Epidemiology

EPIDEMIOLOGY Crisbert I. Cualteros, M.D. http://crisbertcualteros.page.tl

DEFINITION Study of the distribution and determinants of health and disease among populations and the application of such study to the prevention and control of health problems. Determination of the nature, extent and determinants of disease or health problems among populations

COMPONENTS OF EPIDEMIOLOGY Descriptive Epidemiology study of the distribution of disease variables commonly examined are descriptive of person, place and time Analytic Epidemiology use of epidemiologic methods to explain disease occurrence or elucidate causal mechanisms

CHARACTERISTICS / FEATURES OF EPIDEMIOLOGY It is a quantitative science. It is an applied science. Its methods are generally observational. Its focus is the group or community of persons. Its methods are systematic and orderly.

DESCRIPTIVE EPIDEMIOLOGY I. Definition study the amount and distribution of disease within a population II . Uses evaluate trends in health and make comparisons health planning identify problems to be studied by analytical methods = hypothesis

DESCRIPTIVE EPIDEMIOLOGY III. Community Reaction to Disease Absence of disease No cases on current record Disease absent from the beginning or has been eradicated Sporadic Occurrence of few and unrelated cases Endemic Constant occurrence of disease Epidemic Occurrence of a number of cases of disease in excess of the normal expectancy

DESCRIPTIVE EPIDEMIOLOGY IV. Descriptive Variables A. Person 1. Age Different diseases show different age patterns Disease Characteristic Pattern of Magnitude Confers long lasting immunity decreasing with age Degenerative diseases or increasing with age w/ long latency Reflects low resistance high at extreme ages of young and old Reflects high exposure high in middle age groups during middle age

VARIABLES OF PERSON 2. Sex Difference in sexual constitutional, e.g. hormonal balance Greater exposure of males due to habits, recreation, occupation, lifestyle Greater health consciousness of females: Early consultation, diagnosis and treatment Better compliance with treatment More cases recorded – artifactual reason

VARIABLES OF PERSON 3. Civil Status Differences in lifestyle that are causally related to particular diseases Self-selection Concordance between marital partners Greater family support among the married 4. Socio-economic class – affects state of nutrition, level of health awareness or knowledge, etc 5. Genetics

DESCRIPTIVE VARIABLES B. Place 1. Variables Of Place geographic divisions physical environment climate, altitude, soil, vegetation, fauna biological environment: infectious disease agents, animal reservoirs, vectors socio-cultural-political-economic environment related to level of development

VARIABLES OF PLACE population characteristics density urban vs. rural mobility herd immunity

PLACE 2. Etiology of Disease Variations Real Causes - reflect true increase in risk deteriorating or improving environment quality, availability and distribution of medical care Artifactual Causes reliability of diagnosis completeness of reporting and recording of diseases, births and deaths

DESCRIPTIVE VARIABLES C. Time Temporal variations – changes / fluctuations in disease frequency with the passage of time 1. Types of Temporal Variations: Secular Trends Cyclic Fluctuations Short-term Irregular – e.g. Epidemics / outbreaks

TEMPORAL VARIATIONS Secular Trend Cyclic Fluctuations Measure of frequency Mortality rates Incidence Rates Nature of change Increase or Almost regular rises decrease Period of observation 10 years or hours, days, weeks, longer months, years (≤5) Type of Disease Chronic Acute

TIME 2. Reasons for Changes in Mortality Rates Artifactual or non-etiologic Error in the numerator guess diagnosis / misdiagnosis inaccurate counting change in the International Classification of Diseases Error in the denominator over or underestimate of the population

TIME Change in Case Fatality Rate, w/o change in Incidence Rate change in availability or utilization of health care services change in treatment modalities change in risk to superimposed infections

TIME Change in Incidence Rate, w/o change in Case Fatality Rate Artifactual or non-etiologic Real or etiologic change in disease agent change in herd resistance change in the environment

TIME 3. Types of Cyclic Fluctuations Cyclic Intrinsic Fluctuation change/s in the host change in herd resistance accumulation of susceptibles Cyclic Extrinsic Fluctuation (seasonal variation) change in the environment change in the disease agent

Analytic Epidemiology Definition study the determinants of disease or reasons for low and high frequency in specific groups employs Epidemiologic Methods : Definition of the problem Appraisal of existing facts Formulation of hypothesis Testing of hypothesis Conclusion and practical application

ANALYTIC EPIDEMIOLOGY II. Principal Uses Community diagnosis Investigation of epidemics Determination of disease etiology Evaluation of community intervention and programs

COMMUNITY DIAGNOSIS Definition of the Problem Determining the extent and magnitude of the problem using statistical indices Comparison of the statistical indices with those of other places and other diseases computation of economic burden of disease: cost of losses due to disability, death, treatment and prevention

COMMUNITY DIAGNOSIS II. Appraisal of Existing Facts Determining the: state of knowledge of disease or health problem etiology distribution of the disease/ problem in terms of person, place and time factors associated with the disease/ problem

COMMUNITY DIAGNOSIS III. Formulation of Hypothesis Explanations for the existence and magnitude of the disease / problem IV. Testing of Hypothesis V. Conclusion and Practical Solutions to the Problem

EPIDEMICS Definition the occurrence of any number of cases of a disease clearly in excess of the normal expectancy or what usually prevails

EPIDEMICS II. Causes of Epidemics flare up of an old or existing disease increased virulence of existing strain introduction of a new strain of the existing agent increased capacity to multiply decreased resistance of the population dilution of herd resistance with a susceptible population changes in the environment favoring disease transmission, e.g. calamities destroying health facilities, factors favoring survival and multiplication of vectors, changes in climate, temperature, etc.

