Gerstman Chapter 2 1

Epidemiology Kept Simple

Chapter 2

Causal Concepts

Gerstman Chapter 2 2

§2.1 Natural History of DiseaseNatural history of disease Natural history of disease ≡≡ progression of disease in progression of disease in

an individual over time. an individual over time.

Gerstman Chapter 2 3

Natural History of HIV/AIDS

Can you identify the subclinical stage?

Gerstman Chapter 2 4

Definition of Cause

Definition of “cause” • Any event, act, or condition • preceding disease or illness• without which disease would

not have occurred • or would have occurred at a

later time Ken Rothman (contemporary epidemiologist)

Disease results from the Disease results from the cumulative effects of multiple cumulative effects of multiple causes acting together causes acting together ((causalcausal interactioninteraction))

Gerstman Chapter 2 5

Types of Causes• Necessary – found

in all cases• Contributing –

needed in some cases

• Sufficient–a constellation of causes that makes disease inevitable

Gerstman Chapter 2 6

Causal Complement• Causal complement = the

set of factors that completes a sufficient mechanism

• Example: tuberculosis– Necessary agent

Mycobacterium tuberculosis

– Causal complement“Susceptibility”

Susceptibility

M. tuberculosis

Gerstman Chapter 2 7

Epidemiological Iceberg• Only the tip of the iceberg

is easily observable• Dog bite example

– 3.73 dog bites annually

– 451,000 medically treated

– 334,000 emergency room visits

– 13,360 hospitalizations– 20 deaths

Gerstman Chapter 2 8

Epidemiologic Spectrum

• Spectrum of illness - range of severities and manifestations

• Polio example– 95%: subclinical– 4%: flu-like

symptoms– 1%: polio paralysis

Spectrum of Polio

95%

4%

1%

Subclinical

Flu-like

Paralaysis

Gerstman Chapter 2 9

Environment and genetics cannot be separated

• Yellow shank disease occurs only in susceptible chicken strains fed yellow corn

• What would a farmer think if he started feeding yellow corn to a susceptible flock?

• What would a [different] farmer think if he added susceptible chickens to a flock being fed yellow corn?

• Is yellow shank disease environmental or genetic?

How does this apply to arguments about environmental and genetic causes of cancer?How does this apply to arguments about environmental and genetic causes of cancer?

Gerstman Chapter 2 10

Causal WebCausal factors act in a hierarchal web

MI causal web

Gerstman Chapter 2 11

Epidemiologic TriadAgent, host, and environmental interaction

HIV epi triad

Gerstman Chapter 2 12

Homeostatic Balance

E

A H

At equilibriumSteady rate

E

HA

The proportion of susceptibles in population decreases

Environmental changes that favor the agent

EA

H

Environmental changes that favor the host

E

H

A

E

AH

Agent becomes more pathogenic

Gerstman Chapter 2 13

Epi Variables • Descriptive

epidemiology looks at rates according to person, place, and time variables

• First step of investigation

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)

Gerstman Chapter 2 14

“Rate”

00933.098,765

921 rateMortality

sindividual 98,765in death 921 :Example

size population

events of no.Rate

Loosely, the “rate” of an event is the number of events divided by population size

Gerstman Chapter 2 15

Expression of Rate via “Population Multiplier”

• To express a rate with multiplier m, multiply by m

• Example: Let m = 1000 The rate of .00933 =.00933 × 1000 per 1000 = 9.33 per 1000.

• Example: Let m = 100,000 The rate of .00933 = .00933 × 100,000 per 100,000

= 933 per 100,000

Gerstman Chapter 2 16

Person Variables• Person variables are

characteristics, attributes, and behaviors of individuals

• Examples of person variables are listed in Table 2.3 (p. 49)

• Surrogates for many health determinants

Figure: Recreational injuries by age and gender; rates are per 1000 person-years

Gerstman Chapter 2 17

Place Variables• Place variables

describe the locale where people live and work

• Examples of place variables are listed in Table 2.4 (p. 51)

• Differences may be due to genetic or environment

Figure: Age-adjusted breast cancer mortality, 23 countries, 1958–59

Gerstman Chapter 2 18

Time Variables• Examples of time

variables on p. 53• Epidemic curves -

number of cases over time

• Figure illustrates:(A) Sporadic occurrence(B) Endemic occurrence(C) Point epidemic(D) Propagating

epidemic