Transcript ch10

Chapter 10
Field
Epidemiology
What Is Field Epidemiology?
 Field epidemiology has been defined as the
application of epidemiology under a set of general
conditions
 The problem is unexpected
 A timely response may be demanded
 Travel to and work in the field is required by
epidemiologists to solve the problem
 The investigation time is likely to be limited because of
the need for a timely intervention
Field Investigation
 Field investigations involving acute problems
may differ from conventional epidemiologic
studies in three important ways
1. Field investigations often do not start with a clear
hypothesis.
2. Acute problems involve an immediate need to protect
the public and resolve the concern.
3. Field epidemiologists must decide when the available
information is sufficient to take appropriate action.
Steps in Field Investigations
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Establish the existence of an epidemic (or outbreak)
Confirm the diagnosis
Establish criteria for case identification
Search for missing cases
Count cases
Orient data according to person, place, and time
Classify the epidemic
Determine who is at risk of becoming a case
Analyze data
Formulate hypotheses
Test hypotheses
Develop reports and inform those who need to know
Execute control and prevention measures
Administration and planning activities
1. Establish the existence of an
epidemic (or outbreak)
 Attack rates are appropriate statistics for
investigating disease outbreaks because they
describe rapidly occurring new cases of disease
in a well-defined population over a limited time
period
 Attack rates are usually calculated by person
characteristics (e.g., age, sex, race/ethnicity,
and occupation) in order to identify high-risk
groups
2. Confirm the Diagnosis
 Assessment of the clinical findings should be
done to assure correctness and reliability of the
findings
 Clinical diagnosis by appropriately trained
individuals
 Laboratory diagnosis
3. Establish Criteria for Case
Identification
 Standard clinical criteria (what)
 Loose case definition vs. strict case definition
 A case may be further characterized by
 Who
 When
 Where
4. Search for Missing Cases
 Investigation may include
 Physicians
 Clinics
 Health maintenance organizations
 Hospital emergency rooms
 Public health clinics
 Migrant health clinics
 Related facilities
 Asymptomatic persons or mild cases and their
contacts should be evaluated
 Suspected cases vs. probable cases
5. Count Cases
 Exposure status and disease frequency need to
be determined and compared with the
appropriate at-risk population
6. Orient data according to
person, place, and time
 Person
 Inherent characteristics or people (age, race/ethnicity,
sex)
 Acquired characteristics (immunity or marital status)
 Activities (occupation, leisure, use of medications)
 Conditions (socioeconomic state, access to health care)
 Place
 Residence,birthplace, place of employment, school
district, hospital unit, country, state, county, street
address, map coordinates, etc.
 Census tract
 Time
 Epidemic curve
7. Classify the Epidemic
 Common source
 Propagated
 Mixed
8. Determine who is at risk of
becoming a case
 Clinical, medical, and lab findings need to be
confirmed, evaluated, and analyzed for all cases
to substantiate the diagnosis
 Classify cases by exposure status
9. Analyze Data
 The epidemiologist gathers, compiles, tabulates,
analyzes, and interprets the findings
 Analysis often involves statistical methods:
 Frequency tables
 Bivariate analyses
 Multiple regression
10. Formulate Hypotheses
 For example, in a food-borne outbreak, hypotheses
should be developed for the following
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Infection source, vehicle
Suspect foods
Transmission mode
Pathogen type (based on clinical symptoms, incubation
periods)
Time factors in the outbreak and course of the disease
Place factors in the outbreak
Person characteristics and factors in the outbreak
Outside sources of the infection
Transmission of the disease outside of the study population
Exposed, unexposed, well, and ill cases/individuals
11. Test Hypotheses
 Statistical tests should be employed to evaluate
hypotheses
 T-test
 Chi-square test
 F-test
 If established facts or information are lacking to
substantiate a hypothesis, more information should
be gathered or research hypothesis should be
rejected
12. Develop reports and inform
those who need to know
 Narrative of the investigation and review of the
course of the epidemic
 Tables, graphs, charts, or any useful and helpful
illustrations are presented, as well as any pertinent
epidemiologic data, tests, lab reports, information,
and characteristics
 Addresses the information presented under
hypothesis
13. Execute control and prevention
measures
 Immunization programs
 Risk factor prevention
 Behavior change programs
14. Administration and planning
activities
 Organization
 Coordination
 Communication
 Planning
 Funding
 Allocation
The Steps
 Conceptual
 Provide a logical progression for the investigation
 Can/should be taken out of order (with caution)
Oswego
 Oswego — An Outbreak of Gastrointestinal
Illness Following a Church Supper, Adapted from
Centers for Disease Control and Prevention,
Epidemiology Program Office, Case Studies in
Applied Epidemiology, No. 401-303
 http://www.cdc.gov/EIS/casestudies/xoswego.40
1-303.student.pdf
 Gross MB. Oswego county revisited. Public
Health Reports 1976;91:160-70.
