DESCRIPTIVE EPIDEMIOLOGY
PERSON,PLACE & TIMEDESCRIBING THE DISEASE UNDER STUDY:
WHO? WHERE? WHEN?PERSON PLACE
TIME
DESCRIBING THE DISEASE
TimePlace
Person
Year, Season Month, Week Day, Hour of onset Duration
Climatic zones Country, Region Urban/Rural Local community Towns Cities Institutions
Age, Sex Race Marital ,immune state Occupation, Social status, Education
Birth order Family size Height Weight Personal habits
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1) 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.
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PERSON DISTRIBUTION:
AGE BIMODALITY GENDER ETHNIC GROUP OCCUPATION SOCIOECONOMIC STATUS MARITAL STATUS BEHAVIOURa) Age
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
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- Biologic difference like sex linked genetic inheritance 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
Differences in racial and ethnic origin. Eg: Tuberculosis, sickle cell anemia
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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.*
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)*
g) Behaviour
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.*
2) Place distribution
*a) International variation
Descriptive studies have shown that the pattern of a disease is not the same everywhere Eg: Cancer of the stomach is very common in Japan, but unusual in the US.
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b) National variation
There are variations in disease occurrence within countries. Eg: The distribution of endemic goitre, fluorosis, nutritional deficiencies show variations in their distribution .*
c) Rural-Urban Variations
Due to differences in population density, levels of sanitation, deficiencies of medical care, education and environment factors, there exists a rural-urban variation Chronic bronchitis, cardiovascular diseases, accidents 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.*
d) Local distributions
These variations can be studied with the help of “spot or shaded” maps. If the map showed clustering, it may suggest a common source of infection.Eg: Study of Cholera epidemic by John Snow in 1854 *Study by John Snow, 1854
Spot map of deaths from cholera in Golden Square area, London, 1854This pump was later suspected and proved to be a source of infection
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Migration studies
The use of migrant studies is a way of distinguishing genetic and environmental factors. Carried out in 2 ways- Study of genetically similar groups but living under different environmental conditions. Eg: Twins 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*
1) Time distribution
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a) Short term fluctuations
An epidemic is the best known short term fluctuation. It is defined as “ The occurrence of cases of an illness or other health related events in a community or region clearly in excess of normal expectancy” *Epidemic curve
A graph of time distribution of epidemic cases is called epidemic curve.*
2.PERIODIC FLUCTUATIONS
SEASONAL TRENDSCYCLIC TRENDS
b) Periodic Fluctuations
1) Seasonal trend- Seasonal variation is characteristic of many communicable diseases. Eg: Measles, upper respiratory tract infections(seasonal rise during winter). Non-infectious diseases and conditions may sometimes exhibit seasonal variation. Eg: Sunstroke, hay fever.*
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2) Cyclic trend- Some diseases occur in cycles spread over short periods of time (days, weeks, months) . Eg: Influenza pandemics are known to occur at intervals of 7-10 yrs due to antigenic variations. Non-infectious conditions may also occur in this trend. Eg: car accidents in the US are more frequent on weekends.
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c) Long term trends (secular)
It refers to changes in the occurrence of disease over a long period of time. Eg: Coronary disease, diabetes showing consistent upward trend and a decline in TB, polio in developed countries during the past 50 yrs.*
Describe the exposed population characteristics (who?). What is strange in the population structure? Where do highest & lowest death rates occur? How can this population be together in time & place? What are the possible causes of this unusual event?
Describe the exposed population characteristics (who?) What is strange in the population structure? Where do highest & lowest death rates occur? How can this population be together in time & place? What are the possible causes of this unusual event?