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Disease causation.pptx

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Disease causation.pptx

  1. 1. Disease causation Ms. Precious Simushi BSc Biomed, MPH
  2. 2. Objectives • By the end of the session students should; • Be able to define and understand disease causation • Understand the concepts of a causal relationship • Understand the types of disease causation models (triad and causal pies)
  3. 3. Causal Concepts • Effective disease control and prevention depends on understanding the causes of the illness. • In general terms, a cause is something that produces an effect or brings about a result. • important --- prevention, diagnosis and treatment..
  4. 4. • A cause---condition, characteristic or a combination of factors playing an important role in producing a health outcome. • Logically, a cause must precede an outcome • the effect would not have occurred if the cause did not precede it
  5. 5. • Another definition incorporates time factor; • Rothman & Greenland, 1998-defined a cause as; • an event, condition or characteristic that preceded a disease without which the disease event either would not have occurred at all or would not have occurred until some later time.
  6. 6. • we expect that an increase in the level of a causal factor in inhabitants will be accompanied by an increase in the incidence of disease in that population • if the causal factor can be eliminated or diminished, the frequency of disease or its severity will decline.
  7. 7. • Basically what we imply is that: • “if the person who developed disease Y had not been exposed to factor X, then disease Y would not have occurred. Therefore, X is a cause.”
  8. 8. CAUSAL RELATIONSHIPS • A cause is termed sufficient when it inevitably produces or initiates an outcome (radiation or benzene- leukemia) • necessary if an outcome cannot develop in its absence (carcinogenesis/ TB)
  9. 9. • Some diseases are caused completely by genetic factors in the individual: • alpha- and beta-thalassemias, sickle cell anemia, Marfan syndrome • other causes of a disease interact with genetic factors in making certain individuals more vulnerable than others Examples include heart disease, high blood pressure, Alzheimer's disease, arthritis, diabetes, cancer, and obesity.
  10. 10. TYPES OF CAUSAL RELATIONSHIPS  Necessary and sufficient  Necessary, but not sufficient  Sufficient, but not necessary  Neither sufficient nor necessary
  11. 11. Individual Assignment 1 • What is meant by causal relationship? Discuss with examples on the types of causal relationships: 1. Necessary and sufficient 2. Necessary, but not sufficient 3. Sufficient, but not necessary 4. Neither sufficient nor necessary Due date: 25/11/22 printed copies, not more than 3 pages inclusive cover page
  12. 12. Factors in causation • Necessary but they are rarely sufficient • Predisposing factors: • Predisposing Factors Definition Predisposing factors are the risk factors that make a person more susceptible to developing a disease. • such as age, sex, or specific genetic traits that may result in a poorly functioning immune system
  13. 13. • Enabling: • any characteristic of the environment that facilitates action • factors such as low income, poor nutrition, bad housing and inadequate medical care may favour the development of disease
  14. 14. • Precipitating factors • factors that initiate or promote the onset of any illness, disease, accident, or behavioral response • such as exposure to a specific disease agent may be associated with the onset of a disease
  15. 15. • Reinforcing factors • Influencing Behavior Factors • such as repeated exposure, environmental conditions and unduly hard work may aggravate an established disease or injury
  16. 16. Causation of disease • disease and other health events do not occur randomly in a population • occur in some members of the population than others- risk factors that may not be distributed in population. • Epidemiology concepts may be used to identify the factors that place some members at greater risk than others
  17. 17. • A number of models have been developed as to why disease occur • Among the simplest of these is the epidemiologic triad or triangle, the traditional model for infectious disease
  18. 18. • consists of an external agent, a susceptible host, and an environment that brings the host and agent together. • Agent, host, and environmental factors interrelate in a variety of complex ways to produce disease. • Different diseases require different balances and interactions of these three components.
  19. 19. Agent • Can be a virus, bacterium, parasite, or other microbe • presence of that agent alone is not always sufficient to cause disease. • A variety of factors influence whether exposure to an organism will result in disease, including the organism's pathogenicity (ability to cause disease) and dose.
  20. 20. Host • refers to the human who can get the disease. • A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual's exposure, susceptibility, or response to a causative agent. • Opportunities for exposure are often influenced by behaviors such as sexual practices, hygiene, and other personal choices as well as by age and sex.
