Medical Microbiology

RNAviruses 441

Pathogen.Thetermtogavirusesformerlyincludedavarietyofviruses,

includingwhatwenowclassifyastheflaviviruses.Asdefinedtoday,the

togavirusesincludethezoopathicpestiviruses,onespeciesofrubivirus,

therubellavirusandthealphaviruseswith 25 species.Thealphaviruses

mostimportanttotravelersaretheChikungunyavirus(Africa,Asia),the

Sindbisvirus(Africa,Asia,Australia),theRossRivervirus(Australia,Oceania),

andtheMayarovirus(SouthAmerica),whicharetransmittedtohumansby

bloodsuckingmosquitoes.

Togavirusespossessanicosahedralcapsidandaclosefittingenvelope.The

capsidmeasures 35 – 40 nmandtheentirevirion 60 – 65 nm.Thegenomeof

thetogavirusesisasingle-stranded,polyadenylated,senseRNA.Replication

notonlyproducesnew40SgenomicRNA,butasubgenomic26SRNAfrag-

mentaswell,whichcodesforthecapsidproteins.Viralprogenyarereleased

by“budding”atthecellsurface.

Pathogenesisandclinicalpicture.Thearthropodbornealphaviruses,zoo-

nosesofthetropicalandsubtropicalregions,frequentlycauseasymptomatic

orbenigninfectionswithfever,exanthem,andjointpain.Occasionally,how-

ever,persistentarthralgiaandpolyarthritis(lastingmonthsorevenyears)do

occur,sometimesinvolvingjointdestruction.Evenrarer,sequelaeincludeen-

cephalitisandmeningoencephalitiswithhighlethalityrates.

“Germanmeasles”isaharmlessexanthemousinfectioninchildrenand

youths,causedbyarubivirus,therubellavirus,andtransmittedbydirect

contact.Theinfectionsremaininapparentinnearlyhalfthecases.Thevirus

atfirstreplicatesinlymphoidorgansattheportalofentryandinthenaso-

pharyngealspace,afterwhichaviremiadevelopsbeforetheexanthemman-

ifests.Inpregnantwomen,thevirustakesthisroutethroughtheplacentato

theembryo,whereitcancausecongenitaldeformitiesorembryonicdeath,

especiallyinthefirstthreemonthsofpregnancy.Theorgansinthedevelop-

mentalstageinthistrimesteraremostseriouslyaffectedbytherubellain-

fection.Themostfrequentcongenitaldeformitiesaredeafness,cataracts,car-

diacdefects,microcephaly,andspinabifida.Inintrauterineembryodeaths

duetorubellainfectionstheimmediatecauseofdeathisusuallymyocardial

damage.AmeaslesinfectionconfirmedbyIgMdetectionoraraisedantibody

countisthereforeanindicationforafirst-trimesterabortion.

Diagnosis.Serodiagnosisisthemethodofchoiceinsuspectedalphavirusand

rubivirusinfections.EIAmethodsarealsoavailableforIgMdetection.

Prevention.Therearevaccinestoprotectagainstalphavirusinfectionsand

rubella.Themainaimofrubellaprophylaxisistopreventrubella-causedem-

bryopathies.Since 10 – 15 %ofyoungadultsarestillsusceptibletorubellain-

fectionsandalivevaccinewithfewsideeffectsthatconfersreliableimmu-

nityisavailable,serialvaccinationofchildren(boysandgirls!)isdonebefore

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Kayser, Medical Microbiology © 2005 Thieme