EPIDEMIOLOGY

EPIDEMIOLOGYDEFINITIONEPIDEMIOLOGY : The study of the distribution and determinants of health related states (or) Events in specified populations , and the application of this study to the control of health problems -John M.Last (1988)

Concept of disease:Germ theory of disease: The germ theory of disease is generally referred to as a one-to-one relationship or single cause idea between causal agent and disease. Disease Agent Man Disease

Multifactorial causation: Pettenkofer of munich (1819- 1901)

Epidemiological triadAgentEnvironmentHostWeb of causation for myocardial infarction: Changes in lifestyle stressAbundance of food Smoking Emotional disturbances obesity Lack of exercise Hypertension Aging&other factorHyperlipidemia catacholamine and thrombotic tendency Changes in walls of artery

Coronary atherosclerosis Coronary occlusion Myocardial ischemia Myocardial infarctionNATURAL HISTORY OF DISEASEPERIOD OF PRE-PATHOGENESISPERIOD OF PATHOGENESIS

Disease process Before man is involved The course of the disease in the man

In the human host interaction Host re Of host and stimuli

Early pathogenesis early lesions disease convalescenceAgentHostEnvironment( bring agent and host together)Levels of preventionAgent multiplicationTissue and physiological changesrecoverySigns and symptomillnessDisabilityDefect or chronic stageDeathPrimary preventionSecondary preventionTertiary preventionModes of interventionHeath promotion &specific protectionEarly diagnosis and treatment Disability limitation and treatment6Aims of epidemiology1.To describe the distribution and magnitude of health and disease problems in human populations.2.To identify aetiological factors (risk factors) in the pathogenesis of the diseases.3.To provide the data essential to the planning ,implementation and evaluation of services for the prevention,control and treatment of disease and to the setting up of priorities among those services

Uses of EpidemiologyTo study historically the rise and fall of disease in the populationCommunity diagnosisPlanning and evaluationEvluation of individuals risk and chancesSyndrome identificationCompleting the natural history of diseaseSearching for causes and risk factors8MEASUREMENTS IN EPIDEMIOLOGYMeasurement of mortalityMeasurement of morbidityMeasurement of disabilityMeasurement of natalityMeasurement of the presence, absence (or)distribution of the characteristics (or)attributes of the diseaseMeasurement of medical needs,health care facilities,utilization of health services and other health related eventsMeasurement of demographic variables

TOOLS OF MEASUREMENTRATE : The occurrence of some particular events in a population during a given period of time Death rate =Number of deaths in one year x 1000 Mid year population

RATIO : Measure of disease freguency in a ratio .it expresses a relation in size between two random guantities

The number of children with scabies at a certain time ------------------------------------------------------------- The number of children with malnutrition at a certain time

PROPORTIONThe number of children with scabies at a certainTime---------------------------------------------------------------x100The total number of children in the village at the same timeMethods of Epidemiology1.Observational studiesa. Descriptive studiesb. Analytical studies (i) Ecological or correational , with populations as unit of study (ii)cross sectional or prevalence, with individuals as unit of study (iii)case control or case reference with individuals as unit of study

(iv ) Cohort or follow up, with individuals as a unit of study2. Experimental studies intervention studiesa.Randomized controlled trails or clinical trails, patients as unit of studyb.Field trails or community intervention studies, with healthy people as unit of studyc.Community trails , with communities as unit of study

Descriptive EpidemiologyWhen is the disease occuring ? - Time distributionWhere is the disease occuring ?- Place distributionWho is getting the disease ?- person distribution

Procedures in Descriptive studiesDefining the population to be studied Defining the disease under studyDescribe the disease byTimePlacePersonMeasurement of diseaseComparing with known indicesFormulation of an aetiological hypothesisDescription of disease in terms of distribution:Time distributionWeeklyMonthlyYearSeasonduration

Place distributionInternational variationNational variationRural-urban variationLocal distribution

Person distributionAge,sex,occupation,Marital status,habits,social class,Stress,migration.behaviorShort term fluctuationSeasonal trendCyclic trendPeriodic fluctuation Long term fluctuationSlow epidemicPropagated epidemicCommon source epidemicSingle exposureRepeated exposureTypes of EpidemicsThree major types of epidemicsCommon sourse epidemicsSingle exposure or point sourse epidemicsContinuous or multiple exposure epidemics.Propagated epidemics Person to person Arthropod vectorAnimal reservoir C. Slow (modern) epidemicsAnalytical EpidemiologyCASE CONTROL STUDYFactor CasesPresent (or) ControlsAbsent

PROSPECTIVE ( cohort) STUDYIndividual exposed to particular factor

Individual unexposed to particular factor

Individual with particular disease

Individual without particular disease

Presence or absence of particular disease

TIMECASE CONTROL STUDYThe case control method has three distinct features:

Both exposure and outcome have occurred before the start of the studyThe study proceeds backwards from effect to cause and It uses a control or comparison groups to support or refute an inference.

