9/10/2010Dr. Salwa Tayel19/10/2010Dr. Salwa Tayel1.

9/10/2010 Dr. Salwa Tayel 19/10/2010 Dr. Salwa Tayel 1

Dr. Salwa Tayel 2Dr. 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

Dr. Salwa TayelDr. Salwa Tayel 339/10/20109/10/2010 Dr. Salwa TayelDr. Salwa Tayel 33

Learning ObjectivesLearning Objectives

At the end of this lecture you (will) be able to:At the end of this lecture you (will) be able to:

ExplainExplain common definitions and basic concepts used in common definitions and basic concepts used in

epidemiology.epidemiology.

Describe natural history and spectrum of infectious Describe natural history and spectrum of infectious

diseases and their diseases and their implicationsimplications for public health. for public health.

Dr. Salwa TayelDr. Salwa Tayel 44Dr. Salwa TayelDr. Salwa Tayel 44

Infection:Infection:

It is the entry, development and multiplication of an It is the entry, development and multiplication of an

infectious agent in the body of man or animal.infectious agent in the body of man or animal.

Outcome of infection varies.Outcome of infection varies.

Infectious disease:Infectious disease:

A clinically manifest disease of man or animal A clinically manifest disease of man or animal

resulting from infection.resulting from infection.

In-apparent infection:In-apparent infection:

The infection does not become manifest at any The infection does not become manifest at any

stage.stage.


An illness due to a specific infectious agent or its An illness due to a specific infectious agent or its

toxic products that arises through toxic products that arises through transmissiontransmission

of that agent from reservoir to susceptible host.of that agent from reservoir to susceptible host.

Communicable Disease (typhoid, influenza,..Communicable Disease (typhoid, influenza,..

Non- Communicable Disease (DM, cancer,..Non- Communicable Disease (DM, cancer,..

Communicable DiseaseCommunicable Disease


Natural historyNatural history

NaturalNatural historyhistory of disease refers to the progress of a of disease refers to the progress of a

disease process in an disease process in an individualindividual over time, in the over time, in the

absenceabsence of intervention. of intervention.

The natural historyThe natural history of a disease of a disease describes the course describes the course

of the disease in an individual starting from the moment of the disease in an individual starting from the moment

of of exposureexposure to the causal agents till one of the possible to the causal agents till one of the possible

outcomesoutcomes occurs. occurs.


Spectrum of diseaseSpectrum of disease

The idea that an exposure can lead to varying signs, The idea that an exposure can lead to varying signs,

symptoms and severity of the same disease in the symptoms and severity of the same disease in the

populationpopulation is the spectrum of disease. is the spectrum of disease.

Why do we have varying degrees of severity? Prognosis?Why do we have varying degrees of severity? Prognosis?

The outcome will depend on the interactions of host, agent The outcome will depend on the interactions of host, agent

and environmental factors.and environmental factors.


Classification of diseases according to clinical Classification of diseases according to clinical severity (spectrum)severity (spectrum)


Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS

Class A: Inapparent infection


Examples: Measles, Chickenpox

Class B: Classic casesClass B: Classic cases


Examples: Examples: Rabies, HemorrhagicRabies, Hemorrhagic feversfevers caused by Ebola caused by Ebola and Murberg viruses. and Murberg viruses.

Class C: Severe or Fatal infections


Implications for public healthImplications for public health


The relation of severity of illness to disease statistics.


The pyramid and iceberg of diseaseThe pyramid and iceberg of disease

11 Diseased, diagnosed & controlledDiseased, diagnosed & controlled

22 Diagnosed, uncontrolledDiagnosed, uncontrolled

33 Undiagnosed or wronglyUndiagnosed or wronglydiagnosed diseasediagnosed disease

44 Risk factors for diseaseRisk factors for disease

55 Free of risk factors Free of risk factors

Diagnosed Diagnosed diseasedisease

Undiagnosed orUndiagnosed orwrongly diagnosed diseasewrongly diagnosed disease


Cases of illness correctly diagnosed by clinicians in the Cases of illness correctly diagnosed by clinicians in the

community often represent only the “community often represent only the “tip of the icebergtip of the iceberg.”.”

