Epidemiology- IMahmood ur Rahman

Definition of EpidemiologyEpidemiology is the study of the distribution and determinants of diseases or states in population groups and the application of this study to the control of health problems (Last JM ed. Dictionary of Epidemiology, Oxford University Press, 1995)

'Epidemiology is that field of medical science which is concerned with the relationship of various factors and conditions which determine the frequencies and distributions of an infectious process, a disease, or a physiologic state in a human community'

(Lilienfeld 1978)

Historical PerspectiveHippocrates - 5th centuryAssociation between external environment and personal characteristics and health

Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor. (Hippocrates, On airs, waters and places Medical Classics 3:19, 1938).

Historical PerspectiveJohn Graunt 1662 (Hennekins and Buring 1987)The Nature and Political Observations Made Upon the Bills of MortalitySystematic statistical approach Analyzed births and deaths in LondonExcess of males born, higher mortality for malesInfant mortality is very highSeasonal variation for mortalityImportance of routinely collected information for study of human illnessWilliam Farr - 1839Examined mortality and occupation and marital statusIdentified important issues in epidemiological investigations Use of comparison population, influence of multiple factors on disease

Historical PerspectiveJohn Snow (1854) Father of modern epidemiologyEstablished modern epidemiologic methodsCholera epidemic in LondonPlotted geographical location of all cases deaths from cholera

From The Visual Display of Quantitative Data, Edward R. Tufte

John Snow (cont)Went door to door, collecting information on daily habitsSuspected water supply as source of epidemicBroad street pump closed, epidemic stoppedMode of investigation shoe leatherPractical application of epidemiology use epidemiological investigation to impact a health problem

How the Epidemiologist WorksStudies origin and distribution of a health problemCollection of data Constructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to exist Likened to a detective investigating the scene of a crime looking for cluesStarts with examination of sick person(s)Extends investigation to the setting where illness is occurringLooks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled

Epidemiologic Analyses Areas of StudyCausal agents related to disease:Biological agents bacteria, viruses, insectsNutritional agents diet (fats, carbohydrates, food nutrients)Chemical agents gases, toxic agentsPhysical agents climate, vegetation, chemical pollutants (air, water, food)Social agents occupation, stress, social class, lifestyle, location of residence

Epidemiologist studies:

Host characteristics:Biological factorsAge, sex, degree of immunity, other physical attributes that promote resistance or susceptibilityBehavioral factors Habits, culture, lifestyleSocial environment Living conditions such as poverty, crowdingNorms, values and attitudesSocially prescribed standards of livingUse of food and water, food handling practicesHousehold and personal hygiene

Eras of EpidemiologySanitary era early 19th centuryInfectious disease era between late 19th century and early 20th centuryChronic disease era 2nd half of 20th centuryEco-epidemiology era 21st century

Comparing the job of a clinician and the job of an epidemiologistThe clinician Deals with patientsTakes a history Conducts a physicalMakes a diagnosis Proposes a treatmentFollows up the patientThe epidemiologist Deals with populationsFrames the questionInvestigatesDraws conclusions Gives recommendationsEvaluates programmes

Attack rate of acute hepatitis by age and sex, Baripada, Orissa, India, 2004Person

CasesPopulationAttack rate per 1000Age0-4110120.15-91121802210-143774004515-44416513588145+735615313SexMale3411026833.3Female1971016461.9

Uses of epidemiologyExamine causationStudy natural historyDescription of the health status of populationDetermine the relative importance of causes of illness, disability and deathEvaluation of interventionsIdentify risk factors

1. Examine causationGenetic factorsEnvironmental factors(Biological, chemical, physical, psychological factors)Good healthIll healthLife style related factors

To study historical rise and fall of disease in the population

The diseases wax and wane (tuberculosis ) new ones appear (encephalitis, asbestos poisoning, LSD psychosis) old ones are eradicated (smallpox)

Number of countries with smallpox, 1967-1978

A Pertussis by 5-Year Age Groups B. Pertussis by

Lung Cancer Rates in the United States, 19301999

Infant Mortality Rates for 2002, by Race and Ethnicity of Mother

Chipat riverAttack rate of acute hepatitis by zone of residence, Orissa, India, 20040 - 0.9 / 10001 - 9.9 / 100010 -19.9 / 100020+ / 1000Attack rate Place

Common Source (Vehicle), Single (Point) Exposure

Outbreak of Cholera, London, August-September , 1854

Common Source, Continued exposure :

Propogated source

Seasonal fluctuations :

Malaria and JE are commoner during immediate post monsoon season.

