Descriptive Epidemiology

Ahmed Mandil Prof of Epidemiology

KSU College of Medicine

Headlines Definition of Health Definition of Epidemiology Uses and Applications of

Epidemiology Descriptive versus Analytical

Epidemiology Epidemiological triads Related disciplines

Definition of Health

A state of complete physical, mental and social well-being, and not mere absence of infirmity (WHO, 1945)

Definition of Epidemiology

Epidemiology may be defined as: the study of distribution and determinants of health-related states or events in specified populations; and the application of this study to control of health problems (Last, 2000)

  

Uses and Applications of Epidemiology

- Establishing etiological hypothesis (causation)

- Studying natural history of disease

- Describing health status of populations

- Evaluating interventions 

Establishing etiological hypothesis (causation)

This includes the study of the different factors (genetic / environmental / interaction between both), which may be responsible for ill health. This includes investigation of causes of communicable diseases, as well as risk factors for non-communicable diseases and injuries. All this can lead to identification of the most effective preventive methods, which have the overall goal of improving the health of populations.

Studying natural history of disease

This includes the study of the course and outcome (i.e. natural history) of diseases (in individuals and groups). This includes following up the affected people from the time of exposure, through subclinical stages; appearance of signs and symptoms (clinical disease); and final outcome (recovery, chronicity, disability, death)

Describing health status of populations

Epidemiology is also often used to describe the health status of populations groups. Knowledge of disease burden (morbidity, mortality) in populations is essential for health authorities, which seek to use limited resources to the best possible effect by identifying priority health programmes for prevention and care.

Evaluating Interventions

In addition to investigating etiological hypothesis, studying natural history of diseases, describing health status of population groups, epidemiologists have become involved in evaluating interventions. This inlcudes evaluating the effectiveness and efficiency of health promotion and educations programmes; preventive measures as well as public health services including management schemes.

What Is The Unique Skill Of Epidemiologists?

MEASURING DISEASE FREQUENCY IN

POPULATIONS

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

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

Epidemiological Triads

Descriptive Epidemiology Triad:

Person Place Time

Analytical Epidemiology Triad:

Agent Host Environment

The Basic Triad Of Descriptive Epidemiology

THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY:

TIME PLACE PERSON

Time

Changing or stable?

Seasonal variation.

Clustered (epidemic) or evenly distributed (endemic)?

Point source or propagated.

Time Trends Point source (e.g. food-borne outbreaks),

in terms of hours / days Seasonal - cyclicity (e.g. common cold,

influenza), in terms of months Propogative (e.g. water borne epidemics),

in terms of weeks / months Secular (e.g. morbidity / mortality of non-

communicable diseases), in terms of years Cluster in time / place

Place Geographically restricted or

widespread (pandemic)? Relation to water or food supply

(clusters: multiple / one) Residence (rural, urban, sub-urban) Weather (temperature, humidity) Natural / political

Person

Age

Socio-economic status

Gender

Ethnicity/Race

Behavior

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 laboratory evidence

A COMMON ERROR IN EPIDEMIOLOGY IS MOVING TO ANALYTIC EPIDEMIOLOGY WITHOUT HAVING A SOLID BASE IN THE DESCRIPTIVE EPIDEMIOLOGY OF THE CONDITION.

The Basic Triad Of Analytic Epidemiology

THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE:

HOST

ENVIRONMENTAGENT

AgentsAgents

Biological (micro-organisms) Physical (temperature, radiation,

trauma, others) Chemical (acids, alkalis, poisons,

tobacco, others) Environmental (nutrients in diet,

allergens, others) Psychological experiences

Host Factors

Genetic endowment

Immunologic state

Age

Personal behavior

Environment Crowding Atmosphere Modes of communication – phenomena

in the environment that bring host and agent together, such as: Vector Vehicle Reservoir

Epidemiologists are required to have some knowledge of the disciplines of:

• 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.

Further Reading Porta M. A dictionary of epidemiology. 5th

Edition. Oxford, New York: Oxford University Press, 2008.

Gordis L. Epidemiology. 4th edition. Philadelphia, Pennsylvania: Elsevier Science, 2008 

Beaglehole R, Bonita R, Kjellstrom T. Basic epidemiolgy. 2nd edition. Geneva: World Health Organization, 2006

Paneth N. Introduction to epidemiology. University of Pittsburgh.