Transcript 1.Communicable Disease Epidemiology(Definitions). - Home

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Dr. Salwa Tayel
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Infectious Disease Epidemiology
By
Associate Professor
Family and Community Medicine Department
King Saud University
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Learning Objectives
At the end of this lecture you (will) be able to:
Explain common definitions and basic concepts used in
epidemiology.
Apply these definitions in different situations.
Identify the epidemiological approach to study health
problems
Describe natural history and spectrum of infectious
diseases and their implications for public health.
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Definitions
Epidemiology is the study of
distribution and
determinants of
health-related events in specified populations and
the application of this study to prevent and control
of health problems.
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Distribution
Person……….
Place…………..
Time……………
Describe disease frequency
– Incidence Rate……
– Prevalence Rate……
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Disease Determinants
HOST
AGENT
ENVIRONMENT
The Epidemiologic Triad
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The epidemiological approach:
An Epidemiologist’ approach to study healthrelated events involves answering five
questions:
What?
Who?
Where?
When?
Why?
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Infection:
It is the entry, development and multiplication of an
infectious agent in the body of man or animal.
Outcome of infection varies.
Infectious disease:
A clinically manifest disease of man or animal
resulting from infection.
In-apparent infection:
The infection does not become manifest at any
stage.
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Communicable Disease
An illness due to a specific infectious agent or its
toxic products that arises through transmission
of that agent from reservoir to susceptible host.
Communicable Disease (typhoid, influenza,..
Non- Communicable Disease (DM, cancer,..
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Contamination:
The presence of living infectious agents on the
exterior surface of the body or on the clothes or
articles of the person or on any inanimate object in
the environment including water and food.
Contagious diseases
A disease that is capable of being transmitted from
one person to another by contact or close
proximity. e.g. scabies, trachoma and leprosy.
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Endemic
The constant presence of a disease or infectious
agent within a given geographic area or population
group;
It may also refer to the usual prevalence of a given
disease within such area or group.
Epidemic
The occurrence of more cases of a disease than
expected in a given area or among a specific
group of people over a particular period of time.
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Pandemic
An epidemic occurring over a very wide area (several
countries or continents) at the same time and usually
affecting a large proportion of the population.
e.g. Influenza, cholera
Outbreak
A more or less localized epidemic affecting large number of
a group, in the community
e.g. outbreak of food poisoning
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Sporadic
Cases occur irregularly, haphazardly from time to
time and generally infrequently.
Cases are few and separated widely in space and
time showing no connection to each other.
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Nosocomial infection (Hospital acquired infection):
It is an infection occurring in a patient while in a
hospital or other health care facility
It should not be present or incubating at the time of
admission.
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Elimination of disease
Disease incidence is reduced to a minimal level at
which the disease is no longer considered a public
health problem, while infection may still occur
Example:
The aim of elimination of Neonatal tetanus is
reduction of its incidence to less than
one case/1000 live births.
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Eradication:
 It means worldwide disappearance of a disease
(permanent reduction to zero level) with complete
destruction of the agent.
The organism can be present only in laboratories
and no need for interventions.
e.g. smallpox eradication from the world since 1979.
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Natural history
Natural history of disease refers to the progress of a
disease process in an individual over time, in the
absence of intervention.
The natural history of a disease describes the course
of the disease in an individual starting from the moment
of exposure to the causal agents till one of the possible
outcomes occurs.
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Spectrum of disease
The idea that an exposure can lead to varying signs,
symptoms and severity of the same disease in the
population is the spectrum of disease.
Why do we have varying degrees of severity? Prognosis?
The outcome will depend on the interactions of host, agent
and environmental factors.
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Disease Determinants
HOST
AGENT
ENVIRONMENT
The Epidemiologic Triad
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•Agent factors refer to an infectious microorganism—
virus, bacterium, parasite, or other microbes.
•They are necessary but not always sufficient
alone to cause disease.
•Host factors are intrinsic factors that influence an
individual’s exposure, susceptibility, or response to a
causative agent.( age, nutrition, race,…
•Environmental factors are extrinsic factors which
affect the agent and the opportunity for exposure.
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Factors Affecting Disease Transmission
and Symptomatic Clinical Disease
VECTOR
-- Prevalence
-- Portal(s) of entry
Host
-- Susceptibility
-- Immune response
-- Resistance
--- Portal(s) of entry
Vector
Agent
Environment
-- Virulence
Toxigenicity
-- Infectivity
Resistance
-- Pathogenicity Antigenicity
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-- Balance of immune to
susceptible individuals
--- Opportunity for
exposure (e.g. crowding)
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Classification of diseases according to clinical
severity (spectrum)
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Class A: Inapparent infection
Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS
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Class B: Classic cases
Examples: Measles, Chickenpox
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Class C: Severe or Fatal infections
Examples: Rabies, Hemorrhagic fevers caused by Ebola
and Murberg viruses.
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Implications for public health
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The relation of severity of illness to disease statistics.
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The pyramid and iceberg of disease
1
Diseased, diagnosed & controlled
2
Diagnosed, uncontrolled
3
Undiagnosed or wrongly
diagnosed disease
4
Risk factors for disease
5
Free of risk factors
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Diagnosed
disease
Undiagnosed or
wrongly diagnosed disease
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Iceberg phenomenon
Cases of illness correctly diagnosed by clinicians in the
community often represent only the “tip of the iceberg.”
Many additional cases may be too early to diagnose or may
remain asymptomatic.
Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.
The risk is that persons with in-apparent or undiagnosed
infections may be able to transmit infection to others.
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Timelines for Infection and Symptomatic Disease
Dynamics of
infectiousness
latent infectious
period period
susceptible
E
Dynamics of
disease
noninfectious
-- removed
-- dead
-- recovered
time
-- dead
incubation symptomatic -- recovered
-- immune
period
period
-- carrier
susceptible
time
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The pyramid and iceberg of disease
1
Diseased, diagnosed & controlled
2
Diagnosed, uncontrolled
3
Undiagnosed or wrongly
diagnosed disease
4
Risk factors for disease
5
Free of risk factors
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Diagnosed
disease
Undiagnosed or
wrongly diagnosed disease
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Applications of the concepts of natural history
and spectrum of disease
Persons with in-apparent or undiagnosed
infections can transmit infections to others.
Control measures must be directed toward all
infections capable of being transmitted to others;
– both clinically apparent cases and
– those with in-apparent or undiagnosed infections.
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Further reading:
Principles of EPIDEMIOLOGY
in Public Health Practice
Third Edition
An Introduction to
Applied Epidemiology and Biostatistics
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Centers for Disease Control and Prevention (CDC)
Office of Workforce and Career Development
Atlanta, GA 30333 (free Online)
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EPIDEMIOLOGY IN MEDICINE
Charles H. Hennekens, M.D., Dr.P.H.
Julie E. Buring, Sc.D.
Park’s textbook of PREVENTIVE AND SOCIAL
MEDICINE
John E. Park
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The End
Thank You
Website http://faculty.ksu.edu.sa/73234/default.aspx
[email protected]
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