Descriptive
Epidemiology-Part II
Prof. Najam Khalique
Department of Community Medicine
II) Periodic Fluctuations
A) Seasonal Trend:
Seasonal variation is characteristic of many communicable diseases.
Eg: Measles/ Varicella: Early spring,
Upper respiratory tract infections: During winter, Bacterial gastro-enteritis: Summer, etc.
Non-infectious diseases and conditions may sometimes exhibit seasonal variation.
Eg: Sunstroke, hay fever, snakebite
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B) Cyclic trend:
Some diseases occur in cycles spread over short periods of time (days, weeks, months or years)
Eg: Influenza pandemics are known to occur at intervals of 7-10 yrs due to antigenic variations
Measles – In pre vaccination era, major peaks appear every 2 to 3 years
Rubella – Every 6 to 9 years
Non-infectious conditions may also occur in this trend. Eg: Automobile accidents more frequent on weekends
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C) Long Term Trends:
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It refers to changes in the occurrence of disease over a long period of time (several years to
decades)
Coronary disease, diabetes showing consistent upward trend
Decline in TB, polio in developed countries during the past 50 yrs
Implies a consistent tendency to change in a
particular direction or a definite movement in one direction
Interpretation Of Time Trends
The epidemiologist seeks which diseases Increasing,
Decreasing,
Emerging health problems
Effectiveness of measures to control old ones
Tries to formulate aetiological hypotheses
Provide guidelines to the health administrator in matters of prevention or control of disease
Place Distribution
Place variations
1) International variations
2) National variations 3) Rural-Urban
variations
4) Local distributions 25
A) International Variation
Descriptive studies have shown that the pattern of a disease is not the same
everywhere
Cancer of the stomach is very common in Japan, but unusual in the US
Cancer of oral cavity & uterine cervix is common in India, uncommon in industrialized world
Aim is to identify factors which are crucial in the cause and prevention of disease
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B) National Variations
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27Variations in disease occurrence within countries
The distribution of endemic goitre, lathyrism, fluorosis, malaria, nutritional deficiencies
show variations in their distribution in India
Descriptive epidemiology will provide data
regarding the type of disease problems & their magnitude in terms of incidence, prevalence &
mortality rates
C) Rural-urban Variations
Due to differences in population density, social class, levels of sanitation, deficiencies of
medical care, education and environment factors
Chronic bronchitis, cardiovascular diseases, accidents, mental illness are more frequent in urban than rural areas
Skin and zoonotic diseases and soil transmitted helminths may be more frequent in rural than urban areas
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D) Local Distributions
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Variations in disease frequency occur in inner &
outer areas
Can be studied with the help of “spot or shaded” maps
Maps show at a glance areas of high or low
frequency, the boundaries & patterns of disease distribution
If the map shows clustering, it may suggest a common source of infection or risk factor
Spot map of deaths from cholera in Golden Square area, By John Snow, 1854
Epidemiologist looks for geographical variation which may provide evidence of the source of disease and its mode of spread
Relating these variations to agent, host &
environmental factors helps to formulate and test aetiological hypotheses
Geographical distribution may change with change in epidemiological triad
Migration Studies
Migrant studies are a way of distinguishing genetic and environmental factors
Carried out in 2 ways-
1. Study of genetically similar groups but living under different environmental conditions. Eg: Twins
2. Study of genetically different groups living in a similar environment. Eg: Men of Japanese origin living in USA have higher rate of coronary heart disease than the Japanese in Japan
Person Distribution
The disease can be characterised by defining a person who develops a disease based on age, sex, occupation, marital status, social
factors, habits and other host factors.
a. Age
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Certain diseases are more frequent in certain age groups than others.
Eg: Measles in childhood,
cancer in middle age and atherosclerosis in old age.
Many chronic and degenerative diseases show a progressive increase in prevalence with
advancing age.
Bimodality
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There may be two separate peaks instead of one in the age incidence curve of a disease.
This is known as bimodality as seen in Hodgkin’s lymphoma, breast cancer.
It indicates that there are two different sets of causative factors even though the
clinical and pathological manifestations of the disease is the same in all ages.
b. Sex
Variations occur due to-
1. Biologic difference like sex linked genetic inheritance
2. Cultural and behavioural differences between the sexes in social settings.
Eg: 4:1 male to female ratio in lung cancer due to cigarette smoking
C) Ethnicity
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Differences in disease occurrence between population subgroups of different racial and ethnic origin.
Eg: Tuberculosis, sickle cell anemia
d. Marital Status
In a study, the mortality rates were lower for married people than unmarried
It is because according to demographers and
sociologists, marriages are selective with respect to health of the individual.
Healthier the individual, the more likely to get married.
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e. Occupation
Occupation may alter the habit pattern of employees (Sleep, alcohol, smoking, etc)
Workers in a particular occupation are
exposed to certain types of risk. Eg: Workers in coal mines are likely to suffer from
silicosis
f. Social Class
Health and diseases are not equally distributed in social classes
Certain diseases show higher prevalence in upper class (Diabetes, Coronary heart disease, hypertension)
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g. Behaviour
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Behavioural factors such as smoking,
sedentary life, over-eating, drug abuse lead to certain diseases (Coronary heart disease, Cancer, etc.)
Factors like mass movement (eg:
Pilgrimages) may also lead to transmission of infectious diseases.
h. Stress
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The effects of stress are seen based on the patient’s response (susceptibility to disease, exacerbation of symptoms, compliance with medical regimen, etc)
i. Migration
Due to migration of people, there is also
transmission of the disease from one place to another.
4. Measurement Of Disease
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Types-
Cross sectional studies:
a. Prevalence can be obtained.
b. It is based on a single examination of a cross section of population at one point in time.
c. More useful for chronic diseases
Longitudinal studies:
a. Incidence can be obtained.
b. The observations are repeated in the
same population over a prolonged period of time by means of follow up examination.
c. Longitudinal is more useful, but it is time consuming.
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5. Comparing With Known Indices
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Comparisons are made with known indices to arrive at clues to the disease’s etiology
6. Formulation of a Hypothesis
A hypothesis is a supposition, arrived at from observation or reflection
An epidemiologic hypothesis should specify-
a. The population- characteristics of the people to whom the hypothesis applies
b. Specific cause
c. Expected outcome-the disease
d. Dose response relationship-The
amount of the cause needed to lead to the stated incidence of the effect.
e. Time response relationship-Time period between exposure to the cause and
observation of the effect.
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Uses Of Descriptive Epidemiology
It provides data regarding the magnitude of
the disease load and types of disease problems in terms of morbidity and mortality rates and
ratios.
It provides clue to disease etiology and help in the formulation of an etiological hypothesis.
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It provides background data for planning, organizing and evaluating preventive
service.
Contribute to research by describing variations in disease occurrence by time, place and person.
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THANK YOU
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