EPIDEMIC INVESTIGATION Dr. Azmawati Mohammed Nawi Dept. Of Community Health
Definition of outbreak• O Occurrence of more cases of disease than f f di th expected in a given area among a specific group of people over a particular period of time. or• Two or more linked cases of the same illness.
Definitions• Outbreak- more cases in a time and place p(or population) than expected.• Epidemic- same as outbreak or morewidespread/prolonged, more political• Cluster- a group of cases in a certainplace and time suspected to be greaterthan expected. expected
Definitions• Vehicle- non-living intermediary (food,water, fomite) t f it )• V t Vector- li i intermediary (insect, living i t di (i tarthropod): mechanical or biologicaltransmission (part of life cycle)• Reservoir- habitat where the agent growsand multiplies (humans, animals,environment)
Definitions• Modes of transmission– Direct:Direct contact (mucous membranes, skin, fecal-oral)Droplet spread– Indirect:AirborneVehicle borne- food water or fomite borne food,Vectorborne- arthropod• Portals of Entry- ingestion, inhalation,ppercutaneous
Objectives of outbreak investigations• To control ongoing outbreaks outbreaks,• To prevent future outbreaks,• To advance knowledge about a disease.
10 Steps in an Outbreak Investigation A) Preliminary analysis: 1 Prepare for field work p 2 Verify the diagnosis 3 Establish the existence of an outbreak 4 Define and identify cases y 5 Describe the data in terms of time (epidemic curve), place, and person 6 Develop hypotheses 7 Test hypotheses: (compare attack rates between exposed and non exposed) B) further investigation: 8 Carry out additional studies 9 Implementing control and prevention measures 10 Outbreak report
Step.1 Prepare for field work• Review literature• Prepare the supplies and equipments p pp q p• Consult laboratory staff• Arrange for portable computer, camera g p p ,• Consult local staff
EpidemiologistMicrobiologist OutbreakEnvironmental specialistMinistry / Government InvestigationPress officer Team?Others FIELD
Roles in Foodborne Outbreak Investigations Graphic developed by Terry Rabatsky-Ehr, Regional Epidemiologist, CT DPH
Vector Reservoir Dead SickInvestigation SurveillancePrediction Exposed Clinicians Epidemiologist g Coordination Education Laboratory L b t Clinical Health personnel Specimen transfer t f Special General Diagnostic groups population Media Authorities Decisions Infrastructure Regulations Vaccinations etc
Step 2 Verify the diagnosis• Describe cases clinically• Obtain a complete listing of foods served• Collect specimens of feces and vomits and send for laboratory• Submit suspected food for laboratory• Look for the possible source of contamination and periods of inadequate refrigeration and heating• Inquire about the origin of the incriminated food, manner of its preparation and storage before serving• Search for food handlers with skin infections. Culture all purulent lesions and collect nasal swabs from all food handlers
Step 3 Establish Existence of an Outbreak 3. Routine surveillance Clinical / Laboratory 3)Detection of ) f General public outbreak Media
Routine surveillance3)Detection f3)D t ti of Clinical L b Cli i l / Laboratory t General public outbreak Media Is this an outbreak?
Routine surveillance3)Detection f3)D t ti of Clinical L b Cli i l / Laboratory t General public outbreak Media Is this an outbreak? •Compare the current number of cases with the number from the previous weeks or months •Check health department records Check •Consult local data sources
Step 4. Define d Identify CSt 4 D fi and Id tif Cases• Epidemiologists establish a case p de o og sts estab s definition: a standard set of criteria for deciding whether a person should be classified as having the disease or l ifi d h i th di condition under study.• Usually includes : 1. Clinical information about the disease 2. Characteristics about the people who are affected 3. Information about the location or place 4. A specification of time during which the outbreak occurred.
