Presenter – Anil Koparkar Moderator –Dr. Chetna Maliye

DESCRIPTIVE EPIDEMIOLOGY

Presenter – Anil KoparkarModerator –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 Correlational Populations

Cross-sectional Prevalence Individuals

Case -Control Case -Reference Individuals

Cohort Follow Follow-up/ Longitudinal Individuals

B. Experimental/ intervention Studies

Randomized Controlled Studies Clinical Trial Patients

Field Trial Healthy person

Community Trial Community intervention studies 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.


Cross Sectional Descriptive Studies mainly directed to work out the:

Prevalence Mean Pattern surrogate for longitudinal 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.

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)


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


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.


9. Twin Studies : 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.

10.Other Variables:

Various Socio - Demographic, Physiological, Biochemical, Immunological characteristics.


DISTRIBUTION ACCORDING TO TIME

A. Common Source (Vehicle) Epidemics -1. Common Source (Vehicle), Single (Point) Exposure:2. Common Source, Continued exposure3. Common Source, interrupted exposure

B. Propagated Source

C. Seasonal fluctuations

D. Cyclical Changes

E. Secular trends


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.


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 contamination is removed or all possible “susceptible” have become infected.

Has certain characteristic features epidemic curve rises slowly, falls gradually; peak is not sharp but rather plateau - like and duration of epidemic is stretched out.


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.

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.


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.

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’.

International Comparisons (…cont.)

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.

Local distributions : The finding may finally be due to one of the two reasons:1. The inhabitants of that place, OR

2. 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.

DISTRIBUTION ACCORDING TO PLACE (…CONT.)

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. Centers for Disease Control and Prevention. Principles of Epidemiology an Introduction to Applied Epidemiology & Biostatistics. 2nd Ed.16-30.

2. Bhalwar R. Textbook of Public Health and Community Medicine.1st ed.2009.131-

3. Park K. Park’s textbook of preventive and social medicine. 20th edition, 2009. Banarsidas Bhanot publishers, Jabalpur, India. 56-

4. Beaglehole R, Bonita R, Kjellstrom T. Basic Epidemiology.2nd edition. World Health Organization.2006. 4, 6-11, 26-

5. Last JM, ed. Dictionary of Epidemiology, Second edition. New York: Oxford U. Press, 1988:42.

6. MacMahon B, Trichopoulos D. Epidemiology Principles and Methods. Second ed.Little, Brown and company. 1996:

THANK YOU……………..

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Presenter – Anil Koparkar Moderator –Dr. Chetna Maliye