Concepts of disease causation

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.

5
 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.

7
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,…
8

More about agent factors
Living 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.
11

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.

14
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.

15
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.

16
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.

17
 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

18
 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’s
Component 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.

Quiz
1. 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.

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