Outbreak Investigation Outbreak Investigation Step by Step Step by - - PowerPoint PPT Presentation
Outbreak Investigation Outbreak Investigation Step by Step Step by Step Darin Areechokchai MD., DTM&H., MCTM. Surveillance and Investigation Section Bureau of Epidemiology, Department of Disease Control Ministry of Public Health, Thailand
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Darin Areechokchai MD., DTM&H., MCTM. Surveillance and Investigation Section Bureau of Epidemiology, Department of Disease Control Ministry of Public Health, Thailand
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Disease Occurrence Population
by person, place, and time
Study of
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Evaluation of PH measures
*RM Page, et al. “Basic epidemiological methods and biostatistics, 1995 p.32
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The occurrence of cases of an illness,
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More than
Median number of cases in previous 5 years or Average number of cases + 2sd of previous 5 yr
Number of Cholera case by month, District A
10 20 30 40 50 60 70
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec # c a s e s
2003 Median 1998 - 2002
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Cases linked to the same factors
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(outbreak -> sense of urgency, Epidemic -> sense of wide spreading)
countries of regions of the world
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Surveillance data that are collected and
Health care provider or citizen who knows
Media: Newspaper, TV, Internet
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2 deaths and 118 coma cases after eating raw park salad in a cremation
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10 20 30 40 50 60 70 80
Jan Mar May July Sep Nov Jan Mar May July Sep Nov Jan Mar May July Sep Nov Jan Mar May July Sep Nov Jan Mar May July
1995 1996 1997 1998 1999
Reported Cholera cases in Khonkaen, Thailand January 1995 - July 1999
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Common source outbreak Propagated source outbreak
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1 2 3 4 5 6 7 8 9 10
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5 10 15 20 25 30
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Definition
More than normal expectancy Cases with epidemiological linkage New disease (emerging disease)
Detection
Surveillance Health care providers media
Pattern and epidemic curve
Common source Propagated source
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Characterize a public health problem Identify preventable risk factors Recommend control and prevention
20 40 60 80 100 1 5 9 13 17 21 25 29 33 37
Primary Case 1st case at HC Report to DMO Lab result Samples taken Response begins
Days Opportunity for control
20 40 60 80 100
1 3 5 7 9 1 1 1 3 1 5 1 7 1 9 2 1 2 3 2 5 2 7 2 9 3 1 3 3 3 5 3 7 3 9
Days Potential cases prevented
Primary Case Response begins
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control measures
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1.
2.
3.
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Scenario 1 Scenario 1 Scenario 1
Many adults in a remote village were sick with fever, severe joint and muscle pain and rash over the body
Outbreak confirmed Maybe measles, rubella, dengue etc. Investigation warranted Shall we start the vaccination or spray mosquitoes?
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Consider the following factors when deciding
It could be “true” outbreak with common cause It could be unrelated cases of the same disease Severity of illness Transmissibility Local politics Public concern
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Epidemiologist Clinician Microbiologist Environmentalist Government Press officer Others
Team coordinates field investigation
Form Outbreak I nvestigation & Control Team
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control measures
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Standard criteria for deciding if a person should be
Clinical criteria, restrictions of time, place, person Simple, practical, objective Sensitivity versus specificity
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Can emphasize sensitivity or specificity in
SENSITIVITY SPECIFICITY
many false positives many specimens to test low % tested specimens +ve
Sensitivity: Most cases detected, but …
Overload
Specificity: Cases missed, but …
SENSITIVITY SPECIFICITY
few false positives fewer specimens to test high % tested specimens +ve
Underload
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Categories of cases
Suspected
Symptoms reported but not
confirmed
No lab or epidemiologic link
Probable
Symptoms confirmed Epidemiologic link Initial lab test
Confirmed
Lab test Epidemiologic link
> 5 yr in town X
Suspected
With diarrhea symptoms
Probable
Mucous bloody diarrhea WBC, RBC in stool exam
Confirmed
RSC found Shigella sonnei
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(Passive cases) (Active cases)
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There are usually more cases than are
Reported cases may not be representative of all
cases
To identify exposure and/or outcome
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To refine the case definition as more
To define the exposed population for the
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Passive strategies involve review of
OPD, IPD log book in hospitals, Health center Laboratory log book
Active strategies
Screening unit in
Door to door
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Demographic information
Age, gender, race, occupation
Clinical information
Symptoms, date of onset, lab results, severity
risk factor information to collect
Varies by type of outbreak
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Survey of hepatitis B cases in a male juvenile detention, Saraburi, Thailand, November 1999 Date of interview Interviewer's name Patient number Patient's name-Surname Age (in years) Number of domitory (1-5) Study field
Clinical symptoms (sick inside the juvenile detention, since 1 Jan 99) Possible risk factors No Yes Y N Homosexual Y N Injected drug user Y N This hepatitis cases is laboratory confirY N Fatigue Onset of symptoms Tatooing Jaundice Nausia/Vomiting Clinical info. Risk factors Demographic info. I dentifying info.
