Epidemiology DPT- 6 Dr Muhammad Zahid Latif HOD Community Medicine ANMC Date:

Epidemiology

History Epidemiology started with Adam & Eve, both were trying to investigate the qualities of forbidden fruit Foundations were laid in early 19 th century

Meaning Epi = among Demos= People Logos= study

Definition The branch of medicine that deals with the study of the causes, distribution, and control of disease in populations. (

Definition The study of the distribution and determinants of health-related states or events in specified population, and the application of this study to control of health problems. [source: Last (ed.) Dictionary of Epidemiology, 1995]

Simple Old Definitions Oxford English Dictionary THE BRANCH OF MEDICAL SCIENCE WHICH TREATS EPIDEMICS Kuller LH: American J of Epidemiology 1991;134:1051 EPIDEMIOLOGY IS THE STUDY OF "EPIDEMICS" AND THEIR PREVENTION Anderson G. In: Rothman KJ: Modern Epidemiology THE STUDY OF THE OCCURRENCE OF ILLNESS

A Modern Definition Study of the occurrence and distribution of health-related diseases or events in specified populations, including the study of the determinants influencing such states, and the application of this knowledge to control the health problem (Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)

Who is an epidemiologist ? A professional who strives to study and control the factors that influence the occurrence of disease or health-related conditions and events in specified populations and societies, has an experience in population thinking and epidemiologic methods, and is knowledgeable about public health and causal inference in health (Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)

Aims of Epidemiology To eliminate or reduce the health problem or its consequences To promote the health & well being of society as a whole

Aims of Epidemiology To describe the distribution and magnitude of health and disease problems in human population To identify the risk factors in the pathogenesis of disease To provide the data essential for the planning, implementation and evaluation of services for the prevention, control and treatment of disease To decide the priorities

Epidemiological Approach Asking Questions

Epidemiological Approach Making Comparisons

Classically speaking Epi = upon Demos = people Ology = science Epidemiology = the science which deals with what falls upon people….. Bridge between biomedical, social and behavioral sciences

Epidemiologists are required to have some knowledge of: Public health: because of the emphasis on disease prevention Clinical medicine: because of the emphasis on disease classification and diagnosis (numerators) Pathophysiology: because of the need to understand basic biological mechanisms in disease (natural history) Biostatistics: because of the need to quantify disease frequency and its relationships to antecedents (denominators, testing hypotheses) Social sciences: because of the need to understand the social context in which disease occurs and presents (social determinants of health phenomena)

Purposes of Epidemiology 1.To investigate nature / extent of health- related phenomena in the community / identify priorities 2.To study natural history and prognosis of health-related problems 3.To identify causes and risk factors 4.To recommend / assist in application of / evaluate best interventions (preventive and therapeutic measures) 5.To provide foundation for public policy

Classical versus Modern Applications Classical: descriptive, observational, field, analytical, experimental, applied, healthcare, primary care, hospital, CD, NCD, environmental, occupational, psycho-social, etc Modern: risk-factor, molecular, genetic, life- course, CVD, nutritional, cancer, disaster, etc

Broad Types of Epidemiology Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person. We try to formulate hypothesis, look into associations ? Typical study design: community health survey (synonyms: cross-sectional study, descriptive study) Testing a specific hypothesis about the relationship of a disease to a specific cause, by conducting an epidemiologic study that relates the exposure of interest to the outcome of interest (? Cause- effect relationship) Typical study designs: cohort, case-control, experimental design DESCRIPTIVE EPI ANALYTIC EPI

Descriptive Epidemiology Is A Necessary Antecedent of Analytic Epidemiology To undertake an analytic epidemiologic study you must first: Know where to look Know what to control for Be able to formulate / test hypotheses compatible with a- priori lab / field evidence

Basic Triad of Descriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY ARE: PERSON PLACE TIME

Personal Characteristics (whom) Age Gender Socio-economic status (education, occupation, income) Marital status Ethnicity/race/genetic profile Behavior / habits

Place (where ?) Geographically restricted or widespread (outbreak, epidemic, pandemic)? Off-shore (tsunami…) Climate effects (temperature, humidity, combined effects..) Urban / sub-urban-squatter / rural Relation to environmental exposure (water, food supply, etc) Multiple clusters or one?

