Epidemiology Kept Simple

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Epidemiology Kept Simple

• Chapter 2
• Causal Concepts

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2.1 Natural History of Disease

• Natural history of disease progression of
  disease in an individual over time
• Disease defined loosely to refer to any form of
  morbidity or premature death
• We consider
• Single factor
• Multiple causal factors

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Stages in Natural History of Disease (Single
Cause)

• Fig 2.1 (p. 34)

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Natural History of HIV/AIDS

• Fig 2.3 (p. 37)

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Multiple Causal Factors

• Causal factors rarely (if ever) act alone
• Cause is the cumulative effects of multiple
  factors acting together
• interdependence
• interaction
• multi-causality

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Sophisticated view of incubation

• Induction period time between causal action and
  disease initiation
• Latency period time between disease initiation
  and detection
• Empirical induction period induction latency

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Natural History of Heart Attack (Genetic
Environmental Factors)

• Fig 2.5 (p. 38)

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2.2 Spectrum Iceberg

• Every ailment has a broad range manifestations
  severities
• We often see only the tip of the iceberg

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2.3 Causal Concepts

• What do we mean by cause?
• There are several ways to define cause
• Cause in a metaphysical concept
• Rothman Greenlands (1998) definition
• any event, act, or condition
• precedes disease
• without which disease would not have occurred or
  would have occurred at later time
  (counterfactual)

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Sufficient / Component Cause (Causal Pies)

• Necessary factor found in all of cases (e.g.,
  Mycobacteria exposure for TB)
• Contributing factor unnecessary but combines
  with other factors to have an effect (e.g.,
  susceptibility to TB)
• Sufficient cause is achieved when factors combine
  to make disease inevitable (Mycobacteria
  susceptible sufficient for TB)

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Causal Pies Fig. 2.8 (p. 43)
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Causal Complement

• a factor or set of factors that complete a
  sufficient cause

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Sufficient / Component Cause (cont.)

• Interdependence factors working together in
  sufficient causal mechanism
• completed pie
• Helps understand complex epi concepts
• e.g., What is the effect of a factor?
• ANS The effect depends on prevalence of causal
  complements in the population
• The effect of Mycobacterium exposure in a fully
  immune population is nil (increases risk by 0)
• The effect of Mycobacterium exposure in a fully
  susceptible population is extreme (increases risk
  by 100)

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Yellow Shank Metaphor

• Yellow shank disease in chickens occurs only in
  susceptible strains feed yellow corn
• What would the farmer think if you
• Added yellow corn to the diet of a susceptible
  flock?
• Added susceptible chickens to a flock feed yellow
  corn?
• Now tell me, what causes cancer, environmental
  factors or genetic factors?

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Another Model (Causal Web)
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Causal Web Continued

• Interdependence of cause at multiple levels
• Macro (upstream)
• Individual
• Physiologic (downstream)

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Still, another way to think about causality
(Agent, Host, Environment)
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2.4 Epidemiologic Variables

• Person
• Place
• Time

I keep six honest serving men (They taught me all
I know) Their names are what and why and
when And how and where and who. (Kipling)
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Person Variables

• Types of person variables (Table 2.3, p. 49)
• Determines exposure and host susceptibility
• Illustrative example Fig 2.13. (p. 50) Rate per
  1000 sports- and recreational injuries

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Place

• Host and environmental factors associated with
  place variables listed in Table 2.4 (p. 51)
• Illustrative example (Regional Differences in
  Breast Cancer Mortality (Table 2.14, p. 52)
• rate in U.S. 20 per 100,000 in 1962
• rate in Japan 4 per 100,000 in 1962
• rages in Japanese-Americans increases with each
  generation
• reason is unclear see theories, p. 51

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Time

• Table 2.5 Examples of time variables
• Fig 2.15

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