Descriptive Epidemiology - E-Library for the Post

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DESCRIPTIVE EPIDEMIOLOGY
Presenter – Anil Koparkar
Moderator –Dr. Chetna Maliye
FRAMEWORK
Introduction
 Definition of Descriptive epidemiology
 Descriptive and analytical epidemiology
 Types of Descriptive Studies

Case Reports and Case Series
 Cross Sectional and Longitudinal Descriptive
Studies


Epidemiological Descriptions according
Person
 Time
 Place


References
INTRODUCTION

Epidemiology
 Greek
words epi = people
 Logos = the study of
“Epidemiology is the study of the distribution and
determinants of health-related states or events in
specified populations, and the application of this
study to the control of health problems”.
EPIDEMIOLOGICAL STUDIES
Type of study
Alternate name
Unit of study
A. Observational studies
Descriptive studies
Analytical studies
Ecological
Cross-sectional
Case -Control
Cohort
Correlational
Prevalence
Case -Reference
Follow Follow-up/ Longitudinal
Populations
Individuals
Individuals
Individuals
B. Experimental/ intervention Studies
Randomized Controlled Studies
Clinical Trial
Field Trial
Community Trial
Community intervention studies
Patients
Healthy person
Communities
DESCRIPTIVE EPIDEMIOLOGY

Definition
A
study in which only one group, i.e. subjects having
the outcome (disease or any other health related
phenomena of interest) are studied, without any
comparison group, for describing the outcome or
health - related phenomena according to its
frequency or such other summary figures (as
mean), and its distribution according to selected
variables related to person, place and time.
DESCRIPTIVE V/S ANALYTICAL STUDY
Descriptive
Analytical
1 group is studied
At least 2 groups are studied
At the start – no hypothesis
At the start - definite
hypothesis
At the end - possible
hypotheses
At the end - confirms or rejects
the hypothesis.
TYPES OF DESCRIPTIVE STUDIES

Case Reports and Case Series
 based
on reports of a single, or else a series of
cases - treated or untreated - without any specific
comparison (control) group
 describing
signs, symptoms or patho-physiological
parameters in the series of patients
 do
not indicate risk.
TYPES OF DESCRIPTIVE STUDIES
 Cross
Sectional Descriptive Studies
 mainly
directed to work out the:
Prevalence
Mean
Pattern
surrogate
for longitudinal descriptive studies
TYPES OF DESCRIPTIVE STUDIES
 Longitudinal
Descriptive Studies
 follows
up single group of subjects over a
defined period
 objectives:
To
see the incidence
To describe the ‘natural history of a disease’
To describe a health related natural phenomena
To study the ‘trend’ of a disease & ‘health related phenomena
LONGITUDINAL VS CROSS SECTIONAL
DESCRIPTIVE STUDIES
Cross sectional study
Longitudinal study
To know prevalence, mean, pattern
of disease, etc.
To know incidence, natural history of
a disease, health related natural
phenomena, trend of a disease or
health - related phenomena
researcher examines only once
subject examined at least twice
gives us the “prevalence”
gives us the “incidence”.
Less time consuming & easy
Should be preferred when possible ,
but often a difficult way.
EPIDEMIOLOGICAL DESCRIPTIONS ACCORDING
TO PERSON, PLACE AND TIME
DESCRIPTIONS ACCORDING TO PERSON
Age:

Distribution of the disease according to “age specific” rates.

Death rates
 Highest
during infant, preschool age & extreme old age,
 Lowest during 5 - 24 years group
Measles in childhood, cancer in middle age,
atherosclerosis in old age is common
 non - communicable (chronic) diseases - rising trend
during middle age.
 Bimodality

