🦠 VIRUS Defination, morphology, lab diagnosis

Virology

1. ???VIRUS–COMPLETECHAPTER(DrJagroopSinghSidhu)
1.DEFINITIONOFVIRUS
Avirusisasubmicroscopicinfectiousagentthatcanreplicateonly
insidelivinghostcellssuchasbacteria,plants,oranimals.
Virusesareacellular,meaningtheydonothaveacellularstructurelike
othermicroorganisms.
TheycontaineitherDNAorRNAastheirgeneticmaterial(neverboth).
Theylacktheenzymesandmachineryrequiredformetabolismand
dependcompletelyonthehostcellforreplication.
Examples:
HIV(HumanImmunodeficiencyVirus),Influenzavirus,HepatitisBvirus,
Coronavirus,Herpessimplexvirus.
2.MORPHOLOGYOFVIRUS
A.Size
Extremelysmall:20–300nanometers(nm).
Visibleonlyunderanelectronmicroscope.
B.Structure
1.NucleicAcid
oTheviralgenomeismadeupofeitherDNAorRNA,whichcanbe
single-stranded(ss)ordouble-stranded(ds).
oItcarriesgeneticinformationnecessaryforreplication.
2.Capsid
oAproteincoatthatsurroundsandprotectstheviralnucleicacid.
oComposedofsmallersubunitscalledcapsomeres.
oProvidesshapeandrigiditytothevirus.
3.Nucleocapsid
oThecombinationofnucleicacid+capsid.

2. 4.Envelope(insomeviruses)
oAlipoproteinmembranederivedfromthehostcellmembrane.
oContainsglycoproteinspikesthathelpinattachmenttohostcells.
oEnvelopedvirusesarelessresistanttophysicalandchemical
agents.
oNon-envelopedviruses(nakedviruses)aremoreresistant.
5.ShapeofViruses
Shape Example
IcosahedralAdenovirus
Helical Rabiesvirus,Influenzavirus
ComplexPoxvirus,Bacteriophage

3. 3.EFFECTOFPHYSICALANDCHEMICALAGENTSONVIRUSES
Agent EffectonVirus
Heat Inactivatedat56°Cfor30minutes.
Envelopedvirusesaremoreheat-
sensitive.
Cold Virusescansurviveforlongat–70°Cor
inliquidnitrogen.
Drying(Desiccation) Non-envelopedvirusesresistdrying;
envelopedvirusesareeasilydestroyed.
Ultraviolet(UV)Light Damagesnucleicacidandinactivates
viruses.
pH MostvirusesarestableatpH5–9;
extremepHdestroysthem.
EtherandChloroform Dissolvethelipidenvelopeand
inactivateenvelopedviruses.
Disinfectants (Phenol,
Formaldehyde, Sodium
Hypochlorite)
Destroyviralproteinsandnucleicacids.
Detergents Damagetheviralenvelopeandcapsid
proteins.
4.COLLECTIONANDTRANSPORTATIONOFSAMPLESFORVIRAL
DIAGNOSIS
A.GeneralPrinciples
Collectspecimensearlyinthecourseofillnesswhenviralloadis
highest.
Usesterilecontainersandaseptictechniques.
Avoidcontaminationwithbacteriaorfungi.
Labeleachspecimenproperly(patientname,ID,date,specimentype).
B.CommonSpecimens
TypeofInfection SpecimenCollected

4. RespiratoryinfectionNasopharyngealorthroatswab,sputum
CNSinfection Cerebrospinalfluid(CSF)
Hepatitis Blood/Serum
Skinlesions Vesiclefluid,scabs,scrapings
Genitalinfection Swabfromthelesionordischarge
Entericinfection Stoolsample
Eyeinfection Conjunctivalswab
C.TransportMedium
ViralTransportMedium(VTM)isusedforswabsandsecretions.
oContainsbuffer,proteinstabilisers,andantibioticstomaintainviral
viability.
D.StorageandTransportConditions
Transportat2–8°Cifprocessedwithin48hours.
Forlongerstorage,freezeat–70°C.
Avoidrepeatedfreezingandthawing.
E.SafetyPrecautions
UsePPE(gloves,mask,labcoat).
Handlesamplesinbiosafetycabinets.
Followbiosafetylevel-2orlevel-3precautionsdependingonvirustype.
5.LABORATORYDIAGNOSISOFVIRALINFECTIONS
A.Purpose
Toconfirmclinicaldiagnosis.
Toidentifythespecificviruscausingthedisease.
Tomonitorthecourseofinfectionandtreatment.
Tostudyepidemiologyandoutbreaks.
B.StepsinLaboratoryDiagnosis

