This document discusses different models of disease causation in epidemiology. It describes: 1. The epidemiological triad model which views disease as resulting from an interaction between an external agent, susceptible host, and environmental factors. 2. Additional causation models including the wheel/pie model and web model which emphasize multiple interconnected factors. 3. Rothman's component cause model which views diseases as having one or more "sufficient causes" made up of various necessary "component causes". This model represents causes as pieces of a pie where different pie combinations can result in the disease.

1. Concept of disease
occurrence

2. Introduction


3. Causation/Cause
 In epidemiology a cause can be considered to
be something that alter the frequency of
disease, health and production status or
associated factors in population

4. Causation models
 Many models are present in epidemiology but the most common are
1. Epidemiological triad
2. Wheel or pie model
3. Web model
4. The sufficient cause and component causes model (Rothman’s
component cause model)

5. Epidemiological triad
 Traditional model of infectious disease causation
 It has three components
1. An external agent
2. Susceptible host
3. Environmental factors
 These components interrelate in variety of complex ways to produce
disease and alter production

7. Agent factors
 1. infectious agents- agent might be Mos (virus, bacteria, parasites, prions). Generally these
agents must be present for the disease to occur as an essential causal factor.
 2. Nutritive- excessive or deficiencies (cholesterol, vitamins and minerals)
 3. chemical agents- (carbon monoxide, drugs and medication)
 4. Physical agents- (ionizing radiation, non-ionizing radiation)

8. Agent factors
 Living organism: adaptability, host range, virulence, pathogenicity, dose ID50, LD50
 Chemical agents (toxins n pollutants): toxicity dose, penetrability, stability, half life
 Physical agents (radiation, sounds, winds, floods, droughts, soil): composition, magnitude,
exposure time

9. Host factors
 Host factors are intrinsic factors that influence an individual’s exposure, susceptibility or
response to causative agent.
 Host factors that affect an individuals risk of exposure to an agent: age, race, breed, strain,
purpose of domestication, feed and feeding habits, breeding practices, sociological status
etc.
 Host factors which affect the susceptibility and response to an agent: genetic composition,
nutritional and immunogenic status, anatomic structure, presence of other diseases and
medications, purpose and use of domestication, method of rearing and husbandry
practices and psychological makeup.

10. Environmental factors
 These are the extrinsic factors which affect the agent as well as host and
opportunity for exposure.
 Environmental factors include:
1. Physical factors such as geology and climate
2. Biological factors such as insects that transmit an agent
3. Socioeconomic factors such as crowding, sanitation and availability of
health services,

11. Web of causation
 It is devised to address chronic disease and can also be applied to
communicable diseases due to multifactorial/multi-etiological nature of
causation in many diseases.
 Silent features
1. There is no single cause
2. Causes of diseases are interacting in various ways
3. Illustrate the interconnections of possible causes
4. Here the disease is usually well-defined from clinical point of view but the
etiological perspective is more complex

13. Examples

14. The wheel of disease causation
 Mausner and 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 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

17. Rothman’s component causes and
causal pies model
 Rothman’s model has emphasized that the causes of diseases comprise a collection of factors.
 These factors represent a pieces of pie, the whole pie(combinations of factors) is the sufficient
cause for the disease.
 May be several pies for a disease or syndrome
 It shows that disease may have more than one sufficient cause, with each sufficient cause being
composed of several factors

18. Rothman’s component causes and
causal pies model