Introduction to Epidemiology

Learning Objectives

At the end of this session, the resident should be able to:

♦ Define epidemiology♦ Describe the history of epidemiology♦ List the uses of epidemiology♦ Describe the basic epidemiologic methods♦ Describe the tasks of the epidemiologist

Epidemiology is …

♦ For a student– “the worst taught course in Medical school”

♦ For a clinician– “The science of making the obvious obscure”

♦ For the statistician– “The science of long division”

♦ For the average person– “ The study of skin diseases”

♦ And for you ?

The Etymology of Epidemiology

epi = upon demos = people logy = study of

Fundamental axioms (assumptions)

♦ Diseases (or other health events) do not occur at random– They occur in specific population groups exposed in a

particular way (at risk)

♦ Diseases (or other health events) have causal and preventive factors that can be identified– hence researchers investigate these factors and apply

the knowledge to control disease

Definition of epidemiology

« The study of the distribution and determinants

of health related states or events in specified populations

and the application of this studyto the control of health problems. »

Last, 1988

Key words:

♦ Study Basic science

♦ Distribution Time, place, person

♦ Determinants Cause, risk factors

♦ Event / Health status

♦ Population Public health

♦ Application Information for action

Distribution: Descriptive Epidemiology

♦ What, Who, When, and Where

♦ Frequency: – Number, rates, and risk– Quantify diseases to determine magnitude

♦ Patterns: – Time, Place, and Person

Determinants: Analytic Epidemiology

♦ Why and How

♦ Causes and influences

♦ Evidence for control and prevention

♦ Compare between exposure groups to

obtain causal relationships

Health related events

♦ Epidemic Communicable Disease♦ Endemic Communicable Disease♦ Non-communicable Disease♦ Chronic Diseases♦ Injuries♦ MCH♦ Occupational, and Environmental Health♦ Health Behaviors

Clinic vs. Epidemiology

Clinician♦ Person♦ Medical history,

physical examination♦ Differential diagnosis ♦ Diagnostic test ♦ Treatment

Epidemiologist♦ Population ♦ Surveillance

descriptive epidemiology

♦ Comparison♦ Analytical epidemioogy ♦ Intervention

(prevention/control)

History of epidemiology

Origin of epidemiologic concepts500 BC when Hippocrates wrote his book“On Airs, Waters and Places”

The chapter on environmental influences on health was about the following risk factors:

• the hot (weather)• the cold (weather)• the winds (weather)• the water quality (contamination)• drinking and eating habits (behaviour)• indolence• exercise and labour

Therefore, disease does not just happen

Classical theories of disease causation

Miasma theory 1700sbased on notion that bad air causes illnessmalaria = mal aires (bad air)

Contangion Vivum theory 1800sinvolves a living contangion each disease has a specific cause.

two examples strengthened the theoryMarseilles plague in 1720muscardine in 1816 (fungal silkworm disease)

Timeline

400 B.C. Hippocrates (environment)1546 Fracastoro (sci. of microbiology)1662 Graunt, John (Life tables))1747 Lind (Scurvey)1796 Jenner (smallpox)1839 Farr, William (vital statistics)1846 Panum (measles)1847 Semmelweis (childbed fever)1848-54 Snow, John (Cholera)1860-90 Pasteur and Koch (Bacteria-TB)

John Graunt

♦ 1662

♦ Natural & Political Observations Made Upon the Bills of Mortality: weekly tally of deaths by cause in London

William Farr

♦ 1839

♦ Superintendent of the Statistics Department of the General Registry Office for Great Britain

♦ Father of surveillance and vital statistics

John Snow

♦ Cholera Epidemics– Linelist– Epidemic Curve– Spotmap– Table by district– Table by household

Cholera outbreak, London 1854

WORKHOUSE

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Epidemiology and Disease

Levels of Disease

SporadicSporadic

EndemicEndemic

EpidemicEpidemic

PandemicPandemicIncreasing amount of diseaseIncreasing amount of disease

Epidemiologic Triad

AgentAgent

HostHost EnvironmentEnvironment

Host factors

Physiologic

Immunologic

Behavioral

Genetic

•Influence the chance for disease or its severity

Agent Factors

Infectious

Nutritional

Diseases with no agent

Toxic

•Necessary for disease to occur

Environmental Factors

Physical

Socioeconomic

Biologic

Host, Agent, Environment

Host Agent EnvironmentAgeSexRace/EthnicityReligionSESMarital statusLifestyleExerciseBehaviorCo-morbidityGenetic makeup

BiologicMicroorganisms

ChemicalToxins, tobacco,alcohol, drugs

PhysicalTrauma, radiation,fire

NutritionLack of, excess

Disease vectorsPopulation densitySubstances insurroundings andworkplaceAir qualityWeatherNoiseFood and water sourcesSpecial environments:Hospitals, day-care,institutions, bath houses,crack houses, refugeecamps

