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Introduction To Epidemiology

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Lectures 1&2

EVALUATION

First Midterm 20%

Second Midterm 25% 50%

Class participation/assignments 5 %

Final Exam 50%

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Endemic: a disease or pathogen present or usually prevalent

in a given population or geographic region at all times

Hyperendemic: equally endemic in all age groups of a

population

Holoendemic: endemic in most of the children in a population,

with the adults in the same population being less often

affected

Epidemic: a disease occuring suddenly in numbers far

exceeding those attributable to endemic disease; occuring

suddenly in numbers clearly in access of normal expectancy

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TERMINOLOGY

Pandemic: a widespread epidemic distributed or occuring

widely throughout a region, country, continent, or globally

Epizootic: of, or related to a rapidly spreading and widely

diffused disease affecting large numbers of animals in a given

region

Incidence: rate of occurrence of an event; number of new cases

of disease occuring over a specified period of time; may be

expressed per a known population size

Prevalence: number of cases of disease occurring within a

population at any one given point in time

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TERMS ASSOCIATED WITH DISEASE CAUSATION, ETC.

Host

Agent

Environment

Fomites

Vector

Carrier – active

Incubatory

Convalescent

Healthy

Intermittent

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What is epidemiology? What is an epidemiologist?

Greek (English)

epi (among)

demos (people)

logy (study)

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“Epidemiology . . .

is a Greek word that means to put

people to sleep with charts and

graphs.”

- Dr. Mark Johnson

In testimony before the

House Judiciary Committee

on the spread of HIV

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DEFINITIONS OF EPIDEMIOLOGY

Oxford English Dictionary

THE BRANCH OF MEDICAL SCIENCE WHICH TREATS OF

EPIDEMICS

Kuller LH: Am J Epid 1991

EPIDEMIOLOGY IS THE STUDY OF "EPIDEMICS" AND THEIR

PREVENTION

Anderson G: Modern Epidemiology

THE STUDY OF THE OCCURRENCE OF ILLNESS

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DEFINITIONS OF EPIDEMIOLOGY

Lilienfeld A: in Foundations of Epidemiology

THE STUDY OF THE DISTRIBUTION OF A DISEASE OR A

PHYSIOLOGICAL CONDITION IN HUMAN POPULATIONS AND

OF THE FACTORS THAT INFLUENCE THIS DISTRIBUTION.

Last JM: A Dictionary of Epidemiology

The study of the distribution and determinants of

health and disease related states in populations, and the

application of this study to control health problems.

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What is Epidemiology? EPIDEMIOLOGY

is the study of the nature, cause, control and determinants of the frequency

and distribution of disease, disability, and death in human populations.

Epidemiology:

is the study of factors influencing the occurrence, transmission,

distribution, prevention and control of disease in a defined population

“epidemiology’s full value is achieved only when its

contributions are placed in the context of public health action,

resulting in a healthier populace.” (Koplan et al. 1999)

“the product of [epidemiology] is research and information

and not public health action and implementation”.

(Atwood et al. 1997)

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MEASURING DISEASE FREQUENCY

HAS SEVERAL COMPONENTS

Classifying and categorizing disease

Deciding what constitutes a case of disease in a study

Finding a source for ascertaining the cases

Defining the population at risk of disease

Defining the period of time of risk of disease

Obtaining permission to study people

Making measurements of disease frequency

Relating cases to population and time at risk

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EPIDEMIOLOGIST

AN EPIDEMIOLOGIST IS A PUBLIC HEALTH SCIENTIST, WHO IS

RESPONSIBLE FOR CARRYING OUT ALL USEFUL AND EFFECTIVE

ACTIVITIES NEEDED FOR SUCCESSFUL EPIDEMIOLOGY

PRACTICE

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WHAT IS THE UNIQUE SKILL OF EPIDEMIOLOGISTS?

MEASURING DISEASE

FREQUENCY IN

POPULATIONS

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Epidemiologists are required to have some knowledge of the disciplines of public health, clinical medicine, pathophysiology, statistics, and the social sciences.

• Public health, because of the emphasis on disease prevention.

• Clinical medicine, because of the emphasis on disease classification and diagnosis.

• Pathophysiology, because of the need to understand basic biological mechanisms in disease.

• Statistics, because of the need to quantify disease frequency and its relationships to antecedents.

• Social sciences, because of the need to understand the social context in which disease occurs and presents.

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Purpose of Epidemiology

The ultimate purpose of Epidemiology is prevention of

diseases and promotion of health

1. Identify causes and risk factors for disease.

2. Determine the extent حجم / رقعة of disease in the

community.

