2. Natural of the history odf dieseases-1.ppt

Meaningful of slides

1. Natural History of Diseases

2. Contents
•Introduction
•Naturalhistoryofdisease
•Modelsofdiseasecausation
•Factorsofdiseasecausation
•Patterns of Disease Occurrence

3. Introduction
•Knowledgeofthenaturalhistoryofdiseaseranksalongside
causalunderstandinginimportancefordiseasepreventionand
control.
•Naturalhistoryofdiseaseisoneofthemajorelementsof
descriptive epidemiology.

4. Natural history of diseases
Naturalhistoryofdiseasereferstotheprogressofa
diseaseprocessinanindividualovertime,inthe
absenceofintervention.
Theprocessbeginswithexposuretooraccumulation
offactorscapableofcausingdisease.
Withoutmedicalintervention,theprocessendswith
Recovery
Disability
Death.
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5. Natural history of disease
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6. ▶Naturalhistoryofdiseasecanbewell
establishedbycohortstudy
▶Asthesestudiesarecostlyandlaborious,
understandingnaturalhistoryofdiseaseis
largelybasedonotherepidemiologicalstudies
suchascrosssectionalandretrospective
studies.
NATURALHISTORYOFDISEASE

7. Natural history of diseases
Consists of two phases :-
PREPATHOGENESIS
The process in the environment
PATHOGENESIS
The process in the man
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8. Pre-pathogenesis phase
This refers to the period preliminary to the onset of disease in
man.
The disease agent has not yet entered man, but the factors which
favor its interaction with the human host are already existing in
the environment.
Weall are in pre pathogenesis phase many of communicable
and non communicable diseases
This situation is frequently referred to as “man in the midst of
disease” or “man exposed to the risk of disease”.
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9. Pathogenesis phase
Thisphasebeginswithentryofthedisease“agent”inthe
susceptiblehumanhost.
Aftertheentry,agentmultipliesandinducestissueand
physiologicalchanges,thediseaseprogressesthroughthe
periodofincubationandlaterthroughtheperiodofearlyand
latepathogenesis.
Thefinaloutcomeofthediseasemayberecovery,disabilityor
death.
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10. NATURALHISTORYOFDISEASE
DEATH
DISABILITY
RECOVERY
ENTRY OF
PATHOGEN
CLINICALSIGN
ORSYMPTOM
DIAGNOSIS
PRE
PATHOGENESIS PATHOGENESIS
CLINICAL

11. ICEBERGCONCEPTOFDISEASE
DEATH
SEVERE
DISEASE
MILDILLNESS
INFECTION WITHOUT
CLINICAL ILLNESS
EXPOSURE WITHOUT
INFECTION
CLINICAL
DISEASE
SUBCLINICAL
DISEASE

12. MODELSOFDISEASECAUSATION
MIASMA THEORY
GERM THEORY OF DISEASE
EPIDEMIOLOGICAL TRIAD
BEING’S MODEL
WEB OF CAUSATION THEORY
WHEEL THEORY

13. MIASMATHEORY
n
History of disease causation goes back
to Miasma theory
It says “ diseases such as cholera,
chlamydia or black death caused by
Miasma, means bad air
It was most accepted theory till 19th
century when it was replaced by Germ
theory of disease

14. GermTheoryofDisease
•Proposed by Robert Koch and Louis Pasteur.
•Every human disease is caused by a microbe orgerm, which is
specific for that disease and one must be able to isolate the
microbe from the diseased human being.
ROBERTKOCH LOUISPASTEUR

15. GERM THEORY
OF DISEASE
▶Germ theory of disease was first
proposed in 1546
▶Louis Pasteur proved that the diseases
are caused by organisms in 19
th
century.
▶Supported by John Snow

16. Epidemiological Triad
16

17. Factors that may be associated with
increased risk of human disease
17

18. Agentcharacteristics
•Infectivity refers to the proportion of exposed persons who
become infected.
•Pathogenicity refers to the proportion of infected persons who
develop clinical disease.
•Virulence refers to the proportion of persons with clinical
disease who become severely ill or die.

19. Agentcharacteristics
•HepatitisAvirusinchildrenhaslowpathogenicityandlow
virulence,sincemanyinfectedchildrenremainasymptomaticand
fewdevelopsevereillness.
•Inpersonswithgoodnutritionandhealth,measlesvirushashigh
pathogenicitybutlowvirulence,sincealmostallinfectedpersons
developthecharacteristicrashandillnessbutfewdevelopthelife-
threateningpresentationsofmeasles(pneumonia,encephalitis).
•Inpersonswithpoornutritionandhealth,measlesisamorevirulent
disease,withmortalityashighas5-10%.
•Rabiesvirusisbothhighlypathogenicandvirulent,sincevirtually
100%ofallinfectedpersons(whodonotreceivetreatment)
progresstoclinicaldiseaseanddeath.

