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.
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 PERSONAge: 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)
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 : 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.
ACCORDING TO PERSON…. (CONT.)
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
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 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.
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.
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 TIME (…CONT.)
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
REFERENCES1. 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……………..