Concepts of disease causation

The Presentation explains basic models of disease causation, to understand the etiology or causes of disease & altered production and helps to understand the applicability of causal criteria applied to epidemiological studies.

1. 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: [email protected]

2. 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

3. 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.

4. 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. 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

6. 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. 7 The Epidem iological Triad HOST AGENT ENVIRONMENT Genetic make up (breed, strain, variety. Nutritional status Immunological status etc. Virulence Pathovar Biovar ID 50 ,LD 50 Climate Radiations Rainfall Winds

8. 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

9. More about agent factors Living organisms: Adaptability Host range Virulence Pathogenicity Dose: ID 50 , LD 50 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

10. 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

11. 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

12. 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.

13. Web of Causation Disease behaviour Unknown factors genes phenotype workplace social organization microbes environment

14. 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. 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. 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. 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. 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

19. 19 Causal Complement (Causal Pie) Causal complement ≡ t he set of factors that completes a sufficient causal mechanism Example: Typhoid Necessary agent Salmonella enterica ser Typhi Causal complement “Susceptibility”

20. 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

21. 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)

22. 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.

23. Quiz Define the cause and disease. Name the different models to explain the causation of disease. Give Trriad factors of HS and FMD. Define sufficient and necessary & component causes. Differentiate between Mausner & Kramer’s Wheel and Rothman’s causal Pie Model. Construct Pies for Mastitis & Infertility in cows. Construct web model for repeat breeding in buffaloes.