Step 4. Define d Identify CSt 4 D fi and Id tif CasesInvestigators often classify cases as one of the following:• Confirmed: usually has laboratory verification• Probable: usually has clinical features without lab verification• Possible: usually has fewer of typical clinical features
Example case definition• Possible or suspect – Patient with severe diarrhoea• Probable – Patient older than 5 years with severe dehydration or dying of acute watery diarrhoea in town “x” between 1 June x” and 20 July 1998• Confirmed – Isolation of Vibrio cholerae from stool of patient
Step 4. Define and Identify Cases The following information is collected: Risk factors Identifying informationClinical Information:Information: Name, address, SpecificDate of onset, Exposures, Telephone no, …..symptoms, signs, t i Immunityttt, statusHospitalization,death Demographic information: Age, sex, race, occupation ti
Line ListingNext, selected critical items are , abstracted into a table called a “line g listing” – Each column represents an important variable, such as age and sex g – Each row represents a different case, by number• This simple format allows the investigator to scan key information on every case and update it easily
Step 5. Describe the Data• Characterize the outbreak by time, place, and person (descriptive epidemiology)
Characterizing By Time: Epidemic Curve Epidemic curve or “epi curve” = a graph of the number of cases by their date of onset.• Number of cases is plotted on y axis y-axis• Time is plotted on the x-axisAdvantages:• Shows course of epidemic• May enable estimation of probable time period of exposure
Epidemic Curve• X axis- time (of onset, of exposure)• Y axis- number of cases• No gap between data points (X axis iscontinuous) )• X axis unit= ¼ as long as the incubationpe odperiod• Each box usually= 1 case
Epidemic Curve• Magnitude over time• Shape: point source vs propagated p p p p g• Evaluation of intervention• g Outliers recognized
Examples of epidemic curves• Point source epidemic – Shape – a steep Point source up slope, a peak slope and a gradual down-slopep – Interpretation - people are exposed to the same source over a relatively brief l ti l b i f period
Examples of epidemic curves• Continuous common source Continuing common source epidemic – Shape - curve will have a plateau instead of a peak – Interpretation - p p people are exposed to the same source over an extended period
Examples of epidemic curves Multiple waves -person to person p p p• Propagated epidemic – Shape - a series of progressively taller t ll peaksk – Interpretation - person-to- t person spread
Characterizing By Place: Spot Map• Assessment of an outbreak by place provides information on the geographic extent of a problem• A spot map of cases i a t f in community may show clusters or patterns that reflect water supplies, wind currents, or , proximity to a restaurant or grocery store.
Characterizing By Person• Determine the populations at risk by characterizing the outbreak by person• Define populations by – Personal characteristics (Examples: age, race, sex, or medical status) – Exposures ( (Examples: occupation, leisure activities, use of medications, tobacco, drugs)
Step 6 Develop Hypotheses 6.• Hypotheses based on – Descriptive epidemiology - person, place and time
CasesPerson Time Place Evaluate information Pathogen? Source? Transmission?
Step St 7. Test Hypotheses– Use analytic epidemiology to test hypotheses by using a comparison group t quantify relationships to tif l ti hi between various exposures and the disease. disease– Calculate and compare attack rates among those exposed And those not exposed.
Example:• Best for analyzing an outbreak in a small y g well-defined population – Example: gastroenteritis among people p g gp p who attended a wedding• Ask each attendee the same set of questions about potential exposures• Then, the attack rate can be calculated for people who ate a particular item (were exposed) and an attack rate for those who did not eat that item (were not exposed). exposed)
Attack RatesFor the exposed group, the attack rateequals the number of people who ate itemand became ill divided by (÷) the totalnumber of people who ate that item. Total # of people who ate the item and became illExposed Group = Total # of people who ate that item
Attack RatesFor the not exposed group, the attack rateequals the number of people who did noteat item but still became ill divided by (÷)the total number of people who did not eatthat item. item Total # of people who did not eat the item but still became illNot Exposed Group = Total # of people who did not eat that item
• To identify source of outbreak, look for – High attack rate among those exposed and – Low attack rate among those not exposed and In addition – Most of the people who became ill should have consumed the item
Relative RiskRelative risk is calculated by dividing (÷)the attack rate for people who wereexposed to the item by the attack rate forthose who were not exposed. Attack rate for people who were exposed to the item Relative Risk = Attack rate for those who were not exposed
Example: Attack Rates Food Not Exposed Group• Baked Ham 17/29 = 59%• Mashed potatoes 23/37 = 62%• Spinach 20/32 = 62%• Cabbage salad C bb l d 28/47 = 60%• Milk 44/71 = 62%• Ice C I Cream (V ) (Van) 3/21 = 14%• Ice Cream (Choc) 20/27 = 74%• Fruit l d F it salad 42/69 = 61%
Step 7: Testing the hypotheses– Carry out statistical tests to determine the proper source of the outbreak ( e.g chi-square test)– look for “statistical significance” P <0.05 ( i.e the difference between the groups is not due to chance)
Step 8. 8 Carry Out Additional StudiesLaboratory and environmental studies y• While epidemiology can implicate vehicles and guide appropriate public health action, laboratory evidence can clinch the findings• Environmental studies often help explain why an outbreak occurred and may be very important in some settings
Step 9. Implementing Control and Prevention Measures P ti M• Remove source of contamination• Remove persons from exposure• Isolate and/or treat infected persons• Interrupt transmission• Eliminate the susceptibility of individuals by vaccination or prophylactic May M occur at any t time during the outbreak!! ti d i th tb k!! chemotherapy
Step 10 Outbreak reportSt 10. O tb k t• Prepare written report• It include discussion of factors leading to outbreak• Evaluation of the methods used for the control of outbreak• Recommendations for prevention of similar outbreak