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30/10/2010 Dr. Salwa Tayel 2
Associate Professor Associate Professor Family and Community Medicine DepartmentFamily and Community Medicine Department
King Saud UniversityKing Saud University
ByBy
Infectious Disease Infectious Disease EpidemiologyEpidemiology
Chain of infectionChain of infection
Dr. Salwa TayelDr. Salwa Tayel
30/10/201030/10/2010 Dr. Salwa TayelDr. Salwa Tayel 33
Learning objectives:Learning objectives:
By the end of this lecture student will be able to::By the end of this lecture student will be able to::
Describe the infectious disease process (Describe the infectious disease process (Chain of infectionChain of infection))
List the types of reservoir of infectious diseases of manList the types of reservoir of infectious diseases of man
Define a carrier and list its types. Define a carrier and list its types.
Define zoonoses and list examples.Define zoonoses and list examples.
Identify the different modes of transmission of the organisms from the Identify the different modes of transmission of the organisms from the
reservoir to the susceptible host.reservoir to the susceptible host.
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AgentAgent
Susceptible HostSusceptible Host ReservoirReservoir
Mode of transmissionMode of transmission
Cycle of infectionCycle of infection
Portal of ExitPortal of ExitPortal of InletPortal of Inlet
IP
PC
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Animal reservoirsAnimal reservoirs
Zoonoses:Zoonoses: Infectious diseases that are transmissible
under normal conditions from vertebratevertebrate animals to
humans. (with humans as incidental hosts)
ZoonoticZoonotic diseases include:diseases include:
brucellosis (cows and pigs), brucellosis (cows and pigs),
anthrax (sheep),anthrax (sheep),
plague (rodents), plague (rodents),
rabies (bats, dogs, and other mammals).rabies (bats, dogs, and other mammals).
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ZoonosesZoonosesZoonoses are
Human Diseases with
Animal Reservoirs.
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Environmental reservoirs:Environmental reservoirs:
Soil, and waterSoil, and water
• Soil:Soil: Agents Agents live and multiply in the live and multiply in the soilsoil..
Examples: Examples:
-Tetanus-Tetanus spores spores and and
- Fungal agents; (those causing - Fungal agents; (those causing histoplasmosishistoplasmosis))
• Pools of waterPools of water are the primary reservoir of are the primary reservoir of
Legionnaires’ bacillus.Legionnaires’ bacillus.
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3. Portal of exit3. Portal of exit
Portal of exit is the path by which an agent leaves the
source host.
Examples:
•Respiratory tract
•GIT
•Skin and mucous membrane
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4. Modes of transmission4. Modes of transmission• • Direct transmissionDirect transmission
— — Direct contactDirect contact
— — Droplet spreadDroplet spread
• • Indirect transmissionIndirect transmission
— — AirborneAirborne
— — Vehicle borneVehicle borne
— — Vector borne:Vector borne:
• MechanicalMechanical
• BiologicBiologic
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• Direct contactDirect contact occurs through:
Skin-to-skin contact, kissing, and sexual intercourse.
Direct contact refers also to contact with soil or
vegetation harbouring infectious organisms.
Direct transmissionDirect transmission
There is essentially There is essentially immediateimmediate transfer of the agent from transfer of the agent from
a reservoir to a susceptible host by direct a reservoir to a susceptible host by direct contactcontact or or
dropletdroplet spread. spread.
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Contact TransmissionContact Transmission
Rhinovirus?
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Droplet spreadDroplet spread
•Transmission by direct spray of relatively
large, short-range aerosols over a few feet,
before the droplets fall to the ground.
•These aerosols may be produced by sneezingsneezing,
coughingcoughing, or even talkingtalking.
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Droplet TransmissionDroplet Transmission
Less thanone meter
Measles?
