Introduction to Introduction to EpidemiologyEpidemiology

DefinitionDefinition Epidemiology: Epi (upon) + demos (people) + logy Epidemiology: Epi (upon) + demos (people) + logy

(study of)(study of) The study of the distribution of a disease or The study of the distribution of a disease or

condition in a population and the factors that condition in a population and the factors that

influence that distribution.influence that distribution.

Epidemiology is a discipline that describes, Epidemiology is a discipline that describes, quantifies, and postulates causal mechanisms for quantifies, and postulates causal mechanisms for health phenomena in populations. health phenomena in populations.

Epidemiology is concerned with the distribution and Epidemiology is concerned with the distribution and determinants of health and diseases, morbidity, determinants of health and diseases, morbidity, injuries, disability and mortality in populations.injuries, disability and mortality in populations.

Basic Research Basic Research TerminologyTerminology

Retrospective:Retrospective: Refers to time of Refers to time of data collectiondata collection

Case Control Study:Case Control Study: Persons w/ Persons w/ disease & those w/out are disease & those w/out are comparedcompared

Cohort Study:Cohort Study: Persons w/ and/or Persons w/ and/or w/out disease are followed over w/out disease are followed over timetime

Aims & LevelsAims & Levels

As the basic method of public health,As the basic method of public health,

epidemiology is concerned with efforts epidemiology is concerned with efforts to:to:

1.1. Describe the health status of the Describe the health status of the populationpopulation

2.2. Explain the etiology of diseaseExplain the etiology of disease

3.3. Predict the occurrence of disease Predict the occurrence of disease

4.4. Control the distribution of diseaseControl the distribution of disease

Terminology Terminology (Cont.)(Cont.)

Cross-sectional Study:Cross-sectional Study: Presence or Presence or absence of exposure to possible risk absence of exposure to possible risk factor measured at one point in time. factor measured at one point in time. Prevalence obtained.Prevalence obtained.

Prevalence:Prevalence: The # of new cases and The # of new cases and existing cases during specified time existing cases during specified time period.period.

Incidence:Incidence: The # of NEW cases per The # of NEW cases per unit of a population at risk for disease unit of a population at risk for disease occurring during stated time period.occurring during stated time period.

FoundationsFoundations

Human diseases do not occur by Human diseases do not occur by chancechance

Factors that cause or contribute to Factors that cause or contribute to diseases and injuries can be diseases and injuries can be identified by means of systematic identified by means of systematic investigationinvestigation

GoalsGoals

Improve the understanding of the Improve the understanding of the natural history of disease and the natural history of disease and the factors that influence its distributionfactors that influence its distribution

InterventionIntervention

Historical AntecedentsHistorical Antecedents

HippocratesHippocrates: Environment – disease : Environment – disease causationcausation

GrauntGraunt: Vital statistics in the mid 1600: Vital statistics in the mid 1600

SnowSnow: Natural experiments to track cholera : Natural experiments to track cholera outbreak in Londonoutbreak in London

KochKoch: Human disease - micro-organism / a : Human disease - micro-organism / a diseasedisease

Epidemiologic ApproachEpidemiologic Approach

QuantificationQuantification

Use of special vocabularyUse of special vocabulary

Interdisciplinary compositionInterdisciplinary composition

Epidemiologic Approach: Epidemiologic Approach: Special VocabularySpecial Vocabulary

Epidemic – Pandemic – EndemicEpidemic – Pandemic – Endemic

EpidemicEpidemic: Excessive occurrence of a : Excessive occurrence of a disease.disease.

EpidemicityEpidemicity: Relative to usual : Relative to usual frequency of the disease in the same frequency of the disease in the same area, among the specified population, area, among the specified population, at the same season of the year.at the same season of the year.