5. 1.Collectionofspecimen
2.Transportofspecimen
3.Processinganddetectionofvirus/viralcomponents
4.Interpretationofresults
C.MethodsofDiagnosis
1.DirectDemonstrationofVirus/ViralAntigen
Method Principle Examples
ElectronMicroscopy
(EM)
Directvisualizationofviral
particles
Rotavirus,Adenovirus
LightMicroscopy Demonstrationofinclusion
bodiesininfectedcells
Negribodies(Rabies),
Cowdry bodies
(Herpes)
Immunofluorescence
(IFA)
Detectsviralantigensusing
fluorescent-labeled
antibodies
RSV, Influenza,
Rabies
ELISA (Antigen
Detection)
Enzyme-linkedantibodies
detectviralantigen
HepatitisB,HIV(p24
antigen)
LatexAgglutination Antigen-antibodyclumping
visiblemacroscopically
Rotavirusinstool
2.VirusIsolationinCulture
Consideredthegoldstandardmethod.
Requireslivingcellstosupportviralreplication.
SystemsUsed:
1.Animalinoculation(rarelyusedtoday)
2.Embryonatedeggs(usedforInfluenzavirus)
3.Cellculture(mostcommonlyused)
TypesofCellCultures:

6. Type Example Feature
PrimarycelllineMonkeykidneycells Shortlifespan,highly
sensitive
Diploidcellline Human embryonic
fibroblast
Moderatelifespan
Continuouscell
line
HeLa,Hep-2 Immortal,easymaintenance
CytopathicEffect(CPE):
Visiblemorphologicalchangesininfectedcellssuchas:
oRounding
oSyncytiaformation
oInclusionbodies
3.MolecularMethods(DetectionofViralNucleicAcid)
Test Principle Example
PCR (Polymerase
ChainReaction)
AmplifiesviralDNA HBV,Herpesvirus
RT-PCR (Reverse
TranscriptasePCR)
ConvertsviralRNA→cDNA
→amplifies
SARS-CoV-2,
HCV,Influenza
Real-Time PCR
(qPCR)
QuantifiesviralloadinrealtimeHIV,COVID-19
Nucleic Acid
Hybridization
LabeledDNA/RNAprobes
detect complementary
sequences
HPV
Advantages:Highlysensitiveandspecific.
Disadvantage:Expensiveandneedsexpertise.
4.SerologicalTests(DetectionofAntibodiesorAntigens)
Test Principle/Use Example

7. ELISA Detectsantigenorantibody HIV,HBV,HCV
NeutralizationTest Measuresabilityofantibodiesto
neutralizevirus
Enteroviruses
Hemagglutination
InhibitionTest
Detectsantibodythatinhibits
virus-inducedRBCclumping
Influenza
WesternBlot Detectsspecificviralproteins Confirmatory
testforHIV
RapidCardTest Immunochromatography-based
rapiddetection
DengueNS1,
HBsAg,HIV
Interpretation:
IgMantibodies→Recentinfection
IgGantibodies→Pastinfectionorimmunity
5.DetectionofViralEffectsinTissues
HistopathologicalExaminationcanshow:
oInclusionbodies
oCellnecrosis
oMultinucleatedgiantcells(syncytia)
Examples:
Negribodies–Rabies
Owl’seyeinclusion–Cytomegalovirus
CowdryTypeAinclusion–HerpesSimplexVirus
D.ModernRapidDiagnosticTechniques
RT-PCRandReal-timePCR–Mostsensitiveandspecific.
MultiplexPCR–Detectsseveralvirusesatonce.
NextGenerationSequencing(NGS)–Usedfordetailedgeneticstudies.
RapidAntigenTests(RDTs)–Usedforquickdetection(Dengue,
COVID-19).

8. Point-of-CareTests–Forbedsideorfieldtesting.
E.InterpretationofResults
Resultsmustalwayscorrelatewithclinicalfeaturesand
epidemiologicaldata.
False-negativeresults:duetopoorsamplecollectionorlowviralload.
False-positiveresults:duetocontaminationorcross-reactivity.
6.SUMMARYTABLE
DiagnosticMethodDetects Example
DirectmicroscopyVirus/inclusionbodiesRabies,CMV
AntigendetectionViralproteins HIV,HBV
Virusisolation Wholevirus Influenza
MolecularmethodsViralgenome SARS-CoV-2,HCV
Serology Antibodies HIV,Dengue
7.KEYPOINTS
Virusesareacellularobligateintracellularparasites.
TheyhaveeitherDNAorRNA,surroundedbyacapsid,andsometimes
anenvelope.
Envelopedvirusesaresensitivetoheat,ether,anddisinfectants.
Propercollection,transport,andstorageofsamplesareessentialfor
accurateresults.
PCR-basedtechniquesandELISAarethemostcommonlyused
diagnosticmethodsinmodernvirology.