Epidemics occur when host, agent and environmental factors are not in balance

♦ New agent ♦ Change in existing agent (infectivity,

pathogenicity, virulence)♦ Change in number of susceptibles in population♦ Environmental changes affecting transmission of

agent or growth of agent

Reservoir

Habitat in which the agent normally lives and multiplies♦ People

– Symptomatic - Smallpox– Asymptomatic - HIV

♦ Animals (zoonoses)– Brucellosis– Plague

♦ Environmental– Histoplasmosis– Legionnaires’ bacillus

Mode of Transmission

♦ Direct – Contact - Cutaneous Anthrax, hookworm– Droplet –Smallpox

♦ Indirect– Airborne – Histoplasmosis, Inhalation Anthrax– Vehicleborne food or water - Salmonella– Vectorborne

• Mechanical – Shigella by fly appendages• Biological – Malaria (maturation)

Chain of Infection

ReservoirReservoir

Mode of Mode of TransmissionTransmission

Susceptible Susceptible HostHost

Mode of Mode of TransmissionTransmission

Other susceptible hosts

Characteristics of Field Epidemiology

♦ The problem or issue is urgent or a priority.

♦ A prompt response is needed.

♦ Work at the field site is essential

♦ Interventions require communication with diverse groups with different interests.

Applied Epidemiology

♦ Practical, action-oriented, and relevant

♦ Provides data for public health decision-making

♦ Focuses on prevention/ intervention

Epidemiology helps us to:

♦ Determine the magnitude and trends

♦ Identify the aetiology or cause of disease

♦ Determine the mode of transmission

♦ Identify risk factors or susceptibility

♦ Study the natural history of diseases

♦ Determine the role of the environment

♦ Evaluate the impact of the control measures

♦ Provide information for health planning

Basic Epidemiologic Methods

♦ Count cases (events)

♦ Define involved population

♦ Determine rates/proportions

♦ Compare rates

♦ Make inferences

Epidemiology “3 Steps”

♦ Counting number of events or conditions in populations or subgroups of persons. (C)

♦ Dividing the number of events by the number of persons in the population to make rates. (D)

♦ Comparing rates from different populations to make inferences about the cause for the observed differences in rates. (C)

Epidemiology: Step 1 - Counting

♦ Counting number of events or conditions in populations or subgroups of persons.

♦ The first step in descriptive epidemiology – How many persons experienced a

particular condition?– Count = “numerator”.

Hang-gliding

Number of accidents according to time of the dayHang-gliding accidents (before)

0102030405060708090

100

Morning Mid-morning Mid-afternoon Evening

Time of the day

Acci

dent

s

MaleFemale

Number of accidents according to time of the dayHang-gliding accidents (after)

0102030405060708090

100

Morning Mid-morning Mid-afternoon Evening

Time of the day

Acc

iden

ts

MaleFemale

What is inappropriate here?

Epidemiology: Step 2 - Dividing

♦ Dividing the number of events by the number of persons at risk in the population to make rates.

♦ The second step in descriptive epidemiology involves:– What group of persons experienced the event?– Population group = denominator.– Use events and population to make: proportions,

rates, odds.

Epidemiology: Step 3 - Comparing

♦ Comparing rates from different populations to make inferences about possible cause for the observed differences in rates:

– Analytic epidemiology• Cohort (exposed vs. non-exposed)• Case-control (sick vs. healthy)

– Compare rates to make rate ratios or rate differences.– Comparison of odds to make odds ratios.– Use ratios or differences to identify risk factors. – Use statistical tests to determine reliability of ratios or

differences.

The epidemiologic study

♦ in epidemiology we make comparisons

♦ we compare people with regard to certain characteristics or risk factors

♦ we examine where they differ or resemble with each other

♦ we exploit these differences to measure risk for disease or protection from disease by making comparisons

Earliest account of comparison

‘Test your servants please, for ten days, and let us have vegetables to eat and water to drink. And then see how we look and how the boys look who are eating the king’s food, and on the basis of what you see decide what you do with your servants.’

Book of Daniel 1:12-16

Test your servants please

Overseer

Vegetables and water

King’s food and wine

experiment

Earliest account of comparison(cont’d)

The overseer listened to them in this matter and tested them ten days. After ten days they looked healthier and better fed than the other boys who were eating the king’s food. So the caretaker took away the king’s food and wine and gave them vegetables.

Daniel 1:12-16

Test your patients please

Researcher

Drug A Drug

B

experiment

Testing for drug efficacy

Main activities of the epidemiologist

♦ Describe an event in terms of :– Time When?– PlaceWhere?– Person Who?

♦ Analyse the association between the event (disease, death) and its determinants (risk factors)

♦ Make recommendations: preventive actions, control measures

Tasks of Epidemiologist

♦ Surveillance♦ Investigation♦ Analysis and interpretation♦ Communication♦ Evaluation♦ Research and Management

Recap

Now that you have completed this session you should be able to:

♦ Define epidemiology♦ Describe the history of epidemiology♦ List the uses of epidemiology♦ Describe the basic epidemiologic methods♦ Describe the tasks of the epidemiologist

Resources

♦ Gregg, Chapter 2, ♦ Principles of Epi, page 2-10

Acknowledgements

♦ D. Klaucke, E. Maes CDC Atlanta♦ Dennis Coulombier