epid 600 class 13 outbreaks
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An illness due to a specific infectious agent or its toxic
products that arises through transmission of that agent or
its products from an infected person, animal or inanimate
reservoir to a susceptible host; either directly or indirectly
through an intermediate plant or animal host, vector or the
inanimate environment
Infectious disease
Benensen AS, editor. Control of Communicable Diseases Manual. Sixteenth Edition, 1995. 2
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Dynamics of disease transmission
Human disease results from interaction between the host, agent andthe environment. A vector may be involved in transmission.
Host susceptibility to the agent is determined by a variety of factors,including genetic background, nutritional status, vaccination, priorexposure, context
AGENT
HOST
VECTOR
ENVIRONMENT
Epidemiologic
Triad
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Factors associated with increased risk of
human disease
Host Characteristics Agent Environmental Factors
Age Biologic (Bacteria, viruses) Temperature
Sex Chemical (Poison, smoke) Humidity
Race Physical (Trauma, radiation) AltitudeOccupation Nutritional (Lack, excess) Crowding
Marital Status Housing
Genetics Neighborhood
Previous Diseases Water
Immune Status Food
Air Pollution
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The potential for a given agent to cause an outbreak depends
on the characteristics of the agent, including the mode of
transmissionof the agent
Two basic modes of transmission
Direct
Indirect
Certain diseases can be transmitted directly or indirectly
Modes of disease transmission
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In an infectious setting, immediate and direct transfer of an
agent to a host by an infected person or animal
Touching, biting, or sexual intercourse are classic examples
Measles virus: airborne by droplet spread or direct contactwith nasal/throat secretions of infected persons
In a noninfectious setting, the host may have direct contact
with the agent in the environmentChildren ingesting lead paint from playground equipment
Direct mode of disease transmission
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Vehicle-borne
Transmission through contaminated inanimate objects (toys, food,
water, surgical utensils, or biological products such as blood, tissues
or organs)
Vector-borne
Transmission through simple contamination by animal or arthropod
vectors or their actual penetration of the skin or mucous membranes
Airborne
Transmission occurs when microbial, particulate, or chemical agents
are aerosolized and remain suspended in air for long periods of time
Indirect mode of disease transmission
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Interval from receipt of infection to the time of onset of clinical
illness (signs and symptoms)
Different diseases have different incubation periods
No precise incubation period but a range is characteristic for
a disease
What accounts for this delay?
Time needed for the pathogen to replicate to the critical
mass necessary for clinical diseaseSite in the body at which the pathogen replicates
Dose of the infectious agent received at time of infection
Incubation period
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The spectrum of severity varies by disease:
1. Exposure, No infection2. Carrier - Individual harbors the pathogen but does not show
evidence of clinical illness; a potential source of infection
(can transmit the agent)
3. Subclinical Infection - Disease that is not clinically apparent; leadsto immunity, carrier, or non-immunity
4.
Clinical Infection - Apparent disease characterized by signs andsymptoms; results in immunity, carrier, non-immunity, or severe
consequences such as death
Outcomes of exposure to an agent
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Endemic - The habitual presence (or usual occurrence) of a disease
within a given geographic area
Epidemic - The occurrence of an infectious disease clearly in excess
of normal expectancy, and generated
from a common or propagated sourcePandemic - A worldwide epidemic affecting an exceptionally high
proportion of the global population
Endemic
Epidemic
Time
Number
of Casesof
Disease
Endemic, epidemic, pandemic
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Deaths in Greater London;
December 1 15, 1952
NumberofDeaths
December
1
1.000
800
600
400
200
0
Period of Dense Fog
2 3 4 5 6 7 8 9 10 11 12 13 14 15
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Disease Outbreaks
Typically, sudden and rapid increase in the number of cases of a
disease in a population
Common Source
Cases are limited to those who share a common exposure
Food-borne, water
Propagated
Disease often passed from one individual to another
Measles, STDs
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Essential Steps in an Outbreak
Investigation
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Steps of an Outbreak Investigation
1. Establish the existence of an outbreak2. Verify the diagnosis3. Define and identify cases4. Describe and orient the data in terms of person, place
and time
5. Develop hypotheses6. Evaluate hypotheses7. Refine hypotheses and carry out additional studies8. Implement control and prevention measures9. Communicate findings
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Before you decide whether an outbreak exists, you must first
determine the expected or usual number of cases for the given
area and time
Step 1: Establish the existence of an
outbreak
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Data sources
Health department surveillance records for a notifiable
disease
Sources such as hospital discharge records, mortalityrecords and cancer or birth defect registries
for other diseases and conditions
If local data is not available, make estimates using data
from neighboring states or national data
Step 1: Establish the existence of an
outbreak
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Whether or not an outbreak is investigated or control measures are
implemented is not strictly tied to verifying that an epidemic exists
Other factors may come into play, including:
Severity of the illness
Potential for spread
Political considerations
Public concern and pressure from community
Availability of resources
Step 1: Establish the existence of an
outbreak
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Public Health Surveillance
The ongoing and systematic collection, analysis, and
interpretation of outcome-specific data for use in the
planning, implementation, and evaluation of public healthpractice.
