This is important in epidemiology, because rates can be used to measure the speed of a disease event or to make epidemiologic comparisons between populations over time. In this case, the data are “pulled” by the requestor, usually during emerging diseases or significant changes in disease incidence. One of the most famous medical prospective cohort studies is the Framingham Heart Study. Summary. CDC twenty four seven. In animals, time may refer to milking shift, breeding season, lambing/calving season, at weaning, during shipment, on arrival at the feedlot, dry vs wet season, etc. Epidemiologists assume that illness does not occur randomly in a population, but happens only when the right accumulation of risk factors or determinants exists in an individual. Measures of disease frequency: definition and calculation of prevalence, incidence, risk, rate, basic and net reproductive rate, choosing suitable measures, limitations of case and population definitions and their impact on measures of disease frequency. Disadvantages include a potentially high possibility of selection bias, the fact that individuals may have difficulty recalling certain exposures (termed recall bias), and the requirement for the existence of medical and/or exposure records. In the latter case, cause-specific mortality is expressed as the case fatality rate (CFR), which is the number of deaths due to a particular disease occurring among individuals afflicted with that disease in a given time period. Finally, a pandemic occurs when an epidemic becomes global in scope (eg, influenza, HIV/AIDS). Indeed, the term health-related states or events may be seen as anything that affects the well-being of a population. Type I error, also known as a false positive, is when the null hypothesis is rejected when it actually should have been accepted. In this scenario, the prevalence of coccidioidomycosis at the time of the serosurvey would be 237/6,821 or 0.035 (3.5%); the incidence in September would be 97 cases, and the incidence rate would be 97/6,821 or 0.014 (1.4%). An effective case definition is critical because it may be confusing, especially in the absence of definitive diagnostics, to differentiate between actual disease cases and those ill from other causes. Key terms in this definition reflect some of the important principles of epidemiology. • Epidemiology is the basic science of Preventive and Social Medicine. Incidence rates are usually expressed by a multiplier that makes the number easier to conceptualize and compare. Recall bias: Cases and controls may remember an exposure differently (and non-randomly). Greenwood M.Epidemics and crowd-diseases: an introduction to the study of epidemiology, Oxford University Press; 1935. epidemiology is an atheoretical discipline. When the period of the study is from the past to the present, the study is a retrospective cohort study. In clinical medicine, two additional diagnostic test parameters are relevant. A rate is another type of ratio in which the denominator involves the passage of time. Analytical epidemiology is accomplished through either observational studies or interventional studies. Place variables commonly illustrate geographic differences in the occurrence of a particular disease. In other words, there is an important characteristic of the controls that make them different from the general population. The negative predictive value (NPV) is the probability of a patient not having the disease condition when the test is negative. The purpose is to describe and identify opportunities for intervention. epidemiology: The branch of a science dealing with the spread and control of diseases, computer viruses, concepts, etc., ... a basic principle. Among the most salient are to observe historical health trends to make useful projections into the future, discover (diagnose) current health and disease burden in a population, identify specific causes and risk factors of disease, differentiate between natural and intentional events (eg, bioterrorism), describe the natural history of a particular disease, compare various treatment and prevention products/techniques, assess the impact/efficiency/cost/outcome of interventions, prioritize intervention strategies, and provide foundation for public policy. Consider a study comparing library usage between students at two universities. Principles of Epidemiology Public health workers use epidemiologic principles as the foundation for disease surveillance and investigation activities. When analyzing results of an epidemiologic study, there are two categorical types of error when either accepting or rejecting the null hypothesis. PRINCIPLES OF EPIDEMIOLOGY. 8) Analogy—The effect of similar associations between other variables of exposure and outcome may be considered. In evaluating the causality of disease associations, analytical studies address the question of “why” as opposed to the “person/place/time” of descriptive studies. image source: s-media-cache-ak0.pinimg.com There are a number of definitions of epidemiology described by different authors but the most suitable definition is that epidemiology may be defined as the detailed scientific study of the distribution and determinants of disease or disability in society. Typical epi curves from common sources include 1) a common specific point source in which all cases were exposed at the same time and place (eg, a foodborne illness outbreak); 2) a common source with continuous exposure in which although the source is common, cases gradually rise before either peaking or plateauing and declining; and 3) a common source with intermittent exposure in which the peaks occur at irregular times corresponding to the earlier exposures. Type I error (which can never be zero) is generally reported as the P value. Dictionary of epidemiology. The former is more subjective than the latter but can be just as effective, especially in field epidemiologic conditions. Focused studies can help assist epidemiologists to determine why those differences have occurred and to identify specific risk factors. Graph the number of cases of congenital syphilis by year for the country, ____ 5. A proportion is a type of ratio in which the numerator is part of the denominator (A/[A + B]). to study the progression of disease. The odds ratio (OR) represents the odds that an outcome will occur from a particular exposure, compared with the odds of the outcome occurring in the absence of that exposure. PPV and NPV are clinically relevant, because they are directly related to the prevalence of disease. In epidemiology, health surveillance is accomplished in either passive or active systems. Common time variables include secular trends (changes over long periods of time), seasonal/cyclic periods, and specific points in time (eg, outbreaks, epidemics, clusters, etc). 