92 page EPIDEMIOLOGY 101 PowerPoint Presentation on CD

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Contains the following key public domain (not copyrighted) U.S. Government publication(s) on one CD-ROM in both Microsoft PowerPoint and Adobe Acrobat PDF file formats: TITLE: Introduction to Epidemiology, 92 pages (slides) SLIDE TOPICS, SUBTOPICS and CONTENTS: Introduction to Epidemiology Basic Epidemiology Medical Surveillance Outbreak Investigation Epidemiology is The study of the distribution and determinants of health-related states and events in specified populations and the application of this study to the control of health problems. (Last) Special Terms Distribution Person, Place, Time Not randomly distributed Determinants Host factors Environmental factors Agent factors Epidemiologic Triad Epidemiologic Triad Host Factors Genetics Age, Gender, Race, Ethnicity Physiology, Immunology Behavior, Nutrition Epidemiologic Triad Environmental factors Physical environment Geography Climate Toxins Biologic environment Socioeconomic environment Epidemiologic Triad Agent factors Non-Infectious (? = Environment) Physical properties Transfer of energy Infectious agents Virulence Infectivity Uses of Epidemiology Historical study – Worse or better? Population assessment Evaluation of individual health risks Evaluation of interventions Completing the clinical picture Etiology of injury and disease Epidemiologic Method Observe and count (‘tick marks’) Relate events to the population at risk Make comparisons Develop hypothesis Test the hypothesis Make scientific inferences / experiment Intervene and evaluate Epidemiologic Terms Prevalence Incidence Crude or adjusted rates NOTE: P ~ I x D Epidemiologic Terms Relative Risk Odds Ratio Bias Confounding Interaction Descriptive Epidemiology Person, Place and Time Case report Case series Ecologic studies* Cross-Sectional studies* Analytical Epidemiology Tests hypotheses about causation Usually follow descriptive studies Observational or Experimental May be used to make decisions Institute or modify preventive programs Legislative actions Evaluate effectiveness of a preventive program Observational Epidemiology Observation of the occurrence of a condition or disease in people who are already segregated into groups on basis of some experience or exposure Cohort study Case-Control study Cohort Study Subsets of the population can be identified by exposure status before the onset of disease. The subsets are then followed over time to determine rates of disease by exposure status. aka ‘Prospective Study,’ ‘Longitudinal Study,’ ‘Incidence Study,’ etc. Cohort Study Yields rates of disease Measure of association is the Relative Risk Good for looking at all results of an exposure Expensive in time and money Rate of Disease Cohort Study ILL Relative Risk (Rate Ratio) RR = Rate in Exposed Rate in Unexposed Case-Control Study Subjects are selected on the basis of disease status: Those with disease (cases) or those without disease (controls). The PAST exposures of the two groups are compared. aka ‘Retrospective Study’ Measure of association is the Odds Ratio Subject to ‘Recall Bias’ Case-Control Study ILL Case-Control Study Odds that ill were exposed in the past = a/c Odds that controls were exposed = b/d ODDS RATIO (OR) = (a/c)/(b/d) = ad/bc OR approximates RR (if disease is rare) “Things are not always what they seem....” Bias Confounding Interaction Bias Deviation of results or inferences from the truth.... Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth. (Last) Not simply ‘biased’ as used in everyday speech Sources of Bias Selection bias Information bias Comparison bias Confounding A situation in which the effects of two processes are separated. (Last) Confounded? Does gray hair cause heart attacks? Review this study supported by the Grecian Formula Institute of Public Health... Confounding Age < 40 Age > 40 Interaction The interdependent operation of two or more causes to produce or prevent an effect The effect of one or more factors varies as a function of the level of another factor aka synergism (antagonism), effect modification Disease Surveillance Purpose of Surveillance Determine baselines Natural history of disease Describe trends Detect epidemics Plan and set priorities Detect rare / dispersed disease Evaluate interventions Disease Surveillance Cycle Collection of data Consolidation of data Analysis of data Dissemination of reports Action to control and prevent Sources of Surveillance Data Vital statistics Health reports Hospital records Registries Surveys Disease and Non-Battle Injury (DNBI) Surveillance DoD (JCS) directed Routine collection & reporting of all military medical encounters Includes reportable events Data collected at lowest echelon of medical care Analyzed data flow back to reporting units US DoD DNBI Surveillance http://cba.ha.osd.mil/projects/other/depl-surv/depl-surv-main.htm