1. Host Parasite relationship

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18 January 2011 Dr. Salwa Tayel 1


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Associate ProfessorAssociate ProfessorFamily and Community Medicine DepartmentFamily and Community Medicine Department

King Saud UniversityKing Saud University

ByBy

Infectious Disease EpidemiologyInfectious Disease Epidemiology


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1818 JanuaryJanuary 20112011 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:

Understand definitions used in infectiousUnderstand definitions used in infectious

disease epidemiology.disease epidemiology.

Study natural history of infectious diseasesStudy natural history of infectious diseases

and its Implications for public health.and its Implications for public health.


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EpidemiologyEpidemiology is the study ofis the study of

distributiondistribution andand

determinantsdeterminants ofofhealthhealth--related events in specified populations andrelated events in specified populations and

the application of this study to prevent and controlthe application of this study to prevent and control

of health problems.of health problems.

DefinitionsDefinitions


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1818 JanuaryJanuary 20112011Dr. Salwa TayelDr. Salwa Tayel 55

DistributionDistribution

Person.Person.

Place..Place..

TimeTime

Describe diseaseDescribe disease

Incidence RateIncidence Rate

Prevalence RatePrevalence Rate


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The Epidemiologic TriadThe Epidemiologic Triad

HOST

AGENT ENVIRONMENT

Disease DeterminantsDisease Determinants


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DefinitionsDefinitions

Infection:Infection:It is the entry, development and multiplication of anIt 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 resulting fromA clinically manifest disease of man or animal resulting from

infection.infection.

InIn--apparent infection:apparent infection:The infection does not become manifest at any stage.The infection does not become manifest at any stage.


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

toxic products that arises through transmissiontoxic products that arises through transmission

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

Communicable Disease versusCommunicable Disease versus

NonNon-- Communicable DiseaseCommunicable Disease

Communicable DiseaseCommunicable Disease


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Contamination: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 diseasesContagious diseases

A disease that is capable of being transmitted from

one person to another by contact or closeproximity. e.g. scabies, trachoma and leprosy.


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EndemicEndemic

The constant presence of a disease or infectiousThe constant presence of a disease or infectious

agent within a given geographic area or populationagent within a given geographic area or population

group; may also refer to the usual prevalence of agroup; may also refer to the usual prevalence of a

given disease within such area or group.given disease within such area or group.

EpidemicEpidemic

The occurrence of more cases of a disease thanThe occurrence of more cases of a disease than

expected in a given area or among a specificexpected in a given area or among a specific

group of people over a particular period of time.group of people over a particular period of time.


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PandemicPandemic

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,Influenza, choleracholera

OutbreakOutbreak

A more or less localized epidemic affecting large number ofA more or less localized epidemic affecting large number of

a group, in the communitya group, in the community

e.g. outbreak ofe.g. outbreak offood poisoningfood poisoning


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SporadicS

poradic

Cases occur irregularly, haphazardly from timeCases occur irregularly, haphazardly from time

to time and generally infrequently.to time and generally infrequently.

Cases are few and separated widely in space andCases are few and separated widely in space and

time showing no connection to each other.time showing no connection to each other.


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Nosocomial infectionNosocomial infection (Hospital acquired infection):(Hospital acquired infection):

It is an infection occurring in a patient while in aIt is an infection occurring in a patient while in a

hospital or other health care facilityhospital or other health care facility

It should not be present or incubating at the time ofIt should not be present or incubating at the time of

admission.admission.


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Elimination of diseaseElimination of disease

Disease incidence is reduced to a minimal level atDisease incidence is reduced to a minimal level atwhich the disease is no longer considered a publicwhich the disease is no longer considered a public

health problem, while infection may still occurhealth problem, while infection may still occur

Example:Example:

The aim of elimination of Neonatal tetanus isThe aim of elimination of Neonatal tetanus is

reduction of its incidence to less thanreduction of its incidence to less than

one case/one case/10001000 live births.live births.


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Eradication:Eradication:

It means worldwide disappearance of aIt means worldwide disappearance of adisease (permanent reduction to zero level)disease (permanent reduction to zero level)

with complete destruction of the agent.with complete destruction of the agent.

The organism can be present only inThe organism can be present only in

laboratories and no need for interventions.laboratories and no need for interventions.

e.g. smallpox eradication from the world sincee.g. smallpox eradication from the world since19791979..


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Cause of disease.Cause of disease.

A factor (characteristic, behavior, event, etc.) that directlyA factor (characteristic, behavior, event, etc.) that directly

influences the occurrence of disease. A reduction of theinfluences the occurrence of disease. A reduction of the

factor in the population should lead to a reduction in thefactor in the population should lead to a reduction in the

occurrence of disease.occurrence of disease.

Risk factor:Risk factor:

An aspect of personal behavior or lifestyle, anAn aspect of personal behavior or lifestyle, an

environmental exposure, or an inborn or inheritedenvironmental exposure, or an inborn or inherited

characteristic that ischaracteristic that is associatedassociated with an increasedwith an increased

occurrence of disease or other healthoccurrence of disease or other health--related event orrelated event or

condition.condition.


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Natural historyNatural history

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

disease process in an individual over time, in thedisease process in an individual over time, in the

absence of intervention.absence of intervention.

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

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

ofofexposureexposure to the causal agents till one of the possibleto the causal agents till one of the possible

outcomesoutcomes occurs.occurs.


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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 thesymptoms 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?Why do we have varying degrees of severity?

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

and environmental factors.and environmental factors.


