Unit 2 – Public Health Epidemiology Chapter 4 – Epidemiology: The Basic Science of Public Health

Epidemiology The diagnostic discipline of public health A major part of public health’s assessment function Investigates causes of diseases Identifies trends in disease occurrence Evaluates effectiveness of medical and public health interventions An observational science

Patterns of Disease Occurrence Who is getting the disease? When did they get the disease? Where is the disease occurring? From this information, epidemiologists can infer why the disease is occurring.

Epidemic Surveillance Endemic vs. Epidemic Notifiable diseases Recognition of new disease Increased importance with threat of bioterrorism

Outbreak Investigation Verify the diagnosis Construct a working case definition Find cases systematically – active surveillance Ask the who, where, and when questions to describe the epidemic by person, place, and time. Consider the incubation period Look for common source of exposure

Question #1 What has epidemiology contributed to peoples understanding of Heart Disease? Lung Cancer?

Epidemiology and Chronic Diseases Identify risk factors Observe long-term trends Heart disease – leading cause of death in U.S. Framingham Study – started in 1948 Lung cancer and smoking – early 1950s British Physicians’ Study Hammond-Horn study in U.S.

Unit 2 – Public Health Epidemiology Chapter 5 – Epidemiologic Principles and Methods

Definition of Epidemiology Epidemiology is the study of the distribution and determinants of disease frequency in human populations

Step 1: Define the Disease Death is easy to determine – death certificates have cause of death Some diseases need blood tests or stool cultures to verify diagnosis Some diseases are hard to define – e.g. EMS, SARS Sometimes definition changes as more is learned – e.g. AIDS Other health outcomes – injuries, risk factors

Question #2 Explain the interaction between incidence, prevalence and prognosis. Give examples

Disease Frequency Count number of people with disease and relate to the population at risk (PAR) PAR (denominator) may be total population or exposed population, or one gender or age group; often comes from census Two ways to measure frequency: Incidence – number of new cases Prevalence – number of existing cases Incidence is used for studying causes of disease

Disease Frequency, ctd. If causes or risk factors increase, incidence and prevalence increase If ability to diagnose increases, incidence and prevalence increase Prevalence depends on incidence and prognosis

Question #3 Why are the who, when, and where questions useful in determining the causes of disease? Give examples.

Distribution of Disease Who – sex, age, occupation, race When – season, year (long-term trends), elapsed time since an exposure (epidemic curve) Where – neighborhood (e.g. clusters), latitude (climate), urban vs. rural, national variations

Determinants of Disease Why is distribution as it is? Can make inferences from distribution Risk factors

Human Population Epidemiology observes humans, in contrast to biomedical sciences, which can do experiments on laboratory animals

Question #4 Explain the three major types of epidemiologic studies.

Kinds of Epidemiologic Studies Goal is to determine an association between an exposure and a disease or other health outcome May be prospective or retrospective Intervention study Cohort study Case-control study

Question #5 Which is most likely to yield a valid result? Why?

Intervention Study Closest thing to an experiment Start with two groups: experimental group (gets the intervention or exposure) and control group Watch them over time and compare outcomes Experimenter chooses who is in which group Two groups should be as similar as possible so that intervention is the only difference

Intervention Study, ctd. Randomized, double-blind, placebo control is the ideal Pharmaceutical companies conduct many clinical trials for new drugs Physicians’ Health Study – prevention study Aspirin to prevent heart disease Beta carotene to prevent cancer Field trial of polio vaccine – 1954 Kingston-Newburgh study of fluoridation to prevent tooth decay

Cohort Study For situations when doing an intervention study would be unethical or too difficult Considered the next most accurate Choose a large number of healthy people, collect data on their exposures, and track outcomes over time The only difference from intervention is that people choose their own exposures

Cohort Study -- examples Framingham Heart Study Nurses’ Health Study British physicians study on smoking and lung cancer Hammond-Horn study on smoking and lung cancer in U.S.

Question #6 Which is likely to yield an answer in the shortest period of time? Why?

Case-Control Study Choose people who already have disease Choose a healthy control group of individuals as similar as possible to cases Interview them all and ask for their previous exposures Advantage: faster and cheaper Least accurate approach

Unit 2 – Public Health Epidemiology Chapter 6 – Problems and Limits of Epidemiology

Problems with Studying Humans Intervention study: subjects may not follow prescribed behavior throughout study period Cohort study: sometimes hard to isolate which of many factors are responsible for health differences Case-control study: control group may not be truly comparable; also errors in reporting or recall For all studies, must worry about differential drop- outs

Sources of Error Random variation Confounding variables Bias Selection bias Reporting bias or recall bias

Factors that Lend Validity to Results Strong association Dose-response relationship Known biological explanation Large study population Consistent results from several studies

Hormone Replacement Therapy Conflicting results from two major studies Clinical trial is the gold standard Results of cohort study were confounded by associated factors that made women taking HRT healthier, even without the therapy

Ethical Issues Nazi experiments on humans Tuskegee syphilis study New rules – informed consent New rules -- institutional review boards Importance of clinical trials Possibility of conflict of interest by medical providers who stand to profit

Question #7 Look up the Tuskegee Syphilis Study? Why was it unethical? What influence has it had on the conduct of clinical trials?