20. Environment
•Physical environment
–Non living things and physical factors(air, water, soil, housing,
heat, light, etc)
•Biological environment
–Microbial agents, insects, animals, plants and man himself.
•Psychosocial environment
–Lifestyle, poverty, urbanization, community life, income,
education, stress etc.

21. ▶Asaresultofadvanceinpublichealth,communicable
diseasesdeclinedandriseofnoncommunicablediseases
calledmoderndiseasesisseen.
▶Thesediseasescouldnotbeexplainedbysinglecause
idea.
▶Itisnowknownthatthesediseasesareduetomultiple
factorswhichleadstoformationofadvancedmodelof
triangleofepidemiology.
EPIDEMIOLOGICAL TETRAD

22. EpidemiologicalTetrad
•Agent
•Host
•Environment
•Time

23. The “BEINGS” Model of Disease
Causation
•Thisconceptpostulatesthathumandiseaseanditsconsequencesarecaused
bya complex interplayofninedifferentfactors–
–Biologicalfactorsinnateinahumanbeing,
–Behaviouralfactorsconcernedwithindividuallifestyles,
–Environmentalfactorsas physical,chemical andbiologicalaspectsof
environment,
–Immunologicalfactors,
–Nutritionalfactors,
–Geneticfactors,
–Socialfactors,
–Spiritualfactorsand
–Servicesfactors,relatedtothevariousaspectsofhealthcareservices.

24. TheTheoryof“WebofCausation”
•The“epidemiologicaltriadtheory”wasveryeffectivelyused
byLeavelandClarkinexplainingthenaturalhistoryof
diseaseandlevelsofpreventionforobviatingsuchdepartures
fromthestateofhealth.
•Butitcouldnotexplainthecausationofnoncommunicable
diseaseslikeIHDorroadaccidents.

25. Webofcausation
•McMahonandPughforwardedthetheoryof“epidemiological
webofcausation”,
hypercholesterolemia,
whereinthevariousfactors(e.g.
smoking,hypertension)arelikean
interactingwebofaspider.
•Eachfactorhasitsownrelativeimportanceincausing
thefinaldeparturefromthestateofhealth,aswellas
interactswithothers,modifyingtheeffectofeachother.

26. WebofCausationapplied toMyocardialInfarction

27. ExampleofWebofCausation
SusceptibleHost
Infection Tuberculosis
Vaccination
Genetic
Overcrowding
Malnutrition
Tissue Invasion and
Reaction
Exposure to
Mycobacterium

28. PHYSICALENVIRONMENT
SOCIALENVIRONMENT
BIOLOGICALENVIRONMENT
WHEEL THEORY OF
CAUSATION

29. FactorsofDisease Causation
a)Predisposingfactorsarefactorswhichcreateastateofsusceptibility,so
thatthehostbecomesvulnerabletotheagentortonecessarycause,e.g.
age,sex,previousillness.
b)Enablingfactorsarethosewhichassistinthedevelopmentof(orin
recoveryfrom)thedisease;e.g.housingconditions,socio-economic
status.
c)Precipitatingfactorsarethosewhichareassociatedwithimmediate
exposuretothediseaseagentoronsetofdisease,e.g.drinking
contaminatedwater,closecontactwithacaseofpulmonaryTB.

30. FactorsofDiseaseCausation
e)Reinforcingfactorsarethosewhichaggravateanalreadyexisting
disease, e.g. malnutrition, repeated exposures
f)Riskfactors:Ariskfactorisdefinedasacondition,qualityorattribute,
thepresenceofwhichincreasesthechancesofanindividualtohave,
developorbeadverselyaffectedbyadiseaseprocess.Ariskfactorisnot
necessarilythecauseofadiseasebutdoesincreasetheprobabilitythata
personexposedtothefactormaygetthedisease.

31. Exposureto
TB
Infection(10-30%)
Noinfection(70-90%)
ActiveTB(10%ill)
-5%developTBwithin2years
-5%developTBmanyyearslater
LatentTB(90%well)
-NeverdevelopTB
-Notinfectious
Treated
Untreated
Cured
50%diewithin5years
25%remainsick
25%recover

32. Spectrum of Disease
The term “spectrum of disease” is a graphic
representation of variations in the manifestations of
disease.
At the one end of disease spectrum are subclinical
infections which are not ordinarily identified, and
at the other end are fatal illnesses.
In the middle of spectrum lie illnesses ranging in
severity from mild to severe.
These different manifestations are the result of
individuals’ different states of immunity and
receptivity.
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33. Classification of diseases according to their
spectrum
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34. Class A: Inapparent infection
Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS
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35. Class B: Classic cases
Examples: Measles, Chickenpox
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36. Class C: Severe or Fatal
infections
Examples: Rabies, Hemorrhagic fevers caused by Ebola
and Murberg viruses.
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