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It provides organized information about the cases Can be quickly reviewed and updated Create a table in which each row represents a case
Line listing of streptococcus meningitis cases after eating raw pork
ID Sex Age Onset Alcohol 25/4/07 26/4/07 27/4/07 26/4/07 27/4/07 27/4/07 26/4/07 26/4/07 27/4/07 26/4/07 27/4/07 BF LUN DIN BF LUN DIN 1 M 50 Yes Raw Raw
Cook Cook 2 M 41 Yes Raw Raw
M 43 Yes Raw Raw
M 62 Yes Raw Raw Cook
M 71 No
M 56 Yes
M 51 Yes
M 50 Yes Raw Raw
F 49 Yes
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Frequency distributions of demographics
May provide information about exposure
Frequency distributions of potential
May provide information about source or route
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Time:
Epidemic curve
Place:
spot map area map Attack rate (incidence) by place
Person
Frequency Specific attack rate (incidence)
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An epidemic curve (epi curve) is a
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An epi curve can provide information on
Pattern of spread Magnitude Outliers Time trend Exposure and/or disease incubation period
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The overall shape of the epi curve can
Common source Propagated
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Allow information about the time trend of
Consider:
Date of illness onset for the first case Date when the outbreak peaked Date of illness onset for the last case
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Outliers are cases at the very beginning and end
If they are not an error, they may represent
Baseline level of illness Outbreak source A case exposed earlier than the others An unrelated case A case exposed later than the others A case with a long incubation period
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2 4 6 8 10 12 14
Date of onset Number of cases suspect first case active passive
16 21 23 25 28 October
Exposure period = (Onset of the first case – Shortest incubation period) to (Onset of the most cases – the average incubation period)
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Plot the number of cases of disease reported
Plot the time or date of illness onset on the x-axis
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Technical tips
Choice of time unit for x-axis depends upon
Begin with a unit approximately one quarter of
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Epi curves are histograms
There should not be any space between the
Label each axis Provide a descriptive title Include the pre-epidemic period to show
1 2 3 4 5 6 7 8 w k1 w k2 w k3 w k4 w k5 w k6 w k7 w k8 w k9 w k10 w k11 w k12 w k13 w k14
number of cases
13 10 8 9 11 12 14 6 5 7 15 4 3 2 1
Burma Measles case distribution by sector, Nupo camp, Tak, Jan – Mar 2007 Epidemic curve by week of onset (N = 27)
Case in week 2 Case in week 6 Case in week 7 Case in week 8 Case in week 9 Case in week 10 Case in week 11 Case in week 12 Case in week 13
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11 cases in 8 provinces Male : Female = 8 : 3 Median age 7 (2 - 58 years old) 8 died (CFR 72.7%) :
died (N=8) improved (N=3) Area of poultry outbreak
10 20 30 40 50 60 70 80 90 100
Pediatric (N=7) Adult (N=4)
Fever Cough Sputum Dyspnea Rhinorrhea Diarrhea Vomiting Wbc<5000 Platelet<106 Pleural effus. Percent of cases
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Cases
5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 200 400 600 800 1000 1200 0-4 '5-14 '15-44 '45-64 '64+ Age Group
Evaluate information Pathogen? Source?
Transmission?
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control measures
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Examples
water from there.
visitor got sick after drank a glass of juice. (outlier case)
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questionnaire
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1.
2.
3.
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4.
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9.
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We can use analytic studies to test hypotheses. We want to know:
Whether there is an association between exposure and disease, How strong the association is, What proportion of cases are due to exposure, and Whether there is an increased risk of disease with increased
exposure (a dose-response relationship).
Two common types of analytic studies are cohort study
and case-control study.
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A “cohort” is a group of people who have
Can represent the source population—the
Examples of cohorts:
All employees in an office building Everyone who attended a football game All the residents of a neighborhood
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Tend to be retrospective (exposures in the past
Occurrence of disease in exposed group
Risk ratio tells whether disease is associated
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Expose Non-exp
I n a shigellosis outbreak, fermented vegetable was suspected to be the implicated food
A person who ate the fermented vegetable was 6.2 times more likely to be ill than a person who did not eat...
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The most frequently used type of study in outbreaks
Can be quickly implemented Can be used when cohort study might be large and time-
consuming
Identify people with disease (case-patients) and
You already know who is case
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Calculate odds ratio to measure strength of
Compare odds of exposure among case-patients to
Cannot calculate risk ratio in case-control study
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I n a botulism outbreak, home-canned bamboo shoots was suspected to be the implicated food A person who ate the bamboo shoots was 201 times more likely to be ill than a person who did not eat... Cases Controls
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The choice depends on the situation Always think about the source population:
Are members of the group easily identifiable?
Use a cohort study
Is the cohort difficult to identify or too large to
Use a case-control study design
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control measures
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Process used to determine the production and
Helps determine if (and where) you can conduct
Used to clarify the point at which the implicated
Necessary investigation to identify contributing
Simmer with ingredients
Soaking in water & peel
Critical Critical point point
TEXT
Mixed ÷
TEXT TEXT
Boiling pork ~ 1-1.5 hrs Boiling eggs
Slice red pork Splitting egg by thread & knife
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Human clinical specimens from cases, contacts
Blood Serum Urine
Type of specimen depends on the outbreak Specimens from environments, animals
Saliva Hair Feces
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control measures
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May occur at any time during the outbreak!!
Control the source of pathogen Interrupt transmission Modify host response
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Interrupt environmental
transmission
Control vector transmission Improve personal sanitation
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control measures
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Main reference: http://www.sph.unc.edu/nccphp/focus/
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The I nternational Field Epidemiology Training Program, Thailand
Many slides in this presentation are from the World Health Organization, the European Programme for Intervention Epidemiology Training and work of