4th ed. Despite these potential limitations, detailed information regarding the dates and numbers of reported cases is visually useful. Then epidemiologists began to look at behaviors related to health and well-being, such as amount of exercise and seat belt use. verify here. Descriptive methods are commonly applied to little-known diseases; they use preexisting data, address the questions of who/where/when, and identify potential associations for more in-depth analytical studies. Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in defined populations.. A similar concept to incidence is prevalence. Additionally, this type of study is not usually applicable for discovering disease etiologies; observational studies are much better suited for this purpose. The investigators jump back in time to identify a cohort of individuals at a point in time before they developed the outcomes of interest, and try to establish their exposure status at that point in time. One of the most basic ethical principles of medicine and epidemiology is the moral obligation to cause no harm to participants (non-malfeasance), whether physical or psychological. Principles of Epidemiology Lecture 7. They then determine whether the subject subsequently developed the outcomes of interest. As a fundamental medical science to study the frequency and occurrence of disease, historically, epidemiology has been considered the study of infectious diseases in large populations. Individual data are not necessary, only aggregate data for the group(s) under study. The Merck Veterinary Manual was first published in 1955 as a service to the community. Therefore, the clinician and the epidemiologist have different responsibilities when faced with a person with illness. Identify causes or determinants of disease 6. Start studying Principles of Epidemiology. Distribution implies that diseases and other health outcomes do not occur randomly in populations; determinants are any factors that cause a change in a health condition or other defined characteristic; morbidity is illness due to a specific disease or health condition; mortality is death due to a specific disease or health condition; and the population at risk can be people, animals, or plants. Case series are listings of a larger number of cases, usually presented consecutively (eg, a characterization of dog bite incidents in a population of veterinarians and/or technicians over time). Mortality is the corresponding measure of death in a population and can be applied to death from general (nonspecific) causes or from a specific disease. Obviously, each member of the cohort must not have the disease or outcome at the beginning of the study. The unit under study is a group of people or animals versus an individual. To calculate the sensitivity and specificity of a test, consider the following 2 × 2 table (see Table: Calculating Test Sensitivity and Specificity). In other words, the word epidemiology has its roots in the study of what befalls a population. In this type of study, the units are groups (or communities) of participants assigned to treatment or control conditions. For this reason, it can be very difficult to legally use human participants in many such trials. Epidemiology is a scientific discipline with sound methods of scientific inquiry at its foundation. Case definitions are usually based on either clinical signs or diagnostic test results. In this case, the cohort is assumed to share a particular exposure and is followed over time to document the occurrence of new instances of a particular disease or outcome. Dona Schneider, PhD, MPH, FACE Concepts in Infectious Epidemiology. Active surveillance is usually much more expensive and labor intensive; it typically is limited to short-term analyses of high-impact events. In analytical studies, this conjecture is termed the null hypothesis, meaning that the default assumption is that there is no association between the exposure in question and the disease outcome. In a propagated source outbreak, the peaks of cases occur one incubation period apart. Interventional studies typically take one of two forms, either a randomized controlled (clinical) trial or a nonrandomized (community) trial. Public health officials investigate disease outbreaks to control them, to prevent additional illnesses, and to learn how to prevent similar outbreaks from happening in the future. A negative test result, therefore, strongly implies that disease is absent, whereas a positive result may require additional, confirmatory testing. Ecologic studies also have several disadvantages. Because this reporting is voluntary (sometimes referred to as being "pushed" to health agencies), passive surveillance tends to underreport disease, especially in diseases with low morbidity and mortality. The natural history begins before infection (prepathogenesis period) when the agent simply exists in the environment, includes the factors that affect its incidence and distribution, and concludes with either its disappearance or persistence (endemnicity) in that environment. Please confirm that you are a health care professional. False-negative test results do not affect the PPV. Results showed that productivity increased during the course of the study regardless of the changes in light; the workers just performed better because of the attention. Two essential components are the number of new cases and the period of time in which those new cases appear. Tabulate the frequency of clinical signs, symptoms, and laboratory findings among children with chickenpox in Cincinnati, Ohio. The objectives of epidemiology include the following: to identify the etiology or cause of disease. Surgeon General’s now-standard warnings. The numerator and denominator may be independent of each other. to determine