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Epidemiological methods (

(describing the disease by time and place :Presented by

Dr. Noha Hamza

Describing the disease The primary objective of descriptive epidemiology is to describe the occurrence and distribution of disease by time, place and person, and identifying those characteristics associated with presence or absence of disease in individuals. This objective involves systematic collection and . analysis of data

Characteristics frequently examined in descriptive studies Time year, season, month, week, day, hour of onset, duration

Place Person Climatic zones Country, region, urban/rural Local community Towns cities institutions

Age Sex marital status Occupation Social status education

Birth order Family size Height Weight Blood pressure Blood cholesterol Personal habits

Time distribution Time trends : or (fluctuation in disease occurrence) : I. II. III.

Short-term fluctuation Periodic fluctuation Long-term or secular trends

l. Short-term fluctuation The best known Short-term fluctuation in the occurrence of a disease is an epidemic.

Epidemic is defined as : The occurrence in a community or region of cases of an illness or other health-related events clearly in excess of normal expectancy.

Types of epidemics 1.

Common source epidemics. Single exposure or “point source” epidemics Continuous or multiple exposure epidemics

i. ii.


Propagated epidemics i. ii. iii.


Person- to –person Arthropods vector Animal reservoir

Slow (modern) epidemics

Epidemic curve It is a graph of the time distribution of epidemic cases. It suggests : i. ii. iii.

A time relationship with exposure to a suspected source. A cyclical or seasonal pattern suggestive of a particular infection. A common source or propagated spread of the disease.

Common source epidemics. 1 a. Single exposure (point source( epidemics : The exposure to the disease agent is brief and essentially simultaneous. The resultant cases all develop within one incubation period of the disease. (e.g :an epidemic of food poisoning).

Fig 1 illustrates epidemic curve of : Common source, Single exposure epidemics

The main features of a “ point-source� epidemic are : i. The epidemic curve rises and falls rapidly, with no secondary waves. ii. The epidemic tends to be explosive, there is clustering of cases within a narrow interval of time. iii. All cases develop within one incubation period of disease.

Causes of common source epidemics: i. Exposure to an infectious agent. ii. Contamination of the environment (air, water, soil). iii. Industrial chemicals or pollutants (e.g. Minamata disease in Japan resulting from consumption of fish containing high concentration of methyl mercury).

Common source epidemics b( Continuous or repeated exposure : The exposure from the same source is prolonged-continuous, repeated or intermittent, not necessarily at the same time or place. e.g :  a prostitute may be a common source in gonorrhoea out break.  The outbreak of respiratory illness, the legionnaire’s disease in philadelphia (1976).

The main features of continuous exposure epidemics are : i. There is no explosive rise in the number of cases. ii. They continue beyond the range of one incubation period. iii. There are no secondary cases among persons who had contact with ill persons.

Propagated epidemics.2 Propagated epidemic is most often of infectious origin (e.g epidemics of hepatitis A and polio). Usually shows a gradual rise and tails off over a much longer period of time. Transmission continues until the No. of susceptibles is depleted or susceptible individuals are no longer exposed to infected persons or intermediary vectors.

Speed of spread of Propagated epidemic depends upon: i. Herd immunity. ii. Opportunities for contact. iii. Secondary attack rate.

l l. Periodic fluctuation a) Seasonal trends: Seasonal variation is a well known characteristics of many communicable diseases (e.g measles, cerebro-spinal meningitis etc.). It is related to environmental conditions which favour disease transmission (temp., humidity, rainfall, life cycle of vector, etc). Non-infectious diseases sometimes exhibit Seasonal variation (sunstroke, snakebite).

b) Cyclic trends : Some diseases occur in cycles spread over short periods of time (days, weeks, months or years). e.g: measles in the pre-vaccination era appeared in cycles with major peaks every 2-3 years.

Cyclic trends are due to naturally occurring variations in herd immunity. they can be due to antigenic variations (influenza pandemics).

l l l. Long-term or secular trends The term “ secular trend� implies changes in the occurrence of disease (progressive increase or decrease) over a long period of time (several years or decades). It also implies a consistent tendency to change in a particular direction or a definite movement in one direction. e.g( coronary heart disease, diabetes) showing upward trend in the developed countries.

Place distribution ))geographical comparisons

Geographical studies helped in understanding of disease, its nature, its determinants and its relation to subsequent pathology. The world is not a uniform unit; cultures, standards of living and external environment vary greatly.

Geographic differences in disease occurrence are determined by the agent, host and environmental factors. Classic examples of place related diseases include yellow fever, schistosomiasis, sleeping sickness and endemic goitre. Geographic distribution may provide evidence of the source of disease and its mode of spread.

Classification : i. International variations. ii. National variations. iii. Rural-urban differences. iv. Local distribution.

International variations Descriptive studies by place have shown that the pattern of disease is not the same everywhere (e.g cancer, cardiovascular diseases). These variation stimulated epidemiologists to search for cause-effect relationships between the environment factors and disease. The aim is to identify factors which are crucial in the cause and prevention of disease.

National variations Variations in disease occurrence exist within countries or national boundaries (e.g: distribution of endemic goitre, malaria, nutritional deficiency diseases, etc(. Some parts of the country are more affected, others less affected or not affected at all. The data provided by descriptive studies regarding the type of disease problems and their magnitude (incidence, prevalence and mortality rates) is needed to demarcate the affected areas and for providing appropriate health care services.

Rural-urban variations Diseases more frequent in urban than rural areas include : Chronic bronchitis. Mental illness. accidents. Cardiovascular diseases. Drug dependence.

Diseases more frequent in rural than urban areas include : Skin and zoonotic diseases. Soil-transmitted helminths.

These variations are due to : Differences in population density. Social class. Deficiencies in medical care. Levels of sanitation. Education. Environmental factors.

Because of these variations ;epidemiologists seeks to define groups at higher risk of particular disease and providing guidelines to the health administrator for their prevention and control.

Local distributions There are inner and outer city variations in disease frequency. These variations are best studied by the aid of “spot maps” or “ shaded maps”. These maps show areas of high or low frequency. the boundaries and pattern of disease distribution (e.g :if the map shows clustering of cases, it may suggest a common source of infection or a common risk factor shared by all the cases).

Migration studies Large scale migration of human populations from one country to another provides a unique opportunity to evaluate the role of the possible genetic and environmental factors in the occurrence of disease in a population.

Migrants studies can be carried out in 2 ways: Comparison of disease and death rates for migrants with those of their kin who have stayed at home. Comparison of migrants with local population of the host country .

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