How do we know when we have an
excess over what is expected?
Thacker, Berkleman. Epidemiologic Reviews 1988;10:164-90 18
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Disease for which regular, frequent, and timely information
regarding individual cases is considered necessary for the
prevention and control of disease
Notifiable disease
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Two goals in verifying a diagnosis
Ensure that the problem has been properly diagnosed
Ensure that the outbreak really is what it has been reported to be
Review clinical findings and laboratory results for affected people
Visit or talk to several of the people who became ill
For outbreaks involving infectious or toxic chemical agents, be certain
that the increase in diagnosed cases is not the result of a mistake inthe laboratory.
Step 2: Verify the diagnosis
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Establish a case definition- a standard set of criteria for deciding
whether a person should be classified as having the disease under
study
In many outbreaks, a working definition of the disease syndrome
must be drawn up that will permit the identification and reporting of
cases
As the investigation proceeds and the source, mode of transmission
and/or etiologic agent becomes better known, you can modify the
working definition
Step 3: Define and identify cases
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Step 3: Define and identify cases
A case definition includes four components
1. Clinical information about the disease2. Characteristics about the people who are affected
(person)
3. Information about the location (place)4. A specification of time during which the outbreak
occurred (time)
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To increase sensitivity & specificity of reporting, we use three classifications ofcases that reflect the degree of certainty regarding diagnosis:
1. Confirmed
2. Probable
3. Possible
The case definition is used to actively search for more cases beyond the earlycases and the ones that presented themselves.
Confirmed Case Probable Case Possible Case
LaboratoryVerification
Clinical
Features
+
+ ++ +
Step 3: Define and identify cases
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The following information should be collected from every affected
person in an outbreak:
1. Identifying information - name, address, phone
2. Demographic information - e.g., age, sex, race, occupation
3. Risk factor information
4. Clinical information
Verify the case definition has been met for every case
Date of onset of clinical symptoms to create an epidemic curve
Step 3: Define and identify cases
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The first cases to be recognized are usually only a small proportion of
the total number
To identify other cases, use as many sources possible
Passive Surveillance - Relies on routine notifications by healthcare
personnel (recall Notifiable Diseases)
Active Surveillance - Involves regular outreach to potential reporters
to stimulate reporting of specific conditions; investigators are sent tothe afflicted area to collect more information
Contact physician offices, hospitals, schools to find persons with
similar symptoms or illnesses
Send out a letter, telephone or visit the facilities to collect information
Step 3: Define and identify cases
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The time course of an epidemic is shown by the distribution of the
times of onset of the disease, called the Epidemic Curve
Graph of the number of cases of the health event by their date of
onset
Provides a simple visual display of the magnitude and time trend of
the outbreak
May stratify epidemic curves by place (residence, work, school, etc.)
or by personal traits (age, gender, race, etc.) to assess whether time
of onset varies in relation to place or person characteristics
Step 4: Describe and orient the data in
terms of time
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Assessment of the outbreak by place provides
Information on the geographic extent of the problem
A spot map indicating place of occurrence of cases may
show clusters or patterns that provide clues to the natureand source of the outbreak
Patterns reflecting water supply, wind currents, or
proximity to a restaurant, swimming pool, school room or
workplace
If the size of overall population varies between
comparison areas, a spot map of the area may be
misleading because it only shows number of cases
Step 4: Describe and orient the data in
terms of place
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Examine risks in subgroups of the affected population
according to personal characteristics, as well as
interaction between characteristics
Age, race, sex, occupation, social group, medical status
Characterizing an outbreak by person helps to determine
which subgroups of the population are at risk
Step 4: Describe and orient the data in
terms of person
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Examples of epidemic curves
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Though we generate hypotheses from the beginning of
the outbreak, at this point, the hypotheses are sharpened
and more accurately focused.
Use existing knowledge (if any) on the disease, or find
analogies to diseases of known etiologyHypotheses should address
Source of the agent
Mode of transmission
Exposures associated with disease and should beproposed in a way that can be tested
Step 5: Develop hypotheses
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Generally, after a hypothesis is formulated, one should be able to
show that
All additional cases, lab data, and epidemiologic evidence are
consistent with the initial hypothesis; and
No other hypothesis fits the data as well
Observations that add weight to validity
The greater the degree of exposure (or higher dosage of the
pathogen), the higher the incidence of disease
Higher incidence of disease in the presence of one risk factor
relative to another factor
Step 6: Evaluate hypotheses
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Reminder....attack rate
An attack rate is the proportion of a well-defined
population that develops illness over a limited period of
time, such as during an epidemic or outbreak
Useful for comparing the risk of disease in groups with
different exposures
Remember..an attack rate is an incidence proportion
(even though it is called a rate)
Often expressed as a percent
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Attack rate
Attack Rate = Number of new cases occurring in a given time period
Population at risk at the start of the time period
= Number of people at risk who develop a certain illness
Total number of people at risk
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Calculating an Attack Rate in a
food-borne outbreak
In a foodborne outbreak occurring among people attending a social
function or common geographical site
Calculate an attack rate for people who ate a particular item
(exposed) and an attack rate for those who did not eat the item
(unexposed)
The attack rate is calculated by dividing the number who
became ill and consumed the item by the total number
of people who consumed that item
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Identifying the source of an outbreak
Look for an item with
A high attack rate among those exposed
AND
A low attack rate among those not exposed (so the ratio of attack
rates for the two groups is high)
Ideally, most of the people who became ill should have been
exposed to the proposed agent so that the exposure could explain
most, if not all, of the cases.