http://cba.ha.osd.mil/projects/other/depl-surv/depl-surv-policy.htm http://cba.ha.osd.mil/projects/other/depl-surv/depl-surv-forms.htm http://amsa.army.mil/AMSA/amsa_home.htm DNBI Rates An early warning system Focus attention on problems Measure effectiveness of interventions Identify areas of preventable disease and injury The ‘Vital Signs’ of the military unit DNBI Weekly Report Summary of weekly DNBI rates Provides baseline Reliant on a PROPER Sick Call Logbook or Daily Patient Log Form Sick Call Logbook Patient info Name, SSN UNIT & UIC Duty location Chief Complaint Diagnosis Type of visit INITIAL FOLLOW UP REFERRAL Disposition RTD Light Duty / Profile Lost Duty Day / Quarters Admit Injury Classification Recreational / Sports MVA Work / Training Other Confidentiality DNBI Surveillance Worksheet Daily and routine process Simplifies weekly DNBI reporting Admin data at top of form TROOP STRENGTH refers to the number of persons supported by the MTF (by gender) TROOP STRENGTH obtained from S-1/J-1 Record DIAGNOSIS for each INITIAL visit Record LIGHT DUTY, LOST DUTY days Diagnostic Categories Combat/Operational Stress Dermatologic Gastrointestinal, Infectious Gynecologic Heat/Cold Injury Injury, Recreational/Sports Injury, MVA Injury, Work/Training Injury, Other Psychiatric Respiratory STD Fever, unknown origin All other, Medical/Surgical Dental MISC/Admin/Follow-up Definable (open) Rate Calculation Divide number of patients seen that week for each category by average troop population at risk For gynecologic category, divide by number of females Multiply by 100 (to get percent / week) EXAMPLE: 20 DERM cases last week in a unit of 500 soldiers 20/500/wk = 0.04/wk; 0.04/wk X 100 = 4%/wk incidence for DERM encounters Record number of estimated LIGHT / LOST DUTY days DNBI Rate Analysis Compare observed rate with reference rate CINC SURGEON or JTF SURGEON may modify reference rates Look for trends in disease rates Comment on “PROBLEMS IDENTIFIED AND CORRECTIVE ACTIONS” DNBI Surveillance Sum of references rates may NOT equal total DNBI reference rate Rate of one category can be high with low total DNBI rate DNBI rate can be high without busting any one disease category Track DNBI rates over time and compare to historical rates DNBI rates vary with phase of deployment Field Epidemiology: Outbreak Investigations Outbreak Investigation Overview Procedures for outbreak investigations PITFALLS! Data analysis Outbreak Investigation Processes Prepare for field work Establish existence of an outbreak Verify the diagnosis Define and identify cases Perform descriptive epidemiology Outbreak Investigation Processes (cont.) Develop hypothesis Evaluate hypothesis Reconsider/refine hypothesis Implement prevention/control measures Communicate findings 1: Prepare for Field Work Investigation Scientific knowledge--have it or get it! Laboratory consultation Supplies, equipment Administration Review SOPs, local directives Consultation Know your role, especially when other agencies involved 2: Establish the Existence of an Outbreak Determine if disease incidence is higher than expected in this population The primary reason for disease surveillance! Other Reasons for ‘Outbreaks,’ i.e., Observed Cases > Expected Change in reporting procedures Changes in case definition Increased awareness or public interest Improved diagnostics New clinician Population change 3: Verify Diagnosis Ensure proper diagnosis Rule out lab error Visit and interview patients Better understanding of clinical features Mental image of outbreak Set up frequency distributions Frequency Distribution 4a: Establish a Case Definition Critical to success of investigation! Definition composed of: Clinical criteria: simple and objective Epidemiologic variables: person, place, time Case Definitions Clinical criteria examples Titer elevation Fever 101 degrees and above 3 or more loose bowel movements/day Myalgias severe enough to limit activities Epidemiologic variables Persons w/no immunity to or past Hx of rubeola Place: Workers, attendees of child care center Time: Contact period 14-20 February Classification of Case Definitions Definite case Probable case Possible case Classification: DEFINITE CASE “Confirmed” Usually lab verified Last to arrive Example: E coli O157:H7 isolated from stool culture or development of HUS in a school-age child with GI symptoms beginning between 3 November and 8 November 1996 Classification: PROBABLE CASE Case has typical clinical features No lab confirmation (may be pending) Example: Bloody diarrhea with the same person, place, and time restrictions May be only case definition in certain cases Classification: POSSIBLE CASE Fewer typical clinical features Epidemiologic variables: same, but some data may be missing Example: Abdominal cramps and diarrhea (at least 3 stools in a 24-hr. period) in a school-age child with onset during the same period Don’t lose track of them! Case Definition Development Early in investigation: INCLUDE