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EPIDEMIOLOGY - Overview Ronald D. Warner, DVM, PhD; Family & Community Medicine

EPIDEMIOLOGY - Overview Ronald D. Warner, DVM, PhD; Family & Community Medicine. DEFINITIONS : ASSUMPTIONS : GENERAL APPROACH : AREAS of APPLICATION / PRACTICE :. Epidemiology - DEFINITIONS. “old” --- Study of epidemics.

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EPIDEMIOLOGY - Overview Ronald D. Warner, DVM, PhD; Family & Community Medicine

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  1. EPIDEMIOLOGY - OverviewRonald D. Warner, DVM, PhD; Family & Community Medicine • DEFINITIONS : • ASSUMPTIONS : • GENERAL APPROACH : • AREAS of APPLICATION / PRACTICE :

  2. Epidemiology - DEFINITIONS • “old” --- Study of epidemics. • most --- Analyses of the distributions and determinants of disease/ injury ( health-related states ) frequency in populations , and the application of this study to the control of health-related problems. • “new” --- Systematic approach to answer any health-related questions. • Epidemiology is a basic science of medicine.

  3. Epidemiology - Basic ASSUMPTIONS • Health events do not occur randomly. • Natural history of disease/injury is a continuum : includes both causal and preventive factors. [ # susceptible > # exposed > # subclinical > # clinical illnesses ]( # of mild illness > # of severe illness > # of fatalities ) • Hallmark of epidemiologic inference : unbiased comparison • Constant goal : science - based intervention(s)

  4. Tetanus in Oregon : 1982 - 1996 * = fatality YEAR AGE GENDER INJURY Hx of VACC 1996 85 female punctured arm on railroad tie * none 1992 63 male punctured foot on barbed wire none 1992 81 male abrasion to foot * none 1991 91 female punctured leg on an anvil none 1991 90 female punctured finger on rose thorn * none 1990 36 male reinfected healing thumb wound unknown 1987 8 female punctured thumb with a splinter none 1985 62 female punctured foot on barbecue grate none 1982 21 male punctured foot at construction site unknown

  5. Natural history of illness ; the “continuum” TOTAL Population number of “Susceptibles” number Exposed to “sufficient cause” number “ Infected ” number of clinical Illnesses Dead

  6. Prevalence estimates of illness ; pyramid of health care 1000 adults “at risk” in a given geographic region 750 adults report at least one illness or injury / month 250 adults consult doctor each month 9 local hosp referrals ‘out’ 5 1 UMC

  7. the “pyramid” of disease Natural History ;elements involved in causation Environment vector Agent time Host

  8. The “Web” of disease Causation

  9. Rothman’s “Causal Pie” A = necessary cause B, C, D, E, H, I, J, & K = contributing causes K J A E A D B D C C H J E I III A B II Necessary cause(s) + contributing cause(s) = SUFFICIENT CAUSE

  10. Natural history “model” of cervical carcinoma Infection: bacterial &/or viral Normal Invasive cancer Dysplasia : mild moderate severe cancer in situ

  11. “Web” of Causation ; Sufficient Cause(s) [ susceptible ] undiagnosed (subclinical) outcome [ pathology ] Diagnosed (clinical ) outcome [ sequelae ] recovery , disability , death

  12. Levels of Prevention ( Natural History of disease ) PRIMARY Prevention SECONDARY Prevention TERTIARY Prevention clinical horizon SICK ; curative medicine Sickness --------> Health promotion Case finding “Rehab” medicine WELL Time -------->

  13. the general Epidemiologic ( scientific ) Approach • 1. Identify a PROBLEM : clinical suspicion ; case series ; review of medical literature • 2. Formulate a HYPOTHESIS ( asking the right question ) ; good hypotheses are: Specific, Measurable, and Plausible • 3. TEST that hypothesis ( assumptions vs. type of data ) • 4. always Question the VALIDITY of the result(s) : Chance ; Bias ; and Causality

  14. the epidemiologic study: threats to Validity • Chance : role of random error in outcome measure(s)( p - value ; power of the study and the confidence interval )--- largely determined by sample sizeBias : role of systematic error in outcome measure(s) • Selection bias - subjects not representative • Information bias - error(s) in subject data / classification • Confounding - 3rd variable (causal) assoc. w/ both X and Y

  15. Relationship between Bias and Chance:measuring blood pressure by two different methods intra-arterial pressure sphygmomanometer * * * * * * * * * * * * * * * * * * * * * * * * * * * * Chance Bias 80 90 Diastolic blood pressure ( mm Hg )

  16. Interpreting an observed association (study outcome): coffee drinking & pancreatic cancer A. Causal B. due to Confounding Coffee drinking Coffee drinking Smoking Pancreatic Cancer Pancreatic Cancer

  17. important Criteria for Causalinference :-- “bottom line” in most epidemiologic studies • Temporality : if X “causes” Y, then X must precede Y • Strength of association: >> risk estimates imply “more” cause • biologic Plausibility: does the association make bio-“sense” ? • biologic Gradient (dose-response) : “more” X ... “more” Y ? • Consistency with other studies: “same” in other populations ? • Specificity of association : is X uniquely associated with Y ?