Many additional cases may be too early to diagnose or may Many additional cases may be too early to diagnose or may

remain asymptomatic.remain asymptomatic.

Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.

The riskThe risk is that persons with in-apparent or undiagnosed is that persons with in-apparent or undiagnosed

infections may be infections may be able to transmitable to transmit infection to others. infection to others.

Iceberg phenomenon Iceberg phenomenon


Applications of the concepts of natural history Applications of the concepts of natural history and spectrum of diseaseand spectrum of disease

Persons with in-apparent or undiagnosed Persons with in-apparent or undiagnosed

infections can transmit infections to others.infections can transmit infections to others.

Control measures must be directed toward all Control measures must be directed toward all

infections capable of being transmitted to others;infections capable of being transmitted to others;

– both clinically apparent cases and both clinically apparent cases and

– those with in-apparent or undiagnosed infections.those with in-apparent or undiagnosed infections.


Chain of infectionChain of infection


Chain of infectionChain of infection

A process that begins when an agent leaves its

reservoir or host through a portal of exit, and is

conveyed by some mode of transmission, then

enters through an appropriate portal of entry to

infect a 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


The requisites (essentials) for the The requisites (essentials) for the perpetuation of communicable diseases:perpetuation of communicable diseases:

The elements of the cycle of infection:The elements of the cycle of infection:1. Presence of microbiological agent.

2. Presence of reservoir.3. Portal of exit.

4. Mode of transmission.5. Portal of entry (inlet).

6. Presence of susceptible host.


1. Agent: 1. Agent:

Microorganisms are responsible for disease Microorganisms are responsible for disease

production (viruses, bacteria, protozoa, parasites, production (viruses, bacteria, protozoa, parasites,

fungi,.. fungi,..


Mechanisms of disease production Mechanisms of disease production

(pathogenesis):(pathogenesis):

• Invasiveness: (Pneumococcosis, measles).

• Toxicity: (Tetanus, Botulism).

• Hypersensitivity: (Tuberculosis).

• Others: (Immune suppression; AIDS).


Agent factors that affect disease transmission:Agent factors that affect disease transmission:

Infectivity, Infectivity,

Pathogenicity, Pathogenicity,

Virulence,Virulence,

AAntigenicity,…ntigenicity,…


InfectivityInfectivity

The ability of an agent to invade and multiply (produce The ability of an agent to invade and multiply (produce

infection) in a susceptible host.infection) in a susceptible host.

How to measure How to measure ((Infectivity);Infectivity); ease & spread of infection?ease & spread of infection?

Secondary Attack RateSecondary Attack Rate

The proportion of exposed susceptible persons who The proportion of exposed susceptible persons who

become infected.become infected.

Examples: High infectivity: Measles, ChickenpoxExamples: High infectivity: Measles, Chickenpox

Low infectivity: LeprosyLow infectivity: Leprosy

100sec

xessusceptiblofNumber

casesondaryofNumberrateattackSecondary


PathogenicityPathogenicityIs the ability of the organisms to produce specific clinical Is the ability of the organisms to produce specific clinical

reaction after infectionreaction after infection

ItIt refers to the proportion of infected persons who develop refers to the proportion of infected persons who develop clinical disease.clinical disease.

Examples: Examples:

High pathogenicity: Measles, Chickenpox (Class B)High pathogenicity: Measles, Chickenpox (Class B)

Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Meningitis, AIDS (Class A)Meningitis, AIDS (Class A)

It can be measured by:It can be measured by:

caseslSubclinica

casesClinical

Ratio of clinical to sub-clinical caseRatio of clinical to sub-clinical case= =


VirulenceVirulence

It refersIt refers the ability of organisms to produce severe pathological the ability of organisms to produce severe pathological reaction.reaction.

