Investigating the nature of disease (Outbreak investigation) Muhammad Tahir, MPH,MSc Epi & Bio
What is an outbreak? Occurrence of more cases of disease thanexpected in a given area among a specific group of people over a particular period of time
Food or waterborne outbreak (WHO two or more persons similar illness dfiii definition) after ingestion of the same type of food or water from the same source epidemiological evidence the food or the water the source of the illness
Why investigate outbreaks? Stop the outbreak Find and neutralise the source (cause) Prevent additional cases Prevent future outbreaks Improve surveillance and outbreak detection Improve our knowledge Keep the public s confidence Training
Specific demands when investigating outbreaks Unexpected event Act quickly Rapid control Bias caused by media reports Legal and financial pressure Interdisciplinary coordination Work carried out in the field Systematic approach
Steps of an outbreak investigation Confirm outbreak and diagnosis Define a case Identify cases & obtain information Describe data collected and analyse Develop hypothesis Test hypothesis: analytical studies Special studies Communicate results, including outbreak report Implement control measure
Detection Routine surveillance Clinical / Laboratory General public Media
Confirm outbreak and diagnosis Is this an outbreak? More cases than expected? Surveillance data Surveys: hospitals, labs, physicians Caution! Seasonal variations Notification artefacts Diagnostic bias (new technique) Diagnostic errors (pseudo outbreaks)
Cases of legionellosis by week of notification France, January 1996 August 1997 8 7 6 5 4 3 2 1 0 1 4 7 101316192225283134374043464952 3 1996 Week of notification 1997
Cases of legionellosis by week of notification France, January 1996 August 1997 8 National meeting: legionellosis diagnosis and reporting 7 6 5 4 3 2 1 0 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Confirm outbreak and diagnosis Laboratory confirmation serology isolates, typing of isolates toxic agents Contact (visit) the laboratories Meet attending physicians Examine some cases Not alwaysnecessaryto confirm all the cases Not always necessary to confirm all the cases but confirm a proportion throughout the outbreak
Outbreak confirmed Immediate control measures? Further ut investigation? prophylaxis aetiological agent exclusion / isolation mode of transmission publicwarning vehicle of transmission hygienic measures source of contamination others population at risk exposure causing illness
Outbreak confirmed, further investigationswarranted Form Outbreak Control Team? Epidemiologist Microbiologist Clinician Environmentalist Engineers Veterinarians Others Team coordinates Team coordinates field investigation
Descriptive epidemiology Who are the cases? (person) Where do they live? (place) When did they become ill? (time)
Case definition Simple, practical, objective Sensitive? Specific? Multiple case definitions confirmed probable possible
Identify & count cases notifications laboratories hospitals, GPs schools workplace, cases, media, etc
Identify & count cases Obtain information Identifying information Demographic information Clinical details Exposures and known risk factors
Identify & count cases Obtain information Analysis of descriptive data Describe in time place person
Time Epi Curve Histogram Distribution of cases by time of onset of symptoms, diagnosis or identification Cases time interval depends on incubation period 10 9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11 12 Days
Epicurves cases Common point source cases 10 6 9 5 4 3 2 1 0 1 2 3 4 5 6 7 8 hours cases 8 7 6 5 4 3 2 1 0 12 Common persistent source 1 2 3 4 5 6 7 8 9 10 11 12 days Propagated source 10 8 6 4 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 weeks
Epi curve Describe start, end, duration peak importance atypical cases Helps to develop hypotheses incubation i period etiological agent type of source type of transmission time of exposure
Place Place of residence Place of possible exposure work meals travel routes day care leisure activities iii Maps identify an area at risk
Person Distribution of cases by age, sex, occupation,etc (numerator) 60 female 50 male Distribution of these variables in population (denominator) 600 females 350 males Attack rates female: 60/600 Males: 50/350
Develop hypotheses Who is at risk of becoming ill? What is the disease? What is the source and the vehicle? What is the mode of transmission?
Compare hypotheses with facts Test specific hypotheses Analytical studies cohort studies case control studies
Testing hypothesis Cohort attack rate exposed group attack rate unexposed group Case control % of cases exposed % of controls exposed
Verify hypothesis Special investigations/studies Microbiological investigation Environmental investigation Veterinarian investigation Trace back investigations (origin of foods) Entomological investigations
Implement control measures May (must)occur at any time during the outbreak!! At first, general measures According to findings, more specific measures 1) Control the source of pathogen 2) Interrupt transmission 3) Modify host response
Outbreak report Regular updates during the investigation Detailed report at the end communicate public health messages influence public health policy evaluate performance training tool legal proceedings
Steps of an outbreak investigation Confirm outbreak and diagnosis Define case Identify cases and obtain information Descriptive data collection and analysis Develop hypothesis Analytical studies to test hypotheses Special studies Communication, including outbreak report Implement control measures
References http://www.dorak.info Tayal,S. Assistant professor King Saud University Leon Gordis. Epidemiology (3 rd ed.)
Case Definition Clinically compatible case: symptoms fit the clinical presentation of disease Confirmed case: clinically compatible and either lab confirmed or epilinked Probable case: meets the clinical case definition and is not lab confirmed or epi-linked