studies
• When the population in an outbreak is not well defined, a case control study design is used • Both case-patients and controls are asked about their exposures • Controls must not have the disease, but should be from the same population as the case-patients • The measure of association used is called an odds ratio

Source: CDC Excite

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Odds Ratio
• In a case-control study, attack rates cannot be calculated because the total number of people in the community who were and were not exposed to the source of the disease under study is not known • An odds ratio is used to measure of association • To calculate an odds ratio, it is helpful to look at data in a 2 x 2 table.

Source: CDC Excite

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Odds Ratio
Example • Suppose an epidemiologist were investigating an outbreak of hepatitis A in a small town. • The suspected source was a favorite restaurant of the townspeople. • After questioning case-patients and controls about whether they had eaten at that restaurant, the data might look like this in a 2 x 2 table:

Source: CDC Excite

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Odds Ratio: 2 x 2 Table

Source: CDC Excite

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Odds Ratio
• The odds ratio is calculated as: ad/bc Using the example from previous 2 x 2 Table: 30 x 70 ’ 36 x 10 = 5.8 • People who ate at restaurant A were 5.8 times more likely to develop hepatitis A than were people who did not eat there • Now epidemiologists can compare the odds ratio with the odds ratios for other possible sources

Source: CDC Excite

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Step 7: Evaluate Hypotheses Testing Statistical Significance
• Determine how likely the study results could have occurred by chance – Called testing for ―statistical significance‖ • Steps to testing ―statistical significance‖ – State null hypothesis – no association between exposure and outcome – Calculate chi-square test – Look up corresponding p-value in table of chisquares.

Source: CDC Excite

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Interpreting P-Values
• Epidemiologists set in advance a cutoff point above which they will consider that chance is a factor – The common cutoff point is .05 – If the p-value is below the cutoff point, the finding is considered ―statistically significant‖ and the null hypothesis is rejected • The smaller the p-value, the stronger the evidence is for statistical significance

Source: CDC Excite

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Step 8. Refine Hypotheses and Carry Out Additional Studies
• When analytic epidemiological studies do not confirm the hypotheses, they must be reconsidered along with new vehicles or modes of transmission • Even when an analytic study identifies an association, hypotheses will need to be refined • Often, more specific exposure histories or a more specific control group are needed • Epidemiologists consider what questions remain unanswered, and what kind of study might be used in the particular setting to answer some of these questions
Source: CDC Excite

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Step 8. Refine Hypotheses and Carry Out Additional Studies
Laboratory and environmental studies • While epidemiology can implicate vehicles and guide appropriate public health action, laboratory evidence can clinch the findings • Environmental studies often help explain why an outbreak occurred and may be very important in some settings

Source: CDC Excite

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Step 9. Implementing Control and Prevention Measures
• Control measures, which can be implemented early, should be aimed at specific links in the chain of infection, the agent, the source, or the reservoir • In some situations, control measures are directed at interrupting transmission or exposure – Limit airborne spread – Use the method of ―cohorting‖ by putting infected people together in separate area • Some control measures are directed at reducing susceptibility, such as travel immunizations
Source: CDC Excite

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Step 10. Communicate Findings
• • The final task in an investigation is to communicate the findings to others who need to know. This communication usually takes two forms: 1) an oral briefing 2) a written report

Source: CDC Excite

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Role of Non-Epidemiologists in CT Outbreak Investigations
1. Assure outbreak comes to attention ASAP of those with experience and authority to investigate. – Report outbreaks to the state DPH. – Joint investigations by the LHD and DPH. 2. Provide assistance, as needed, depending on expertise. – Sanitarians can help to develop questionnaires, conduct environmental investigation. – Surge capacity needed to: answer phones/public inquiry; conduct interviews for analytic studies.
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Source: CDC Excite

Roles in Foodborne Outbreak Investigations

Source: CDC Excite

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