Epidemiology Epidemiology Kept SimpleKept Simple

Chapter 2Chapter 2

Causal ConceptsCausal Concepts

§2.1 Natural History of §2.1 Natural History of DiseaseDisease

• Natural history of disease = Natural history of disease = progression of disease in an progression of disease in an individual over timeindividual over time

• ““Disease” defined loosely to refer Disease” defined loosely to refer to any form of morbidity or to any form of morbidity or premature death premature death

• We considerWe consider• Single factor Single factor • Multiple causal factorsMultiple causal factors

Stages in Natural History of Stages in Natural History of Disease (Single Cause)Disease (Single Cause)

Fig 2.1 (p. 34)Fig 2.1 (p. 34)

Natural History of HIV/AIDSNatural History of HIV/AIDSFig 2.3 (p. 37)Fig 2.3 (p. 37)

Multiple Causal FactorsMultiple Causal Factors

• Causal factors rarely (if ever) Causal factors rarely (if ever) act aloneact alone

• Cause is the Cause is the cumulative effects cumulative effects of multiple factors acting of multiple factors acting togethertogether• interdependenceinterdependence• interactioninteraction• multi-causalitymulti-causality

Sophisticated view of Sophisticated view of “incubation”“incubation”

• Induction period Induction period = time between causal = time between causal action and disease initiation action and disease initiation

• Latency period Latency period = time between disease = time between disease initiation and detectioninitiation and detection

• Empirical induction period Empirical induction period = induction + = induction + latencylatency

Natural History of Heart Natural History of Heart Attack (Genetic + Attack (Genetic +

Environmental Factors)Environmental Factors)Fig 2.5 (p. 38)Fig 2.5 (p. 38)

§2.2 Spectrum & §2.2 Spectrum & “Iceberg”“Iceberg”• Every ailment has a broad range Every ailment has a broad range

manifestations & severities manifestations & severities • We often see only the tip of the icebergWe often see only the tip of the iceberg

§2.3 Causal Concepts§2.3 Causal Concepts

• What do we mean by What do we mean by causecause? ? • There are several ways to define “cause”There are several ways to define “cause”• Cause in a metaphysical concept Cause in a metaphysical concept

• Rothman & Greenland’s (1998) definition:Rothman & Greenland’s (1998) definition:• any event, act, or condition any event, act, or condition • precedes diseaseprecedes disease• without which without which disease would not have disease would not have

occurred or would have occurred at later time occurred or would have occurred at later time (counterfactual) (counterfactual)

Sufficient / Component Sufficient / Component Cause (Causal Pies)Cause (Causal Pies)

• Necessary factorNecessary factor – found in all of cases – found in all of cases (e.g., (e.g., MycobacteriaMycobacteria exposure for TB) exposure for TB)

• Contributing factorContributing factor – unnecessary but – unnecessary but combines with other factors to have an combines with other factors to have an effect (e.g., susceptibility to TB)effect (e.g., susceptibility to TB)

• Sufficient cause Sufficient cause is achieved when is achieved when factors combine to make disease factors combine to make disease inevitable (inevitable (Mycobacteria Mycobacteria + susceptible = + susceptible = sufficient for TB)sufficient for TB)

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

Causal ComplementCausal Complement

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

Mycobact

Susceptibility

Sufficient / Component Cause Sufficient / Component Cause (cont.)(cont.)

• InterdependenceInterdependence = factors working = factors working together in sufficient causal mechanism together in sufficient causal mechanism • completed piecompleted pie

• Helps understand complex epi concepts Helps understand complex epi concepts • e.g., What is the effect of a factor? e.g., What is the effect of a factor? • ANS: The effect depends on ANS: The effect depends on prevalence of causal prevalence of causal

complements complements in the populationin the population• The effect of The effect of Mycobacterium Mycobacterium exposure in a fully exposure in a fully

immune population is nil (increases risk by 0%)immune population is nil (increases risk by 0%)• The effect of The effect of Mycobacterium Mycobacterium exposure in a fully exposure in a fully

susceptible population is extreme (increases risk susceptible population is extreme (increases risk by 100%)by 100%)

Yellow Shank MetaphorYellow Shank Metaphor

• Yellow shank disease in chickens occurs Yellow shank disease in chickens occurs only in susceptible strains feed yellow only in susceptible strains feed yellow corn corn

• What would the farmer think if you:What would the farmer think if you:• Added yellow corn to the diet of a susceptible Added yellow corn to the diet of a susceptible

flock?flock?• Added susceptible chickens to a flock feed Added susceptible chickens to a flock feed

yellow corn? yellow corn?

• Now tell me, what causes cancer, Now tell me, what causes cancer, environmental factors or genetic factors?environmental factors or genetic factors?

Another Model (Causal Another Model (Causal Web)Web)

Causal Web ContinuedCausal Web Continued

• Interdependence of cause at Interdependence of cause at multiple levelsmultiple levels• Macro (“upstream”)Macro (“upstream”)• IndividualIndividual• Physiologic (“downstream”)Physiologic (“downstream”)

Still, another way to think about Still, another way to think about causality (Agent, Host, & causality (Agent, Host, &

Environment)Environment)

§2.4 Epidemiologic §2.4 Epidemiologic Variables Variables

• Person Person • Place Place • Time 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 VariablesPerson Variables• Types of person variables (Table 2.3, p. Types of person variables (Table 2.3, p.

49)49)• Determines exposure and host Determines exposure and host

susceptibilitysusceptibility• Illustrative example: Fig 2.13. (p. 50) Illustrative example: Fig 2.13. (p. 50)

Rate per 1000 sports- and recreational Rate per 1000 sports- and recreational injuriesinjuries

PlacePlace

• Host and environmental factors associated Host and environmental factors associated with place variables listed in Table 2.4 (p. with place variables listed in Table 2.4 (p. 51)51)

• Illustrative example (Regional Differences in Illustrative example (Regional Differences in Breast Cancer Mortality (Table 2.14, p. 52) Breast Cancer Mortality (Table 2.14, p. 52) • rate in U.S. = 20 per 100,000 in 1962rate in U.S. = 20 per 100,000 in 1962• rate in Japan = 4 per 100,000 in 1962rate in Japan = 4 per 100,000 in 1962• rages in Japanese-Americans increases with rages in Japanese-Americans increases with

each generationeach generation• reason is unclear – see theories, p. 51reason is unclear – see theories, p. 51

Time Time • Table 2.5: Examples of time variablesTable 2.5: Examples of time variables• Fig 2.15Fig 2.15