What is meant by epidemic, types of epidemics, steps used to investigate the occurrence of epidemics

1. Types of Epidemics
and
Epidemic Investigations
ProfessorTarek Tawfik Amin
Public Health Dept. Faculty of Medicine
Cairo University
amin55@myway.com

2. 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

3. 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”

4. 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.

5. 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

6. 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

7. 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

8. Epidemic curve of repeated exposure
No. of cases
Time
1I.P
Exposure
1-Waterwell exposure.
2-A Nationally distributed
food orotherbrands.

9. 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

10. II- Propagated Epidemics
Initialperiod Height Terminationphase
Failed to infect
Infected person
Susceptible
population
Susceptible/Immune
Mostly immune
Primary
case

11. 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.

12. Epidemic Investigations

13. 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).

14. 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

15. 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.

16. 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.

17. 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).

18. 2-Confirm existence of
epidemic
-Point source epidemic (HAV, cholera, food
poisoning) are evident.
-Modern (slow) epidemic (cancer, CVD) are
difficult to recognize.

19. 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.

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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.

27. 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

28. 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.

29. 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

30. 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.

31. 8-Evaluation of ecological factors
-Atmospheric changes temperature,
humidity, and airpollution
-Population dynamics of vectors &animal
reservoirs.
-Breakdown in water supply system

32. 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

33. 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.
.

34. 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

35. 10-Report writing
***Methodology of investigations:
- Case definition
– Questionnaire used in investigation
– Survey method (household, retrospective)
- Prospective surveillance
- Data collected laboratory specimens and
techniques

36. ****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

37. 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

38. ******Evaluation:
*******Significance of results
********cost/effectiveness
********Preventive measures
10-Report writing

39. Review questions:
-Enumerate types of epidemics and their
characters
-What are the objectives of epidemic
investigation?
-Enumerate steps of epidemic investigation.

40. 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.

41. 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%.

42. 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

43. 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.

44. 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.

45. 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%

46. 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.

47. 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.

48. 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.

49. 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.

50. 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

51. Foods Responsible forthe Outbreak,
Searching forthe Source

52. 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.

53. 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???????????