BASIC STEPS

Selection of cases and controlsMatchingMeasurement of exposure Analysis and interpretation

Elements of a Cohort studyThe elements of a cohort study are: Selection of study subjectsObtaining data on exposureSelection of comparison groups Follow upAnalysis

Relative RiskIncidence of disease (or death) among exposedRR = ________________________________Incidence of disease (or death) among non - exposedAttributable RiskIncidence of disease rate among exposedminus incidence of disease rate among non -exposed= ------------------------------------------------------------------------ x 100 Incidence rate among exposedExperimental EpidemiologyTwo types are:Randomized controlled trailsNon- randomized trails

Randomized controlled trails

The basic steps in conducting a RCT include the following:

Drawing up a protocolSelecting reference and experimental populationRandomization Manipulation or intervention Follow upAssessment of outcome.Select suitable population(Reference or target population) Select suitable Sample(Experimental or study population) Make necessary exclusionsRANDOMIZE Experimental group Control group Manipulation & follow -up Assessment Those not eligibleThose who do not wish to give consentDESIGN OF RANDOMIZED CONTROLLED TRIALMEASUREMENT OF MORBIDITY :INCIDENCE RATEPREVALENCE RATE

INCIDENCE RATE:

Incidence rate refers to frequency of new cases of specific disease during a given period in a defined population(population at risk ) during that period .= No.of new cases of specific disease during a given time period x1000 Population at risk during that period

Prevalence rate:Point prevalencePeriod prevalencePoint prevalence : It is the number of all current new and old cases of a disease at one point of time in relation to a defined population Number of current new and old cases of a specified disease existing at a given point of time x100

Estimated population at the same point of time Period prevalence: It is the frequency of all current new & old cases during a defined period of time in relation to a defined population .= No.of current new& old cases of a specified disease during a given period of time interval x 100

Estimated mid- interval population at risk

MEASUREMENT OF MORTALITY:1.Crude Death Rate: It measures the frequency of deaths occuring from various causes in a given population during a specified period .CDR = No.of deaths during the year x 1000 Mid year population2.Specific Death Rates:It can be a. Disease specific b. Group specificDisease specific : =No.of deaths from cancer during a calender year x1000 Mid year populationGroup specific: =No.of deaths among females during a calender year x1000 Mid year population of femalesSpecific death rate in a age group of 15-45 years. =No.of deaths of persons aged 15-45 years during a calender year x1000 Mid year population of persons aged 15- 45 years

AgeSex Dynamics of disease transmission: SourceOrreservoirMoSusceptiblehostModes OftransmissionSource: The person,animal,object or substance from which an infectious agent passes or is disseminated to the host.Reservoir : Any person,animal,arthropod,plant,soil or substance in which an infectious agent lives and multiplies,on which it depends primarily for survival ,and where it reproduces itself in such manner that it can be transmitted to a susceptible host .

Source or reservoir Human reservoirAnimal reservoirReservoir in non- living things

Human reservoir

cases - A person in the population or study group identified as having the particular disease,health disorder or condition under investigation

1.Clinical case:Mild or moderate,typical or atypical ,severe or fatal2.Subclinical cases: Inapparent, covert,missed,abortive3.Latent cases : Have infectious agent,dont show symptoms,the infectious agent lies dormant.

casescarriersClinical casesSub clinical caseslatentInvestigator:Primary case- It is the first case of a communicable disease introduced into the population being studied.Secondary case Those cases which develop from the contact with primary case .Index case This term refers to first case which has come in attention of investigator.

Carriers - An infected person or animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others.Carriers may be classified as below ,Incubatory Those who shed the infectious agent during the incubation period of diseaseEx : measles,mumps,polio

2.Convalescent carriers To shed the disease agent during the period of convalescence.Ex: Typhoid fever ,cholera,diptheria3.Healthy carriers A person whose infection remains subclinical may or may not be a carrier.Duration :Temporary carriers Those who shed the infectious agent for short periods of time .Chronic carriers - A chronic carrier is one who excretes the infectious agent for indefinite periods .Portal of Exit :UrinaryIntestinalRespiratoryOthersMode of transmission:1.Direct transmission:Direct contactDroplet infectionContact with soilInoculation into skin or mucosaTransplacental(vertical )2.Indirect transmission :Vehicle borneVector borne