It is the proportion of persons with clinical disease who become It is the proportion of persons with clinical disease who become severely ill or die.severely ill or die.

Examples: Examples: Rabies, Hemorrhagic feversRabies, Hemorrhagic fevers caused by Ebola and caused by Ebola and Murberg viruses. (Class C)Murberg viruses. (Class C)

100disease that of cases ofnumber Total

disease a from deaths ofnumber Total ratefatality Case x

It can be measured by: Case fatality rate


Antigenicity (Immunogenicity)Antigenicity (Immunogenicity)

The ability of the organism to produce specific immunity

(antibodies or antitoxin).

It can be measured by:

Second attack frequency:Second attack frequency:

Second attacks are rare in measles, mumps and measles, mumps and

chickenpox. chickenpox.

Re-infection occurs as in case of common cold,common cold, syphilis syphilis

and gonorrhea.and gonorrhea.


Human reservoirsHuman reservoirs

2. Reservoir of infection2. Reservoir of infection

The The reservoir reservoir of an agent is the of an agent is the habitathabitat in which an in which an

infectious agent normally lives, grows, and multiplies. infectious agent normally lives, grows, and multiplies.

Types of reservoirs:Types of reservoirs:

Humans, animals, and the environment.Humans, animals, and the environment.

Two types of human reservoir exist:Two types of human reservoir exist:• • Cases:Cases: persons with symptomatic illness persons with symptomatic illness• • CarriersCarriers


Pat

ho

gen

Res

ervo

irs

Pat

ho

gen

Res

ervo

irs

Humans are the most important

reservoir of human

infectious disease.


CarrierCarrier

A person or animalA person or animal withoutwithout apparent disease who apparent disease who

harbors a specific infectious agent and is harbors a specific infectious agent and is

capable of transmitting the agent to others.capable of transmitting the agent to others.


Carriers are dangerous because:Carriers are dangerous because:

1. They do not show any clinical manifestation so they

carry normal life.

2. The carrier and his contacts are not aware of their

condition so, they take no precautions.

3. It is difficult to discover them.

4. It is not always possible to deal with them.

5. Chronic carriers can remain infectious for a long time

leading to repeated introduction of the disease to

contacts.


Types of Carriers:Types of Carriers:1. Asymptomatic (In-apparent) carrier: The carrier state that may occur in an individual with an

infection that is in-apparent throughout its courseExamples: Polio virus, meningococcus, hepatitis A Examples: Polio virus, meningococcus, hepatitis A virusvirus

2. Incubatory, Convalescent, Post-Convalescent carriers:

The carrier state may occur during the incubation period, convalescence, and post convalescence of an individual with a clinically recognizable disease.Examples of Examples of Incubatory carrierIncubatory carrier: Measles, chickenpox: Measles, chickenpox


Convalescent carriers:Convalescent carriers:

Examples:Examples: Diphtheria, hepatitis B viruses and Salmonella speciesDiphtheria, hepatitis B viruses and Salmonella species

****According to duration of carriage:According to duration of carriage:

The carrier state may be (The carrier state may be (transienttransient carrier or carrier or chronicchronic carrier). carrier).

Chronic carriers:Chronic carriers:

They continue to harbour an agent for anThey continue to harbour an agent for an extended time (months extended time (months

or years) following the initial infection.or years) following the initial infection.

Examples: HExamples: Hepatitis B virus and Salmonella typhiepatitis B virus and Salmonella typhi


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.


ThanThank youk you

Bibliotheca Alexandrina

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

[email protected]

9/10/2010Dr. Salwa Tayel19/10/2010Dr. Salwa Tayel1.

Documents

Transcript of 9/10/2010Dr. Salwa Tayel19/10/2010Dr. Salwa Tayel1.