Time (when ?) Changing or stable? Clustered (epidemic) or evenly distributed (endemic)? Time-trends: Point source, propagated, seasonal, secular, combinations

What designs do epidemiologists use ? Qualitative designs Quantitative designs – Observational – Experimental Building evidence

What measures do Epidemiologists use ? Frequency measures Effect measures Impact fractions

Among Unique Skills of Epidemiologists: MEASURING DISEASE FREQUENCY IN POPULATIONS

Measuring Disease Frequency Has Several Components Measuring Disease Frequency Has Several Components Classifying and categorizing disease Deciding what constitutes a case of disease in a study Finding a source for ascertaining the cases Defining the population at risk of disease Defining the period of time of risk of disease Obtaining permission to study people Making measurements of disease frequency Relating cases to population and time at risk

HOST ENVIRONMENTAGENT

Epidemiological Triad A state of equilibrium between agent host and environment results in health, if this equilibrium disturbs disease will occur Disease is the combination of 1. A harmful agent 2.A susceptible host 3.An appropriate environment

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When the disease occurs? Disease occurs only when the host environmental factors make the agent sufficient enough to cause disease D Agent (SEED) factors Host Factors (SOIL) Environmental Factors (SHOWER )

Agents Agents Biological (micro-organisms) Physical (temperature, radiation, trauma, others) Chemical (acids, alkalis, poisons, tobacco, others) Environmental (nutrients in diet, allergens, others) Psychological experiences

Host Factors Host Factors Genetic endowment Immunologic status Personal characteristics Personal behavior Definitive versus intermediate (in vector- borne diseases)

Environment Environment Living conditions (housing, crowding, water supply, refuse, sewage, etc) Atmosphere / climate Modes of communication: phenomena in the environment that bring host and agent together, such as: vector, vehicle, reservoir, etc)

Importance of natural history Each disease has its own natural history; but it is not necessarily the same in all individuals. If the phase of natural history is known, appropriate level of prevention can be applied.

Disease Agents The disease agent is defined as 'a living or non- living substance or the excessive presence or absence of a force which may initiate a disease process‘.

Agent

Host The human host is supposed to be 'soil' and the disease agent as the 'seed'. The host factors are: 1. Demographic characteristics such as age sex and ethnicity. 2. Biological characteristics such as genetic factors, blood groups, enzymes, immunological factors etc. 3. Social and economic characteristics such as education, occupation, income, housing etc. 4. Life-style factors such as nutrition, exercise, use of alcohol, drug abuse, smoking etc.

Host

Environment The environment in which man lives is an important factor in the causation of diseases. Environment is classified as: 1. Physical environment 2. Biological environment 3. Psychological environment.

Environment

Epidemiologic Methods Ecological AnalyticalDescriptive Experimental / Interventional Observational Community Trials Field Trials RCT Cross sectional Case Control Cohort

Analytical Studies Descriptive studies look at the entire population In Analytical studies the subject of interest is the individual with in population Two Types Case Control Study Cohort Study

Case Control Study A case-control study is a type of study design used widely, originally developed in epidemiology, although its use has also been advocated for the social sciences. It is a type of observational study in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute. Case-control studies are often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition/disease (the "cases") with patients who do not have the condition/disease but are otherwise similar (the "controls").study designepidemiologyobservational study

Case Control Studies Often called retrospective studies Distinct Features – Both exposure and outcome (disease) have occurred before the start of study – The study proceed backward from effect to cause – It uses a control or comparison group to support or refute and inference

Case Control Studies These are comparison studies Case & Control must be comparable with respect to – Age – Sex – Occupation – Social status etc – Immunized & Un immunized children

Basic Steps There are four basic steps in conducting a case control study; 1. Selection of cases and controls 2. Matching 3. Measurement of exposure and 4. Analysis and interpretation

 Selection of cases and controls Selection of cases Definition of case Diagnostic Criteria Eligibility Criteria Source of Cases Hospitals General Population Selection of Controls Hospital controls Relatives Neighborhood controls General population

Steps  Matching  Measurement of Exposure  Analysis Exposure rate among cases and controls to suspected factor Estimation of disease risk associated with exposure(odds ratio)

Advantages 1. Relatively easy to carry out. 2. Rapid and inexpensive( compared with cohort studies) 3. Require comparatively few subjects. 4. Particularly suitable to investigate rare disease or diseases about which little is known but a disease which is rare in the general population (e.g. leukaemia in adolescents) may not be rare in special exposure group (e.g. prenatal X-rays) 5. No risk to subjects. 6. Allows the study of several different aetiological factors(e.g. smoking, physical activity and personality characteristics in myocardial infarction) 7. Risk factors can be identified. Rational prevention and control programmes can be established. 8. No attrition problems, because case control studies do not require follow up of individuals into the future. 9. Ethical problems minimal.

Disadvantages 1. Problems of bias relies on memory or past records, the accuracy of which may be uncertain; validation of information obtained is difficult or sometimes impossible. 2. Selection of an appropriate control group may be difficult. 3. We cannot measure incidence, and can only estimate the relative risk. 4. Do not distinguish between causes and associated factors. 5. Not suited to the evaluation of therapy or prophylaxis of disease 6. Another major concern is the representativeness of cases and controls.