Triad , Wheel, Web

and Pie concepts of

disease causation

By

Dr. Bhoj R Singh

Principal Scientist & Head Division of Epidemiology,

ICAR-Indian Veterinary Research Institute, Izatnagar-

243122, Bareilly, UP, India.Email: brs1762@gmail.com

Objectives

To explain basic models of disease causation.

To understand the etiology or causes of disease & altered production.

To understand the applicability of causal criteria as applied to epidemiological studies

A cause?

Webster’s dictionary defines a cause as

‘something which has an effect’.

In epidemiology a cause can be considered

to be something that alters the frequency of

disease, health & production status or

associated factors in a population.

CONCEPT OF DISEASE

A condition of the body or some part

or organ of the body in which its

functions are deranged.

It is a mal-adjustment of an

organism to the environment.

It is deviation from normal function &

purpose.

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In epidemiology, there are several models of disease

causation that help understand disease process.

The most widely applied models are:

The epidemiological triad (triangle),

the wheel, and

the web. And

The sufficient cause and component causes models

(Rothman’s component causes model)

General Models of Causation

The epidemiologic triad Model

The epidemiologic triangle or triad is the traditional

model of infectious disease causation.

It has three components: an external agent, a

susceptible host, and environmental factors that

interrelate in a variety of complex ways to produce

disease & alter production & utility in animals.

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The Epidemiological Triad

HOST

AGENT ENVIRONMENT

Genetic make up (breed,

strain, variety.

Nutritional status

Immunological status etc.

Virulence

Pathovar

Biovar

ID50,LD50

Climate

Radiations

Rainfall

Winds

Agent factors

•Infectious agents: agent might be microorganism—virus,

bacterium, parasite, prions, other microbes and others

(poisonous creatures). Generally, these agents must be present

for disease to occur as essential causal factor.

•Nutritive: Excesses or deficiencies (Cholesterol, vitamins,

proteins)

•Chemical agents: (carbon monoxide, drugs, medications)

•Physical agents (Ionizing radiation,…

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More about agent factorsLiving organisms:

Adaptability

Host range

Virulence

Pathogenicity

Dose: ID50, LD50 etc.

Chemical agents (Toxins and pollutants)

Toxicity dose,

Pnetrability,

Stability

Half-life etc.

Physical agents (Radiations, sound, winds, floods, draughts, soil etc)

Compositions

Magnitude,

Exposure time

Host factors

•Host factors are intrinsic factors that influence an individual’s

exposure, susceptibility, or response to a causative agent.

•Host factors that affect a individual's risk of exposure to an

agent:

•Age, race, sex, breed, strain, purpose of domestication, feed

and feeding habits, breeding practices sociological status, etc..

•Host factors which affect susceptibility & response to an agent:

•Genetic composition, nutritional and immunologic status,

anatomic structure, presence of other disease or medications,

purpose & use of domestication, method of rearing & husbandry

practices and psychological makeup.10

Environmental factors

Environmental factors are extrinsic factors which affect the agent

as well as the host and the opportunity for exposure.

Environmental factors include:

Physical factors such as geology, climate,..

Biologic factors such as insects that transmit an agent; and

Socioeconomic factors such as crowding, sanitation, and the

availability of health services.

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Web of Causation Web of Causation is devised to address chronic

disease – can also be applied to communicable

disease) due to multi-factorial/ multi-etiologic nature

of causation in many diseases

There is no single cause / multi-factorial causes

Causes of disease are interacting in various pathways

Illustrates the interconnectedness of possible causes

Here the disease is usually well-defined from a clinical point

of view (e.g. lung cancer, Mastitis) but the etiologic

perspective is more complex.

Web of Causation

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The Wheel of Disease Causation

Mausner & Kramer,1985

The Wheel of Causation de-emphasizes the

agent as the sole cause of disease,

It emphasizes the interplay of physical, biological

and social (production) environments. It also

brings genetics into the mix.

A disease model which discriminates between

'necessary' and 'sufficient' factors.

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Necessary and sufficient causes

A necessary cause is a causal factor whose presence is

required for the occurrence of the effect. If disease does

not develop without the factor being present, then we term

the causative factor “necessary”.

Sufficient cause is a “minimum set of conditions, factors or

events needed to produce a given outcome.

The factors or conditions that form a sufficient cause are

called component causes.

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Example

The tubercle bacillus is required to cause

tuberculosis but, alone, does not always

cause it.

Thus tubercle bacillus is a necessary,

not a sufficient cause.

This true for most the infectious causes.

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Rothman's model has emphasised that the causes of disease

comprise a collection of factors.

These factors represent pieces of a pie, the whole pie

(combinations of factors) is the sufficient cause for a

disease. May be several pies for a disease or syndrome.

It shows that a disease may have more that one sufficient

cause, with each sufficient cause being composed of several

factors.

Rothman’s Component Causes and

Causal Pies Model

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The factors represented by the pieces of the pie in this model

are called component causes.

Each single component cause is rarely a sufficient cause by

itself, but may be necessary for causation of the disease.

Control of the disease could be achieved by removing one of

the components in each "pie" and if there were a factor

common to all "pies“ (necessary cause) the disease would be

eliminated by removing that alone.

Rothman’sComponent Causes and Causal Pies

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Causal Complement

(Causal Pie) Causal complement ≡

the set of factors that completes a sufficient causal mechanism

Example: Typhoid

Necessary agent Salmonella enterica ser Typhi

Causal complement“Susceptibility”

Causal pies representing all sufficient causes of a

particular disease

20 A given disease can have multiple sufficient mechanisms

Necessary cause =found in all cases (B)Contributing cause =needed in some cases (A, C, D, E, F) but not in all cases.Sufficient cause = the set of necessary & contributing causes that make disease inevitable in an individual

Applications

Epidemiologic Triad (devised to enhance

search for understanding communicable

disease). Model works with both infectious or

non-infectious diseases.

Web of Causation (devised to address chronic

disease – can also be applied to communicable

disease)

Applications

The purpose of studying cause and effect

in epidemiology is to generate knowledge

to prevent and control disease.

Distinguishing natural from other causes of

death & loss or reduced production/ utility.

Establishing modes of transmission &

spread.

Quiz1. Define the cause and disease.

2. Name the different models to explain the

causation of disease.

3. Give Trriad factors of HS and FMD.

4. Define sufficient and necessary & component

causes.

5. Differentiate between Mausner & Kramer’s Wheel

and Rothman’s causal Pie Model.

6. Construct Pies for Mastitis & Infertility in cows.

7. Construct web model for repeat breeding in

buffaloes.