Epidemiology- I Mahmood ur Rahman
Definition of Epidemiology Epidemiology is the study of the distribution and determinants of diseases or states in population groups and the application of this study to the control of health problems (Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)
'Epidemiology is that field of medical science which is concerned with the relationship of various factors and conditions which determine the frequencies and distributions of an infectious process, a disease, or a physiologic state in a human community' (Lilienfeld 1978)
Historical Perspective • Hippocrates - 5th century • Association between external environment and personal characteristics and health
“Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical Classics 3:19, 1938).
Historical Perspective • John Graunt – 1662 (Hennekins and Buring 1987) The Nature and Political Observations Made Upon the Bills of Mortality • Systematic statistical approach • Analyzed births and deaths in London • Excess of males born, higher mortality for males • Infant mortality is very high • Seasonal variation for mortality • Importance of routinely collected information for study of human illness • William Farr - 1839 • Examined mortality and occupation and marital status • Identified important issues in epidemiological investigations • Use of comparison population, influence of multiple factors on disease
Historical Perspective • John Snow (1854) – Father of modern epidemiology • Established modern epidemiologic methods • Cholera epidemic in London • Plotted geographical location of all cases – deaths from cholera
John Snow (cont) • Went door to door, collecting information on daily habits • Suspected water supply as source of epidemic • Broad street pump closed, epidemic stopped • Mode of investigation – “shoe leather” • Practical application of epidemiology – use epidemiological investigation to impact a health problem
How the Epidemiologist Works • Studies origin and distribution of a health problem • Collection of data • Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist • Likened to a detective investigating the scene of a crime looking for clues • Starts with examination of sick person(s) • Extends investigation to the setting where illness is occurring • Looks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled
Epidemiologic Analyses – Areas of Study Causal agents related to disease: • Biological agents – bacteria, viruses, insects • Nutritional agents – diet (fats, carbohydrates, food nutrients) • Chemical agents – gases, toxic agents • Physical agents – climate, vegetation, chemical pollutants (air, water, food) • Social agents – occupation, stress, social class, lifestyle, location of residence
Epidemiologist studies: • Host characteristics: • Biological factors • Age, sex, degree of immunity, other physical attributes that promote resistance or susceptibility • Behavioral factors • Habits, culture, lifestyle • Social environment • Living conditions such as poverty, crowding • Norms, values and attitudes • Socially prescribed standards of living • Use of food and water, food handling practices • Household and personal hygiene
Eras of Epidemiology • Sanitary era – early 19th century • Infectious disease era – between late 19th century and early 20th century • Chronic disease era – 2nd half of 20th century • Eco-epidemiology era – 21st century
The clinician Deals with patients Takes a history Conducts a physical Makes a diagnosis Proposes a treatment Follows up the patient The epidemiologist Deals with populations Frames the question Investigates Draws conclusions Gives recommendations Evaluates programmes Comparing the job of a clinician and the job of an epidemiologist
Person Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004
Uses of epidemiology • Examine causation • Study natural history • Description of the health status of population • Determine the relative importance of causes of illness, disability and death • Evaluation of interventions • Identify risk factors
1. Examine causation Genetic factors Good health Ill health Environmental factors (Biological, chemical, physical, psychological factors) Life style related factors
To study historical rise and fall of disease in the population • The diseases wax and wane (tuberculosis ) new ones appear (encephalitis, asbestos poisoning, LSD psychosis) old ones are eradicated (smallpox)
A Pertussis by 5-Year Age Groups B. Pertussis by <1, 4-Year, Then 5-YearAge Groups
Underground water supply Pump from river bed Place Attack rate of acute hepatitis by zone of residence, Orissa, India, 2004 Attack rate 0 - 0.9 / 1000 1 - 9.9 / 1000 10 -19.9 / 1000 20+ / 1000 Chipat river
Seasonal fluctuations : • Malaria and JE are commoner during immediate post monsoon season. • Asthma shows highest incidence during spring and autumn suggesting specific environmental factors in its causation
Searching for causes and risk factors • Theories in Primitive and Middle Ages • Supernatural Causes e.g. being possessed by evil spirits, wrath of gods, punishment for evil deeds during previous births “bad air” • Contagion Theory • William Farr -Miasma Theory • Germ Theory - Henle-Koch postulates • Epidemiological Wheel Theory • The Theory of “Necessary” and Sufficient” cause
Role of the host, the agent and the environment in the occurrence of disease Biologic, Chemical, Physical (injury, trauma) Social Psychological AGENT VECTOR Sanitation Weather Pollution Socio-Cultural Political Genotype Nutrition Immunity Behaviour ENVIRONMENT HOST
Syndrome identification • Epidemiologists are called “lumpers and splitters” • The differentiation of hepatitis A from hepatitis B and the distinction between several varieties of childhood leukemia. • “lumping” include the identification of many manifestations of tuberculosis. • Patterson-kelly syndrome of association between dysphagia and iron deficiency anaemia but when association was tested by epidemiological studies , it was not found.
Individual risk and chances: Life table-The ‘risks’ the average male in England
Measures of disease frequency • Prevalence • Number of cases of a disease in a defined population at specified point of time • Incidence • Number of new cases, episodes or events occurring over a defined period of time
Incidence rate Number of people who get the disease or condition in a specified time X Factor I = Total population at risk
Population at risk All women (age groups) Total population Population at risk 0-25 years 25-69 years 25-69 years Men Women 70+ years Eg. Population at risk in a study of carcinoma of cervix
Prevalence Number of people with the disease or condition at a specified time P = X Factor Total population at risk
How to remember this: think about a bathtub Cure incidence prevalence Death
Community diagnosis : • The definition of indicators is a pre-requisite Indicators - to estimate the burden of illness and the strategies for control. • The main health indicators are expressed in terms of crude age-adjusted or age-specific mortality rates (such as infant mortality rates, mortality for children under 5, or maternal mortality rates), disease-specific morbidity rates, and life expectancy at birth.
3. Study natural history Death Sub-clinical disease Clinical disease Good health Recovery
4. Evaluation of interventions Treatment, Medical care Good Health Ill Health Health promotion Preventive measures Public health services
Evaluation • Evaluation is the process of determining, as systematically and objectively as possible, the relevance, effectiveness, efficiency, and impact of activities with respect to established goals.
Effectivenessrefers to the ability of a program to produce the intended or expected results in the field; effectiveness differs from efficacy, which is the ability to produce results under ideal conditions. • Efficiency refers to the ability of the program to reproduce the intended results with a minimum expenditure of time and resources.
Descriptive Epidemiology Collecting all data describing occurrence of disease Person Disease predominates in smokers Disease predominates immuno-compromised individuals Place Disease is associated with an arid climate Disease is associated with a tropical climate Time Disease associated with the date of company picnic A particular year
Disease surveillance • Frequency with which the disease occurs • Definition of the disease • Size of the population from which the cases develop • Completeness of the reporting of the cases.
Epidemiologic triad HOST DISEASE AGENT ENVIRONMENT