PandemicPandemic: Epidemic on a worldwide scale: Epidemic on a worldwide scale

EndemicEndemic: Characterize a disease that is : Characterize a disease that is habitatually present in a particular habitatually present in a particular geographical region. (Malaria and Cholera)geographical region. (Malaria and Cholera)

SurveillanceSurveillance: Systematic collection, : Systematic collection, analysis, interpretation, dissemination of analysis, interpretation, dissemination of data. data.

Descriptive Variables for the Descriptive Variables for the Health of the CommunityHealth of the Community

Demographic & Social VariablesDemographic & Social Variables Age and SexAge and Sex Socioeconomic StatusSocioeconomic Status Family structureFamily structure Racial, ethnic and religious compositionRacial, ethnic and religious composition

Variables related to community Variables related to community infrastructureinfrastructure

Availability of social and health servicesAvailability of social and health services Quality of housing stockQuality of housing stock Social stabilitySocial stability

Health Related VariablesHealth Related Variables Homicide and suicide ratesHomicide and suicide rates Infant Mortality RateInfant Mortality Rate Mortality from Specific ConditionMortality from Specific Condition Magnitude of chronic and infectious Magnitude of chronic and infectious

diseasesdiseases Alcoholism and drug abuse ratesAlcoholism and drug abuse rates Teenage pregnancy ratesTeenage pregnancy rates Birth RateBirth Rate

Population Dynamics and Population Dynamics and EpidemiologyEpidemiology

Three factors affect the size of populations: Three factors affect the size of populations:

BirthBirth

DeathDeath

MigrationMigration1.1. When is the Po in equilibrium?, When is the Po in equilibrium?,

2.2. What is a fixed Po?, What is a fixed Po?,

3.3. What is a dynamic Po?, What is a dynamic Po?,

4.4. When a Po is in steady state?When a Po is in steady state?

Epidemiologic Triad: Traditional Model Epidemiologic Triad: Traditional Model of Infectious Disease Causationof Infectious Disease Causation

The epidemiologic triangle recognized The epidemiologic triangle recognized three factors in the pathogenesis of three factors in the pathogenesis of disease: disease:

HostHost

Agent Environment

The AgentThe Agent Agent: must be present for an infection Agent: must be present for an infection

to occur: Microbial agents to occur: Microbial agents

Characteristics of Infectious disease Characteristics of Infectious disease agents:agents:

1.1. InfectivityInfectivity2.2. PathogenicityPathogenicity3.3. VirulanceVirulance4.4. ToxigenicityToxigenicity5.5. ResistanceResistance6.6. AntigenicityAntigenicity

Agent Agent Host Host After exposure: from sub-clinical infection After exposure: from sub-clinical infection

(inapparent) to active case of the disease. (inapparent) to active case of the disease.

End Result: Recovery, disability, End Result: Recovery, disability, disfigurement, death.disfigurement, death.

Ability to fight infections, comprises 2 Ability to fight infections, comprises 2 broad categories:broad categories:

1.1. Non-specific defense mechanismsNon-specific defense mechanisms2.2. Disease specific defense mechanismsDisease specific defense mechanisms

The EnvironmentThe Environment Domain in which the disease-causing agent may Domain in which the disease-causing agent may

exist, survive or originate. exist, survive or originate.

Acts as a reservoir or niche that fosters the Acts as a reservoir or niche that fosters the survival of infectious disease agents. The survival of infectious disease agents. The reservoir may be a part of the physical reservoir may be a part of the physical environment or may reside in animals or insects environment or may reside in animals or insects (vectors) or other human beings (human (vectors) or other human beings (human reservoir – host)reservoir – host)

External Environment: physical, biologic, social, External Environment: physical, biologic, social, economic componentseconomic components

