Epidemiology Kept Simple Chapter 2 Causal Concepts

§2.1 Natural History of Disease Natural history of disease = progression of disease in an individual over timeNatural history of disease = progression of disease in an individual over time “Disease” defined loosely to refer to any form of morbidity or premature death“Disease” defined loosely to refer to any form of morbidity or premature death We considerWe consider Single factorSingle factor Multiple causal factorsMultiple 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 aloneCausal factors rarely (if ever) act alone Cause is the cumulative effects of multiple factors acting togetherCause is the cumulative effects of multiple factors acting together interdependenceinterdependence interactioninteraction multi-causalitymulti-causality

Sophisticated view of “incubation” Induction period = time between causal action and disease initiation Induction period = time between causal action and disease initiation Latency period = time between disease initiation and detection Latency period = time between disease initiation and detection Empirical induction period = induction + latency 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 & severitiesEvery ailment has a broad range manifestations & severities We often see only the tip of the icebergWe often see only the tip of the iceberg

§2.3 Causal Concepts What do we mean by cause ?What do we mean by cause ? There are several ways to define “cause”There are several ways to define “cause” Cause in a metaphysical conceptCause in a metaphysical concept Rothman & Greenland’s (1998) definition:Rothman & Greenland’s (1998) definition: any event, act, or conditionany event, act, or condition precedes diseaseprecedes disease without which disease would not have occurred or would have occurred at later time (counterfactual) 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) 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) 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) 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

Sufficient / Component Cause (cont.) Interdependence = factors working together in sufficient causal mechanism Interdependence = factors working together in sufficient causal mechanism completed piecompleted pie Helps understand complex epi conceptsHelps 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 prevalence of causal complements in the populationANS: 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 immune population is nil (increases risk by 0%) The effect of Mycobacterium exposure in a fully susceptible population is extreme (increases risk by 100%)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 cornYellow shank disease in chickens occurs only in susceptible strains feed yellow corn What would the farmer think if you:What would the farmer think if you: Added yellow corn to the diet of a susceptible flock?Added yellow corn to the diet of a susceptible flock? Added susceptible chickens to a flock feed yellow corn?Added susceptible chickens to a flock feed yellow corn? Now tell me, what causes cancer, environmental factors or genetic factors?Now tell me, what causes cancer, environmental factors or genetic factors?

Another Model (Causal Web)

Causal Web Continued Interdependence of cause at multiple levelsInterdependence of cause at multiple levels Macro (“upstream”)Macro (“upstream”) IndividualIndividual Physiologic (“downstream”)Physiologic (“downstream”)

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

§2.4 Epidemiologic Variables PersonPerson PlacePlace TimeTime 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)Types of person variables (Table 2.3, p. 49) Determines exposure and host susceptibilityDetermines exposure and host susceptibility Illustrative example: Fig (p. 50) Rate per 1000 sports- and recreational injuriesIllustrative example: Fig (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)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)Illustrative example (Regional Differences in 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 each generationrages in Japanese-Americans increases with each generation reason is unclear – see theories, p. 51reason is unclear – see theories, p. 51

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