definite, probable, and possible cases Later: sharper focus --tighten definition to increase specificity 4b: Identify & Count Cases “Cast a wide net”--expand investigation Use multiple sources; be diligent Ask case-patients Information Collection Identifying info: name, SSN, address, phones Demographic info: age, sex, race, worksite, occupation Clinical info: verify against case definition; get onset date Risk factor info: disease-specific (e.g., HA: inquire about exposure to risky foods/water; overeats travel) Reporter info: staff member or person providing case report Information Collection (cont.) Collect on standard case report form or questionnaire Use line listing! Add new cases as they become known Contains key information on every interview Classify as case or non-case/control 5: Perform Descriptive Epidemiology Person Place Time Descriptive Epi: Person Host characteristics: age, race, sex Exposures: occupation, leisure activities, medications, drugs Disease-specific risk factors: sexual exposure, IV drug use, etc.. Descriptive Epi: Place Spot map: simple and effective Identifies: Geographic extent of outbreak Clustering or patterns Applications Communities: residence, work, environ. Facilities: hospitals, dining hall seating arrangements, worksites, surgical suites Descriptive Epi: Time Draw epidemic curve No. of cases by time of onset or DX Tells us: “Where are we now?” “What’s the forecast?” Probable time of exposure (maybe) Epidemic pattern Epidemic Curves Epidemic Curves Epidemic Curves Descriptive Epidemiology Summarize your findings Use the typical, predominant descriptors Generalize Don’t worry about political correctness! 6: Develop Hypotheses Should address: Source Mode of transmission Exposures that caused disease Requires familiarity with disease--consider what you know Case-patient interviews give insight Hypothesis should be testable Develop Hypotheses (cont.) Discuss with local HD Still clueless? Convene group-pt. meeting Visit homes--plunder Don’t forget outliers MAY give best clues because they’re somehow different from norm-- 7: Evaluate Hypotheses Two methods: Compare hypothesis with the established facts Analytic epidemiology Quantify relationships Explore the role of chance Hypothesis Evaluation: Comparative Method Clinical, laboratory, environmental, and/or epidemiologic evidence overwhelming Example: All cases had bloody diarrhea; E. coli (EIEC) isolated from stools All had same ready-to-eat food; no other common exposures Salad preparer at restaurant had same strain E. coli 2 wks earlier; known not to practice good hygiene Hypothesis Evaluation: Analytic Method Cohort study--best for small, well-defined outbreaks Population at risk easily located/recognized Ill/well interviewed Calculate attack rates--”ate” vs “didn’t eat” Put on food-specific attack rate table Analytic Method (cont) Relative Risk Calculation Analytic Studies Case-Control studies Use when population not well defined Must select controls as comparison group Should not have disease in question Representative of population Examples: random sample, friends, neighbors, family, other patients Use odds ratio as measure of association between exposure and disease Analytic Method (cont) Statistical significance testing Explores role of chance Chi-square (X2) most common Greater risk difference = Larger chi-square Sample size/denominator big influence Analytic Method (cont) p-value Probability of finding a difference in risk at least as strong as the one you just observed if there was actually no difference in risk. Large X2 => small p-value De-emphasize in small outbreaks Analysis: Bottom Line Rely primarily upon . . . Differences in risk between exposed and non-exposed Biologic plausibility Common sense 8: Refine Hypothesis / Additional Study Unrevealing analytic studies likely if hypothesis not well-founded If descriptive epi unrevealing, analysis useless Reconsider; work another angle Consider new vehicles or modes of transmission Meet with case-patients; visit homes Additional Studies Laboratory Studies Lab evidence can clinch findings Sometimes absolutely necessary to close investigation Environmental Investigations Often answer “why” outbreak occurred 9: Implementation of Control & Prevention Measures Aim at weak link(s) in chain of infection Apply to agent, source, or reservoir Destroy contaminated food Disinfecting contaminated water Destroy vector breeding sites Control & Prevention Measures (cont.) Interrupt transmission or exposure Personal protective countermeasures Reducing host susceptibility Immunizations Malaria chemoprophylaxis 10: Communicate the Findings Include descriptive epidemiology Be objective Serves as a reference and historical document Outbreak Investigation Summary Use a SYSTEMATIC approach Establish the case definition early Descriptive Epi is important Statistics are only a tool Always COMMUNICATE finding

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