30/10/2010Dr. Salwa Tayel1. 30/10/2010Dr. Salwa Tayel2 Associate Professor Family and Community...

30/10/2010 Dr. Salwa Tayel 1

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.


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


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).


ZoonosesZoonosesZoonoses are

Human Diseases with

Animal Reservoirs.


ZoonosesZoonosesToxoplasmosis


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.


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


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


Pat

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Pat

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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.


Contact TransmissionContact Transmission

Rhinovirus?


Dir

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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.


Droplet TransmissionDroplet Transmission

Less thanone meter

Measles?


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


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


Wat

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Cryptosporidium parvum?


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Cholera!The Broad Street Pump.


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)?


Indirect-Contact TransmissionIndirect-Contact Transmission

Door knobs are another good example of a

fomite.

Influenza virus?


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 nucleidroplet 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.


Airborne TransmissionAirborne Transmission

More than

one meter

Mycobacterium tuberculosis?


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.


Po

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En

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Po

rtal

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als

of

En

try “Many

organisms that cause

one disease if they enter

one body site are harmless if they enter

another, e.g., various enteric

urinary-tract pathogens.


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.


Susceptibility of a host depends on:

1. Genetic factors

2. General factors

3. Specific acquired immunity


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.


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, ...


Specific acquired immunity:Specific acquired immunity:

It refers to protective antibodies that are

directed against a specific agent.


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.


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).


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).


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”.


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.


AgentAgent

Susceptible HostSusceptible Host ReservoirReservoir

Mode of transmissionMode of transmission

Cycle of infection

Portal of ExitPortal of ExitPortal of InletPortal of Inlet

IP

PC


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.


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.


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.


ThanThank youk you

Bibliotheca Alexandrina

Website http://faculty.ksu.edu.sa/73234/default.aspx

[email protected]

30/10/2010Dr. Salwa Tayel1. 30/10/2010Dr. Salwa Tayel2 Associate Professor Family and Community...

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