  18. APPLICATIONS of epidemiologyin the practice of Medicine • … disease Surveillance (medical intelligence) • … searching for Causes • …comparing Diagnostic Techniques • …discovering the Natural History of a medical condition • … recognizing Prognostic Factors • … evaluating new Procedures / Treatments

  19. possible Explanations for epidemiologic Observations • BIAS ( systematic error )Selection : subjects not representative of population of interest Information : miscoding , misclassification ; recall & interviewer bias Confounder : i.) assoc w/disease of interest, in absence of exposure ii.) assoc w/exposure of interest, but not as result of exposure • CHANCE ( random error ) • TRUTH : The observation is correct. ( accept TRUTH only after excluding the others !! )

  20. Descriptive epidemiology :Patterns of Disease Occurrence • distribution of disease in populationsnumerator ( “event” count ) / denominator ( group “at risk” ) • by “person” : age , race / ethnicity , gender , occupation , education , marital status , genetic marker , sexual preference • by “place” : residence (urban vs. rural) , worksite , social event • by “time” : week , month , year ; sporadic , seasonal , trends --- incubation period ; latency

  21. Pattern of “All - cause” Mortality ; by “person” :Age groupings Age-specific death rates for deaths from all causes. USA;1991Source: National Center for Health Statistics;1993

  22. Pattern of disease Occurrence ; by “Person”:race / ethnicity Tuberculosis ; incidence rates / 100,000 ; United States , 1992 CDC. MMWR 1993; 42:696

  23. Pattern of disease Occurrence ; by “place”Rocky Mountain Spotted Fever Reported cases Rocky Mountain Spotted Fever. Counties reporting cases: U.S.; 1993CDC Summary of notifiable diseases. USPHS; 1994

  24. Pattern of disease Occurrence :by “Place”

  25. Pattern of disease Occurrence :by “Time”

  26. Patterns of disease Occurrence :Correlation of Population statistics • Ecologic ( correlation ) studies --- plot : disease (population) burden [ Y axis ] vs. prevalence of “risk factor” [ X axis ] • -- correlation coefficient : r ; + or - -- r-squared : % variability in Y “explained” by X • is only a hypothesis-generating study design* beware of ecologic fallacy when considering “results”

  27. Scatterplot of incidence of reported AIDS & TB. 15 states ; US , 1993correlation (ecologic) study: CDC, MMWR. 1993; 42:4 25 20 15 10 5 0 * * * * * * * * * * * * * * * TB incidence (per 100,000) 0 10 20 30 40 50 60 70 80 90 100 Incidence of AIDS per 100,000

  28. Relationship between # dental carries & fluoridecontent of public wateradapted from - Dean HT , et al. 1942. Pub Hlth Rep 57:1155-79 * * * * * * ** * * * * * * * * ** * * * * * 100 90 80 70 60 50 40 30 20 10 Dental carries per 10 children examined 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Fluoridecontent (ppm) of public water supply

  29. Patterns of disease Occurrence :Migration and disease • Geographic migration : • Social migration : • Biologic migration :

  30. Relative risk of cancer mortality : various Japanese men ,compared with white men living in California Relative risk ( compared with California whites ) Japanese Japanese immigrants Immigrant Japanese sons cancer Sites in Japan in California in California Stomach 8.4 3.8 2.8 Liver 4.1 2.7 2.2 Colon 0.2 0.4 0.9 Adapted from - Buell P and Dunn JE. 1965. Cancer 18:656.

  31. Descriptive epidemiology : pattern of occurrencePrevalence of HIV+ and community Mosquito index r = .83 r - squared = .92 * p < .001 p < .001 * * * * * * * * * * * * * * 20 15 10 5 0 HIV+ 0 2 4 6 8 10 12 14 16 18 20 22 Index of community mosquito infestation

  32. Analytic epidemiology :Case-control study; HIV “carried” by mosquitoes ? Mosquito exposure No exposure HIV + Controls O.R. = 5.38

  33. Analytic epidemiology : stratification for confounding ;Case-control study. HIV “carried” by mosquitoes ? Mosquito Exposure No exposure FemalesHIV + 3 2 166 133 Mosquito Exposure Males HIV + 155 15 304 controls 81 10 O.R. = 1.21 O.R. = 1.27 261

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