epidemiology of communicable disease
TRANSCRIPT
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EPIDEMIOLOGY OF
COMMUNICABLE DISEASE
Dr. Nelson E. Hora
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Chain of Infection
6 links
I. Etiologic agent
II. Reservoir
III. Portal of Exit
IV. Mode of Transmission
V. Portal of Entry
VI. Susceptible Host
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I. Etiologic Agents
seven categories of biological agentsthat can cause infectious diseases:
1. Metazoa
2. Protozoa3. Fungi
4. Bacteria
5. Rickettsia6. Viruses
7. Prions
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I. Etiologic Agent
Host-Parasite (Infectious Agent) Interactions
Infectivity
Ability of an agent to invade and multiply (produce infection)
in a host
Pathogenicity
Ability to produce clinically apparent illness
Virulence
Proportion of clinical cases resulting in severe clinical
manifestations (including sequelae)
Immunogenicity
Infections ability to produce specific immunity
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I. Etiologic Agent
Pathogenetic Mechanisms
1. direct tissue invasion
2. production of toxin
3. immunologic enhancement or allergic
reaction leading to damage to the host
4. persistent or latent infection
5. enhancement of host susceptibility to drugsof otherwise minimal toxicity
6. immune suppression
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II. Reservoir
Human
Animals
Environment
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II. Reservoir
Human Reservoir
Levels1. Colonization
2. Inapparent infection (covert or subclinical infection)3. Infectious disease
All infected persons, including those with
colonization only, are potential sources ofinfection to others
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II. Reservoir
Human reservoir
Carrier an infected person who does nothave apparent clinical disease but is,
nevertheless, a potential source of infection toothers
Type of Carrier Inapparent throughout
Incubatory carrier Convalescent carrier
Chronic carrier
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II. Reservoir
Animal Reservoirs
Zoonoses - infections transmissible under
natural conditions form vertebrate animals to
man
Environmental Reservoirs
Plant, soil and water
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III. Portal of Exit
The route by which the disease agent
may escape from the human or animal
reservoir
1. Respiratory
2. Genitourinary
3. Alimentary
4. Skin
5. Transplacental
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IV. Mode of Transmission
Necessary to bridge the gap between the
portal of exit form the reservoir and the
portal of entry into the host
Two Basic Modes
1. Direct
2. Indirect
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IV. Mode of Transmission
. Direct transmission
Consists of essentially immediate transfer of
an infectious agent form an infected host or
reservoir to an appropriate portal of entry
Include spray by droplets
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IV. Mode of Transmission
. Direct transmission
Person-to-person spread of disease
Three important aspects
Generation time
Herd immunity
Secondary Attach Rate
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IV. Mode of Transmission
Secondary attack
rate =
number of new cases in
group minus initialcase(s)
during specified time
periodnumber of susceptible
persons in group minus
initial case(s)*
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IV. Mode of Transmission
Indirect transmission
Vehicle borne
Vector borne
Air borne
2 types of particles implicated
Dust
Droplet nuclei
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V. Portal of Entry
Usually the same as the portal of exit form
the reservoir
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VI. Susceptible Host
Susceptibility is affected by:
Genetic factors
General resistance factors
Specific acquired factors
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THE INFECTIOUS DISEASE
SPECTRUM
Some do not become infected at all
Some become infected but develop no
symptoms
Some become infected and develop mild
or moderate symptoms
Some become infected and developsevere symptoms
Some die as a result of their infection
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THE INFECTIOUS DISEASE
SPECTRUM
Part of this variation is due to the capacity
of the agent to produce disease and to
differing levels of resistance of the hosts.
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THE INFECTIOUS DISEASE
SPECTRUM
The existence of the infectious disease spectrum
can make it challenging to find out the extent of
transmission in a particular population.
Most cases with inapparent or mild symptomswill never be discovered or reported, since these
people will not seek health care.
So when moderate or severe cases are reported
they may represent the tip of the iceberg
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THE INFECTIOUS DISEASE
SPECTRUM
Another challenge is posed by the fact that
many diseases look alike.
A variety of agents may produce
essentially similar clinical syndromes.
That is why laboratory identification of the
specific disease agent is so important in
any epidemiological investigation.