Natural History and Spectrum of Disease lecture 4.ppt

Notes

1. EPIDEMIOLOGY / HIKMET
QUBEILAT
Natural History and Spectrum of Disease

2. Natural history of disease refers to the progress of a
disease process in an individual over time, in the absence
of intervention.
The process begins with exposure to or accumulation of
factors capable of causing disease.
Without medical intervention, the process ends with
recovery ,disability, or death
EpidemiologyC. Kinya

3. Epidemiology C. Kinya

4. Susceptible
host
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent Infectious Non-infectious
Infection
No infection
Clinical disease

5. Latent period
the time interval from exposure to development of
infectiousness
Infectious period
the time during which the host can infect another
susceptible host
Non-infectious period
the period when the host’s ability to transmit disease
to other hosts ceases
Incubation period
the time interval between exposure to development of
clinical disease

6. Infectivityreferstotheproportionofexposedpersons
whobecomeinfected.
Pathogenicityreferstotheproportionofinfectedpersons
whodevelopclinicaldisease.
Virulencereferstotheproportionofpersonswithclinical
diseasewhobecomeseverelyillordie
Epidemiology C. Kinya

7. HepatitisAvirusinchildrenhaslowPathogenicityandlowvirulence,since
manyinfectedchildrenremainasymptomaticandfewdevelopsevereillness.
Inpersonswithgoodnutritionandhealth,measlesvirushashigh
Pathogenicitybutlowvirulence,sincealmostallinfectedpersonsdevelop
thecharacteristicrashillnessbutfewdevelopthelife-threatening
presentationsofmeasles,pneumonia,orencephalitis.
Inpersonswithpoorernutritionandhealth,measlesisamorevirulent
disease,withmortalityashighas5-10%.Finally,rabiesvirusisbothhighly
pathogenicandvirulent,sincevirtually100%ofallinfectedpersons(whodo
notreceivetreatment)progresstoclinicaldiseaseanddeath.
Epidemiology C. Kinya

8. Thenaturalhistoryandspectrumofdiseasepresents
challengestotheclinicianandtothepublichealth
worker.
Becauseoftheclinicalspectrum,casesofillness
diagnosedbycliniciansinthecommunityoftenrepresent
onlythe“tipoftheiceberg.”Manyadditionalcasesmay
betooearlytodiagnoseormayremainasymptomatic.
Epidemiology C. Kinya

9. Forthepublichealthworker,thechallengeisthat
personswithinapparentorundiagnosedinfections
maynonethelessbeabletotransmitthemtoothers.
Suchpersonswhoareinfectiousbuthavesubclinical
diseasearecalledcarriers.
Epidemiology C. Kinya

10. Frequently,carriersarepersonswithincubating
diseaseorinapparentinfection.Personswithmeasles,
hepatitisA,andseveralotherdiseasesbecome
infectiousafewdaysbeforetheonsetofsymptoms.
Ontheotherhand,carriersmayalsobepersonswho
appeartohaverecoveredfromtheirclinicalillness,
suchaschroniccarriersofhepatitisBvirus
Epidemiology C. Kinya

11. Levelofdisease;Theamountofaparticulardiseasethatis
usuallypresentinacommunityisthebaselinelevelofthe
disease.
Thislevelisnotnecessarilythepreferredlevel,whichshould
infactbezero;ratheritistheobservedlevel.
Theoretically,ifnointerventionoccurredandifthelevelislow
enoughnottodepletethepoolofsusceptiblepersons,the
diseaseoccurrenceshouldcontinueatthebaselinelevel
indefinitely.
C. kinya

12. Thus,thebaselinelevelisoftenconsideredtheexpected
levelofthedisease.
Forexample,overthepast4yearsthenumberof
reportedcasesofpoliomyelitishasrangedfrom5to9.
Therefore,assumingthereisnochangeinpopulation,we
wouldexpecttoseeapproximately7reportedcasesnext
year.
C. kinya

13. Differentdiseases,indifferentcommunities,showdifferent
patternsofexpectedoccurrence:
apersistentlevelofoccurrencewithalowtomoderate
diseaselevelisreferredtoasanendemiclevel
apersistentlyhighlevelofoccurrenceiscalleda
hyperendemiclevel
anirregularpatternofoccurrence,withoccasionalcases
occurringatirregularintervalsiscalledsporadic.

14. Occasionally,thelevelofdiseaserisesabovetheexpected
level.Whentheoccurrenceofadiseasewithinanareais
clearlyinexcessoftheexpectedlevelforagiventime
period,itiscalledanepidemic.
Publichealthofficialsoftenusethetermoutbreak,which
meansthesamething,becauseitislessprovocativetothe
public.
Whenanepidemicspreadsoverseveralcountriesor
continents,affectingalargenumberofpeople,itiscalleda
pandemic.
C. KINYA

15. Epidemicsoccurwhenanagentandsusceptiblehostsarepresentin
adequatenumbers,andtheagentcaneffectivelybeconveyedfroma
sourcetothesusceptiblehosts.Morespecifically,anepidemicmay
resultfromthefollowing:
1.Arecentincreaseinamountorvirulenceoftheagent
2.Therecentintroductionoftheagentintoasettingwhereithasnot
beenbefore
3.Anenhancedmodeoftransmissionsothatmoresusceptibleare
exposed
4.Somechangeinthesusceptibilityofthehostresponsetotheagent

16. Epidemics can be classified according to their manner of
spread through a population:
Common-source
 Point
 Continuous
 Intermittent
Propagated
Mixed
Other