11 Concept Of Cause

Concept of cause Alick Mwambungu [email protected] Session 11 27/03/09

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Cause • An understanding of the cause of disease is important in the health field not only for prevention but also in diagnosis and the application of correct treatment. • A cause of a disease is an event,condition,characteristic or a combination of these factors which plays an important role in producing the disease. 27/03/09

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Causality • We are interested in finding the causes of diseases because we want to be able to intervene to prevent disease from occurring. • The ideas about why, where and how are influenced by concepts of diseases and the wider frame of reference in which we operate. 27/03/09

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Causality • Logically a cause must precede a disease. • A cause is termed sufficient when it inevitably produces or initiates a disease • termed necessary if a disease can not develop in its absence. • A sufficient cause is not usually a single factor, but often comprises several components. 27/03/09

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Types of Causes Causes that act together are classified as necessary and sufficient.

• Necessary cause–a factor found in all cases • Sufficient cause– a combination of factors that makes disease inevitable 27/03/09

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• 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 yrs without developing lung cancer; other factors mostly unknown are required. • However, the cessation of smoking reduces the number of cases of lung cancer in a population even if the other component are not altered. 27/03/09

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• Each sufficient cause has a necessary cause as a component eg.In a study of an outbreak of food borne infection, it may be found that chicken salad and creamy dessert were both sufficient causes of salmonella diarrhoea. • The occurrence of salmonella is the necessary cause of this disease. • Similarly, there are different components in the causation of tuberculosis, but the tubercle bacillus is a necessary cause. 27/03/09

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• A single component cause is rarely a sufficient cause by itself. • For example, even exposure to a highly infectious agent such as measles virus does not invariably result in measles disease-the host must be susceptible; other host factors may also play a role.

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• An agent which rarely causes disease in healthy persons may be pathogenic when other conditions are right. • Pneumocystits carinii is one such organism, harmlessly colonizing some healthy persons but causing potentially lethal pneumonia in persons whose immune systems have been weakened by HIV. • Presence of Pneumocystitis carinii organisms is therefore a necessary but not sufficient cause of pneumocystis pneumonia. 27/03/09

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Factors in causation • Three types of factor play a part in causation of disease. All may be necessary but they are rarely sufficient to cause a particular disease or state. • Predisposing factor :sex,age,previous illness • Enabling factors: Low income, poor nutrition, bad housing, inadequate medical care. • Precipitating factors: Exposure to a specific agent or noxious agent. 27/03/09

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Establishing the cause of diseases • Causal inference: This is the term used for the process of determining whether observed associations are likely to be causal.

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Interaction • The effect of two or more causes acting together is often greater than would be expected on the basis of summing the individual effects: Lung cancer=Smoking + Asbestos exposure.

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Sanitary movement • Cause: a combination of unfavourable living conditions thought to lead to the general burden of disease. • Where: factors operating at community level were the causes of disease. • How: Documented mortality rates in different communities and found many associations between mortality rates and these population based variables. • They took these associations to be proof of causation. 27/03/09

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Germ theory • Cause: A single specific agent that inevitably produced specific disease outcome-physical entity, that could be isolated in lab • Where: Efforts at disease prevention were concentrated at the individual level. • How:- the agent must be found in every case -the agent must not be found in other disease. -should be able to reproduce disease in experimental animals. 27/03/09

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• Pasteur's work on microorganisms led to the formulation of the following rules by Koch for determining whether a specific living organism causes a particular disease.

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Koch’s Postulates

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Multi-causality • Cause: any factor that plays an essential role in producing an occurrence of diseases i.e. a number of factors act together. • Where: there are no upper or lower limits to the level at which we could search for causes. • How: find association between exposure and outcome. • Decide whether the association is valid considering alternative explanations. 27/03/09

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Multi-causality • We do not make judgement about causality on the basis of one association • We need to take into account other evidence from a number of different sources and the coherence of the association with existing theory and knowledge.

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Hills Criteria of Causation • Outlines the minimal conditions needed to establish a causal relationship between two items. • These criteria were originally presented by Austin Bradford Hill (1897-1991). • A British medical statistician • As a way of determining the causal link between a specific factor (e.g. cigarette smoking) and a disease (such as emphysema or lung cancer). 27/03/09

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• Hill's Criteria form the basis of modern epidemiological research, which attempts to establish scientifically valid causal connections between potential disease agents and the many diseases that afflict humankind.

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Sir Austin Bradford Hill(1897-1991)

-Temporality -Strength -Consistency -Dose-response relationship – Specificity – Biological plausibility – Coherence – Reversibility (Experimental) – Analogy 27/03/09

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1.Temporality or Time Sequence • For judgement of causality to be reasonable, the exposure of interest should precede the outcome by a period of time consistent with biologic mechanism

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Bradford Hill Criteria • Temporality Exposure must precede outcome

Exposure

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Time

Outcome

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2.Strength • The strength of an association is measured by risk ratio, or odds ratio. • The stronger the association the more likely it is to be causal.

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3.Consistency • Cause-effect relationship can be supported if the outcome is consistency. • The outcome has been supported by a number of studies conducted by different investigators at various times using various epidemiological study designs (cross-section, case-control or cohort studies) among different populations and cultural setting. • Have similar results been shown in other studies? 27/03/09

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4.Biological gradient • Risk of an outcome increases with an increase in the dose of exposure (doseresponse relationship). e.g. the more cigarettes smoked per day, the higher the risk of developing lung cancer.

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5.Specificity • This is established when a single putative cause produces a specific effect. • This is considered by some to be the weakest of all the criteria. • The diseases attributed to cigarette smoking, for example, do not meet this criteria. • When specificity of an association is found, it provides additional support for a causal relationship. • However, absence of specificity in no way negates a causal relationship. 27/03/09

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6.Biological Plausibility • Postulated biologic mechanism by which the exposure might reasonably alter the risk of developing the disease. • For example the belief that the daily consumption of small to moderate amounts of alcohol reduces the risk of developing CHD. • This is enhanced by the fact that there is a plausible biologic mechanism that alcohol raises high-density lipoprotein (HDL) Cholesterol. • Increased levels of which decrease risk of CHD. 27/03/09

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7.Coherence • A causal relationship is more likely if it does not conflict with current knowledge about natural history and biology of disease.

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8.Experimental(Reversibility) • Causality is supported if removal of the exposure leads to a reduction in the risk of the outcome. • Provides very strong evidence in favour of causal relationship.

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9.Analogy • If we already know that a particular exposure causes a specific outcome, then we are more likely to accept that a similar exposure is the cause for a similar outcome. • when one class of causal agents is known to have produced an effect, it can easily be accepted that another agent of that class produces a similar effect. 27/03/09

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• Human papilloma virus causes Cervical cancer. • Other DNA tumour viruses can induce cancers in humans, and species-specific papillomaviruses can induce cancers in animals.

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