Published on April 26, 2014
Types of Epidemics and Epidemic Investigations ProfessorTarek Tawfik Amin Public Health Dept. Faculty of Medicine Cairo University firstname.lastname@example.org
Describing the disease • Pattern of disease may be described by the time of occurrence, week, month, year, etc… • There are three kinds of trends or fluctuations in disease occurrence: 1-Short term fluctuations (Epidemic) 2-Periodic fluctuations 3-Long term orseculartrends
Types of Epidemics Common-SourceEpidemics: - Single exposure or“point source” epidemics. - Continuous ormultiple exposure epidemics. PropagatedEpidemics: - Person to person. - Arthropod vector - Animal reservoir MixedEpidemics. Slow ‘modern’ Epidemics: NC “non-communicable diseases”
I- Common Source Epidemics (A) Single-exposure ‘point’ epidemics -Exposure is Brief and simultaneous (immediate or concurrent) exposure. -All cases develop within one incubation period (food poisoning epidemics). Features of epidemic curve: 1-Rises and falls rapidly, no secondary waves. 2-Tends to be explosive, with clustering of cases within narrow interval of time. 3-All cases develop within one incubation period.
Epidemic curve of point source I.P Time Exposure Median I.P No. of cases 1-Commonly due to infectious diseases 2-May be from environmental pollution
Single exposure or“point source” epidemics (B) Continuous orrepeated exposure • Frequently not always due to exposure to an infectious agent • They can result from contamination of the environment (air, water, food, soil) by industrial pollutants • Minamata disease in Japan from consumption of fish containing high concentrationsof methyl mercury
Common Source Epidemics (B) Continuous orrepeated exposure 1-The exposure from the same source may be prolonged-continuous, repeated or intermittent 2-No explosive rise in numberof cases. 3-Cases occurovermore than one incubation period. Outbreak of respiratory illness, the Legionnaire disease in 1976 in USA, was a common source, continuous or repeated exposure, no evidence of secondary cases
Epidemic curve of repeated exposure No. of cases Time 1I.P Exposure 1-Waterwell exposure. 2-A Nationally distributed food orotherbrands.
Common Source Epidemics (B) Continuous orrepeated exposure • Epidemic may start from a common source and then continue as a propagated epidemic, Water borne epidemic as example the epidemic reaches a sharp peak, tails (end) off gradually over longer time of period
II- Propagated Epidemics Initialperiod Height Terminationphase Failed to infect Infected person Susceptible population Susceptible/Immune Mostly immune Primary case
Propagated Epidemics ® Of infectious origin, with person to person transmission (hepatitis A,Eand polio epidemics). ® Gradual rise and tails off overlongerperiod of time. ® Transmission continues till depletion of susceptible orsusceptible individuals are no longerexposed to source of infection. ® Communicability (speed of spread) depends on herd immunity among exposed and opportunities forcontact with infective dose and secondary attack rate.
Objectives of epidemic investigations 1-Define magnitude of epidemic (time, person, place) (When, Whom, Where). 2-Determine factors responsible forepidemic (Why). 3-Identify cause, sources of infection and modes of transmission (How). 4-Implement control and preventive measures at commence of epidemic (? Modification).
Epidemic Investigations • Frequently, epidemic investigation are called forafterthe peak of the epidemic has occurred, retrospective investigation • No step by step approach is applicable like “cook book” It is not necessary to follow the 10 steps in order. Several tasks can be carried out simultaneously. Saving time
10 steps of epidemic investigation 1-Verify diagnosis. 2-Confirm existence of epidemic. 3-Define population at risk. 4-Search forcases &theircharacteristics. 5-Data analysis. 6-Hypothesis formulation. 7-Testing hypothesis. 8-Evaluation of ecological factors. 9-Furtherinvestigation of at-risk population. 10-Report writing.
