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Epidemiology Overview and Concepts

Epidemiology Overview and Concepts. Definition. Epidemiology Greek Epi – about or upon Demos – populace or people of districts Logos – word Study of that which is upon the people The study of disease in populations.

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Epidemiology Overview and Concepts

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  1. Epidemiology Overview and Concepts

  2. Definition • Epidemiology • Greek • Epi – about or upon • Demos – populace or people of districts • Logos – word • Study of that which is upon the people • The study of disease in populations

  3. Study of the frequency, distribution, and determinants of health and disease in populations • Analogous to pathogenesis of disease in individuals • Epidemiology is a fundamental science for medicine in populations. Martin et al., 1987

  4. Environment Model of Disease Host Agent Epidemiological Approaches • Ecological Epidemiology • Medical Epidemiology • Understanding how disease agents are transmitted and are maintained in environment • Life cycle or natural history of disease • Foundation for disease eradication programs

  5. Epidemiological Approaches • Etiologic Epidemiology • Determining the cause of disease • “Medical detection” epidemiology • “Shoe leather” epidemiology • Outbreak investigation

  6. Epidemiological Approaches • Clinical Epidemiology • Answers questions asked in practice of veterinary and human medicine • Normality/Abnormality • Diagnosis • Frequency • Risk/Prevention • Prognosis • Treatment • Cause

  7. Epidemiological Approaches • Quantitative Epidemiology • Mathematically describe diseases and associated factors • Explore potential “cause and effect” associations

  8. Epidemiological Approaches • Preventive Medicine • Design optimal management, control or preventive strategies • Use all available epidemiological approaches to accomplish this • Cost-effectiveness or cost-benefit is important component

  9. Application of Epidemiology • It integrates well with basic science • by testing the application of experimental models in the real world • by discovering relationships between outcomes and risk factors which may generate hypotheses for mechanisms of disease.

  10. Environment Model of Disease Host Agent Ecology of Disease

  11. Environment Host Agent Ecology of Disease Model of Disease

  12. Environment Host Agent Ecology of Disease Model of Disease

  13. Model of Disease • Agent • A factor whose presence is required for the occurrence of a disease.

  14. Model of Disease • Host • Animal that supports the replication or development of an agent or is affected by an agent under natural conditions.

  15. Genotype Age Sex Species and Breed Nutritional status Immune status Size and Conformation Coat Color Host Determinants

  16. Model of Disease • Environment • Physical surroundings and management factors that affect hosts and agents

  17. Environmental Determinants • Location • Climate • Macroclimate • Microclimate • Management • Housing • Diet • Husbandry – density, pig-flow, etc.

  18. Model of Disease • Changes in relationships result in different outcomes • Agent overcomes host • Host overcomes agent • Agent and host maintained in equilibrium

  19. Measuring and Expressing Occurrence of Disease • Epidemic • An increase in the number of subjects affected by a disease over the EXPECTED rate of occurrence • Epizootic • Term used to express an epidemic in a population of animals

  20. Measuring and Expressing Occurrence of Disease • Pandemic • An epidemic that occurs over a large geographical area or the world

  21. Measuring and Expressing Occurrence of Disease • Outbreak • Localized epidemic

  22. Measuring and Expressing Occurrence of Disease • Endemic • Occurrence of disease at a constant or expected level

  23. Measuring and Expressing Occurrence of Disease • Sporadic • Pattern of disease in which the disease occurs rarely and without regularity

  24. Frequency of Clinical Events • Mathematically describing occurrence of events such as disease and death • Rates • Ratios • Proportions

  25. Number of Cases Prevalence = Total Number of Animals Frequency of Clinical Events • Prevalence – Proportion of animals within a population that have a condition of interest at a given point in time

  26. No. of new cases over a time period Incidence Rate = Average population at risk during time period (e.g. animal-months) Frequency of Clinical Events • Incidence Rate – Proportion of animals that develop a condition of interest over a specific period of time

  27. Frequency of Clinical Events • Prevalence represents the risk of being a case, whereas incidence represents the risk of becoming a case (Smith, 1995)

  28. Frequency of Clinical Events • Morbidity rate – • As measure of prevalence • Proportion of animals that are affected with disease at a point in time • As measure of incidence • Number of new cases of disease that occur in the average population at risk during a specified time period

  29. Frequency of Clinical Events • Mortality rate – Number of animals that die during a period of time

  30. Frequency of Clinical Events • Attack rate • Special kind of incidence rate • Numerator is the number of new cases • Denominator is the number of individuals exposed at the START of an outbreak • Of the individuals exposed to an agent, how many acquired the disease

  31. Frequency of Clinical Events • Factors Affecting Incidence and Prevalence • Temporal Sequences • Disease Duration • Case Definition • Dangling Numerators • Population at Risk • Crude vs Adjusted Rates • Real vs Apparent Prevalence

  32. Factors Affecting Incidence and Prevalence • Real vs Apparent Prevalence • No test is 100% accurate • Tests give us apparent prevalence not the true prevalence • Need to know the sensitivity and specificity of the test to calculate true prevalence

  33. Test Outcomes

  34. a+c n True Prevalence =

  35. a+b n Apparent Prevalence =

  36. Accuracy • How close is a test result to the truth • Proportion of all tests, both positive and negative, that are correct

  37. a+d n Accuracy =

  38. What is Truth? • Gold Standard • The test that is used todetermine if a disease is truly present or not

  39. What is Truth? • Gold Standard • The test that is used todetermine if a disease is truly present or not • Other tests are compared to it to determine their accuracy

  40. Test (Diagnostic) Sensitivity • Ability to correctly detect diseased animals • Not the same as analytical sensitivity which denotes the detection limits of a test • 100-200 KNOWN diseased animals needed to establish diagnostic sensitivity

  41. a a+c Sensitivity =

  42. Sensitivity False Negatives False Negative Rate • Likelihood of a negative result when patient actually has disease

  43. False Negatives Sensitivity False Negative Rate • Likelihood of a negative result when patient actually has disease

  44. False Negatives Sensitivity False Negative Rate • False negative rate increases with decreased sensitivity • Likelihood of a negative result when patient actually has disease

  45. Reasons for False Negative Reactions • Natural or induced tolerance • Improper timing • Improper selection of test • Analytically insensitive tests • Non-specific inhibitors e.g. anticomplementary serum; tissue culture toxic substances • Antibiotic induced immunoglobulin suppression • Incomplete or blocking antibody

  46. Test (Diagnostic) Specificity • Ability to correctly detect non-diseased animals • Not just analytical specificity • ability to measure the correct substance • 2000 KNOWN non-diseased animals needed to establish

  47. d b+d Specificity =

  48. Specificity False Positives False Positive Rate • Likelihood of a positive result when patient does not have the disease

  49. False Positives Specificity False Positive Rate • Likelihood of a positive result when patient does not have the disease

  50. False Positive Rate • False positive rate increases with decreased specificity • Likelihood of a positive result when patient does not have the disease False Positives Specificity

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