Medical Microbiology

RNAviruses 447

CoronavirusReplicationandViralMaturation

Thecoronavirusgenomeconsistsofthelongestknown,senseRNAstrandex-

ceeding 30 kb,whichisintegratedintheenvelopeintheformofahelicalribonu-

cleoprotein.AhallmarkofcoronaviralRNAreplicationistheproductionofseven

subgenomicmRNAs,eachofwhichcodesforoneviralstructuralprotein.Thesynth-

esisofprogenyviralRNAtakesplaceinassociationwithspecializedmembrane

structures,characterizedasdouble-membranevesicles.Viralmaturationtakes

placeintheroughendoplasmicreticulumafterreplacementofcellularproteins

byviralproteinsinthemembranes.ThevirusesarethentransportedtotheGolgi

apparatus.Theensuingvirusreleasemechanismisunknown.Recently,thereceptor

involvedintheentryoftheSARSvirusintothecellwasreportedtobetheangio-

tensin-convertingenzyme 2 (ACE2).

Pathogenesisandclinicalpicture.Commoncold-coronavirusescausean

everydayvarietyofrespiratoryinfections,whicharerestrictedtotheciliated

epitheliaofthenoseandtrachea.Theyareresponsibleforabout 30 %ofcom-

moncoldinfections.

Theimmunityconferredbyinfection,apparentlyIgA-dependent,isshort-

lived.Reinfectionsarethereforefrequent,wherebytheantigenicvariabilityof

thevirusmaybeacontributingfactor.Variousenteralcoronaviruseswith

morphologiessimilartotherespiratorytypeshavealsobeendescribedin

humans.Theirpathogenicity,andhencetheircontributiontodiarrhea,has

notbeenclarified.

TheSARSvirusistransmittedaerogenicallywithanincubationtimeof

twoto 10 days.Clinically,feverandamarkedshortnessofbreathisnoted,

developingintoasevereatypicalpneumoniawithnewpulmonaryinfiltrates

onchestradiography.Sheddingofvirusisbyrespiratorydischarges.Whether

theviruspresentinotherbodyfluidsandexcretaplaysadecisiverolefor

virustransmissionisnotyetclear.

Diagnosis.Thecommon-coldcoronaviruscanbegrowninorgancultures

ofhumantrachealtissueorinhumandiploidcells.Isolatingtheviruses

fordiagnosticpurposesisnotroutine.Serodiagnosis(complement-binding

reaction,immunofluorescenceorenzymeimmunoassay)andelectronmicro-

scopyarefeasiblemethods.

TheSARSviruscanbeidentifiedbyPCRorisolatedintheVerocellline.

Epidemiologyandprevention.InNovember 20 02,anoutbreakofatypical

pneumonia,latertermedSARS,occurredinthesouthernChinesecityof

Guangzhou(GuangdongProvince).OnlyinFebruaryof 20 03,theworld

wasalertedaboutthelungdisease,shortlybeforeitescapedChina,when

aGuangdongresidentinaHongKonghoteltransmittedittootherguests

whospreadittoToronto,Hanoi,Singapore,andelsewhere.Transmission

ofthevirusisbydroplets,butclosecontact(“householdtransmission”)

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Kayser, Medical Microbiology © 2005 Thieme