1-Verify diagnosis - First step in investigation as sometimes the epidemic report could be spurious (fake) - Misinterpretation of signs and symptoms by the lay public - Confirm diagnosis (clinical, laboratory and radiological) quickly on spot on few cases. -Start epidemiologic investigation.
2-Confirm existence of epidemic - Epidemic exists when the numberof cases (observed frequency) is in excess of the expected frequency forthat population, based on past experience - An arbitrary limit of 2 SDfrom the endemic occurrence is used to define the epidemic threshold forcommon diseases as flu. - Compare with past experience in same locality (2 SDabove mean).
2-Confirm existence of epidemic -Point source epidemic (HAV, cholera, food poisoning) are evident. -Modern (slow) epidemic (cancer, CVD) are difficult to recognize.
3-Defining population at risk 1-obtaining a map of the area: with water collection, residential areas, designated numberto houses. 2-Population censuses (counting): denominatorof attack rates.
3:1-Obtaining a map of the area • Before beginning investigation, a detailed and current map of the area is needed • It should contain information concerning natural land marks, roads and the location of dwelling units along each road orin isolated areas • Area into segments divided by natural landmarks, then into smallersections then houses
3:2-Population censuses (counting): denominatorof attack rates. • Denominatormay be related to the entire population orsubgroups of a population • If denominatoris entire population, a complete census of the population by age and sex should be carried out by house to house visit • This helps in computing the much needed attack rates in groups orsubgroups of population
4-Search forcases &theircharacteristics 1-Medical survey: examine all population (sample). 2-Epidemiological case sheet: filled forall population (sample). Includes: socio-demographics – history of exposure – S&S – special event – sources of suspected vehicle 3-Search formore cases: 2ry cases during IPfrom last case till area is declared free
4:1-Medical survey • Concurrently, medical survey should be carried out in a defined area to identify all including those who have not sought medical care and those at risk • Complete survey will pick all affected individuals with symptoms and signs of the disorder
4:2- Epidemiological case sheet • Interview case sheet, designated according to the preliminary rapid inquiry to collect relevant information • Name, age, sex, occupation, social class, travel, history of previous exposure, time of onset of disease, signs and symptoms, personal contact, events as parties, exposure to vehicles as food, water, milk, history of injections, blood products received
4:3- Search formore cases • Patient asked if knew othercases at home, work, neighborhood, school, • Search of new cases (secondary cases) should be done everyday till the area declared free of epidemic • This should be twice the incubation period of disease since occurrence of last case
5-Data analysis - Purpose: Identify common event or experience and define group involved. -Time (epidemic curve): suggests time of exposure and time clustering of cases The epidemic curve may suggest: - A time relationship with exposure to a suspected source - Whether is a common-source or propagated epidemic - Whether it is a seasonal or cyclic pattern suggestive of a particular infection.
5-Data analysis Place (spot map): shows clustering of cases (common-source) – provide evidence of source, mode of spread like John Snow in the cholera outbreak in London - Person’s characteristics: age, sex, occupation, exposed to specific event. - Determine the attack rates/case fatality rates forthose exposed and non exposed and according to host factors
6-Hypotheses formulation On basis of host-agent-environment formulate hypothesis to explain epidemic in terms of: -Possible source. -Causative agent. -Possible modes of spread. -Predisposing environmental factors.
7-Testing hypotheses All reasonablehypothesesneed to beconsidered and weighed by comparing attack ratesin various groupsfor thoseexposed and non exposed to each suspected factor Consider&test alternative hypotheses to find which hypothesis is consistent with all the facts
8-Evaluation of ecological factors Epidemiologist concern is to relate the disease to environmental factors to know source, reservoirand modes of transmission -Investigate possible ecological factors: -Sanitary status of Eating establishment. -Water&milk supply. -Population movement.
