Transcript Otbreak Investigation - Michigan State University

Outbreak
Investigation
Objectives
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Determine if an outbreak is occurring
Characterise the outbreak
Identify additional cases
Identify causative agent
Identify the source
Initiate steps to terminate transmission
Learn for the future
Surveillance for disease
 Certain infectious diseases pose
significant threats to the health of the
public
 It is important that public health know
about them
 States, federal and international health
authorities develop lists of reportable
disease.
Surveillance
 Physicians, hospitals and clinical laboratories
are required to report, usually within a specified
time period
 Serious diseases are often categorized
separately
 Basic functions of communicable disease
control at the state and local levels is the
gathering and analysis of reportable disease
data
Examples of Reportable
Diseases in Michigan
 Enteric diseases
 Salmonellosis, shigellosis, Campylbacter, hepatitis A
 Sexually Transmitted Diseases
 Syphilis, gonorrhea, HIV
 CNS diseases
 Bacterial and viral meningitis, Arboviral – WNV, EEE
 Vaccine Preventable Diseases
 Measles, mumps, diphtheria, polio, hepatitis B
Collection and analysis of
data
 LHD receives reports
 LHD logs and reviews data
 Number of cases within a specific time period e.g a
week
 Geographic distribution of cases
 Determination if the cases need further investigation
 LHD send s reports to MDCH via LHDSurv
(soon to be replaced by MDSS)
 MDCH sends reports to CDC via NETSS
Data analysis
 Need to know background level
 Track weekly occurrence of disease over a
number of years to establish an average
number of cases per week for that particular
week
 Need to determine when the number of
cases is above the background
Establish when an
outbreak is occurring
 For some diseases, a single case is an
outbreak e.g measles, smallpox
 In others, use the background as a threshold,
but take into account other variables
 e.g. is the gender or age frequency similar to
previous years,
 are the cases clustered geographically
 Sometimes the illness is reported before
a disease is identified
 Many calls from physicians and the public
about persons becoming ill with diarrhea,
vomiting
 Using symptoms try to ascertain the likely
disease
 Get specimens to send to the laboratory
if not already obtained
Investigation
 Characterize the data that you have
 Try to identify other similar cases
occurring
 How would you do this?
 Fax to ERs
 Notices on electronic boards
Case definition
 What would you need in a case definition
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Time
Place
Symptoms
E.g. All persons who experienced vomiting,
diarrhea, nausea or abdominal cramping
and who ate at Joe’s Greasy Spoon
between Dec 10 and Dec 14.
Data Analysis
 Determine which cases fit the case definition
 Analyse by variables
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Age
Gender
Race
Geography
Risk factor
 Food
 Drink
 Behavior e.g. smoking, outdoor activity, attends day care
Data analysis
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Frequencies
Incubation periods
Laboratory tests
Epidemic curve
Attack rates
 By person
 By place
 By risk factor
Hypothesis
 Using the data analysis, formulate
hypotheses
 Select the hypothesis which fits the
picture
 Hypothesis should address
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Source of the disease
Etiologic agent
Method of transmission
Control methods
Hypothesis generation
 Hypotheses can be generated at any
time during the investigation, and refined
as more data becomes available. But
beware of making false assumptions
 But don’t leave it too late, as hypothesis
generation should lead to control
measures
Hypothesis testing
 Laboratory results can confirm the
etiologic agent
 Prevent further consumption of
implicated food can confirm the
transmission/source
 Data analysis –
 Case – control study using matched or
unmatched controls.
Control methods
 Initiate control methods based upon
hypothesis
Report of findings
 So others can learn from our experience