Transcript Epidemiology Kept Simple Causal Concepts Chapter 2

Epidemiology Kept
Simple
Chapter 2
Causal Concepts
§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
Stages in Natural History of Disease
(Single Cause)
Fig 2.1 (p. 34)
Natural History of HIV/AIDS
Fig 2.3 (p. 37)
Multiple Causal Factors
• Causal factors rarely (if ever) act
alone
• Cause is the cumulative effects of
multiple factors acting together
•interdependence
•interaction
•multi-causality
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
Natural History of Heart Attack
(Genetic + Environmental Factors)
Fig 2.5 (p. 38)
§2.2 Spectrum & “Iceberg”
• Every ailment has a broad range
manifestations & severities
• We often see only the tip of the iceberg
§2.3 Causal Concepts
• What do we mean by cause?
• There are several ways to define “cause”
• Cause in a metaphysical concept
• Rothman & Greenland’s (1998) definition:
• any event, act, or condition
• precedes disease
• without which disease would not have occurred or
would have occurred at later time (counterfactual)
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)
Causal Pies Fig. 2.8 (p. 43)
Causal Complement
a factor or set of factors that complete a
sufficient cause
Mycobact
Susceptibi
lity
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%)
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?
Another Model (Causal Web)
Causal Web Continued
• Interdependence of cause at multiple
levels
• Macro (“upstream”)
• Individual
• Physiologic (“downstream”)
Still, another way to think about
causality (Agent, Host, & Environment)
§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)
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
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
Time
• Table 2.5: Examples of time variables
• Fig 2.15