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Unformatted text preview: Chapter Chapter 2
Practical Applications of Epidemiology Seven Uses of Epidemiology Seven Uses of Epidemiology To study the history of the health of populations. To diagnose the health of the community. To study the working of health services operations research. To estimate the individual risks of disease and other conditions. Uses, continued... Uses, continued... To identify syndromes. To complete the clinical picture of chronic diseases. To search for causes of health and disease. Historical Use of Epidemiology Historical Use of Epidemiology Refers to the study of past and future trends in health and illness. Secular trendschanges in disease frequency over time. Examples of Trends Examples of Trends Chronic diseases have replaced acute infectious diseases as the major causes of morbidity and mortality. Leading causes of U.S. deaths are heart disease, cancer, and stroke. Check out Figure 2.1, page 49 Check out Figure 2.1, page 49 Compare the leading causes of death in the USA between 1900 and 1996, about one hundred years later. In 1900, more than half of all deaths were due to infectious diseases such as TB, Influenza, Gastroenteritis (eg, diarrhea), etc. In 1996, roughly 2/3 of all deaths are caused by cardiovascular diseases and cancer. What are the major shifts? By 2100, who knows whether the LCD will appear more like 1900 or 1996? Factors Affecting Reliability of Factors Affecting Reliability of Observed Changes Lack of comparability over time due to altered diagnostic criteria. Aging of the general population. Changes in the fatal course of the condition. Predictions About the Future Predictions About the Future About 20 percent of the U.S. population in 2030 will be age 65 and older. The need for health and agingrelated services will grow. Population Dynamics Population Dynamics Three factors that affect the size of populations are births, deaths, and migration. When these factors do not contribute to net increases or decreases, the population is in equilibrium. Population Terms Population Terms Fixed populationadds no new members and, as a result, decreases in size due to deaths only. Dynamic populationadds new members through migration and births or loses members through emigration and deaths. Demographic Transition Demographic Transition Shift from high birth and death rates found in agrarian societies to lower birth and death rates found in developed countries. Steady Population Steady Population A population is in steady state when the number of members exiting equals the number entering. Epidemiology and the Health of Epidemiology and the Health of the Community Provides a key to the types of problems requiring attention. Determines the need for specific health services. Demographic and Social Demographic and Social Variables Age and sex distribution Socioeconomic status Family structure Racial, ethnic, and religious composition Variables Related to Variables Related to Community Infrastructure Availability of social and health services Quality of housing stock Social stability (residential mobility) HealthRelated Outcome HealthRelated Outcome Variables Homicide and suicide rates Infant mortality rate Selected mortality rates Drug and alcohol abuse rates Teen pregnancy rates Birth rate Healthy People 2010, Goal 2 Healthy People 2010 “ . . . To eliminate health disparities among segments of the population, including differences that occur by gender, race, or ethnicity, . . .” Epidemiology and Policy Epidemiology and Policy Evaluation Using epidemiologic methodologies to evaluate public health policies Examples: tobacco control policies, automobile safety laws, food sold to schoolchildren Epidemiology and Epidemiology and Disease Etiology Applications include:
Search for causes Individual risks Specific clinical concerns Modern Concepts of Causality Modern Concepts of Causality Five criteria for causality (source: 1964 Surgeon General’s Report):
Strength of association Temporality/Time sequence Consistency of results upon repetition Specificity Coherence of explanation Further Consideration of Further Consideration of Causality Sir Austin Bradford Hill expanded the list of criteria to include: Biologic gradient Plausibility Experiment Analogy Causality Causality
