01 Introduction to Epidemiology and health
- 1. Introduction to Public Health Kehabtimer Shiferaw 1
- 2. Health • The WHO defines health as ‘a state of complete physical, mental, and social well- being and not merely the absence of disease, or infirmity’. • Public health is the science and art of preventing disease, prolonging life and promoting health 2
- 4. Learning objective • At the end of the session students should be acquainted with: – The definition and scope of Epidemiology, – Basic assumption and unique characteristics of Epidemiology, – History and evolution of Epidemiology, – Theories of disease causation, – Levels of disease prevention 4
- 5. What’s Epidemiology? • The term "epidemiology" springs directly from ‘epidemic’ which originally referred to communicable disease outbreaks in humans. • Epidemic is derived from the Greek words epi (upon) and demos (people). • i.e. Epidemiology is the study of what's ‘upon the people’ • Classically Epidemiology is defined as: ‘The study of the frequency, distribution and determinants of diseases and health-related problems in human populations and the application of this study to the control of health problems’ 5
- 6. What’s Epidemiology? Cont…. • Frequency: Quantification of the occurrence of diseases/health- related problems. • Distribution: Pattern of occurrence by place, person and time. – Geographical distribution – Temporal variation (seasonal, short period, secular trend) – Pattern according to the type of person most /least affected • Determinants: Answers the question ‘why specific individuals are affected/not affected by the condition?’ What ‘risk factors’ are ‘causing’ this? • Health related problems: This includes injuries, malnutrition, vital events, health related behaviors , life style, and pertinent social factors. 6
- 7. What’s Epidemiology? Cont…. • Risk Factor: An individual attribute (intrinsic characteristic of the individual) or exposure (external environmental situation) that can negatively affect health • Cause: Factor necessary for the disease to occur and in its absence that specific disease can’t occur. • It is also known as Etiologic agent. 7
- 8. Classification of Epidemiology • Descriptive Epidemiology: – Is concerned with describing the frequency of the disease/health-related phenomena by place, person and time. – It is important for planning, priority setting and resource allocation for health interventions. • Analytic Epidemiology: – Investigates the etiology and risk factors of disease/health related conditions. – Also helps to investigate how specific therapies/interventions prevent /control health problems (Experimental Epidemiology). – Also helps to evaluate preventive/curative interventions (Evaluative Epidemiology). – Important for designing of prevention and control strategies. 8
- 9. Purpose/Objectives of Epidemiology 1. To determine the extent /burden of disease in a community or beyond 2. To study the natural history and progression of disease 3. To identify the cause and risk factors of disease 4. To evaluate both existing and new preventive and therapeutic measures for a disease or condition 5. To provide the foundation for providing services and developing programs and policies 9
- 10. Stages of epidemiological reasoning • Specially in analytic epidemiology, epidemiologists use a two-stage sequence of reasoning : 1. Determination of a statistical association b/n the characteristics and the outcome (statistical inference) 2. The derivation of causal inference from such pattern of statistical association 10
- 11. The Discipline of Epidemiology • Because health is multifaceted, epidemiology is interdisciplinary. • It is extensively connected to the biomedical sciences such as medicine, microbiology, physiology and pathology; • Its quantitative nature connect it with statistics (more precisely Biostatistics) and Demography. • Many diseases have environmental origin, the study of life in relation to the environment is ecology, so epidemiology is also the science of the ecology of disease. • Recent advancements are linking it with laboratory sciences • In the search for solutions to health problems, epidemiology is often cast beyond these traditional boundaries to social and behavioral sciences. • Hence epidemiology is bio-social-environmental science focusing on disease in populations 11
- 12. Why we study Epidemiology? • Epidemiology provides comprehensive information for: – Understanding of diseases – Identification of priority health problems – Hypothesis generation and testing – Provision of evidence based care – Designing of disease control and prevention strategies – Evaluation of health programs – Provides the methodology for understanding and planning, conducting, analyzing, interpreting and presenting scientific data. 12
- 13. Basic Assumptions of Epidemiology • The two fundamental assumptions of epidemiology are: – Human disease doesn’t occur at random and – Human disease has causal and preventive factors that can be identified through scientific investigation. 13
- 14. Unique Characteristics of Epidemiology 1. Studies are conducted on human population 2. Allows the quantification of the magnitude of relationship between exposure and disease 3. Provide information on how to prevent disease and alter risk through interventions 14