Asthma shows highest incidence during spring and autumn suggesting specific environmental factors in its causation

Secular trend

Searching for causes and risk factors

Theories in Primitive and Middle Ages Supernatural Causes e.g. being possessed by evil spirits, wrath of gods, punishment for evil deeds during previous births bad air Contagion Theory William Farr -Miasma TheoryGerm Theory - Henle-Koch postulatesEpidemiological Wheel TheoryThe Theory of Necessary and Sufficient cause

Role of the host, the agent and the environment in the occurrence of diseaseVECTORAGENTHOSTENVIRONMENTBiologic, Chemical, Physical (injury, trauma)Social PsychologicalGenotypeNutritionImmunityBehaviourSanitationWeatherPollutionSocio-CulturalPolitical

Causes of Tuberculosis

Syndrome identification

Epidemiologists are called lumpers and splittersThe differentiation of hepatitis A from hepatitis B and the distinction between several varieties of childhood leukemia. lumping include the identification of many manifestations of tuberculosis. Patterson-kelly syndrome of association between dysphagia and iron deficiency anaemia but when association was tested by epidemiological studies , it was not found.

Individual risk and chances: Life table-The risks the average male in England

Measures of disease frequencyPrevalenceNumber of cases of a disease in a defined population at specified point of time IncidenceNumber of new cases, episodes or events occurring over a defined period of time

Incidence rateNumber of people who getthe disease or condition in a specified timeTotal population at risk X FactorI =

Population at riskMenWomen0-25 years25-69 years70+ years25-69 yearsTotal populationAll women(age groups)Population at riskEg. Population at risk in a study of carcinoma of cervix

PrevalenceNumber of people with the disease or condition at a specified timeTotal population at risk X FactorP =

How to remember this: think about a bathtub

Community diagnosis :The definition of indicators is a pre-requisite Indicators - to estimate the burden of illness and the strategies for control.The main health indicators are expressed in terms of crude age-adjusted or age-specific mortality rates (such as infant mortality rates, mortality for children under 5, or maternal mortality rates), disease-specific morbidity rates, and life expectancy at birth.

3. Study natural history Good healthSub-clinical diseaseClinical diseaseRecoveryDeath

Natural history of disease timeline

4. Evaluation of interventionsGood HealthIll HealthTreatment, Medical careHealth promotionPreventive measuresPublic health services

Evaluation Evaluation is the process of determining, as systematically and objectively as possible, the relevance, effectiveness, efficiency, and impact of activities with respect to established goals.

Effectiveness refers to the ability of a program to produce the intended or expected results in the field; effectiveness differs from efficacy, which is the ability to produce results under ideal conditions. Efficiency refers to the ability of the program to reproduce the intended results with a minimum expenditure of time and resources.

Descriptive Epidemiology

Collecting all data describing occurrence of disease Person Disease predominates in smokers Disease predominates immuno-compromised individuals Place Disease is associated with an arid climate Disease is associated with a tropical climate Time Disease associated with the date of company picnic A particular year

Disease surveillanceFrequency with which the disease occurs Definition of the disease Size of the population from which the cases develop Completeness of the reporting of the cases.

Types of epidemiology

Distribution Frequency of health events By person, time and placeDescriptive epidemiologyDeterminant Search for causes or risk factors Response to a study hypothesis Use various epidemiologic methodsAnalytic epidemiologyHealth, disease or injuryDisease-specific epidemiologyApplication Applied epidemiology

Epidemiologic triadENVIRONMENTAGENTHOSTDISEASE

Factors Associated with Increased Risk of Human Disease

HOST (Intrinsic)AGENTSENVIRONMENT (Extrinsic)AgeGenderEthnicityReligionCustomsOccupationHeredityMarital statusFamily backgroundPrevious diseasesBiological (bacteria, etc.)Chemical (poison, alcohol, smoke)Physical (auto, radiation, fire)Nutritional (lack, excess)TemperatureHumidityAltitudeCrowdingHousingNeighborhoodWaterMilkFoodRadiationAir pollutionNoise

EPIDEMIOLOGYScientific basis of public healthMeasure of mortality and morbidityRatesRatioProportionRisk (Cumulative incidence) / Prevalence / Incidence

Ratio is one number divided by another number (numerator doesnt have to be included in denominatorand vice versa)Proportion is a ratio in which the numerator is included in the denominatorIt has no dimension because the unit of the numerator cancels out the unit of the denominatorRate is a ratioThe numerator is the number of events The unit is event (i.e., number of cases)The denominator is the sum of follow-up time contributed by the people at risk of the eventThe unit is time or, more accurately, person-time to account for duration of time of follow-up

*************Population at risk