8-Evaluation of ecological factors -Atmospheric changes temperature, humidity, and airpollution -Population dynamics of vectors &animal reservoirs. -Breakdown in water supply system
9-Furtherinvestigation of at-risk population *Prospective orretrospective collection of additional information through: Clinical examination – screening test – examination of food, stool or blood specimen – biochemical studies – assessment of immunity status. *Detect sub-clinical cases – classify population according to exposure and illness status into: 1-exposure to specific potential vehicle 2-wetherill ornot
10-Report writing * Background: Geographical location 1– Climate condition 2– Demographic status (population pyramid) 3– Socioeconomic status 4– Organization of health services 5– Surveillance and early warning systems 6– Normal disease pattern. .
10-Report writing **Historical data: - Previous epidemic (same orother dis. In same orotherlocalities) – Discovery of first case of the present outbreak. -Occurrence of related diseases in the same area orin otherareas
10-Report writing ***Methodology of investigations: - Case definition – Questionnaire used in investigation – Survey method (household, retrospective) - Prospective surveillance - Data collected laboratory specimens and techniques
****Data analysis: - Clinical (S &S, course, DD, outcome) – Epidemiologic: (time, place &person distribution) sources &modes of transmission - Modes of transmission: Source of infection Routes of excretion and portal of entry Factors influencing transmission 10-Report writing
10-Report writing – Laboratory data: (agent isolation, sero-diagnosis and significance of results) - Data interpretation: - Comprehensive picture of the outbreak - Formulation &testing hypothesis ***** Control measures: Definition of strategies and methods of implementation :constraints - results
******Evaluation: *******Significance of results ********cost/effectiveness ********Preventive measures 10-Report writing
Review questions: -Enumerate types of epidemics and their characters -What are the objectives of epidemic investigation? -Enumerate steps of epidemic investigation.
Case Study A 23-year old male student presented at 10.30 PM on January17 at the college infirmary complaining of sudden onset of abdominal cramping, nausea, and diarrhea. Although the patient was not in severe distress and had no fever or vomiting, he was weak. A number of other students, all with the same symptoms, visited the college infirmary over the next 20 hours. All patients were treated with bed rest and fluid replacement therapy. They recovered fully within 24 hours of the onset of illness.
Calculation of the Attack Rate Existing information was gathered: The index case presented 10.30 PMon January 17 and by 8 PM on January 18, 47 affected students were examined, the attack rate was: Attackrate(AR)= Numberof newcases ÷Persons atrisk* 100 Attack Rate (allstudents) = 47/1164 X100 = 4.0% Furtherinvestigation revealed that: About 2/3 of the students lived in dormitories, one third lived outside (not at risk), so a more precise estimate of the attack rate will be: Attack rate (Dorm. Residents) = 47/756 X100 = 6.2%.
The dormitory of residence of the 47 cases and the attack rate, as well as the population and sex of the occupant of each dormitory. Dormitory Sex Population at risk Numberof cases Attack rate (AR%( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Total F F F F F M M F M M F M M M - 80 62 89 61 53 35 63 103 35 37 34 62 32 10 756 19 2 0 1 5 0 0 4 1 0 1 13 1 0 47 23.8 3.2 0 1.6 9.4 0 0 3.9 2.9 0 2.9 21.0 3.1 0 6.2
Calculation of Risk Ratio Population of dormitories number1 and 12 were more at risk as theirresidents showed the highest attack rate compared to the remaining dormitories. Attack rate (dorms. 1,12) = (19+13) /(80+62) X 100 = 22.5% Attack rate (remaining dorms) = (47-32) /(756-142) X100 = 2.4% A ratio of these attack rate may be calculated as follows: Risk Ratio = = = 9.4 AR (dorm 1,12( AR (remaining dorm( 22.5% 2.4% It means that the AR in dormitories 1.12, was 9.4 times greater than in the remaining 12 dormitories.
Searching forMore Cases Visits to some of the campus dormitories by the investigators soon revealed that not all students who became ill had visited the infirmary. Theextentof theoutbreakis biasedbythedifferent care-seekingbehavior. Questionnaires were prepared and distributed by hand to all students living in seven dormitories chosen randomly to be a representative sample. Adifferentpictureof theepidemic emergedfromtheresults.