- 15. History of Epidemiology • Hippocrates (460-377 BC): – The epidemiologic way of thinking originated in writings ascribed to the Greek philosopher-physician Hippocrates. – In his book entitled ‘Air, Waters and Places’, he displayed an extraordinary awareness of the impact of environment (geography, water, climate, housing) on personal well-being. – In doing so, he identified forces that epidemiologists today recognize as major determinants of human health. 15
- 16. History Cont…. • Claudius Galen (129-199): – Known as father of Experimental Physiology. – In epidemiology he described the importance of ‘innate qualities of human body’, lifestyle and personality in disease causation. – He also argued that diseases were caused by Miasma (Bad air) from waste, stagnant water and dead animals. 16
- 17. History Cont…. • Gerolamo Francastorius (1478-1553): – An Italian presented the first coherent germ theory of disease – He theorized that a variety of diseases are caused by transmissible, self-propagating entities – He thought that these agents were specific to each disease and could spread from person-to-person or through infected articles (fomites) or at a distance – He argued that treatment should consist of the destruction of the entities by either heat or cold or of their evacuation from the body, or by neutralizing them by antagonistic substances 17
- 18. History Cont…. • John Graunt (1620–1674): – Regarded as the founder of Demography. – He was the first to take a major step forward to quantify patterns of birth, death and disease occurrence. – His observations upon the Bills of Mortality (1662) were based on a series of weekly bills covering individual deaths and their causes in the London area back to 1603. 18
- 19. History Cont…. • James Lind (1716-1794): – Carried out experimental study on the etiology and treatment of scurvy. – While he was a British navy physician he observed sailors were developing a deadly bleeding disorder (scurvy) on long voyages. – He selected 12 men from the ship suffering from scurvy, and divided them into six pairs, giving each group different additions to their basic diet – Treatments were: fermented alcohol made from apple juice, sulfuric acid solution, seawater, spicy paste of garlic and mustard, vinegar (acetic acid solution) and citric fruits. – Demonstrated that citrus fruits (orange and lemon) cure scurvy 19
- 20. History Cont…. • Alexandre Louis (1787-1872): – A French physician, known for introducing the use of the ‘numerical method’ in the field of medicine. – He argued that the concept that knowledge about a disease (natural history, clinical presentation and treatment) could be derived from aggregated patient data. – He also numerically demonstrated that bloodletting was an ineffective treatment for the various fevers. 20
- 21. History Cont…. • William Farr (1807–1883): – He introduced a national system of recording causes of death in Britain. – He himself first analyzed the data with great skill. – He was instrumental in building up a classification of diseases for statistical purposes. 21
- 22. History Cont…. • John Snow (1813–1858): – Known as the father of Modern Epidemiology – He investigated the two epidemics of Cholera occurred in London in 1849 and 1854. – Based on descriptive data he suspected the epidemic has something to do with the drinking water source of the city. – He postulated that the cholera was transmitted by a contaminated water through unknown mechanism. – He compared the Cholera mortality pattern across the drinking water sources of London. – He found out that the mortality rate was 10 times lower for the districts supplied by the Lambeth company compared to those supplied by Southwark and Vauxhall company. 22
- 25. History Cont…. – He also demonstrated that the epidemics was particularly high in districts supplied by both companies using water drawn from the heavily polluted part of Themes river. – Accordingly the Lambeth co changed it water source to the less contaminated part of the river and that was followed by reduction in the occurrence of the epidemic. – In doing so he demonstrated how hypothesis can be generated from descriptive epidemiology and how it can be tested and applied for public health benefit. – Note that J Snow’s observation came before the identification of Vibrio cholerae. 25
- 26. History Cont…. • Joseph Goldberger (1874–1929): – An American physician was a well known advocate for scientific and social recognition of the links between poverty and disease. – In 1914 he investigated the cause of Pellagra and demonstrated the disease was associated with diet rather than infection agents. – He conducted restricted-diet experiments on prisoners to show that individuals who heavily consumed corn-based diets were at a greatly increased risk of contracting pellagra. – The investigation was made two decades before the identification of niacin as the cause of pellagra. 26
- 27. History Cont…. • Elizabeth Lane-Claypon (1877–1967): – An English physician and one of the founders of the science of epidemiology, – Pioneered the use of retrospective Cohort and Case-control studies. – In 1912, she published a ground-breaking study of two cohorts of babies, fed cow’s milk and breast milk respectively to demonstrate the importance of BF for child growth. – In 1923, she studied factors associated with breast cancer by comparing 500 women with a history of breast cancer with another 500 women who were free of the disease. – The technique nowadays is known as Case-control Study. 27
- 28. History Cont…. • Streptomycin trial for treatment of TB (1946–1947): – The British Medical Research Council conducted one of the first modern experimental studies on the use of streptomycin to treat pulmonary tuberculosis. – The trial was both double-blind and placebo-controlled. – Generally accepted as the first randomized clinical trial. 28
