OLUWADARE T DEPARTMENTS OF COMMUNITY MEDICINE AND PUBLIC HEALTH Introduction What is health What is a disease disease state Concept of dx causation A cause of a disease is an event condition characteristic or combination of these factors which plays an important role in producing the ID: 998369
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1. CONCEPT OF DISEASE CAUSATION AND MEASURES OF DISEASE FRQUENCYOLUWADARE, TDEPARTMENTS OF COMMUNITY MEDICINE AND PUBLIC HEALTH
2. IntroductionWhat is health?What is a disease/ disease state?
3. Concept of dx causationA cause of a disease is an event, condition, characteristic, or combination of these factors which plays an important role in producing the disease.A cause could be sufficient or necessary
4. Concept of dx causation contd.Why study causation?It deepens our understanding of diseases and how they are causedIt provides entry point(s) for prevention, control and treatmentIt contributes to reduction in morbidity and mortalityIt is helpful in diagnosis and the application of treatment
5. Concept of dx causation contd. To infer causation, the following conditions must have been met:The study has an adequate sample sizeThe study is free of biasAdjustment for possible confounders has been doneThere is an association between exposure of interest and the disease outcome
6. Concept of dx causation contd.Causal relationshipMervyn Susser proposes that in epidemiology, a causal relationship has the following attributesAssociation;Time order; andDirection
7. Concept of dx causation contd.Causal relationshipAssociation: A cause must be associated with the effectTime order: The cause must be present before or at least at the same time as its effect (OR, RR)Direction: Risk of disease increases or decreases as the dose of risk factor increases or decreases (i.e. dose-response relationship).
8. Concept of dx causation contd.Causal inferenceCausal inference is the term used for the process of determining whether observed associations are likely to be causal
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11. Concept of dx causation contd.Factors in causation Predisposing factorsThey create a state of susceptibility to a disease agent e.g. age, sex, family history. These may have no direct bearing on the cause of the disease, but they aid other risk factorsEnabling factorsEnvironmental conditions which favourthe development of disease e.g. poor nutrition, low income, inadequate medical facility
12. Concept of dx causation contd.Factors in causation Precipitating factorsSpecific agent or exposure to which can be associated with the onset of the a disease e.g. pollens in asthmatic attackReinforcing factorsThey aggravate an already established disease, state or condition e.g. repeated exposure
13. Concept of dx causation contd.Germ theory of diseaseThe germ theory of disease states that some diseases are caused by microorganisms.
14. Concept of dx causation contd.Web of causation theoryDisease never depends upon single isolated cause rather it develops from a chain of causation in which each link itself is a result of complex interaction of preceding events .This chain of causation which may be the fraction of the whole complex is known as web of causation.
15. Concept of dx causation contd.Epidemiological triadEnvironmentAgentHost
16. Concept of dx causation contd.Epidemiological triadIt is the traditional model for infectious diseasesAgentHost Environment
17. Concept of dx causation contd.Multifactorial causationThis theory helps to understand the various associated causative factors, which suggests preventive and plan measures to control the disease.Risk factor is a characteristic (socio-demographic, behavioral, lifestyle, environmental) which based on epidemiologic evidence is known to be associated with health related condition or diseases
18. Concept of dx causation contd.Multifactorial causationRothman’s causal piesThe individual factor that contributes to cause disease is shown as a piece of a pieAfter all the pieces of the pie fall into place, the disease occurs. The individual factors are called component causes.The complete pie, might be considered a causal pathway called sufficient cause
19. Concept of dx causation contd.Multifactorial causationRothman’s causal piesSufficient causeA disease may have more than one sufficient causeIt inevitably/certainly produces or initiates a diseaseIt is not usually a single factor, but often comprises several components. E.g. cigarette smoking is one component of the sufficient cause in lung cancer
20. Concept of dx causation contd.Sufficient causeIt 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.
21. Concept of dx causation contd.Rothman’s causal piesA component that appears in every pie or pathway s called a necessary causeNecessary causeThe disease cannot develop in its absence e.g Mycobacterium tuberculosis in TBEach sufficient cause has a necessary cause as a component.
22. EADCBHABFGRothman’s causal piesSufficient cause ISufficient cause II
23. Concept of dx causation contd.Unifactorial theoryThis theory states that is one single specific cause of every disease.
24. Concept of dx causation contd.Koch’s postulateThe organism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.The organism must be isolated from a diseased organism and grown in pure culture.
25. Concept of dx causation contd.Koch’s postulate contd.The cultured organism should cause disease when introduced into a healthy organism.The organism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
26. Concept of dx causation contd.Limitation of Koch’s postulateThe causative organism may disappear when the disease developsCertain microorganisms cannot (at the present time) be grown in pure culture
27. Concept of dx causation contd.Limitation of Koch’s postulateNot all organisms exposed to infectious agent will acquire the infection.Not all diseases are caused by biological agents
28. Measures of disease frequencyThe basic measure of disease frequency are rates, ratio and proportion.Rates are essential for comparing disease frequency in different populations or subgroups of the same population in relation to suspected causal factors.