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The Epidemiologic TriadThe Epidemiologic Triad

HOST

AGENT ENVIRONMENT

Disease DeterminantsDisease Determinants


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The epidemiologic triad ModelThe epidemiologic triad Model

The epidemiologic triangle or triad is the traditionalThe epidemiologic triangle or triad is the traditional

model of infectious disease causation.model of infectious disease causation.

It has three components: an external agent, aIt has three components: an external agent, a

susceptible host, and environment factors thatsusceptible host, and environment factors that

interrelate in a variety of complex ways to produceinterrelate in a variety of complex ways to produce

disease in humans.disease in humans.


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Agent factorsAgent factors

Agent refer to an infectious microorganismAgent refer to an infectious microorganismvirus,virus,

bacterium, parasite, or other microbe.bacterium, parasite, or other microbe.

Generally, these agents must be present for disease toGenerally, these agents must be present for disease to

occur.occur.

That is, they areThat is, they are necessarynecessarybutbut notnot alwaysalways sufficientsufficient

to cause disease.to cause disease.


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Host factorsHost factors

Host factors are intrinsic factors that influence an individuals

exposure, susceptibility, or response to a causative agent.

Host factors which affect a persons likelihood of exposure:Host factors which affect a persons likelihood of exposure:

e.g. Age, race, sex, socioeconomic status, and behaviors

(smoking, drug abuse, lifestyle, sexual practices and

contraception, eating habits)

Host factors which affect susceptibility and response to an agent:Host factors which affect susceptibility and response to an agent:

Age, genetic composition, nutritional and immunologic status,anatomic structure, presence of disease or medications, and

psychological makeup.


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Environmental factorsEnvironmental factors

Environmental factors areEnvironmental factors are extrinsicextrinsic factors which affect thefactors which affect theagent and the opportunity for exposure.agent and the opportunity for exposure.

Environmental factors include:Environmental factors include:

physical factors such as geology, climate, and physicalphysical factors such as geology, climate, and physical

surroundings (e.g., a nursing home, schools);surroundings (e.g., a nursing home, schools);

biologic factors such as insects that transmit the agent; andbiologic factors such as insects that transmit the agent; and

socioeconomic factors such as crowding, sanitation, and thesocioeconomic factors such as crowding, sanitation, and the

availability of health services.availability of health services.


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Factors Affecting Disease Transmissionand Symptomatic Clinical Disease

Host

Vector

Agent Environment

-- Susceptibility-- Immune response-- Resistance--- Portal(s) of entry

-- Virulence Toxigenicity-- Infectivity Resistance-- Pathogenicity Antigenicity

VECTOR-- Prevalence-- Portal(s) of entry

-- Balance of immune tosusceptible individuals

--- Opportunity forexposure (e.g. crowding)


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Terms used to describe an infectious diseaseTerms used to describe an infectious disease

1.1. InfectivityInfectivity refers to the proportion of exposed persons whorefers to the proportion of exposed persons who

become infected (secondary attack rate).become infected (secondary attack rate).

2.2. PathogenicityPathogenicity refers to the proportion of infected personsrefers to the proportion of infected persons

who develop clinical disease (clinical/subclinical ratio).who develop clinical disease (clinical/subclinical ratio).

3.3. VirulenceVirulence refers to the proportion of persons with clinicalrefers to the proportion of persons with clinical

disease who become severely ill or die (case fatality rate).disease who become severely ill or die (case fatality rate).


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

(Low pathogenicity and low virulence)

Classification of diseases according to spectrumClassification of diseases according to spectrum

Class A: Inapparent infection


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Examples: Measles, Chickenpox

(High pathogenicity & low virulence)

Class B: Classic casesClass B: Classic cases


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

((High pathogenicity & high virulence)High pathogenicity & high virulence)

Class C: Severe or Fatal infections


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Implications for public healthImplications for public health


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Cases of illness correctly diagnosed by clinicians in theCases 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 mayMany 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 inis that persons with in--apparent or undiagnosedapparent or undiagnosed

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

Iceberg phenomenonIceberg phenomenon


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Timelines for Infection and Symptomatic DiseaseTimelines for Infection and Symptomatic Disease

Dynamics of

infectiousness latentperiod

infectiousperiod

noninfectious-- removed-- dead

-- recoveredsusceptible

time

Dynamics of

disease

susceptible

time

incubationperiod symptomaticperiod

-- dead

-- recovered-- immune-- carrier

E


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The pyramid and iceberg of diseaseThe pyramid and iceberg of disease

11 Diseased, diagnosed & controlledDiseased, diagnosed & controlled

22 Diagnosed,Diagnosed, uncontrolleduncontrolled

33 Undiagnosed or wronglyUndiagnosed or wrongly

diagnosed diseasediagnosed disease

44 Risk factors for diseaseRisk factors for disease

55 Free of risk factorsFree of risk factors

DiagnosedDiagnosed

diseasedisease

Undiagnosed orUndiagnosed or

wrongly diagnosed diseasewrongly diagnosed disease


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Applications of the concepts of natural historyApplications of the concepts of natural history

and spectrum of diseaseand spectrum of disease

Persons with inPersons with in--apparent or undiagnosed infections canapparent or undiagnosed infections can

transmit infections to others.transmit infections to others.

Control measures must not be directed solely for clinicallyControl measures must not be directed solely for clinically

apparent cases.apparent cases.

Control measures must be directed toward all infectionsControl measures must be directed toward all infections

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

both clinically apparent cases andboth clinically apparent cases and

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


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The EndThe End

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Documents

1. Host Parasite relationship