Almost impossible to prove in epidemiology & medicine Causal Relationships Causal Relationships Two Factors X Z Consistency • When the same variables, factors or When events appear over and over in different over circumstances circumstances • More studies, more evidence Sedentary behavior and heart attack; cigarette smoking and lung cancer Strength When the association shows that a given When factor makes disease occurrence more likely likely Lung cancer occurs 810 times more often in smokers than nonsmokers Specificity • When the cause-effect association of a When disease is specific to one or two connected diseases (increases likelihood of true negatives) diseases Lung cancer and nonsmokers Lung cancer and nonsmokers Not everyone who develops heart disease is overweight Obesity multiple causal pathways Time relationship (Temporality) (Temporality) • Exposure to the “cause” occurs before Exposure the development of the disease the • Tricky with clinical and subclinical Tricky disease disease Congruence (coherence) • Does the association fit with existing Does knowledge? Make sense scientifically? knowledge? • Does it conflict with what we DO know, Does and if so, are there special conditions which alter scientific processes? which Sensitivity • Does the cause-effect analysis hold Does true? Can you correctly identify those with the disease? the Cervical cancer screening STI screening Biological Gradient/DoseResponse Biological Gradient/DoseResponse
• Association is based upon the virility of the Association pathogen and the level of susceptibility of the host host • Seen most clearly with toxins and een carcinogens • More exposure, more risk Higher frequency & quantity of alcohol consumption – more likely to develop alcoholism eg; radiation exposure eg; Plausibility • Association should be proven to be Association causal based on current biological, medical, and scientific knowledge. medical, Feasible and theoretically possible Feasible Experiments and research Experiments and research
• Knowledge and inferences of causeeffect associations based on research and effect experiments • True (double-blind study), natural True (Snow’s cholera epidemic), or interventions (vaccinations, seat belt laws) interventions Chicken pox can be prevented by immunization Chicken pox can be prevented by immunization Analogous Factors Analogous Factors
• Siimilar relationships have been shown to milar be causal be • Weakest evidence of causality
Example: Scrapie never transmitted to humans; Example: therefore, unlikely that Mad Cow Disease would either. therefore, When there is a third factor When Mediating Moderating Confounding Necessary & sufficient Mediating Factors Mediating Explains the relationship between the variable of interest and the outcome variable A significant path of influence Mediating variables Mediating
“y mediates the relationship between x and z” Y X Z What is a mediating variable? What Exercise selfefficacy mediates the relationship of exercise goals and exercise behaviors The impact of dietary knowledge on eating behavior is mediated by …… Eg; social support, perceived barriers, motivation Moderating Factors Moderating
Influence the relationship between the variable of interest (risk or exposure) and the outcome variable (disease or health) Moderating Variables Moderating
“Y moderates the relationship between x and z” X Y Z What is a Moderating variable? variable? Gender moderates the effect of stress on the immune system The relationship between communication skill & marital satisfaction is moderated by stress Confounding factors Confounding “Extraneous factors” that could be responsible for what we observe or do not observe Ex: influence of air pollution & electrical wires A problem of comparison when factors are distributed differently throughout groups Ex: age, academic level Necessary & Sufficient Necessary Cause: an event, condition or characteristic that plays an essential role in producing the occurrence of disease Or a constellation of components that act in concert Necessary and Sufficient Necessary Sufficient cause Necessary cause A set of “minimal” conditions that inevitably produce a disease A causal component that must be present for the disease to occur Ex: phenylketonuria (PKU), phenylaline, genetic defect Risk Factors Risk Factors Due to the uncertainty of “causal” factors the term risk factor is used. Definition: exposure that is associated with a disease. Example of a risk factor: smoking. Three Criteria for Risk Factors Three Criteria for Risk Factors The frequency of the disease varies by category or value of the factor, e.g., light smokers vs. heavy smokers. The risk factor precedes onset of the disease. The observation must not be due to error. Natural History of Disease Natural History of Disease Prepathogenesisbefore agent reacts with host. Pathogenesisafter agent reacts with host. Later stages include development of active signs and symptoms. Clinical end points are: recovery, disability, or death. Primary Prevention Primary Prevention Occurs during prepathogenesis phase. Designed to reduce the occurrence of disease. Examples are education and immunization. Secondary Prevention Secondary Prevention Occurs during pathogenesis phase. Designed to reduce the progress of disease. Examples are screening programs for cancer and diabetes. Tertiary Prevention Tertiary Prevention Designed to limit disability from disease. Also directed at restoring optimal functioning. An example is physical therapy for stroke patients. ...
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- Spring '08