the extent of disease. When corrected for the effects of this confounder, no association was found between alcohol consumption and heart disease. Back to top 10.1: Principles of Epidemiology Dr. Peter Davies of the Veterinary Manual Editorial Board discusses the importance of veterinarian... © 2020 Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA, Epidemic curve common intermittent source, Calculating Test Sensitivity and Specificity. Epidemiology is applied in many areas of public health practice. Retrospective studies are conceived after some individuals have already developed the outcomes of interest. Epidemiology is also derived from the word epidemic. NPV is calculated as true negatives divided by the sum of the true negatives and the false negatives: d/(c + d). Observational studies – we do not interfere in the process of the disease, but simply observe the disease and the associated factors. If they are negatively associated, the value of one variable increases when the other decreases. In one notable example, a very large case-control study in 1950 studied people with lung cancer and demonstrated a very positive association between smoking and lung cancer. Again using the influenza example from above, if 7 students had influenza at the same time during the academic quarter, the prevalence would be 7/102 or 0.069 cases per class (or 6.9%). The case definition defines a case in terms of person, place, and time. For instance, animal rabies is endemic in the USA. What is important about your exposure? It is usually associated with public health because it studies disease patterns and their impact… The other category of studies that comprise analytical epidemiology are interventional studies. For animal populations, “place” may refer to housing (eg, indoors vs outdoors, pen number or stall) or type of herd management (eg, intensive feedlot confinement vs extensive grazing). Although it did not prove causality alone, it was instrumental in the U.S. Rates are typically expressed as a measure of the frequency with which an event occurs in a defined population in a defined time (eg, the number of foodborne Salmonella infections per 100,000 people annually in the USA). Raccoon rabies was epidemic throughout the eastern USA for much of the 1980s and 1990s. Public health surveillance is defined as the ongoing systematic collection, analysis, interpretation, and dissemination of outcome-specific data essential to the planning, implementation, and evaluation of public health practice. The overall shape of the epi curve can give clues to the type of exposure that resulted in the outbreak. Evaluate programs 10. The proportion of female dogs among those found to have parasite infections would be 643/(889 + 643), or 0.42. Why would it be important to find the right cause of the disease? The first step in an analytic study is to form some conjecture regarding observed exposures and health outcomes. 2) Consistency—Repeatedly similar findings observed by different persons in different places with different samples strengthen the likelihood of a causal effect. At its core, nutritional epidemiology seeks to understand the underlying biological basis of nutrition and the mechanisms by which diet can influence health. As the investigation ensues and more information becomes available, it may be necessary to revise the case definition. If the class had only a few people above average, but these students were significantly above average, and the rest of the students were only slightly below average, the distribution would be skewed toward a higher IQ when, in actuality, many members of the class would be below average. Cates W. Epidemiology: Applying principles to clinical practice. An epidemic occurs when a disease occurs in larger numbers than expected in a given population and geographic area. New York: Oxford University Press; 2001. p. 61. Sensitivity is the probability of a positive test result when the disease is actually present. Establish priorities for allocating resources 8. Match the term to the activity that best describes it. Passive surveillance occurs when individual health care providers or diagnostic laboratories send periodic reports to the public health agency. In this type of study, subjects are selected as either having a particular outcome (cases) or not having the outcome (controls). The difference is that epidemiologists tend to use synonyms for the 5 W’s: diagnosis or health event (what), person (who), place (where), time (when), and causes, risk factors, and modes of transmission (why/how). Which of the following diseases is associated with direct exposure to affected animals or their grooming tools? An incidence rate takes the population at risk into account. A major advantage of randomized controlled clinical trials is an inherently high validity for identifying differences in therapeutic efficacy of various interventions. Epidemiology is data-driven and relies on a systematic and unbiased approach to the collection, analysis, and interpretation of data. Innumerable factors influence the temporal waxing and waning of disease. In 1924–1932, workers at the Hawthorne Works (an electric company near Chicago) were studied to see whether productivity was greater depending on how much light was provided at work. Koch’s postulates are four criteria designed to establish a causal relationship between a causative microbe and a disease. An example of passive surveillance is the system of officially notifiable diseases routinely reported to CDC by select health departments across the USA. 5) Biological gradient—Greater exposure generally results in greater incidence of the outcome. An example of the former was a spurious conclusion drawn from a study of the relationship between alcohol consumption and heart disease. Once defined, the group is analyzed against some exposure to see what outcome(s) ensue. Identify risk factors for developing diseases 5. Finally, if an association exists (positive or negative), it is either causal or noncausal related to the outcome. For example, suppose that, among domestic dogs testing positive for internal parasites in Glendale, Arizona, 889 were male and 643 were female. What is important about your disease? However, epidemiology also draws on methods from other scientific fields, including