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Indirect transmission:Indirect transmission:
An agent is carried from a reservoir to a susceptible host An agent is carried from a reservoir to a susceptible host
by: by:
Vehicle borne:Vehicle borne: inanimate inanimate vehiclevehicle
Vector borne:Vector borne: animate animate vectorvector
Airborne transmission:Airborne transmission: suspended air particlessuspended air particles
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Vehicle borne:Vehicle borne:
An infectious agent is carried from a reservoir to a
susceptible host by an inanimate intermediary.
Vehicles include:Vehicles include:
1. Contaminated food and water, typhoid, paratyphoid, food
poisoning, dysentery and cholera.
2. Biologic products (blood), viral hepatitis, AIDS, syphilis
and malaria.
3. Fomites (inanimate objects such as: door knobs, toys,
handkerchiefs, bedding, or surgical instruments).
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Hepatitis A
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Cryptosporidium parvum?
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Cholera!The Broad Street Pump.
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Indirect-Contact TransmissionIndirect-Contact Transmission
Beddings are an example of a Fomite, an inanimate object that can transmit
pathogens between people.
Methicillin-resistant
Staphylococcus aurius
(MRSA)?
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Indirect-Contact TransmissionIndirect-Contact Transmission
Door knobs are another good example of a
fomite.
Influenza virus?
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VectorsVectors are arthropods such as mosquitoes, fleas, and ticks are arthropods such as mosquitoes, fleas, and ticks
• Mechanical transmission:Mechanical transmission:
the agent does not multiply or undergo physiologic the agent does not multiply or undergo physiologic
changes in the vector.changes in the vector.
For example, flies carry For example, flies carry Shigella Shigella on appendages.on appendages.
• Biologic transmission:Biologic transmission:
When the agent undergoes changes and/or When the agent undergoes changes and/or
multiplication within the vector before it is transmitted. multiplication within the vector before it is transmitted.
((Extrinsic incubation periodExtrinsic incubation period). Example: Malaria, Filariasis). Example: Malaria, Filariasis
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Airborne transmissionAirborne transmission occurs by particles that are occurs by particles that are
suspended in air. suspended in air.
There are two types of these particles: There are two types of these particles:
- dust- dust and and
- droplet nuclei- droplet nuclei..
1.1. Dust particlesDust particles::
-result from re-suspension of particles that have settled -result from re-suspension of particles that have settled
on floor or bedding,on floor or bedding,
- infectious particles blown from the soil by the wind. - infectious particles blown from the soil by the wind.
Example: Fungal spores.Example: Fungal spores.
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Airborne TransmissionAirborne Transmission
More than
one meter
Mycobacterium tuberculosis?
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2.2. Droplet nucleiDroplet nuclei
They represent the dried residue of droplets that have
been coughed or sneezed into the air.
They are very tiny particles less than 5 µ (microns) in
size and may remain suspended in the air for long
periods.
Examples:Examples:
TuberculosisTuberculosis is transmitted more often indirectly, through
droplet nuclei, than directly, through direct droplet spread.
Legionnaires’Legionnaires’ disease and histoplasmosishistoplasmosis also spread
through airborne transmission.
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5. Portal of entry5. Portal of entry
An agent enters a susceptible host through a portal of
entry.
-The portal of entry must provide access to tissues
in which the agent can multiply or a toxin can act.
- Often, organisms use the same portal to enter a
new host that they use to exit the source host.
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Po
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En
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organisms that cause
one disease if they enter
one body site are harmless if they enter
another, e.g., various enteric
urinary-tract pathogens.
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6. Host6. Host
A susceptible hostA susceptible host is t is the final link in the chain of infection.he final link in the chain of infection.
The hostThe host is a person or other living organism that can be infected is a person or other living organism that can be infected
by an infectious agent under normal conditions.by an infectious agent under normal conditions.
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Susceptibility of a host depends on:
1. Genetic factors
2. General factors
3. Specific acquired immunity
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General factors which defend against infection:General factors which defend against infection:
• the skin,
• mucous membranes,
• gastric acidity,
• cilia in the respiratory tract,
• the cough reflex, and
• nonspecific immune response.