3. Study natural history and prognosisتشخيص of disease.

4. Evaluate preventive and therapeutic measures

5. Provide foundation for public policy سياسة عامة

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Uses of Epidemiology

To study the cause (or etiology) of disease(s), or conditions, disorders,

disabilities ……… etc.

determine the primary agent responsible or ascertain تأكد من

causative factors

determine the characteristics of the agent or causative factors

define the mode of transmission

determine contributing factors

identify and determine geographic patterns 30

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Uses of Epidemiology

To determine, describe, and report on the natural course

of disease, disability, injury, and death.

To aidلمساعدة in the planning and development of health

services and programs

To provide administrative and planning data

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Example: Use of Epidemiology

In the United States, the National Center for Health

Statistics is a data source for information on health and

disease (Centers for Disease Control

&prevention)

http://www.cdc.gov/nchs

Globally, the World Health Organization is a data

source for information on health and disease

http://www.who.int 32

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LEADING CAUSES OF DEATH, USA, 2000

(number of deaths)

1. Heart Disease

2. Neoplasms (a new and abnormal growth of tissue in a part of the

body, especially as a characteristic of cancer.

3. Cerebrovascular Disease

4. Chronic Pulmonary Disease

5. Accidents/Injuries

6. Diabetes mellitus

7. Influenza and pneumonia

8. Alzheimer’s Disease

9. Nephritis, nephrotic syndrome

10. Septicemia

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PAKISTAN: LEADING CAUSES OF PREMATURE DEATH, 1990

1. Diarrhea

2.

3.

Lower Respiratory Infections - child

4. Rheumatic heart disease

5. Chronic liver disease

6. Congenital malformations

7. Birth Diseases

8. Ischemic heart disease

9. Child Septicemia

10. Injuries

Tuberculosis

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LEADING CAUSES OF DEATH WORLDWIDE, 2000

(Based on number of global deaths)

1. Ischemic Heart Disease

2. Cerebrovascular Disease

3. Lower Respiratory Infections

4. HIV/AIDS

5. COPD (Chronic obstructive pulmonary disease)

6. Perinatal Conditions

7. Diarrhoeal Diseases

8. Tuberculosis

9. Road Traffic Injuries

10. Lung Cancers

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Basic features of Epidemiology

1. Studies are conducted on human population

2. It examines patterns of events in people

3. Can establish cause-effect relationship without the

knowledge of biological mechanism

4. It covers a wide range of conditions

5. It is an advancing science

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Forms of Epidemiology

• Clinical Epidemiology

• Descriptive Epidemiology

• Predictive Epidemiology

• Etiologic Epidemiology

• Genetic Epidemiology

• Occupational Epidemiology

• Social Epidemiology

• Spatial Epidemiology

etc…

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What?

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EPIDEMIOLOGIC APPROACHES (TYPES)

DESCRIPTIVE Health and disease in the community

What? How many? Who? When? Where?

What are the

health problems

of the

community?

What are the

attributes of

these illnesses?

How many people

are affected?

Over what

period of time?

Where do the

affected people

live, work or

spend leisure

time?

ANALYTIC Etiology, prognosis and program evaluation

Why? How?

What are the

causal agents?

What factors

affect outcome?

By what mechanism

do they operate?

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What are the

attributes of

affected persons?

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TWO BROAD TYPES OF EPIDEMIOLOGY

Examining the distribution of a disease

in a population, and observing the basic

features of its distribution in terms of

time, place, and person.

Typical study design:

community health survey (approximate

synonyms - cross-sectional study,

descriptive study)

Testing a specific hypothesis about

the relationship of a disease to a

putative cause, by conducting an

epidemiologic study that relates the

exposure of interest to the disease of

interest.

Typical study designs: cohort, case-

control

DESCRIPTIVE EPIDEMIOLOGY ANALYTIC EPIDEMIOLOGY

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THE BASIC TRIAD OF DESCRIPTIVE EPIDEMIOLOGY

THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR

IN DESCRIPTIVE EPIDEMIOLOGY:

Think of this as the standard dimensions used to track the occurrence of

a disease.

In Descriptive Epidemiology:

Who? - person

Where? - place

When? - time

TIME PLACE

PERSON

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TIME WHEN does the disease occur?

• Temporal

• Range from hours to decades

Changing or stable?

Time since an event

Calendar Time

Age (time since birth)

Characteristics Relating to Time

Secular change (long-term)

Point epidemics (short-term)

Cyclic trends

Seasonal variation

Graphic format often used

• y-axis (vertical) - frequency

• x-axis (horizontal) - time

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SECULAR CHANGE (LONG-TERM)

Incidence Rates of Cancers in Women Incidence Rates of Cancers in Men 44

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POINT EPIDEMICS (SHORT-TERM)

• Short-term changes occur over limited time frames

Hours

Days

Weeks

Months

• Used for short-term exposures or diseases with

short incubation and/or illness durations

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CYCLIC TRENDS

• Cyclic trends may be either long-term or short term events.