29. Measures of disease frequency contd.Measurements in epidemiologyMeasurement of mortalityMeasurement of morbidityMeasurement of disabilityMeasurement of natalityMeasurement of demographic variables
30. Measures of disease frequency contd.Measurements in epidemiology Measurement of the presence, absence or distribution of the characteristic or attributes of the diseaseMeasurement of medical needs, health care facilities, utilization of health services and other health-related eventsMeasurement of the presence, absence or distribution of the environmental and other factors suspected of causing the disease
31. Measures of disease frequency contd.Basic requirements of measurements areValidityReliability AccuracySensitivity Specificity
32. Measures of disease frequency contd.Rate It measures the occurrence of some particular event in a population during a given time period. It is a statement of the risk of developing a condition. It indicates the change in some event that takes place in a population over a period of time.Death rate= Number of deaths in one year x 1000Mid-year population
33. Measures of disease frequency contd.Categories of rates are :Crude rates: These are the actual observed rates such as the birth and death rates. Crude rates are also known as unstandardized rates.Specific rates: These are the actual observed rates due to specific causes ; or occurring in specific groups or during specific time periods.Standardized rates: These are obtained by direct or indirect method of standardization or adjustment, e.g., age and sex standardized rates
34. Measures of disease frequency contd.RatioIt expresses a relation in size between two random quantities.The numerator is not a component of the denominator. The numerator and denominator may involve an interval of time or may be instantaneous in time. Broadly, ratio is the result of dividing one quantity by another.
35. Measures of disease frequency contd.RatioIt is expressed in the form of: z:y or z yExample A: A city of 4,000,000 persons has 500 clinics. Calculate the ratio of clinics per person.500 ⁄ 4,000,000 = 1 clinics per 8000 persons
36. Measures of disease frequency contd.RatioExample B: Delaware’s infant mortality rate in 2001 was 10.7 per 1,000 live births. New Hampshire’s infant mortality rate in 2001 was 3.8 per 1,000 live births. Calculate the ratio of the infant mortality rate in Delaware to that in New Hampshire.10.7 ⁄ 3.8 × 1 = 2.8:1Thus, Delaware’s infant mortality rate was 2.8 times as high as New Hampshire’s
37. Measures of disease frequency contd.Proportion It is a ratio which indicates the relation in magnitude of a part of the whole. The numerator is always included in the denominator.A proportion is usually expressed as a percentage.The number of children with scabies at a certain timeExample A: Number of children with scabies at a certain time x 100 The total number of children in the village at the same time
38. Measures of disease frequency contd.Example BCalculate the proportion of deaths among men.Numerator = deaths in men= 100 deaths in diabetic men + 811 deaths in nondiabetic men= 911 deaths in menNotice that the numerator (911 deaths in men) is a subset of the denominator.Denominator = all deaths= 911 deaths in men + 72 deaths in diabetic women + 511 deaths in nondiabetic women= 1,494 deathsProportion = 911 ⁄ 1,494 = 60.98% = 61%
39. Measures of disease frequency contd.Morbidity ratesThere are three aspects of morbidity namely, the frequency, duration and severityIncidence ratePrevalence rate
40. Measures of disease frequency contd.Morbidity rateincidence rateIt is the number of new cases occurring in a defined population during a specific period of timeThey are used for acute conditionsincidence= Number of new cases of specific disease during a given time period X 1000Population at-risk during that period It is always expressed as follows 32.8 per 1000 per year
41. Measures of disease frequency contd.Morbidity rateIncidence rateSpecial incidence ratesAttack rateIt is used only when the population is exposed to risk for a limited period of time such as during an epidemic Attack rate = Number of new cases of specific disease during a specified time interval X 100Total Population at-risk during that same time interval Secondary attack rateNumber of exposed persons developing the disease within the range of the incubation period X 100Total number of exposed/ susceptible contacts
42. Measures of disease frequency contd.Morbidity rateIncidence rateUsesTo control diseasesTo research into aetiology and pathogenesis, distribution of disease and efficacy of preventive and therapeutic measures.
43. Measures of disease frequency contd.Morbidity ratePrevalence ratePrevalence rate= Total number of people who have an attribute/ disease during at a given period of time Population at-risk of having the attribute/disease at this point in time/ midway through the period
44. Measures of disease frequency contd.Morbidity rateTypes of prevalence ratePoint prevalence Period prevalenceUses It helps to estimate the magnitude of health/disease problems in the community and identify potential health risksThey are useful for administrative planning purposes
45. Measures of disease frequency contd.Mortality rates and ratio Crude death rateSpecific death rateCase fatality rateSurvival rateAdjusted and standardized rates
46. Assignment Mortality rates and ratioCrude death rate……Specific death rate…Case fatality rate…..Survival rate…….Adjusted and standardized rates..The formulas for point and period prevalence….The relationship between Prevalence and incidence.
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