PART I - Background
 On April 19, 1940, the local health officer in the village of
Lycoming, Oswego County, New York, reported the occurrence
of an outbreak of acute gastrointestinal illness to the District
Health Officer in Syracuse. Dr. A. M. Rubin, epidemiologist-intraining, was assigned to conduct an investigation. When Dr.
Rubin arrived in the field, he learned from the health officer that
all persons known to be ill had attended a church supper held on
the previous evening, April 18. Family members who did not
attend the church supper did not become ill. Accordingly, Dr.
Rubin focused the investigation on the supper. He completed
interviews with 75 of the 80 persons known to have attended,
collecting information about the occurrence and time of onset of
symptoms, and foods consumed. Of the 75 persons interviewed,
46 persons reported gastrointestinal illness.
Would you call this an epidemic?
Would you call it an outbreak?
 An outbreak, if we consider an outbreak to reflect
a more localized situation and an epidemic to
refer to more widespread and possibly more
prolonged situations. In addition, the term
outbreak may be perceived as less alarming than
epidemic.
Review the steps of an
outbreak investigation
 The advantage of a list such as this, or an
alternative version of this list, is to make sure
that important steps are not missed while
conducting a field investigation
 However, the order of these steps is not fixed
(e.g., it may be appropriate to implement control
measures immediately)
 In addition, the steps are often dynamic; that is,
case definitions, line listings, hypotheses, and so
on, can and sometimes should change with
further information
Clinical Description
 The onset of illness in all cases was acute,
characterized chiefly by nausea, vomiting,
diarrhea, and abdominal pain. None of the ill
persons reported having an elevated temperature;
all recovered within 24 to 30 hours. Approximately
20% of the ill persons visited physicians. No fecal
specimens were obtained for bacteriologic
examination.
Broad Categories of Diseases
 List the broad categories of diseases that must
be considered in the differential diagnosis of an
outbreak of gastrointestinal illness
 Pathogen (bacterial, viral, parasitic), toxin (e.g., heavy
metals, drugs, insecticides), other (sociogenic,
radiation)
 The investigators suspected that this was a
vehicle-borne outbreak, with food as the vehicle.
What is a vehicle? What is a vector?
 Vehicle – a nonliving intermediary such as food,
water, biologic product, or fomite (inanimate object)
that conveys the infectious agent from its reservoir
(habitat where an infectious agent lives, grows, and
multiplies) to a susceptible host
 Vector – a living intermediary, most often an insect
or arthropod (e.g., mosquito, flea, or tick), that
conveys the infectious agent from its reservoir to a
susceptible host
What are other modes of
transmission?
 Transmission – when an infectious agent leaves its reservoir or
host through a portal of exit, is conveyed by some mode of
transmission, and enters through an appropriate portal of entry
(i.e., oral, respiratory, reproductive, intravenous, urinary, skin,
gastrointestinal, conjuctival, transplacental) to infect a susceptible
host. Transmission involving a vector may be mechanical (i.e., the
agent does not multiply or undergo physiologic changes in the
vector) or biological (i.e., the agent undergoes part of its life cycle
inside the vector prior to transmission into the new host).
 Direct transmission – direct contact (e.g., mucous membrane to
mucous membrane, skin-to-skin, across placenta, fecal-oral),
droplet spread (e.g., sneezes, coughs)
 Indirect transmission – airborne, vehicle-borne, vector-borne
If you were to administer a questionnaire
to the church supper participants, what
information would you collect?
 Identifying information (e.g., name, address, phone
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number)
Demographic information (e.g., age, sex, occupation)
Clinical information (e.g., clinical signs and symptoms,
severity or outcome, time of onset, duration, documented
medical care, pre-existing medical conditions,
medications)
Epidemiology information (e.g., specific exposures and
contacts, such as foods eaten at the picnic, how much,
when, foods eaten before the picnic and before illness,
activities, and contact with ill persons)
Abstractor or interviewer information
What is a line listing?
What is the value of a line listing?
 Looks like a spreadsheet where each row
represents data for a single case and each
column represents a variable (such as identifying
information, demographic information, clinical
information, epidemiologic information, etc.)
Description of the Supper
 The supper was held in the basement of the village
church. Foods were contributed by numerous
members of the congregation. The supper began at
6:00 p.m. and continued until 11:00 p.m. Food was
spread out on a table and consumed over a period
of several hours. Data regarding onset of illness
and food eaten or water drunk by each of the 75
persons interviewed are provided in the attached
line listing. The approximate time of eating supper
was collected for only about half the persons who
had gastrointestinal illness.
What is the value of an epidemic
curve?
 A histogram that shows the course of a disease outbreak or
epidemic by plotting the number of cases by time of onset.