ACCORDING TO PERSON…. (CONT.)
Sex:
 Some
diseases more common in females- gall
bladder and thyroid; CHD, AIDS,IHD, peptic
ulcer, inguinal hernia, accidents and lung
cancer is less common.
 The
sex related differences may be due to
hormonal or other biological differences or
due to differences in attitude towards life.
ACCORDING TO PERSON…. (CONT.)
Ethnic Group
 group of persons who have a greater degree of
homogeneity than the population at large in
respect of biologic inheritance and present day
customs
 categories of variables
 Race
- e.g. Mongoloid, Caucasian & Negroid.
 Nativity - e.g. European, Indian, Chinese etc.
 Religion
 Local reproductive and social units(Cast)
ACCORDING TO PERSON…. (CONT.)
4. Social Class :
 independent risk factor for the disease or it may be indirectly
associated.
 Commonly used scales
 Prasad’s scale based on per capita per month income &
 Kuppuswamy scale which takes an ordinally scaled
combination of education, occupation and income.
5. Occupation :
 The stress of occupation and exposure to various physical,
chemical and biological disease agents therein, may be
associated with high occurrence of such diseases.
 On the other hand, entry into occupation is itself likely to be
related to particular physical (e.g. soldiers) and mental (e.g.
Doctors) capabilities
ACCORDING TO PERSON…. (CONT.)
6. Education :
 Education - improved level of knowledge - reduced risk of disease.
 level of formal education illiterate, just literate (upto 5th standard),
upto matriculation, upto college, graduate, and post - graduate or
Doctoral level.
7. Marital Status :
 In general, mortality rates - married < single < widowed < divorced.
8. Family Variables :
 Depending on the scope of the epidemiological investigations at
hand, various family variables as family size, birth order, maternal
age, parental deprivation during childhood, familial aggregation of
disease, and so on, are studied.
ACCORDING TO PERSON…. (CONT.)
9.
Twin Studies :



10.
Very powerful methods for evaluating the genetic
background of a disease. Working premise monozygotic twins carry identical genes, while
dizygotic twins are simply like two different siblings
from genetic point of view.
Concordance between monozygotic & dizygotic twins genetic background.
discordance in monozygotic twins - environmental
etiology.
Other Variables:
Various Socio - Demographic, Physiological,
Biochemical, Immunological characteristics.
DISTRIBUTION ACCORDING TO TIME
A.
Common Source (Vehicle) Epidemics 1.
2.
3.
Common Source (Vehicle), Single (Point) Exposure:
Common Source, Continued exposure
Common Source, interrupted exposure
B.
Propagated Source
C.
Seasonal fluctuations
D.
Cyclical Changes
E.
Secular trends
DISTRIBUTION ACCORDING TO TIME
A. Common Vehicle Epidemics 1. Common Source (Vehicle), Single (Point) Exposure:
 The
infective material remains present in the vehicle for
a brief period of time
 Has certain characteristic features
All cases occur within one known incubation period of the
disease.
 The epidemic curve has a sharp onset and an equally abrupt
decline.
 The peak of the epidemic is sharp and coincides with the
median incubation period of the disease.

DISTRIBUTION ACCORDING TO TIME
A. Common Vehicle Epidemics 2. Common Source, Continued exposure
 when
an infectious agent persists in the common vehicle for
some amount of time
 The final decline of the epidemic occurs due to


 Has




contamination is removed or
all possible “susceptible” have become infected.
certain characteristic features
epidemic curve rises slowly,
falls gradually;
peak is not sharp but rather plateau - like and
duration of epidemic is stretched out.
DISTRIBUTION ACCORDING TO TIME
A. Common Vehicle Epidemics 3. Common Source, interrupted exposure
 source introduces the infection into the vehicle only
interruptedly
DISTRIBUTION ACCORDING TO TIME (…CONT.)
B. Propagated Source: In such an epidemic, the
source itself propagates, i.e. multiplies
The fall of the epidemic occurs due to
 development of enough herd immunity
 The epidemic curve rises slowly, in waves
 Reaches a flat plateau and then declines slowly.
DISTRIBUTION ACCORDING TO TIME (…CONT.)
C. Seasonal fluctuations

Malaria and JE - immediate post monsoon season;

Airborne / droplet - winters when people tend to congregate and
overcrowd.

Asthma spring and autumn suggesting specific environmental
factors in causation.

