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EPIDEMIOLOGY

Epidemiology is the basic science of preventive and social medicine that studies disease distribution, causation, prevention, and health-related events in human populations. This article provides an overview of epidemiology and its major contributions to improving population health, as well as the importance of epidemiological evidence in shaping health policy.

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EPIDEMIOLOGY

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  1. EPIDEMIOLOGY • Epidemiology is the basic science of preventive and social medicine. It deals with the study of disease distribution and causation, prevention, health and health related events occurring in human population.

  2. KEY MESSAGES • Epidemiology is a fundamental science of public health. • Epidemiology has major contributions to improving population health. • Epidemiology is essential to the process of identifying and mapping emerging diseases. • There is often a frustrating delay between acquiring epidemiological evidence and applying this evidence to health policy.

  3. MEANING • Epidemiology is derived from the word epidemic ( epi= among; demos = people; logos = study) • The foundation of epidemiology was laid in 19th century. • In 1927 W.H. Frost became the first professor of epidemiology in U.S

  4. DEFINITION • That branch of medical science which treats epidemics. ( Parkin, 1873) • The science of the mass phenomena of infectious diseases. ( Frost, 1927) • The study of disease, any disease, as a mass phenomenon (Greenwood, 1934)

  5. JOHN M. LAST Epidemiology has been defined as • 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 heath problems.

  6. AIMS OF EPIDEMIOLOGY • To describe the distribution and magnitude of health and disease problems in human populations. • To identify the aetiological factors. • To provide the data essential to the planning, implementation and evaluation of services for the prevention, control and treatment of disease and to the setting up of priorities among those services.

  7. To eliminate or reduce the health problem or its consequences. • To promote the health and well being of society as a whole.

  8. EPIDEMIOLOGICAL APPROACH • Epidemiological approach to problems of health and disease is based on 2 major foundations. a) Asking question b) Making comparison

  9. ASKING QUESTIONS • Epidemiology is a means of learning or asking questions a) What is the event? (the problem) b) What is the magnitude? c) Where did it happen? d) When did it happen? e) Who are affected? f) Why did it happen?

  10. RELATED TO HEALTH ACTION • A) What can be done to reduce this problem and its consequences? • B) How can it be prevented in the future? • C) What action should be taken by the community? • D) What resources are required? • E) What difficulties may arise?

  11. MAKING COMPARISONS • The basic approach in epidemiology is to make comparisons and draw inferences. This may be comparison of two or more groups – one group having the disease and the other not having the disease or comparison between individuals.

  12. The epidemiologist find the differences in the host and the environment, between those affected and not affected.

  13. Before making comparisons is to ensure what is known as “ comparability ”between the study and control groups. • Both the groups should be similar so that “like can be compared with like”. • For example – the study and control groups should be similar with regard to age and sex composition and similar other pertinent variables.

  14. MEASUREMENTS IN EPIDEMIOLOGY • Measurements of mortality, morbidity, disability, measurements of the presence, absence or distribution of the characteristics or attributes of the disease, measurements of medical needs, measurements of the presence absence or distribution of the environmental and other factors suspected of causing the disease, measurement of demographic variables.

  15. TOOLS OF MEASUREMENT • RATES • RATIOS • PROPORTIONS

  16. RATE • A rate measures the occurrence of some particular event in a population during a given time period. eg: Death rate= number of deaths in 1 yr X 1000 Mid – Year population Numerator, denominator, time specification and multiplier.

  17. RATIO • It expresses a relation in size between two random quantities. The numerator is not a component of the denominator. Eg: sex ratio

  18. A proportion is a ratio which indicates the relation in magnitude of a part of the whole. The numerator is always included in the denominator as a percentage. Eg: The no of children with scabies at a certain time The total no of children in the village X100 at the same time PROPORTIONS

  19. MEASUREMENT OF MORTALITY • Crude death rate The simplest measure of mortality is the ‘crude death rate’. It is defined as the number of deaths (from all causes) per 1000 estimated mid year population in one year, in a given place.

  20. Specific death rates • The specific death rates may be disease specific, age specific and sex specific. EG: Specific death rate due to TB Number of death from TB during a calenderyr X 1000 Mid-year population

  21. CASE FATALITY RATE(RATIO) • It is the ratio of death to cases. Total number of deaths due to a particular disease X 100 Total number of cases due to the same disease

  22. PROPORTIONAL MORTALITY RATE(RATIO) • It is to know what proportion of total deaths are due to a particular cause or age group. Proportional mortality rate expresses the number of deaths due to a particular cause (or in a specific age group) per 100 or 1000 total deaths.

