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Pakistan Field Epidemiology and

Laboratory Training Program

Introduction to Epidemiology

King Cholera dispenses contagion:

the London Cholera Epidemic of 1866

George John Pinwell

Short Training Course on Avian Influenza Surveillance

6-8 February, Islamabad, Pakistan

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Learning Objectives

♦ Define Epidemiology

♦ Summarize the epidemiologicapproach

♦ List the uses of Epidemiology♦ Describe the interactions between

host, agent, and environment

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Epidemiological Principles

♦ Diseases (or other health events)

don’t occur at random

♦ Diseases (or other health events)have causal and preventive factorswhich can be identified

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Epidemiology

Epi = upon

Demos = population

Logos = study of 

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Definition

The study of the distribution anddeterminants of health related states orevents in specified human populations and

its application to the control of healthproblems

Last, 1988 

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Key Words:

The study of the distribution anddeterminants of health related states orevents in specified human populations and

its application to the control of healthproblems.

Distribution   Time, place, person

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Key Words:

The study of the distribution anddeterminants of health related states orevents in specified human populations and

its application to the control of healthproblems.

Determinants   Cause, risk factors

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Key Words:

The study of the distribution anddeterminants of health related states orevents in specified human populations and

its application to the control of healthproblems.

Population   Public health

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Key Words:

The study of the distribution anddeterminants of health related states orevents in specified human populations and

its application to the control of healthproblems.

 Application   Information for

action

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Uses of Epidemiology

♦ Determine the magnitude and trends

♦ Identify the etiology or cause of disease

♦ Determine the mode of transmission♦ Identify risk factors or susceptibility

♦ Determine the role of the environment

♦ Evaluate the impact of the controlmeasures

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Epidemiologist Core Functions

♦ Public health surveillance

♦ Investigation

♦ Data analysis

♦ Evaluation

♦ Communication

♦ Management and teamwork 

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Public Health Cycle

Evaluate population

health and program

output, repeat cycle Target/Evaluate

Analyze

Intervene

Communicate

Descriptive

epidemiology, surveys,

surveillance, Quality

assurance

Analytic epidemiology

defines risk factors

Communications to

decision makers, media, on

 problem and risk factors.

Management, advocacy,

communications

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Basic Epidemiologic Approach

♦ Observe

♦ Count cases (events)

♦ Describe

 – Time, place, person

 – Calculate rates,

♦ Compare rates♦ Develop hypothesis

♦ Test hypothesis

♦ Implement actions (control, prevention)

Descriptive

Epidemiology

 Analytical

Epidemiology

d d l

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Medicine Versus Epidemiology

ClinicalMedicine

Epidemiology

Focus Individuals Populations

Main Goal Diagnosis andTreatment

Prevention andControl

Questions What is wrong

with this patient?What treatment isappropriate?

What are the

leading causes ofdeath or disabilityin this population?

What can be doneto reduce/preventthem?

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Epidemiology and Disease

L l f Di

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Levels of Disease

EndemicEndemic

EpidemicEpidemic

PandemicPandemic

Increasing amount of disease

SporadicSporadic

E id i l i T i d

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Epidemiologic Triad

H t A t E i t

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Host, Agent, Environment

Host

 AgeSex

Religion

Exercise

Behavior 

Co-morbidity

Genetics

Food

H t A t E i t

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Host, Agent, Environment

Host Agent

 AgeSex

Religion

Exercise

Behavior

Co-morbidityGenetics

Biologicmicroorganisms

Chemical toxins

Physical trauma

Nutrition

Host Agent En i onment

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Host, Agent, Environment

Host Agent Environment

 AgeSex

Religion

SES

Exercise

Behavior

Co-morbidity

Genetics

BiologicMicroorganisms

Chemical Toxins

Physical Trauma

Nutrition

Disease vectorsPopulation density

 Air quality

Weather

Noise

Food and watersources

Chain of Infection

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Chain of Infection

Reservoir 

Mode ofTransmission

Susceptible

Host

Mode ofTransmission

Reservoir

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Reservoir

Habitat in which the disease normally

lives and multiplies • People

- Symptomatic - Smallpox

-  Asymptomatic - HIV

•  Animals (Zoonoses)

- Brucellosis- Plague

• Environmental

- Histoplasmosis- Legionnaires’ bacillus

Mode of Transmission

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Mode of Transmission

♦ Direct

 – Contact - Cutaneous Anthrax, Hookworm – Droplet –Smallpox

♦ Indirect – Airborne – Histoplasmosis, Inhalation Anthrax

 – Vehicle-borne food or water - Salmonella

 – Vectorborne

• Mechanical – Shigella by fly limbs

• Biological – Malaria (maturation)

Chain of Infection

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Chain of Infection

Reservoir 

Mode ofTransmission

SusceptibleHost

Mode ofTransmission

Do you need to identify the agent to control it?

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Do you need to identify the agent to control it?

Disease Controlmeasure

Year Agent Year

Scurvy Diet 1753 Vitamin C 1928

Pellagra Diet 1754 Niacin 1924

Smallpox Vaccination 1798 Orthopoxvirus

1958

Cholera Water

quality

1849 Vibrio

cholerae

1893

Yellowfever

Mosquitocontrol

1901 Flavivirus 1928

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The art of epidemiological thinking

is to draw conclusions

from imperfect data

George W. Comstock

Acknowledgements

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 Acknowledgements

♦ Presentations

 – M. Valenciano and D. Coulombier, WHO Lyon

 – C. Whalen, Case Western Reserve

 – E. Simoes, CDC

♦ Reference books

 – M. Gregg, Field Epidemiology, 2 nd ed.

 – R. Timmreck, An Introduction to Epidemiology