Principles of Epidemiology

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Introduction to Epidemiology, Epidemiology is considered the basic science of public health and with good reason. Epidemiology is a a quantitative basic science built on a working knowledge of probability. statistics and sound research methods b a method of causal reasoning based on developing. and testing hypotheses pertaining to occurrence and prevention of morbidity and mortality and. c a tool for public health action to promote and protect the public s health based on science. causal reasoning and a dose of practical common sense 2. As a public health discipline epidemiology is instilled with the spirit that epidemiologic. information should be used to promote and protect the public s health Hence epidemiology. involves both science and public health practice The term applied epidemiology is sometimes. used to describe the application or practice of epidemiology to address public health issues. Examples of applied epidemiology include the following. the monitoring of reports of communicable diseases in the community. the study of whether a particular dietary component influences your risk of developing. evaluation of the effectiveness and impact of a cholesterol awareness program. analysis of historical trends and current data to project future public health resource. Objectives, After studying this lesson and answering the questions in the exercises a student will be able. to do the following,Define epidemiology, Summarize the historical evolution of epidemiology. Describe the elements of a case definition and state the effect of changing the value of. any of the elements, List the key features and uses of descriptive epidemiology. List the key features and uses of analytic epidemiology. List the three components of the epidemiologic triad. List and describe primary applications of epidemiology in public health practice. List and describe the different modes of transmission of communicable disease in a. population,Page 2 Principles of Epidemiology,Introduction.
The word epidemiology comes from the Greek words epi meaning on or upon demos. meaning people and logos meaning the study of Many definitions have been proposed but. the following definition captures the underlying principles and the public health spirit of. epidemiology, Epidemiology is 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 health problems 17, This definition of epidemiology includes several terms which reflect some of the important. principles of the discipline As you study this definition refer to the description of these terms. Study Epidemiology is a scientific discipline sometimes called the basic science of public. health It has at its foundation sound methods of scientific inquiry. Distribution Epidemiology is concerned with the frequency and pattern of health events in a. population Frequency includes not only the number of such events in a population but also the. rate or risk of disease in the population The rate number of events divided by size of the. population is critical to epidemiologists because it allows valid comparisons across different. populations, Pattern refers to the occurrence of health related events by time place and personal. characteristics, Time characteristics include annual occurrence seasonal occurrence and daily or even. hourly occurrence during an epidemic, Place characteristics include geographic variation urban rural differences and location.
of worksites or schools, Personal characteristics include demographic factors such as age race sex marital. status and socioeconomic status as well as behaviors and environmental exposures. This characterization of the distribution of health related states or events is one broad aspect. of epidemiology called descriptive epidemiology Descriptive epidemiology provides the What. Who When and Where of health related events It is discussed in more detail beginning on page. Determinants Epidemiology is also used to search for causes and other factors that. influence the occurrence of health related events Analytic epidemiology attempts to provide the. Why and How of such events by comparing groups with different rates of disease occurrence and. with differences in demographic characteristics genetic or immunologic make up behaviors. environmental exposures and other so called potential risk factors Under ideal circumstances. epidemiologic findings provide sufficient evidence to direct swift and effective public health. control and prevention measures,Lesson 1 Introduction to Epidemiology Page 3. Health related states or events Originally epidemiology was concerned with epidemics of. communicable diseases Then epidemiology was extended to endemic communicable diseases. and noncommunicable infectious diseases More recently epidemiologic methods have been. applied to chronic diseases injuries birth defects maternal child health occupational health and. environmental health Now even behaviors related to health and well being amount of exercise. seat belt use etc are recognized as valid subjects for applying epidemiologic methods In these. lessons we use the term disease to refer to the range of health related states or events. Specified populations Although epidemiologists and physicians in clinical practice are both. concerned with disease and the control of disease they differ greatly in how they view the. patient Clinicians are concerned with the health of an individual epidemiologists are. concerned with the collective health of the people in a community or other area When faced. with a patient with diarrheal disease for example the clinician and the epidemiologist have. different responsibilities Although both are interested in establishing the correct diagnosis the. clinician usually focuses on treating and caring for the individual The epidemiologist focuses on. the exposure action or source that caused the illness the number of other persons who may. have been similarly exposed the potential for further spread in the community and interventions. to prevent additional cases or recurrences, Application Epidemiology is more than the study of As a discipline within public health. epidemiology provides data for directing public health action However using epidemiologic. data is an art as well as a science Consider again the medical model used above To treat a. patient a clinician must call upon experience and creativity as well as scientific knowledge. Similarly an epidemiologist uses the scientific methods of descriptive and analytic epidemiology. in diagnosing the health of a community but also must call upon experience and creativity. when planning how to control and prevent disease in the community. Page 4 Principles of Epidemiology, Although epidemiologic thinking has been traced from Hippocrates circa 400 B C through. Graunt 1662 Farr Snow both mid 1800 s and others the discipline did not blossom until the. end of the Second World War The contributions of some of these early and more recent thinkers. are described below, Hippocrates circa 400 B C attempted to explain disease occurrence from a rational instead.