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Additional epidemiologic studies
What questions remain unanswered about the disease?
What kind of study used in a particular setting would answer these
questions?When analytic studies do not confirm the hypotheses reconsider the
original hypotheses orlook for new vehicles or modes of
transmission
Step 7: Refine hypotheses and carry out
additional studies
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Laboratory and environmental studies
Epidemiologic studies can implicate the source of infection,
and
guide appropriate public health action
But sometimes laboratory evidence can clinch the
findings
Environmental studies often help explain why an outbreak
occurred and may be very important in certain settings
Step 7: Refine hypotheses and carry out
additional studies
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Case control methods applied to a
food-borne outbreak
The usual approach is to apply the case-control
methodology to determine what exposures ill people had
that well people did not have
List all of the relevant items on the menuDetermine the proportions of ill and of non-ill persons who
ate each of the items by questionnaire
Identify the food item with the largest difference in attack
rates between cases (ill) and controls (non-ill)
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The practical objectives of an epidemic investigation are to
stop the current epidemic and establish measures that would
prevent similar outbreaks in the future
Preliminary control measures should be implemented as soon
as possible
Step 8: Implementing control and
prevention measures
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Ro = cD
Ro = Reproductive Rate
(number of secondaryinfections/infected case)
= average probability susceptible partner will be infected
over duration of relationship
c = average rate of acquiring new partners
D = average duration of infectiousness
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To sustain an epidemic
Ro > 1 but also
> 0 (transmission must be possible)can block with barriers
c > 0 (new susceptibles) can reduce contacts
D > 0 (maintain infectiousness)
can treat infection
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Therefore, elements of epidemic control
The elements of epidemic control include:
Controlling the source of the pathogen (if known)
e.g., Remove or inactivate the pathogen
Interrupting the transmission
e.g., Sterilize environmental source of spread; vector controlControlling or modifying the host response to exposure
e.g., Immunize the susceptibles; use prophylactic
chemotherapy
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At the end of the investigation, communicate findings to
others who need to know
Prepare a final report
Provide information on the nature, spread, and controlmeasures employed
The report can take several forms:
1.An oral briefing for local health authorities2.A written report to a journal [note: MMWR]3. Formal presentation of recommendations
Step 9: Communicate findings
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Diarrhea at a high school dinner
In November 2006, between 200-300 students and teachers reported
gastroenteritis after attending a school function in Denmark
The cause was determined to be primarily enterotoxigenic E. coli.
Enterotoxigenic E. coliis transmitted through fecally contaminated food or
water, and mainly diagnosed as travelers diarrhea in industrialized
countries
So how did this happen at a dinner in Denmark?
Pakalniskiene et al. A foodborne outbreak of enterotoxigenic E. coli and Salmonella Anatum infection after a high-school dinner in Denmark, November 2006.
Epidemiol Infect. 2008; 1-6
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Diarrhea at a high school dinner
Pasta salad with pesto has the highest
number of exposed cases (98%), and a
high attack rate; bread rolls come in at a
close second: exposure=96%
Pakalniskiene et al. A foodborne outbreak of enterotoxigenic E. coli and Salmonella Anatum infection after a high-school dinner in Denmark, November 2006.
Epidemiol Infect. 2008; 1-6
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Diarrhea at a high school dinner
So which is it? Bread rolls or pesto?
Eating more
portions ofpasta had a
dose responseeffect on the
risk ratio; this
was not true ofthe bread rolls
Pakalniskiene et al. A foodborne outbreak of enterotoxigenic E. coli and Salmonella Anatum infection after a high-school dinner in Denmark, November 2006.
Epidemiol Infect. 2008; 1-6
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Diarrhea at a high school dinner
None of the food preparers had a history of recent illness or
foreign travel; nor did their stool samples test positive forE coli
Investigators concluded that basil in the pesto was the likely
culprit
The basil had been imported from a country that uses surface
and run-off water for irrigation
Basil from that same country has been linked to various other
outbreaks as well
[aside: the purported producer of the basil denies having grown
basil in the four years prior to the investigation]
Pakalniskiene et al. A foodborne outbreak of enterotoxigenic E. coli and Salmonella Anatum infection after a high-school dinner in Denmark, November 2006.
Epidemiol Infect. 2008; 1-6
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