Means of Transmission – Means of Transmission – Directly or Indirectly from Directly or Indirectly from

reservoirreservoirDirect TransmissionDirect Transmission: Spread of infection through : Spread of infection through

person to person contactperson to person contact

Portals of exitPortals of exit: sites where infectious agents : sites where infectious agents may leave the body (respiratory passages, the may leave the body (respiratory passages, the alimentary canal, the openings in the alimentary canal, the openings in the genitourinary system, and skin lesions. Also genitourinary system, and skin lesions. Also through insect bites, the drawing of blood, through insect bites, the drawing of blood, surgical procedures and accidents)surgical procedures and accidents)

Portal of entryPortal of entry: respiratory system (influenza, : respiratory system (influenza, cold), the mouth & digestive system (hepatits cold), the mouth & digestive system (hepatits A), mucous membranes or wounds in the skin. A), mucous membranes or wounds in the skin.

Indirect TransmissionIndirect Transmission: : through an intermediary sourcethrough an intermediary source

VehicleVehicle: contaminated H2O, infected : contaminated H2O, infected blood on used hypodermic needles.blood on used hypodermic needles.

FomitesFomites: inanimated objects: doorknob : inanimated objects: doorknob or clothing.or clothing.

Vectors:Vectors: animate, living insect or animal animate, living insect or animal that is involved with transmission of the that is involved with transmission of the disease agent.disease agent.

Some Measures Used in Some Measures Used in EpidemiologyEpidemiology

Counts:Counts: Number of individuals with a certain Number of individuals with a certain disease/exposuredisease/exposure

Proportions:Proportions: (P) (P) One’s status in relation to groupOne’s status in relation to group

Ratio:Ratio: A fraction in which there is not necessarily any A fraction in which there is not necessarily any specified relationship between the numerator & specified relationship between the numerator & denominator.denominator.

Rate:Rate: A ratio in which time forms part of the denominator A ratio in which time forms part of the denominator

Prevalence:Prevalence: # of existing cases of a disease / health # of existing cases of a disease / health condition in a Po at some designated time (specified or condition in a Po at some designated time (specified or not). not).

Infant Mortality RateInfant Mortality Rate: (IMR): (IMR) risk of dying during the first year risk of dying during the first year of life among infants born alive (aged 0-365 days)of life among infants born alive (aged 0-365 days)

Perinatal MRPerinatal MR:: measures risk of dying among late fetal death + measures risk of dying among late fetal death + infant deaths within the 7 days of birth. (28 weeks gestation to 7 infant deaths within the 7 days of birth. (28 weeks gestation to 7 days of life)days of life)

Noenatal MRNoenatal MR:: measures risk of dying among infants younger measures risk of dying among infants younger than 28 daysthan 28 days

Postneonatal MRPostneonatal MR:: measures risk of dying among older infants measures risk of dying among older infants (28 days to 12 months)(28 days to 12 months)

Incidence RateIncidence Rate: (IR): (IR) rate of development of rate of development of new casesnew cases

Crude Mortality Rate:Crude Mortality Rate: (CMR) (CMR)

# of annual death # of annual death PoPo

Maternal Mortality Rate:Maternal Mortality Rate: MMR number of MMR number of maternal death ascribed to childbirthmaternal death ascribed to childbirth

Age-Specific Mortality Rate:Age-Specific Mortality Rate: ASMR ASMR# deaths in specific group per total number of # deaths in specific group per total number of

people in that age grouppeople in that age group

Proportional Mortality Rate:Proportional Mortality Rate: PMR # of death PMR # of death within a Po due to a specific diseasewithin a Po due to a specific disease

Specialized Epidemiological Specialized Epidemiological MeasuresMeasures

1.1. Years of Potential Life LostYears of Potential Life Lost (YPLL) (YPLL) Measures # of years lost due to a causeMeasures # of years lost due to a cause It is based on projected life expectancyIt is based on projected life expectancy

2.2. Dependency RatioDependency Ratio (DR) (DR) Proportion of dependents in a society to Proportion of dependents in a society to

the number of adults (19-63)the number of adults (19-63)DR= DR= # under 18 and over 64# under 18 and over 64

# between 18 and 65# between 18 and 65