of a supernatural viewpoint In his essay entitled On Airs Waters and Places Hippocrates. suggested that environmental and host factors such as behaviors might influence the. development of disease, Another early contributor to epidemiology was John Graunt a London haberdasher who. published his landmark analysis of mortality data in 1662 He was the first to quantify patterns of. birth death and disease occurrence noting male female disparities high infant mortality urban. rural differences and seasonal variations No one built upon Graunt s work until the mid 1800 s. when William Farr began to systematically collect and analyze Britain s mortality statistics Farr. considered the father of modern vital statistics and surveillance developed many of the basic. practices used today in vital statistics and disease classification He extended the epidemiologic. analysis of morbidity and mortality data looking at the effects of marital status occupation and. altitude He also developed many epidemiologic concepts and techniques still in use today. Meanwhile an anesthesiologist named John Snow was conducting a series of investigations. in London that later earned him the title the father of field epidemiology Twenty years before. the development of the microscope Snow conducted studies of cholera outbreaks both to. discover the cause of disease and to prevent its recurrence Because his work classically. illustrates the sequence from descriptive epidemiology to hypothesis generation to hypothesis. testing analytic epidemiology to application we will consider two of his efforts in detail. Snow conducted his classic study in 1854 when an epidemic of cholera developed in the. Golden Square of London He began his investigation by determining where in this area persons. with cholera lived and worked He then used this information to map the distribution of cases on. what epidemiologists call a spot map His map in shown in Figure 1 1. Because Snow believed that water was a source of infection for cholera he marked the. location of water pumps on his spot map and then looked for a relationship between the. distribution of cholera case households and the location of pumps He noticed that more case. households clustered around Pump A the Broad Street pump than around Pump B or C and he. concluded that the Broad Street pump was the most likely source of infection Questioning. residents who lived near the other pumps he found that they avoided Pump B because it was. grossly contaminated and that Pump C was located too inconveniently for most residents of the. Golden Square area From this information it appeared to Snow that the Broad Street pump was. probably the primary source of water for most persons with cholera in the Golden Square area. He realized however that it was too soon to draw that conclusion because the map showed no. cholera cases in a two block area to the east of the Broad Street pump Perhaps no one lived in. that area Or perhaps the residents were somehow protected. Lesson 1 Introduction to Epidemiology Page 5,Figure 1 1. Distribution of cholera cases in the Golden Square area. of London August September 1854, Upon investigating Snow found that a brewery was located there and that it had a deep well. on the premises where brewery workers who also lived in the area got their water In addition. the brewery allotted workers a daily quota of malt liquor Access to these uncontaminated rations. could explain why none of the brewery s employees contracted cholera. To confirm that the Broad Street pump was the source of the epidemic Snow gathered. information on where persons with cholera had obtained their water Consumption of water from. the Broad Street pump was the one common factor among the cholera patients According to. legend Snow removed the handle of that pump and aborted the outbreak. Page 6 Principles of Epidemiology,Figure 1 2, Water contaminated with deadly cholera flowed from the Broad Street pump. Figure not shown,Lesson 1 Introduction to Epidemiology Page 7.
Snow s second major contribution involved another investigation of the same outbreak of. cholera that occurred in London in 1854 In a London epidemic in 1849 Snow had noted that. districts with the highest mortalities had water supplied by two companies the Lambeth. Company and the Southwark and Vauxhall Company At that time both companies obtained. water from the Thames River at intake points that were below London In 1852 the Lambeth. Company moved their water works to above London thus obtaining water that was free of. London sewage When cholera returned to London in 1853 Snow realized the Lambeth. Company s relocation of its intake point would allow him to compare districts that were supplied. with water from above London with districts that received water from below London Table 1 1. shows what Snow found when he made that comparison for cholera mortality over a 7 week. period during the summer of 1854,Mortality from cholera in the districts of London. supplied by the Southwark and Vauxhall and the Lambeth Companies. July 9 August 26 1854,Cholera Death, Districts with Water Population Deaths from Rate per. Supplied by 1851 Census Cholera 1 000 Population,Southwark and Vauxhall. Co only 167 654 844 5 0,Lambeth Co only 19 133 18 0 9. Both companies 300 149 652 2 2, The data in Table 1 1 show that the risk of death from cholera was more than 5 times higher.
in districts served only by the Southwark and Vauxhall Company than in those served only by. the Lambeth Company Interestingly the mortality rate in districts supplied by both companies. fell between the rates for districts served exclusively by either company These data were. consistent with the hypothesis that water obtained from the Thames below London was a source. of cholera Alternatively the populations supplied by the two companies may have differed on a. number of other factors which affected their risk of cholera. To test his water supply hypothesis Snow focused on the districts served by both companies. because the households within a district were generally comparable except for water supply. company In these districts Snow identified the water supply company for every house in which. a death from cholera had occurred during the 7 week period Table 1 2 shows his findings. Principles of Epidemiology Second Edition Written by RICHARD C DICKER Selected Lessons Lesson 1 Introduction to Epidemiology Lesson 6 Investigating an Outbreak 12 92 U S Department of Health and Human Services Public Health Service Centers for Disease Control Epidemiology Program Office Public Health Practice Program Office Atlanta Georgia 30333 1 Lesson 1 Introduction to Epidemiology

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