Types of Epidemics and

Epidemic Investigations

Professor Tarek Tawfik AminPublic Health Dept. Faculty of Medicine

Cairo Universityamin55@myway.com

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 fluctuations3-Long term or secular trends

Types of Epidemics Common-Source Epidemics: - Single exposure or “point source”

epidemics. - Continuous or multiple exposure

epidemics. Propagated Epidemics: - Person to person. - Arthropod vector - Animal reservoir Mixed Epidemics. 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.PTime

Exposure

Median I.PNo. of cases

1 -Commonly due to infectious diseases

2 -May be from environmental pollution

Single exposure or “point source” epidemics

(B) Continuous or repeated 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 concentrations of methyl mercury

Common Source Epidemics(B) Continuous or repeated

exposure1-The exposure from the same source

may be prolonged-continuous, repeated or intermittent

2-No explosive rise in number of cases.3-Cases occur over more 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

Time1 I.P

Exposure

1 -Water well exposure.2 -A Nationally distributed

food or other brands.

Common Source Epidemics(B) Continuous or repeated

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 EpidemicsInitial period Height Termination phase

Failed to infect

Infected person

Susceptiblepopulation

Susceptible/ImmuneMostly immune

Primary case

Propagated Epidemics®Of infectious origin, with person to person

transmission (hepatitis A,E and polio epidemics).®Gradual rise and tails off over longer period of

time.®Transmission continues till depletion of

susceptible or susceptible individuals are no longer exposed to source of infection.

®Communicability (speed of spread) depends on herd immunity among exposed and opportunities for contact with infective dose and secondary attack rate.

Epidemic Investigations

Objectives of epidemic investigations

1-Define magnitude of epidemic (time, person, place) (When, Whom, Where).

2-Determine factors responsible for epidemic (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 for after the 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 for cases & their

characteristics.5-Data analysis.6-Hypothesis formulation.7-Testing hypothesis.8-Evaluation of ecological factors.9-Further investigation 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 number of cases (observed frequency) is in excess of the expected frequency for that population, based on past experience

- An arbitrary limit of 2 SD from the endemic occurrence is used to define the epidemic threshold for common diseases as flu.

- Compare with past experience in same locality (2 SD above 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 number to houses.

2-Population censuses (counting): denominator of 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 or in isolated areas

• Area into segments divided by natural landmarks, then into smaller sections then houses

3:2-Population censuses (counting): denominator of attack

rates.

• Denominator may be related to the entire population or subgroups of a population

• If denominator is 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 or subgroups of population

4-Search for cases & their characteristics

1-Medical survey: examine all population (sample).

2-Epidemiological case sheet: filled for all population (sample).

Includes: socio-demographics – history of exposure – S&S – special event – sources of suspected vehicle

3-Search for more cases: 2ry cases during IP from 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 for more cases•Patient asked if knew other cases

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 for those 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 reasonable hypotheses need to be considered and weighed by comparing attack rates in various groups for those exposed 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, reservoir and 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 air pollution

-Population dynamics of vectors & animal reservoirs.

-Breakdown in water supply system

9-Further investigation of at-risk population

*Prospective or retrospective 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 vehicle2-wether ill or not

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 or

other dis. In same or other localities)

– Discovery of first case of the present outbreak.

-Occurrence of related diseases in the same area or in other areas

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 infectionRoutes of excretion and portal of entryFactors 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 StudyA 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 PM on January 17 and by 8 PM on January 18, 47 affected students were examined, the attack rate was:

Attack rate (AR) = Number of new cases Persons at risk * 100Attack Rate (all students) = 47/1164 X100 = 4.0%Further investigation 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.DormitorySex

Population at risk

Number of cases

Attack rate (AR%)

123456789

1011121314

Total

FFFFFMMFMMFMMM-

80628961533563

103353734623210

756

192015004101

1310

47

23.83.20

1.69.400

3.92.90

2.921.03.10

6.2

Calculation of Risk RatioPopulation of dormitories number 1 and 12 were more at risk as their residents 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 for More Cases

Visits to some of the campus dormitories by the investigators soon revealed that not all students who became ill had visited the infirmary.The extent of the outbreak is biased by the different care-seeking behavior.Questionnaires were prepared and distributed by hand to all students living in seven dormitories chosen randomly to be a representative sample.

A different picture of the epidemic emerged from the results.

Responses to the questionnaire surveyed by dormitory.

DormitoryPopulation

Questionnaire returnedNumber%

No. of illstudents

56789

12Nurses*

residenceUnidentified**

Total

533563

103356260-

411

4926286519446013

304

92.574.344.463.154.371.0100

-74.0

131315215

22174

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 AR is lower than 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 EpidemicMore 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

528987

5610678

100150150

105145130

152239237

161251208

34.237.236.7

34.842.237.5

512023

423

31

944444

894964

1456467

1315295

35.231.334.3

32.15.8

32.6The risk ratio of lunch meal on January 17 was:RR (1/17 lunch) = AR eaters / AR non-eaters = 42.2%/5.8 % = 7.3Those who eat this meal were more than 7 times to have becomeIll 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 number of cases by time from eating suspect meal to

development of symptoms.

222211

3318

51

8

5942

101

020406080

100120

Time in hours

8 9 10 11 12number of cases

number ofstudentscumulative

Foods Responsible for the Outbreak, Searching for the

Source

Foods Responsible for the Outbreak, Searching for the Source

Food or beveragesStudents who ate specific meals

Students who did not

Ill Well Total AR% Ill Well Total AR% Fish chowderLamb stew pieTuna noodle casserolPineapple jellFruit saladCabbage saladJill with vanillaJill without MilkCoffeeTea

16951258324

1962911023

36565754395

2977

1273119

5215169

112719

481392184142

30.862.917.451.845.144.439.644.641.724.454.8

877

923963958039128978

10382806982

1261025613

103114

19089

1721081452211829525

192192

45.87.9

53.536.143.443.044.041.148.046.440.6

The risk ratio for certain food was more than 1:8 for 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???????????