Causes of Epidemics new disease introduction of a disease not previously present in the community disease previously affecting lower animals affecting man for the first time recognition for the first time of previously occurring disease known by another name

EPIDEMICS III. Classification of Epidemics according to: 1. Onset (of epidemic) explosive , abrupt, sudden – majority of cases occurring within one incubation period staggering , insidious, gradual 2. Exposure (of cases) mass or simultaneous – exposure occurred about the same time progressive – cases were exposed one after the other from a primary case

CLASSIFICATION OF EPIDEMICS 3. Transmission common vehicle – single or multiple exposure propagated contact-transmitted: person to person vector-transmitted

CLASSIFICATION OF EPIDEMICS 4. Epidemic Curve classical – short ascending, long descending limbs; water-borne inverted – long ascending, short descending limbs; vector-borne

Epidemic Curve bell-shaped – ascending and descending limbs about equal, peak is rounded; contact-transmitted point - ascending and descending limbs about equal, peak is pointed; food poisoning

EPIDEMICS IV. Termination of Epidemics eradication / killing of disease agents at the source or reservoirs interruption or closure of transmission exhaustion of susceptibles

EPIDEMICS V. Steps in the Investigation of Epidemics 1. Definition of the problem verify the diagnosis establish existence of an epidemic 2. Appraisal of existing facts characterize the distribution of cases by person, place and time

INVESTIGATION OF EPIDEMICS 3. Formulation of hypothesis as to source of infection, mode of transmission, factors that may have given rise to the epidemic 4. Testing of hypothesis conduct an epidemiological investigation (case control) 5. Conclusion and recommendations for control and prevention

DETERMINATION OF DISEASE ETIOLOGY I. Types of Epidemiologic Studies A. Descriptive Studies 1. Uses determination of distribution of disease according to person, place and time delineation of syndrome as a disease entity establishment of the natural history of disease classification of disease manifestational: pathologic and symptomatic experiential: based on similarity of experience

Descriptive Studies 2. Types Case report unit of study: single person with a disease limitation: based on experience of a single person provides first clues in the identification of a disease or adverse effects of exposure

TYPES OF DESCRIPTIVE STUDIES Case series unit of study: group of persons with a similar disease Uses: formulation of criteria for diagnosis formulation of indications for treatment identification of prognostic factors determination of survival rates

CASE SERIES Limitation limited generalizability because of unrepresentativeness of subjects and absence of comparison group

TYPES OF DESCRIPTIVE STUDIES Prevalence/Cross-sectional/ Surveys measures prevalence of disease or an event information about exposure and outcome are obtained simultaneously in a well-defined population

PREVALENCE/CROSS-SECTIONAL/ SURVEYS Uses determination of prevalence of risk factors determination of frequency of prevalent cases determination of health status and health needs formulation of hypothesis

PREVALENCE/CROSS-SECTIONAL/ SURVEYS Advantages quick and easy to perform Disadvantages temporality cannot be ascertained selects for longer-lasting and indolent cases

TYPES OF DESCRIPTIVE STUDIES Ecological Studies crude way of exploring relationship between environment or occupation and disease unit of study: populations or groups of people rather than individuals hypothesis generating rather than hypothesis testing

ECOLOGICAL STUDIES Advantage simple to conduct Disdavantage individual link between exposure and effect cannot be made (ecologic fallacy)

Types of Epidemiologic Studies B. Analytic Studies Use to determine whether a factor is causally associated with disease to test epidemiologic hypotheses 2. Categories observational/ non-experimental observes natural course of events case control study, cohort study, cross-sectional study

CATEGORIES OF ANALYTIC STUDIES Experimental/ Interventional exposure to the factor or treatment under study controlled by investigator randomized clinical trial (rct), community trial, laboratory trial

ANALYTIC STUDIES 3. Types Case-control Studies cases (with disease) and controls (no disease) are selected from a chosen population both are questioned or records are reviewed about presence or absence of a suspected cause/risk factor in the past

CASE-CONTROL STUDIES 1. Uses to test risk factors preferred if disease is rare preferred if several factors are associated with disease of interest

CASE-CONTROL STUDIES 2. Requirements for valid results Cases must be representative of all those with disease and clearly defined. Controls must be representative of all those without the disease and come from same community or source as the cases .

