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 1
Outline � Introduction: Epidemiology is … � Outbreak: Definition, Type, Detection � Investigation: Steps 2
What is Epidemiology? Study of Disease Disease Occurrence Population Control • Distribution of disease by person, place, and time • Determinants (Risk Factors) 3
Roles of Epidemiology in Public Health � Surveillance � Outbreak Investigation � Epidemiological Study � Evaluation of PH measures * RM Page, et al. “Basic epidemiological methods and biostatistics, 1995 p.32 4
Definition of Outbreak � The occurrence of cases of an illness, e ? specific health-related behaviour, or other h y t c s n i a t t c health-related events clearly in excess of a e h p W x e l normal expectancy. The area and the a m r o n period in which the cases occur are specified precisely. 5
Excess of Normal Expectancy � 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 70 60 50 s e 40 s a c 30 # 20 10 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2003 Median 1998 - 2002 6
7 Cases linked to the same factors Judged to be an outbreak
Judged to be an outbreak A single case of disease that has never been occurred before. 1997: A 3-year old boy, case of Avian Flu (H5N1) in Hong Kong alerted the public health people around the world to start a full scale investigation. 8
Terms � Endemic = Disease that routinely occurs in a given place � Epidemic = Outbreak (outbreak -> sense of urgency, Epidemic -> sense of wide spreading) � Cluster = An aggregation of cases in a given place & time � Pandemic = Epidemic that spreads over many countries of regions of the world 9
1918 flu pandemic (Spanish flu) 50 – – 100 million deaths worldwide in 18 months 100 million deaths worldwide in 18 months 50 10
Detection of the outbreak Sources of outbreak news: � Surveillance data that are collected and analyzed timely � Health care provider or citizen who knows of “several cases” � Media: Newspaper, TV, Internet 11
Media reports outbreak 2 deaths and 118 coma cases after eating raw park salad in a cremation 12
Surveillance for outbreak detection Reported Cholera cases in Khonkaen, Thailand January 1995 - July 1999 80 70 60 50 40 30 20 10 0 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 13
14 � Propagated source outbreak � Common source outbreak Outbreak patterns (Person-to-person)
15 Date of onset Common source Common Source Outbreak 10 9 8 7 6 5 4 3 2 1 0 # case Food
16 Date of onset Propagated Source Outbreak 30 25 20 15 10 5 0 # cases
Outbreak: the basics � 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 17
Steps of Steps of Outbreak Investigations Outbreak Investigations 18
Why investigate an outbreak? � Characterize a public health problem � Identify preventable risk factors � Recommend control and prevention measures 19
“Usual” sequence of events Primary 1st case Report Samples Lab Response Case at HC to DMO taken result begins 100 80 Opportunity 60 for control 40 20 0 1 5 9 13 17 21 25 29 33 37 Days
14 cases prevented “Ideal” sequence of events Potential 9 3 7 3 5 3 3 3 1 3 9 2 7 2 5 2 3 2 Days 1 2 9 1 7 1 5 Response 1 begins 3 1 1 1 9 7 5 Primary 3 Case 1 100 80 60 40 20 0
Steps of an outbreak investigation Steps of an outbreak investigation Steps of an outbreak investigation Implement control measures 1. Prepare for Field Work : Rapid Response Team 2. Confirm outbreak and diagnosis 3. Define case and start case-finding 4. Descriptive data collection and analysis 5. Develop hypothesis 6. Analytical studies to test hypotheses 7. Special studies (e.g. environmental study) 8. Communicate the conclusion and recommend control measures 9. Follow-up the control implementations 22
I. Preparing for field works Team members and roles 1. Necessary Knowledge and equipments; 2. specimen collection & transport method, etc. Lines of communication 3. 23
II. Confirm outbreak and diagnosis What is the I s this an diagnosis? diagnosis outbreak? outbreak Link between cases? Clinical manifestation Higher than expected? Laboratory result 24
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 • Is this an outbreak? • What is the likely diagnosis? • Should we start the investigation? • Which intervention should be started? Outbreak confirmed � Maybe measles, rubella, dengue etc. Investigation warranted Shall we start the vaccination or spray mosquitoes? 25
To Investigate or not to Investigate � Consider the following factors when deciding whether or not to investigate an outbreak � 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 26
Outbreak confirmed, further investigations warranted Form Outbreak Epidemiologist Clinician I nvestigation & Control Microbiologist Team Environmentalist Government Press officer Others Team coordinates field investigation 27
Steps of an outbreak investigation Steps of an outbreak investigation Steps of an outbreak investigation Implement control measures 1. Prepare for Field Work : Rapid Response Team 2. Confirm outbreak and diagnosis 3. Define case and start case-finding 4. Descriptive data collection and analysis 5. Hypothesis generation 6. Analytical studies to test hypotheses 7. Special studies (e.g. environmental study) 8. Communicate the conclusion and recommend control measures 9. Follow-up the control implementations 28
29 Which one is longer? Standard measurement
Case Definition � Standard criteria for deciding if a person should be classified as suffering from the disease under investigation � Clinical criteria, restrictions of time, place, person � Simple, practical, objective � Sensitivity versus specificity 30
Case definition: example Patient older than 5 years with severe dehydration or dying of acute watery diarrhoea in town “x” between 1 June and 20 July 1999. 31
Case Definitions � Can emphasize sensitivity or specificity in case definition Sensitivity: Most cases detected, but … SPECIFICITY many false positives many specimens to test Overload SENSITIVITY low % tested specimens +ve Specificity: Cases missed, but … SENSITIVITY few false positives SPECIFICITY Underload fewer specimens to test high % tested specimens +ve 32
Case Definition � Categories of cases � > 5 yr in town X � Suspected � Suspected � Symptoms reported but not confirmed � With diarrhea symptoms � No lab or epidemiologic link � Probable � Probable � Symptoms confirmed � Mucous bloody diarrhea � Epidemiologic link � WBC, RBC in stool exam � Initial lab test � Confirmed � Confirmed � Lab test � RSC found Shigella sonnei � Epidemiologic link 33
34 (Active cases) (Passive cases)
Why is Case Finding Important? In an outbreak: � There are usually more cases than are reported to the health department � Reported cases may not be representative of all cases � To identify exposure and/or outcome investigator needs information from persons who represent all cases 35
Why is Case Finding Important? � To refine the case definition as more information is garnered � To define the exposed population for the purposes of developing control measures 36
How to Find Cases � Passive strategies involve review of information available � OPD, IPD log book in hospitals, Health center � Laboratory log book � Active strategies � Screening unit in the affected area � Door to door 37
Information to Collect during Case Finding Process � Demographic information � Age, gender, race, occupation � Clinical information � Symptoms, date of onset, lab results, severity of illness � risk factor information to collect � Varies by type of outbreak 38
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