Unit 3: Descriptive Epidemiology

1. Unit 3: Descriptive Epidemiology

2. Unit 3 Learning Objectives: • 1. Characterize the major dimensions of descriptive epidemiology: Person, Place, Time • 2. Recognize how measurement and quantification of health outcomes by person, place, and time can assist in planning health services. • 3. Recognize how measurement and quantification of health outcomes by person, place, and time can provide clues to etiology of health-related events. • 4. Recognize the characteristics, strengths, and limitations of ecologic (aggregate) studies, case reports, and case series.

3. Unit 3 Learning Objectives (cont.): 5. Understand the “ecologic fallacy.” 6. Understand the concepts of cohort effects and clustering. 7. Understand the design features and information provided by cross-sectional surveys. 8. Recognize the strengths and limitations of cross-sectional surveys. 9. Demonstrate knowledge of cross-sectional surveys conducted at the national level.

4. Assigned Readings: Textbook (Gordis): Chapter 13, pages 204-206 (Ecologic studies) Koepsell and Weiss: Person place, and time. In Epidemiologic Methods, Chapter 7, pages 147-178.

5. Descriptive epidemiology Purpose: To characterize the amount and distribution of disease within a population. In other words …. To identify health problems and patterns of disease that exist. Descriptive studies generally precede analytic studies designed to investigate determinants of disease. Thus, descriptive studies often help to generate research hypotheses.

6. Person Since disease not does occur at random: What kinds of people tend to develop a particular disease, and who tends to be spared? What’s unusual about those people?

7. Person • Age – the most fundamental factor to consider when describing disease occurrence. --- The incidence of most chronic diseases increases with age. --- However, the incidence of many infectious diseases is highest in childhood. --- Some disorders show bi-modal (two peak) distributions (i.e. Hodgkin’s disease). This may reflect different underlying etiologies.

8. Person Gender – biological and non-biological factors related to gender may impact disease risk. --- In all developed countries, life expectancy is higher in females and males – principally due to lower heart disease mortality. --- However, many chronic diseases occur more frequently in women (depression, lupus, etc.) --- As lifestyles continue to become more similar, a question is whether mortality rates will become more similar (i.e. environment vs. biology).

9. Person Race/ethnicity – difficult to define, and to identify which characteristics may relate to disease occurrence. Remarkable variation exists in rates of disease occurrence across racial and ethnic groups. --- Genetics? --- Socioeconomic status? --- Environmental exposures? --- Access to health care? --- Lifestyle factors?

10. Person Social class – summarizing variable (SES), unreliably measured, that links: --- Occupation --- Education --- Area of residence --- Income --- Lifestyle Despite its unreliability, SES is consistently associated with mortality in a gradient fashion.

11. Discussion Question 1 What hypotheses might explain the highest incidence of severe mental illness among the lowest social classes?

12. Discussion Question 1 • Social causation hypothesis: membership (and factors) in low social classes produces schizophrenia and other mental illness. • Social drift hypothesis: mental disorders are disabling – stigma and impaired income earning ability that occur with mental illness results in downward mobility.

13. Place Since disease not does occur at random: Where is the disease especially common or rare, and what is different about those places? Investigation by place includes: • Across countries (international) • Within country variation • Urban/rural differences • Localized areas

14. Place • Infectious and chronic diseases show great variation from one country to another. • Some differences may be attributed to: --- Climate --- Cultural factors --- Diet --- Genetics

15. Place • Infectious and chronic diseases also show considerable variation within a country (i.e. multiple sclerosis varies by latitude in the U.S.). • Some differences may be attributed to: --- Climate --- Geology --- Latitude --- Environmental pollution --- Race/ethnicity

16. Place • Some differences in disease occurrence between urban and rural locations may be attributed to: --- Diet --- Physical activity --- Housing conditions (i.e. lead paint) --- Crowding (i.e. spread of infection) --- Pollution

17. Place • Some localized differences in disease occurrence may be attributed to: --- Carcinogenic exposure (i.e. radon) --- Geologic formations (i.e. water hardness) --- Lifestyle

18. Discussion Question 2 Regarding cross-country variation in disease occurrence, what is a likely impact of migrating from one’s native land to a geographically and culturally different location?

19. Discussion Question 2 For many disorders, particularly chronic diseases, migrants begin to assume disease rates of the host country in just in a few generations. This provides strong evidence for the influence of environmental factors since genetics are relatively stable over time.

20. Time Since disease not does occur at random: How does disease frequency change over time, and what other factors are temporally associated with those changes? The occurrence of health-relate events can vary by time: • Secular trends • Cyclic fluctuations • Point epidemics

21. Time • Secular trends refer to gradual changes in disease occurrence over long periods of calendar time. --- Example: In the U.S., mortality from heart disease has been gradually declining, whereas cancer mortality has been gradually increasing.

22. Time • Cyclic fluctuations refer to shorter-term increases and decreases in disease occurrence over a period of years, or within a year. --- Fluctuations in respiratory infection deaths over a few years --- Seasonal variation of infections, heart attacks, etc.

23. Time • Point epidemic refers to increased disease occurrence among a group of people exposed almost simultaneously to an etiologic factor (i.e. pathogen, contaminant). --- Despite exposure at a common point in time, the actual time of disease onset may vary.

24. Discussion Question 3 In the U.S., heart attacks tend to occur more frequently in the early morning hours, and on Mondays. What are some possible etiological factors associated with this phenomenon?

25. Discussion Question 3 • Perhaps: • Daily hormonal fluctuations • Conditioned responses (i.e. stress associated with return to work on Monday)

26. Cohort Effects Cohort effect: Long-term variation in disease occurrence among a group of persons who share something in common. i.e. • Occupational exposures during a specific time period. • Birth year or era and changes in lifestyle characteristics such as smoking habits.

27. Clustering Clustering: An unusual aggregation of health events grouped together in space or time. i.e. • Adverse reactions to vaccines • Outbreak of legionnaires’ disease in 1970’s • Early 1980’s – high number of cases of Kaposi’s sarcoma in young homosexual men

28. Clustering Clustering: Be careful where to identify a cluster because of chance variation. * * * * * * * * * * * * * *