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General factors that may increase susceptibilityGeneral factors that may increase susceptibility
areare:
• malnutrition,
• alcoholism, and
• disease or therapy which impairs the immune
response (Cortisone, cytotoxic drugs, ...
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Specific acquired immunity:Specific acquired immunity:
It refers to protective antibodies that are
directed against a specific agent.
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Specific acquired immunity:Specific acquired immunity: two types: two types:
1.1. Active immunity:Active immunity: Resistance developed in response Resistance developed in response
to stimulus by an to stimulus by an antigenantigen either; either;
NaturallyNaturally by by infecting agentinfecting agent or or
ArtificiallyArtificially by by vaccinevaccine and usually and usually
characterized by the presence of antibody produced by characterized by the presence of antibody produced by
the host.the host.
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2. Passive immunity:2. Passive immunity:
Immunity conferred by an antibody produced in another Immunity conferred by an antibody produced in another
host and may be acquired;host and may be acquired;
NaturallyNaturally by an infant from its mother or by an infant from its mother or
ArtificiallyArtificially by administration of an antibody by administration of an antibody
containing preparation (antiserum or immune containing preparation (antiserum or immune
globulin).globulin).
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Specific Acquired immunity:Specific Acquired immunity:
This depends on antibodies production. Immunity This depends on antibodies production. Immunity
may be:may be:
1.1. Active immunity.Active immunity.
a)a) Natural (Post-Infection). Natural (Post-Infection).
b)b) Artificial (Post-Vaccination). Artificial (Post-Vaccination).
2.2. Passive immunity.Passive immunity.
a)a) Natural (Trance-placental). Natural (Trance-placental).
b)b) Artificial (Immune-sera). Artificial (Immune-sera).
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Herd immunityHerd immunity
It is the state of immunity of a group or a community. It is the state of immunity of a group or a community.
Also it is; Also it is;
““The resistance of a group to invasion and spread of an The resistance of a group to invasion and spread of an
infectious agent, based on the immunity of a high infectious agent, based on the immunity of a high
proportion of individual members of the group”.proportion of individual members of the group”.
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Factors Affecting Herd Immunity:Factors Affecting Herd Immunity:
The extent of coverage of the immunization program.The extent of coverage of the immunization program.
The degree of resistance to infection afforded by the The degree of resistance to infection afforded by the vaccine.vaccine.
Duration and degree of infectivity of the organism.Duration and degree of infectivity of the organism.
Past experience with different infections.Past experience with different infections.
Overcrowding and environmental sanitation.Overcrowding and environmental sanitation.
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AgentAgent
Susceptible HostSusceptible Host ReservoirReservoir
Mode of transmissionMode of transmission
Cycle of infection
Portal of ExitPortal of ExitPortal of InletPortal of Inlet
IP
PC
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Period of Communicability Period of Communicability
– The time during which an infectious agent my be The time during which an infectious agent my be
transmitted directly or indirectly from an infected person transmitted directly or indirectly from an infected person
to a susceptible person or animal.to a susceptible person or animal.
– Its length varies from one disease to another.Its length varies from one disease to another.
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Incubation period (IP)Incubation period (IP)
The period from exposure to infection to the onset of The period from exposure to infection to the onset of
symptoms or signs of infectious disease.symptoms or signs of infectious disease.
The length of incubation period depends on:The length of incubation period depends on:
– The portal of entry.The portal of entry.
– The rate of growth of the organism in the host.The rate of growth of the organism in the host.
– The dosage of the infectious agent.The dosage of the infectious agent.
– The host resistance.The host resistance.
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Extrinsic incubation period:Extrinsic incubation period:
This is the period taken by the infectious agent outside
the human body until it becomes infective again to a
new individual.
e.g.e.g. Rickettsia takes five days in the intestinal villi of
louse.
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ThanThank youk you
Bibliotheca Alexandrina
Website http://faculty.ksu.edu.sa/73234/default.aspx