The epidemic curve is a basic tool of field epidemiologists
because it is informative in the following ways:
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Shows the magnitude of the epidemic over time
Can distinguish epidemic from endemic disease
Potentially correlated events can be noted on the graph
Its shape can provide clues as to whether a point source,
intermittent source, or propagated source is responsible
 Shows where we are in the course of the epidemic (i.e., still on
the upswing, down slope, or end)
 Can indicate how long it took the health department to identify
the problem
 Outlying cases are easily recognized
What does the graph tell you?
 One very early and one very late case
 Most cases are clustered tightly within a six-hour
interval
 The tight clustering around a single peak is
consistent with a point source outbreak
Case Inconsistencies
 Are there any cases for which the times of onset
are inconsistent with the general experience? How
might they be explained?
 Subject #52 was an 8-year-old boy who ate early (11:00
a.m.). The boy may have eaten early and the vehicle
already prepared and contaminated by 11:00 a.m.
 Subject #16 was a 32-year-old woman. Perhaps she had
a longer incubation period or ate later. Other possibilities
are that she had an unrelated illness, the information was
incorrect, there was a data coding error, or she was a
secondary case.
How could the data in the line
listing be better presented?
 Data could be separated according to illness
status or time of onset
 Military time could have been used
Determine the mean, median, and
range of the incubation period
 Range: Minimum = 3 hours and maximum = 7
hours. The range is 4 hours
 The median incubation period is 4 hours
Differential Diagnosis
 How does the information on incubation period,
combined with the data on clinical symptoms, help in
the differential diagnosis of the illness?
 Each food-borne illness has a characteristic incubation
period, specific symptoms, and foods with which it is most
likely associated.
 The observed incubation period is too long for heavy
metals and too short for viral agents and botulism.
 Referring to a partial list of incubation periods for major
communicable diseases (see Merrill, RM. Introduction to
Epidemiology, 5th edition. Sudbury, MA: Jones and Bartlett
Publishers; 2010, Table 3-7). Staphylococcal food
poisoning has an average incubation period of 2-4 hours.
Hypothesis Testing
HO: RR =1
HA: RR ≠ 1
α = 0.05, n = 75
43 /(43  11)
Risk Ratio (RR) 
 5.6;
3 /(3  18)

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
11/43
18/3
  1.95, 16 .10
95% CI(5.6)  exp ln5.6  1.96 
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43  11 3  18 

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Note
 Three ill people denied eating the vanilla ice cream.
They may have not remembered eating the vanilla
ice cream, there could be multiple vehicles, or there
could have been cross-contamination between
foods by way of dishes, spoons, and servers.
Outline further investigations that
should be pursued
 Detailed review of source, ingredients, preparation,
and storage of incriminated food
 Try to explain cases with atypical time of onset
 Laboratory examination
 Determine if secondary spread in family members
occurred
 Additional calculations (e.g., age- or genderspecific attack rates)
What control measures would you
suggest?
 Prevent consumption of remaining vanilla ice
cream
 Prevent recurrence of similar events in the future
by educating food handlers
 Ascertain whether a commercial product is involved
 Eliminate any contaminated sources of food
Why was it important to work up this
outbreak?
 Rule out contamination of a commercial product. If a commercial
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product is involved, immediate intervention may prevent a
substantial number of further cases.
Prevent future outbreaks by identifying infected food handler,
specific gaps in education or food handling techniques
Public health officials need to respond to such problems in a
timely manner to maintain a cooperative relationship with local
health departments, private physicians, and the community
An epidemiologic explanation of the cause of the outbreak may
allay community fears and concerns
The outbreak investigation may provide opportunities for
investigators to answer questions about the agent, host,
environment, incubation period, and so on
How does this investigation fit the steps for
conducting an outbreak investigation?
 Prepare for field work – Not discussed
 Establish the existence of an epidemic or outbreak – Done
 Confirm the diagnosis – Diagnosis based on signs and
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symptoms. No fecal specimens were obtained for bacteriologic
examination.
Establish criteria for case identification – No case definition was
given in the case study
Search for missing cases – Not discussed
Count cases – Done
Orient the data according to person, place, and time – Not done,
with the exception that time was characterized by the epidemic
curve
Classify the epidemic – Done: point source
Fitting Outbreak Investigation?
 How does this investigation fit the steps for conducting an
outbreak investigation?
 Determine who is at risk of becoming a case – Done: those at the
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church super
Analyze the data – Done: risk ratios involving the different foods
eaten
Formulate hypotheses – Not formally done
Test hypotheses – Not formally done
Develop reports and inform those who need to know – A report was
prepared and disseminated (i.e., see Part IV of the case study)
Maintain surveillance to monitor trends and execute control and
prevention measures – On May 19, all remaining ice cream prepared
by the Petrie sisters from raw milk from the Petrie farm was
condemned. The method of contamination of ice cream is not clearly
understood. Whether positive Staphylococcus nose and throat
cultures occurring in the Petrie family was involved is unclear.
Carry out administration and planning activities – Not specified in the
case study