  21. 21. • Susceptibility and response to an agent are influenced by factors such as • genetic composition • nutritional • immunologic status • presence of disease or medications
  22. 22. Environment • refers to extrinsic factors that affect the agent • Climate change: Changes in temperature and rainfall can alter the survival, distribution, and behavior of insects and other species that can lead to changes in infectious diseases • socioeconomic factors such as crowding, sanitation • availability of health services.
  23. 23. • While the epidemiologic triad serves as a useful model for many diseases, it has proven inadequate for some • cardiovascular disease • cancer • other diseases that appear to have multiple contributing causes without a single necessary one
  24. 24. Component Cause (Causal Pies) • Because the agent-host-environment model did not work well for many non-infectious diseases, several other models that attempt to account for the multifactorial nature of causation have been proposed. • One such model was proposed by Rothman in 1976, and has come to be known as the Causal Pies.
  25. 25. • Most diseases are caused by the cumulative effect of multiple causal components acting (“interacting”) together. • Thus, a causal interaction occurs when two or causal factors act together to bring about an effect.
  26. 26. • Causal interactions applies to both infectious and noninfectious diseases • explains, for example, why two people exposed to the same cold virus will not necessarily experience the same outcome: one person may develop a cold while the other person may experience no ill effects.
  27. 27. • Rothman’s (1976) causal pies helps clarify the contribution of causal components in disease etiology. • Figure 1.17 displays 3 causal mechanism for a disease. • Let us assume these are the only 3 mechanisms that cause this ailment.
  28. 28. • Wedges of each pie represent components of each causal mechanism • corresponding to risk factors (we hope to identify and diminish in the population)
  29. 29. • Each pie represents a sufficient causal mechanism, defined as a set of factors that in combination makes disease occurrence inevitable. • Each casual component (wedge) plays an essential role in a given causal mechanism (pie), and a specific disease may result from a number of different causal mechanisms.
  30. 30. • A cause is said to be necessary when it is a component cause member of every sufficient mechanism. • In other words, the component cause is necessary if the disease cannot occur in its absence. • Component A is a necessary cause, since it is evident in all possible disease mechanisms, and the disease cannot occur in its absence.
  31. 31. • For example, the tubercular bacillus Mycobacterium tuberculosis is a necessary cause of tuberculosis • However, it is not sufficient by itself to cause disease • It is common for a person to harbor the Mycobacterium in their body while remaining disease-free.
  32. 32. • Some individuals are not susceptible to tuberculosis; they are resistant • There are complementary factor that encourage disease to manifest • Include familial exposure, immunosuppression, genetic susceptibility, poor nutrition, overcrowding, and high environmental loads of the agent
  33. 33. • A sufficient cause is not usually a single factor, but often comprises several components (multi-factorial causation) • it is not necessary to identify all the components of a sufficient cause before effective prevention can take place, since the removal of one component may interfere with the action of the others and thus prevent the disease or injury
  34. 34. • For example, cigarette smoking is one component of the sufficient cause of lung cancer • Smoking is not sufficient in itself to produce the disease: some people smoke for 50 years without developing lung cancer. • Other factors, are involved and genetic factors may play a role. However, the cessation of smoking reduces the number of cases of lung cancer greatly in a population even if the other component causes are not altered
  35. 35. • Each sufficient cause has a necessary cause as a component. • For example, in a study of an outbreak of foodborne infection, it may be found that chicken salad and creamy dessert were both sufficient causes of salmonella diarrhoea. • However, the ingestion of Salmonella bacteria is a necessary cause of this disease. • Similarly, there are different components in the causation of tuberculosis, but the infection with Mycobacterium tuberculosis is a necessary cause
  36. 36. • Causal components that do not occur in every sufficient mechanism yet are still essential for some of the causal mechanisms are said to be contributing component causes. • For example, cigarette smoking is a contributing but not necessary cause of lung cancer, since it contributes to the cause of the (vast majority) lung cancer, but is not necessary in every case
  37. 37. Recommended books • Epidemiology - Leon Gordis 5th edition • Basic epidemiology by R Bonita 2nd edition