• Some are “seasonal” while others are cyclic due to other

factors:

School year

Military deployment

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Cyclic Trends

Migratory Birds?

Seasonal Variation

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PLACE WHERE are the rates higher? or lower?

Geographically restricted or widespread (pandemic)?

Geographic location of reservoir

Relation to water or food supply.

Multiple clusters or one?

presence or agents or vectors

climate

geology

population density

economic development

nutritional practices

medical practices

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Dr. John Snow and Cholera

CHARACTERISTICS RELATING TO PLACE

International

Variation within countries

• Urban-rural

• Local

Building Maps

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PERSON

WHO is getting the disease?

Many variables are involved and studied, but factors such as

sex, age & race often have a major effect.

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CHARACTERISTICS OF PERSON

Age

Sex

Ethnic group

Socio-economic status

Gender

Concurrent disease مرض متزامن

Diet, exercise, smoking

Nativity (place of birth)

Religion

Marital status

Behavior

Occupation 50

Age

Sex

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EXAMPLE

You have been asked to investigate an event in

which 2,220 people were exposed and 1,520 of

them died.

Your role as an epidemiologist is to ask

questions about person, place and time.

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HOW DO WE ASK QUESTIONS?

SURVEYS

- OF SURVIVORS

- OF NEXT-OF-KIN ذوى القربى

- OF OTHER RELATED PERSONS

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WITH QUESTIONS YOU LEARN

THAT ...

Person: Men, women and children were all exposed and

at risk. The majority of people who died were

wealthy ثري and young men between 18-50 years

(when compared to survivors).

Place: All those exposed were within 1 block of one

another, the climate was cold.

Time: Mid April, people died within hours of the

precipitating exposure.

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DESCRIPTIVE EPIDEMIOLOGY IS A NECESSARY ANTECEDENT OF ANALYTIC

EPIDEMIOLOGY

To undertake an analytic epidemiologic study you

must first:

Know where to look

Know what to control for

Be able to formulate hypotheses compatible with فرضيات

laboratory evidence

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A COMMON ERROR IN EPIDEMIOLOGY IS MOVING TO ANALYTIC EPIDEMIOLOGY

WITHOUT HAVING A SOLID BASE IN THE DESCRIPTIVE EPIDEMIOLOGY OF THE

CONDITION.

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Three essential characteristics that are examined

to study the cause(s) for disease in analytic

epidemiology are...

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Host Environment

Agent

The Basic Triad Of Analytic Epidemiology

AGENTS

Nutrients

Poisons

Allergens

Radiation

Physical trauma

Microbes

Psychological experiences

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• Necessary for disease to occur

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HOST FACTORS

Genetic endowment

Immunologic state

Age

Personal behavior

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ENVIRONMENT

External conditions

Physical or biologic or social

Crowding

Atmosphere

Modes of communication – phenomena in the

environment that bring host and agent together,

such as:

• Vector

• Vehicle

• Reservoir

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• Contribute to the disease process

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EPIDEMICS ARISE WHEN HOST,

AGENT, AND ENVIRONMENTAL

FACTORS ARE NOT IN BALANCE

• Due to new agent

• Due to change in existing agent (infectivity, pathogenicity,

virulence)

• Due to change in number of susceptibles in the population

• Due to environmental changes that affect transmission of

the agent or growth of the agent

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DIFFERENCES BETWEEN LABORATORY SCIENCES AND FIELD SCIENCES

In the Laboratory:

• Mostly experimental

• Variables controlled by the investigator

• All variables known

• Replication easy

• Results valid

• Meaning of results for humans uncertain.

• Little need for statistical manipulation of data.

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DIFFERENCES BETWEEN LABORATORY SCIENCES AND FIELD SCIENCES

In the Field:

• Mostly observational

• Variables controlled by nature

• Some variables unknown

• Replication difficult; exact replication impossible

• Results often uncertain

• Meaning of results for humans clear

• Statistical control often very important

Men Women Children Total

1st class 67% 3% 0 38%

2nd class 92% 14% 0 59%

3rd class 84% 54% 66% 62%

Total 82% 26% 48% 62%

EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC

CHARACTERISTIC

DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE

WHAT CAUSE OF DEATH IS THIS?

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THE PREVIOUS SLIDE SHOWS DEATH

RATES BY CLASS OF TICKET ON THE

TITANIC, A LARGE OCEAN LINER THAT

SANK AFTER COLLIDING WITH AN

ICEBERG IN 1912