Air borneMechanicalbiologicaldropletdustFomite borneUnclean hands and fingers 1.Direct transmission:Direct contact: skin to skin,mucosa to mucosa or mucosa to skin of the same person or another.Droplet infection: Direct projection of spray of saliva or naso pharyngeal secretions Contact with soil:Ex: hook worm infestations,tetanus etcInoculation into skin or mucosa: ex: rabies virus by dog bite,heptitis B by contaminated needles

Transplacental transmission:Ex:TORCH(Toxoplasma gondi,rubella virus, cyto megalo virus, herpes virus)2.Indirect transmission:( 5 Fs flies,fingers,fomites,food,fluid)a.Vehicle born Transmission of infectious agent through water,food,raw vegetables,fruits,milk,ice,blood,serumb.Vector born (Arthropod, vertebrate, non-vertebrate)Invertebrate type: (Arthropod) Flies and mosquitoes,fleas,cockroaches,sucking lice,bugs,ticks,mites,cyclops.)vertebrate type:Mice,rodents,bats

By transmission chain:A . Man and a non-vertebrate host:Man-arthropod-man (malaria)Man-snail-man (schistosomiasis)B . Man, vertebrate host ,and a non vertebrate host:Mammal arthropod man(plaque)Bird arthropod man(encephalitis)c. Man and 2 intermediate hosts :Man cyclops fish Man (fish tape worm)By methods in which vectors transmit agent: Biting, regurgitation, scratching in of infective faeces, contamination of host with body fluids of vectors

Airborne : Droplet,dust(1 -10 micron range)Fomite borne :other than water or food (soiled clothes,towels,linen,glasses etc)Unclean hands and fingersIII.The vector born transmission on the basis of involvement of vector and propagation of parasites,is of 2 types,

Mechanical transmission: The infectious agent doesnot develop or multiply on or within the vector and is transmitted mechanically by arthropods through soiling of its feet.

Biological transmission: The infectious agent multiply or replicate or both occur in the vector before transmission.

Biological transmission is classified into 3 types,

CHARACTERISTICSPropagativeCyclo propagativeCyclodevelopmentalMULTIPLICATION

yesyesNoDEVELOPMENT NoyesyesEXAMPLEPlaque bacilli in rat fleaMalaria prasitesMicrofilaria in mosquito

Epidemiology : Terminologies1.Infection : The entry and development or multiplication of an infectious agent in the body of man or animals .2.Contamination : The presence of an infectious agent on a body surface ,also on or in clothes, beddings ,toys , surgicl instruments or inanimate articles or substances including water, milk ,food.

3.Infestation: For persons or animals the lodgement ,,development and reproduction of arthropods on the surface of the body or in the clothing .Ex :lice, itch mite Host : A person or animal, including birds and arthropods ,that affords subsistence or lodgement to an infectious agent under natural conditions .Obligate host - the only host Ex : man in measles ,typhoidDefinite host or primary - Host in which the parasite attains maturity or passes its sexual stage .Intermediate or secondary - Larval or Asexual stage.Transport host - Carrier

Infectious disease : A clinically manifest disease of man or animals resulting from an infection .Contagious disease : A disease that is transmitted through contact.Ex: Scabies,trachomaCommunicable Disease : An illness due to a specific infectious agent or its toxic products capable of being directly or indirectly transmitted from man to man ,animal to animal or from the environment( air,dust,soil).Epidemic: Epi = upon , demos = people,the unusual occurrence in a community or region of disease ,specific health related behaviour or health related events clearly in excess of expected occurrence.Endemic : En = in ,demos = people ,It refers to the constant presence of a disease or infectious agent within a given geographic area or population group .Sporadic : The word sporadic means scattered about ,the cases occur irregularly ,haphazardly from time to time ,and generally infrequently . Ex :polioPandemic: An epidemic usually affecting a large proportion of the population Ex : Section of a nation, the entire nation Ex :chleora. Exotic: Diseases which are imported into a country in which they do not other wise occur.Ex: rabiesZoonoses : An infection or infectious disease transmissible under natural conditions from vertebrate animals to man .Ex: rabies ,anthraxEpizootic : An outbreak(epidemic) of disease in an animal population .Ex: Anthrax, JE

Epizootic : An outbreak (epidemic) of disease in an animal population .:Ex: influenzaEpornithic : An outbreak (epidemic) of disease in a bird population.Enzootic : An endemic occuring in animals Ex: anthraxNoso comial Infection: Nosocomial (hospital acquired )infection is an infection originating in a patient while in a hospital or other health care facility.Opportunistic Infection: This is infection by an organism that takes the opportunity provided by a defect in host defence to infect the host and hence cause disease.Ex:AIDSIatrogenic (physician Induced)disease: Any untoward or adverse consequence of a preventive ,diagnostic or therapeutic regimen or procedure ,that causes impairment ,handicap, disability or death resulting from a physicians professional activity.