  23. PROPORTIONAL MORTALITY FROM SPECIFIC DISEASE Number of deaths from the specific disease in a year X 100 Total death from all causes in that year

  24. UNDER – 5 PROPORTIONATE MORTALITY RATE • Number of deaths under 5 years of age in the given year X 100 Total number of deaths during the same period

  25. PROPORTIONAL MORTALITY RATE FOR 50 YEARS & ABOVE • Number of deaths of persons aged 50 years and above X 100 Total deaths of all age groups in that year

  26. SURVIVAL RATE • It is the proportion of survivors in a group Survival Rate = Total number of patient alive after 5 years X 100 Total number of patients diagnosed or treated

  27. MEASUREMENT OF MORBIDITY • Morbidity has been defined as “ any departure, subjective or objective, from a state of physiological well being. The term is used equivalent to such terms as sickness, illness, disability etc.

  28. INCIDENCE • Incidence rate is defined as the number of new cases occurring in a defined population during a specified period of time. INCIDENCE= Number of new cases of specific disease during a given time periodX1000 Population at risk during that period

  29. PREVALENCE • The term disease prevalence refers specifically to all current cases (old and new) existing at a given point in time in a given population.

  30. Point prevalence • The number of all current cases (old and New) of a disease at one point in time in relation to a defined population is called point prevalence. Number of all current cases of a specified disease existing at a given point in time X 100 Estimated population at the same point in time

  31. PERIOD PREVALENCE • It measures all the current cases (old and new) existing during a defined period of time ( eg: annual prevalence) expressed in relation to a defined population.

  32. Period prevalence is given by the formula • Number of existing cases (old and new) of a specified disease during a given period of time interval X 100 Estimated mid – interval population at risk

  33. EPIDEMIOLOGICAL METHODS • Epidemiological studies can be classified as OBSERVATIONAL STUDIES EXPERIMENTAL STUDIES

  34. OBSRVATIONAL STUDIES a) Descriptive studies b) Analytical studies Ecological Cross – sectional Case – control Cohort

  35. EXPERIMENTAL STUDIES • Randomized • Field trials • Community trials

  36. DESCRIPTIVE EPIDEMIOLOGY • Descriptive studies are the first phase of an epidemiological investigation. These studies are concerned with observing the distribution of disease or health – related characteristics in human populations and identifying the characteristics with which the disease in question seems to be associated.

  37. QUESTIONS BASICALLY ASKED • When is the disease occurring ? Time distribution • Where is it occurring? Place distribution • Who is getting the disease? Person distribution

  38. PROCEDURES IN DESCRIPTIVE STUDIES > Defining the population to be studied > Defining the disease under study > Describing the disease by a) Time b) Place c) Person

  39. 4) Measurement of disease 5) Comparing with known indices 6) Formulation of an aetiological hypothesis

  40. Defining the population • Descriptive studies are investigations of populations, not individuals. The first step is to define the population base not only in terms of number but also in terms of age, sex, occupation, cultural characters and similar information needed for the study.

  41. The defined population can be the whole population in a geographical area or a representative sample taken from it. • The defined population can be a specially selected group such as age and sex groups, occupational groups, hospital patients, school children, small communities as well as wider groupings.

  42. The defined population needs to be large enough so that age, sex and other specific rates are meaningful. • The community selected should be stable.

  43. Defining the disease under study • After the population to be studied is defined or specified, then the disease or condition being investigated must be defined.

  44. The clinician may not need a precise definition of the disease for immediate patient care. • But the epidemiologist need an accurate estimate of disease in a population. • The epidemiologist look out for an operational definition.

  45. 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.

  46. TIME DISTRIBUTION • The pattern of disease may be described by the time of its occurrence i.e., by week, month, year, the day of the week, hour of onset etc. • It shows whether the disease is seasonal and shows periodic increase or decrease and whether it follows a time trend.

  47. Types of time trends • Short term fluctuations • Periodic fluctuation • Long term or secular trends

  48. SHORT TERM FLUCTUATIONS • The best known short term fluctuation in the occurrence of disease is an epidemic .

  49. An epidemic is defined as the occurrence in a community or region of cases of an illness or other related events clearly in excess of normal expectancy.

  50. TYPES OF EPIDEMICS • A Common – source epidemics a) Single exposure or “point source” epidemic b) Continuous or multiple exposure epidemics B Propagated epidemics a) Person – person b) Arthropod vector c) Animal reservoir C Slow ( modern) epidemics

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