Seasonal fluctuations are usually demonstrated by line diagrams.
They may help differentiating two similar – appearing illnesses
like JE and meningococcal meningitis - the former having a peak
during post monsoon and the latter manifesting a peak during
peak winters.
DISTRIBUTION ACCORDING TO TIME (…CONT.)
D. Cyclical Changes: These are periodic peaks in
disease frequencies occurring every 3 - 5 years.
Ex. Measles- epidemics tend to occur in cycles of
2 – 3 years.
E. Secular trends : These are time trends occurring
over a period of decades. Ex. Cancers of various
sites
stomach and uterus - declining trend in death rate
 cancers of lung and pancreas - rising trend
 breast cancer mortality rate - no change.

DISTRIBUTION ACCORDING TO PLACE

Many diseases have typical spatial
relationships;
goiter - foothill regions,
 Anthrax and brucellosis - rural areas
 CHD - affluent countries


Differences in the distribution of a disease
political boundaries - international comparison,
regional comparison within countries
 natural boundaries - rural - urban differences,
altitude, or local distribution of disease

INTERNATIONAL COMPARISONS

Japan has very low CHD mortality rates but high
rates for cerebro - vascular accidents,
Hypertension and gastric CA;

UK has high lung CA rates while USA has high
CHD rates.

“Migrant Studies” is good method of dissecting
this fact out.
International Comparisons
(…cont.)
GROUP OF PEOPLE A FROM COUNTRY X
Countries X
Y
 Disease(D)pattern- x
y
 Now let ‘m’ be the disease pattern of the
Group of people A in country Y, then

 If
disease D is due to genetic factor, then
 ‘m’

will approximate to ‘x’. And
If disease D is due to environmental factor, then
 ‘m’
will approximate to ‘y’.
DISTRIBUTION ACCORDING TO PLACE (…CONT.)
Regional Variations within countries :
 e.g. goiter -in the foot hill areas in India.


Rural - Urban differences :
 point
out towards possible environmental factors;
 e.g. IHD, STDs, Hypertension etc. are more common
in the urban areas while
 oro - faecal infections are more common in rural
areas.
DISTRIBUTION ACCORDING TO PLACE (…CONT.)


1.
2.
Local distributions :
The finding may finally be due to one of the two reasons:
The inhabitants of that place, OR
Some etiologic factors, characteristic in the place are
present. If this is the reason, then :
(i) High rates of disease will be observed in all ethnic groups in
that area.
(ii) High rates are not observed in persons of similar ethnic
groups living in other areas.
(iii) Healthy persons entering that area become ill with a
frequency similar to the indigenous inhabitants.
(iv) Inhabitants who have left that area do not show high rates.
(v) Some evidence of the disease may also be found in
animals in the same area.
METHODS OF DISPLAYING AND ANALYZING
PLACE RELATED DISEASE
common methods used:
 Spot Mapping :



simplest, yet a very productive method of displaying the
place - related distribution of a disease
Map - on - map:
we combine two maps to bring disease frequencies,
plotted as colored dots, into visual approximation with
other variables like roads, rivers, indices of poverty etc.
 This technique may also be used for studying
“movement” of a disease in both time and place.

STUDY BY JOHN SNOW, 1854

Spot map of deaths from cholera in Golden Square area, London, 1854
This pump was
later suspected
and proved to
be a source of
infection
REFERENCES
1.
2.
3.
4.
5.
6.
Centers for Disease Control and Prevention. Principles of
Epidemiology an Introduction to Applied Epidemiology &
Biostatistics. 2nd Ed.16-30.
Bhalwar R. Textbook of Public Health and Community
Medicine.1st ed.2009.131Park K. Park’s textbook of preventive and social medicine. 20th
edition, 2009. Banarsidas Bhanot publishers, Jabalpur, India.
56Beaglehole R, Bonita R, Kjellstrom T. Basic Epidemiology.2nd
edition. World Health Organization.2006. 4, 6-11, 26Last JM, ed. Dictionary of Epidemiology, Second edition. New
York: Oxford U. Press, 1988:42.
MacMahon B, Trichopoulos D. Epidemiology Principles and
Methods. Second ed.Little, Brown and company. 1996:
THANK YOU……………..

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