biostatistics and informatics, with biologic, economic, social, and behavioral sciences. The Hawthorne Effect: Participants in a study may act or behave differently because they know they are being studied. What is important about your exposure? The applications of epidemiology are diverse and include identifying causal mechanisms of disease, diagnostic testing, determining prognosis, and testing new treatments. Agent, host, environment Classification of human infections by modes of transportation Incubation period Spectrum of disease. Below are three key terms taken from the definition of epidemiology, followed by a list of activities that an epidemiologist might perform. To make the proper diagnosis and prescribe appropriate treatment for a patient, the clinician combines medical (scientific) knowledge with experience, clinical judgment, and understanding of the patient. Epidemiology is a discipline that studies the distribution and determinants of disease. This concept seems simple at first, but misinterpretations can lead to erroneous study results and conclusions. • Epidemiology is scientific discipline of public health to study diseases in the community to acquire knowledge for health care of the society. In a propagated source outbreak, person-to-person transmission occurs, often through several cycles before declining. Principles of Epidemiology: A Self-Teaching Guide consists of a series of problem-solving exercises designed to introduce and guide readers toward an understanding of the principles and methods of epidemiology, rather than the epidemiology of specific diseases or subject areas such as ""infectious disease"" or ""chronic disease"" epidemiology. Case-control studies have several advantages. In general, bias in descriptive studies is not as prevalent or significant as bias in analytical studies. It provides the background for interventions to reduce transmission of infecting organisms, reduce the number of healthcare-associated infections, and protect healthcare providers from infection. The same is true in characterizing epidemiologic events, whether it be an outbreak of norovirus among cruise ship passengers or the use of mammograms to detect early breast cancer. It is a cornerstone of public health, and shapes policy decisions and evidence-based practice by identifying risk factors for disease and targets for preventive healthcare. Specificity, in contrast, is the probability of a negative test result in the absence of disease, thereby correctly classifying an individual as disease-free (regarding that particular condition). Selection bias results from the identification of subjects/cases from a subset that is not representative of the entire population at risk. , DVM, MPH, DACVPM, Lincoln Memorial University; , DVM, MPVM, DACVPM, Department of Pathobiology, College of Veterinary Medicine, Auburn University. 2. The natural history of a disease in a population, sometimes termed the disease’s ecology, refers to the course of the disease from its beginning to its final clinical endpoints. Therefore, if the study groups are comprised of differing fractions of employed and unemployed people, the results may very well be skewed. They assess whether groups with different rates of disease differ in their demographic characteristics, genetic or immunologic make-up, behaviors, environmental exposures, or other so-called potential risk factors. They are relatively quick, easy, and inexpensive. For example, in a class of veterinary students in which 88 are female and 14 are male, the sex ratio of female students to male students is 88/14, or 6.3 to 1. Significant differences might result if students from one university were queried over the phone regarding library visits, whereas students at the other university were directly observed for actual usage. Selection bias: This occurs when selected controls are not representative of the population from which the cases were selected. PPV is calculated as true positives divided by the sum of the true positives and the false positives: a/(a + b). In an example regarding the class of veterinary students, if 13 of them developed influenza over the course of 3 mo (one quarter), the incidence would be 13 cases per quarter. For instance, the group could be a kennel of dogs, a class of veterinary students, or the citizens of an entire country. Confounding: In epidemiologic studies, a confounder is a variable that is not considered in the study design but is associated with the exposure and exerts an effect on the outcome. Place may also relate to risk of exposure to infectious animals at sale barns or during shipment or to external factors such as severe weather and natural disasters. First, the measurement of many exposures is imprecise, especially of large groups in which the influence(s) of those exposures is difficult to define or not equally exerted. There are two broad types of epidemiological studies: 1. Although epidemiologists and direct health-care providers (clinicians) are both concerned with occurrence and control of disease, they differ greatly in how they view “the patient.” The clinician is concerned about the health of an individual; the epidemiologist is concerned about the collective health of the people in a community or population. Start studying Chapter 5: Principles of Epidemiology. Active surveillance, in contrast, occurs when an epidemiologist or public health agency seeks specific data from individual health care providers or laboratories. Merck & Co., Inc., Kenilworth, NJ, USA is a global healthcare leader working to help the world be well. Type II error, also known as a false negative, is when the null hypothesis is accepted when it actually should have been rejected. In another example, consider a large veterinary practice in the southwest USA that frequently sees dogs with coccidioidomycosis. In fact, epidemiology is often described as the basic science of public health, and for good reason. Epidemiology was originally focused exclusively on epidemics of communicable diseases (3) but was subsequently expanded to address endemic communicable diseases and non-communicable infectious diseases. Therefore, strongly implies that disease is present are unaware whether they are typically when... 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