CASE-CONTROL STUDIES 3. Analysis Odd’s Ratio (OR) proportion of those with history of exposure to the factor among the cases (a/a+c) is compared to those with history of exposure (b/b+d) to the factor among the controls OR = ad/bc

ANALYSIS OF CASE CONTROL STUDIES Outcome (Disease) + - + a b Exposure (Factor) - c d a+c b+d * statistical association between factor and outcome exists if (a/a+c) ≠ (b/b+d) * association is probably causal , if OR > 1

CASE-CONTROL STUDIES 4. Advantages more economical smaller sample size required suitable for rare diseases suitable for diseases associated with multiple exposures

CASE-CONTROL STUDIES 5. Disadvantages more susceptible to bias of recall estimate of risk is indirect controls more difficult to assemble temporal relationship between factor and outcome cannot be ascertained

TYPES OF ANALYTIC STUDIES Cohort Studies groups of subjects are chosen on the basis of having been exposed to a factor or not groups are followed up to identify those who develop the disease or outcome

COHORT STUDIES Uses to test prognostic factors to directly measure risk of development of disease or outcome provide more definitive information about disease etiology preferred for study of rare exposures

COHORT STUDIES 2. Requirement for valid results Similarity of comparison groups 3. Types concurrent Subjects are free of disease or outcome of interest at the time of initiation of the study. Investigator follows-up the groups or cohorts from exposure to appearance of disease or outcome.

TYPES OF COHORT STUDIES . Non-concurrent Subjects who are free of the disease or outcome of interest at some point in the past are identified in terms of their exposure level. Disease or outcome status is determined through existing records. At the time the study is conducted, the specified follow-up period has elapsed.

COHORT STUDIES 4. Analysis Relative Risk or Risk Ratio(RR) proportion of subjects with the disease or outcome among the exposed (a/a+b) is compared to proportion of subjects with the disease or outcome among the unexposed (c/c+d) RR = a/a+b ÷ c/c+d

ANALYSIS OF COHORT STUDIES Outcome (Disease) + - + a b Exposure (Factor) - c d a+c b+d * statistical association between factor and outcome exists if (a/a+b) ≠ (c/c+d) * association is probably causal , if RR > 1

ANALYSIS OF COHORT STUDIES Attributable Risk (AR) estimate of the amount of risk that is attributable to the risk factor AR = a/(a+b) - c/(c+d)

COHORT STUDIES 5. Advantages provides direct estimate of risk temporality can be ascertained (for concurrent studies) less biases of recall and observation allows for determination of population-based rates controls easier to assemble variations in exposure can be followed-up unsuspected effects of the exposure may be observed

COHORT STUDIES 6. Disadvantages more expensive follow-up period may be long high attrition rate large sample size required change in exposure rates over long periods of time

COMPARISON OF CHARACTERISTICS OF CASE CONTROL AND COHORT STUDIES Case Control Cohort Starting population diseased group exposed group Control Group non-diseased unexposed Information Sought frequency of disease rate exposure to risk factor Principal bias knowledge of knowledge of disease influences exposure influences report of exposure diagnosis

COMPARISON OF CHARACTERISTICS OF CASE CONTROL AND COHORT STUDIES Case Control Cohort Time to Complete short usually long Study Measure of Odd’s Ratio Relative Risk Association

TYPES OF ANALYTIC STUDIES Experimental Studies Requirement for validity: complete comparability of comparison groups 1. Types Clinical Trial - Randomized Controlled Trial (RCT) investigator randomly places the subjects to one of the intervention groups ex. drug or surgical trials used if strong evidence for association already exists

TYPES OF EXPERIMENTAL STUDIES Field or community trials subjects are people in the general population who are disease-free but are presumed to be at risk ex. trials of preventive measures, e.g. immunization

FIELD OR COMMUNITY TRIALS Requirements high incidence of disease under study availability of facilities for observation accessibility of subjects availability of resources for precise diagnosis and follow-up

EXPERIMENTAL STUDIES 2. Analysis comparison of disease or outcome rate in experimental (P 1 ) = (a/a+c) and control groups (P 2 ) = (b/b+d)

ANALYSIS OF EXPERIMENTAL STUDIES Therapeutic / Preventive Measure + - + a b Disease/ Outcome - c d a+c b+d

ANALYSIS OF EXPERIMENTAL STUDIES Protective Value = P 2 – P 1 P 2

EXPERIMENTAL STUDIES 3. Advantage Provide the strongest evidence for testing hypothesis 4. Limitation ethical issues, especially for clinical trials

Determination of Disease Etiology II. Assessment of Results 1. determine if statistical association between factor and outcome occurs 2. if association exists, determine if due to: chance perform significance testing extraneous or confounding variables matching specification or restriction standardization of rates stratified analysis

Assessment of Results if association exists, determine if due to: causal relationship criteria: 1. measures of strength of association – OR, RR, Protective Value 2. temporality – exposure occurred prior to outcome 3. dose-response relationship

CRITERIA FOR CAUSAL ASSOCIATION 4. specificity – factor associated with only 1 or limited number of diseases 5. consistency of association – distribution of factor and disease is similar in different sub-groups 6. biologic plausibility – consistency with existing knowledge

Epidemiology

More Related Content

What's hot

Viewers also liked

Similar to Epidemiology

More from Crisbert Cualteros

Recently uploaded

Epidemiology