Responses to the questionnaire surveyed by dormitory. Dormitory Population Questionnaire returned Number % No. of ill students 5 6 7 8 9 12 Nurses* residence Unidentified** Total 53 35 63 103 35 62 60 - 411 49 26 28 65 19 44 60 13 304 92.5 74.3 44.4 63.1 54.3 71.0 100 - 74.0 13 13 15 21 5 22 17 4 110 Dormitories 1-4, 10,11,13, and 14 were not surveyed. Nurse * dormitory was located off campus. Unidentified ** residence was not entered on 13 questionnaire. The overall attack rate now will be = 110/304 X100 = 36.2%
Why infirmary ARis lowerthan the survey AR? Variation in the severity of illness ‘ whose with mild disease may not seek medical care’. Other may thought care elsewhere including severe cases. Access to medical care in the sense of distance, money and availability of the services.
Features of the Epidemic More wide spread and explosive nature of the outbreak as almost 1/3 of the students are affected. The clustering of cases in relation to time suggested a common-source exposure. Data collected during survey indicated that no large gatherings of students ‘parties, sports events’ had recently occurred. Attention then was directed at meals, as most students ate at college cafeteria, included in the survey were questions concerning the source of meals eaten on January 16 and 17.
Analysis of meal-specific exposure histories of the respondents to the questionnaire. Students who ate specific meals Students who did not Ill Well Total AR% Ill Well Total AR% January 16 breakfast lunch dinner January 17 breakfast lunch dinner 52 89 87 56 106 78 100 150 150 105 145 130 152 239 237 161 251 208 34.2 37.2 36.7 34.8 42.2 37.5 51 20 23 42 3 31 94 44 44 89 49 64 145 64 67 131 52 95 35.2 31.3 34.3 32.1 5.8 32.6 The risk ratio of lunch meal on January 17 was: RR(1/17 lunch) = AReaters /ARnon-eaters = 42.2%/5.8 % = 7.3 Those who eat this meal were more than 7 times to have become Ill compared to non-eaters.
Calculation of the incubation period. Having identified the meal at which the students most probably were exposed to the casual pathogen and knowing each student’s time of onset of symptoms, it was possible to calculate the incubation period (the time between eating the lunch meal on January 17 and the onset of symptoms(. The median incubation period is the time by which 50 % of the cases have occurred.
Distribution of numberof cases by time from eating suspect meal to development of symptoms. 2222 11 33 18 51 8 59 42 101 0 20 40 60 80 100 120 Time in hours 8 9 10 11 12 number of cases number of students cumulative
Foods Responsible forthe Outbreak, Searching forthe Source
Foods Responsible forthe Outbreak, Searching for the Source Food or beverages Students who ate specific meals Students who did not Ill Well Total AR% Ill Well Total AR% Fish chowder Lamb stew pie Tuna noodle casserol Pineapple jell Fruit salad Cabbage salad Jill with vanilla Jill without Milk Coffee Tea 16 95 12 58 32 4 19 62 91 10 23 36 56 57 54 39 5 29 77 127 31 19 52 151 69 112 71 9 48 139 218 41 42 30.8 62.9 17.4 51.8 45.1 44.4 39.6 44.6 41.7 24.4 54.8 87 7 92 39 63 95 80 39 12 89 78 103 82 80 69 82 126 102 56 13 103 114 190 89 172 108 145 221 182 95 25 192 192 45.8 7.9 53.5 36.1 43.4 43.0 44.0 41.1 48.0 46.4 40.6 The risk ratio for certain food was more than 1: 8for lamb stew pie, which may indicted the source of infection.
Conditions Favoring Infection The lamb stew pie through further investigations, it was revealed that it was prepared on the previous day (January 16), refrigerated and warmed on the morning it was served. What was the causative agent???????????
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