- 29. History Cont…. • Framingham Study (1947 till date): – The study began with 5,000 adult subjects from Framingham, Massachusetts and its now on 3rd generation of participants. – Widely acknowledged as a premier longitudinal study. – Focuses on risk factors of cardiovascular disease. – It is undisputedly the foundation stone for current ideas about risk factors of CVD. – Like the work of Doll and Hill, the Framingham Study is notable for bringing about a shift in the focus of epidemiology from infectious to noninfectious diseases. 29
- 30. History Cont…. • Richard Doll (1912–2005) and Bradford Hill (1897-1991): – In 1950 they showed that smoking was an important cause of lung cancer through a case-control study. – Later they also conducted a long-term prospective study on smoking and health – Bradford Hill pioneered the first randomized clinical trial (Streptomycin Tuberculosis trial) – In 1965 he developed criteria for assessing evidence of causation. – Richard Doll did pioneering work on the relationship between radiation and leukemia as well as that between asbestos and lung cancer, and alcohol and breast cancer. 30
- 31. History Cont…. • Field trial of the Salk polio vaccine (1954): – The biggest epidemiological experiment till date – Headed by Dr. Jonas Salk who developed the dead polio virus – More than 600,000 children were injected with vaccine or placebo, – And more than a million others participated as “observed” controls. – Firmly establishing the efficacy of the killed virus vaccine and paving the way for eradication of the disease, – Later Dr. Albert Sabin conducted another large scale polio trial on his attenuated vaccine (1957) 31
- 32. Theories of Disease Causation 1. Supernatural theory: – Stated that all disease caused by supernatural forces – i.e. either the curse of god or due to evil force of demons – Disease prevention was based on religious practices. 2. The Hippocrates theory – Environmental factors have effects on ‘personal well-being’ 3. The miasma theory: – Attributed disease to bad odor or air. – This model had been used to explain epidemics of diseases like cholera, plague, and malaria. 32
- 33. Theories of Disease Causation Cont… 4. Contagion theory: – Assumes that all diseases are contagious (communicable) in nature. – Isolation of people, and avoiding contact with cases were the prominent preventive measures. 5. The germ theory: – Is a theory which proposes that microorganisms are the cause of many (if not all) diseases. – Although highly controversial when first proposed, it has led to the innovations and development of antibiotics, vaccines and hygienic practices. 33
- 34. Theories of Disease Causation Cont… 6. The epidemiological triad theory: – This theory states that causation of disease is the function of the environment, causative agents and host factors. – Still an important model for infectious diseases. 7. Multi-causal theory: – The most recent model of disease causation – Has different sub-variants including the causal pie and the web-causal theory – Web-causal theory states that diseases are caused by a function of different factors like socio-cultural factors, political factors, environmental factors etc.. beyond the direct causes of diseases. – Example: see the next slide. 34
- 35. Theories of Disease Causation Cont… 35
- 36. Level of Disease Prevention • Disease prevention is any intervention that seeks to reduce or eliminate diagnosable conditions. • However in a broader sense, it is defined as any measure which is directed to interrupt or slow progress of disease. • Disease prevention is classified into the following four levels. 36
- 37. Level of Disease Prevention Cont… 1. Primordial Prevention: – This aims at the establishment and maintenance of conditions that minimize hazard to health. – Its actions are measures that inhibit the emergence and establishment of social and economic conditions that are known to contribute to an elevated risk of disease. – It is directed before the initiation of the disease. – Its target can be the general population or selected groups. 37
- 38. Level of Disease Prevention Cont… 2. Primary Prevention: – This aims at the prevention of the incidence of disease. – Its actions are focused on the protection of health by personal and community wide efforts (example: preserving good nutritional status, immunization, environmental hygiene, modification of risk factors of chronic diseases, etc.). – Its directed before the initiation of disease. – Its target can be the general population or selected risky groups. 38
- 39. Level of Disease Prevention Cont… 3. Secondary Prevention: – This aims at cure of disease and prevention of more serious consequences through early detection and treatment. – Example: early detection and treatment of trachoma patients, screening for chronic diseases. – In PH context, secondary prevention also results in reduction of prevalence of the disease and risk of transmission. – It is directed at the earlier stage of the disease (before usual time of diagnosis). – Its target is cases at pre-diagnosable condition. 39
- 40. Level of Disease Prevention Cont… 4. Tertiary Prevention: – This is intended to reduce, mitigate or limit progress or exacerbation of diagnosed disease – Is an important aspect of therapeutic and rehabilitative medicine. – Its target population is established patients. 40