Medical Microbiology

460 8 VirusesasHumanPathogen

Epidemiology.InfluenzaAvirusesaregeneticallyvariable.Slightantigenic

changesarethegeneralrule(antigenicdrift,quasispecies,p. 39 1)andare

explainedbyselectionofpointmutantsinthehemagglutininunderimmu-

nologicalpressure.Moreprofoundchanges(antigenicshifts)explainthe

periodicoccurrenceofinfluenzaAepidemicsandpandemics(Table8. 5 ).

AntigenicShift

Itisassumedthatanantigenicshiftoccurswhengenesegmentsareexchanged

betweendifferentinfluenzastrainsasfollows:therearetwomajorreservoirsof

influenzaAviruses,humansandcertain(aquatic)birdspecieswhereby,inthelatter,

influenzavirusesoccurwith 13 hemagglutinintypesandnineneuraminidasetypes

innearlyallpossiblecombinations.Mixedinfectionswithavianandhumanvirus

strainsareobservedinpigs,madepossiblebycertainfarmingpractices,e.g.,in

Asiawhereduckand/orpighusbandryarepracticedtogetherwithfishbreeding.

Thismakesitpossiblefordifferentviralstrainstoinfectthesamehostandfortwo

strainstoinfectthesamehostcell,whichcanresultinarecombinationofgene

elementsfromdifferentinfluenzaAstrains.Table8. 5 showsantigenicchanges

inhemagglutininandneuraminidaseobservedinthehumaninfluenzaAvirussince

the 1 930s.

Preventionandtherapy.Aninactivatedadsorbatevaccineandsomesplit

vaccines(new:intranasalapplication)areavailableforinfluenzaprophylaxis.

Thevaccineisrecommendedespeciallyforpersonswhoseoccupationex-

posesthemtosuchinfectionsaswellaspersonswithcardiovascularprob-

lemsintheirmedicalhistories.

Thetherapeuticoptionsincludeamantadine,whichinhibitstheviralun-

coatingprocess,andmorerecentlyneuraminidaseinhibitors.Thesesub-

stancesshortenthedurationofillnessbyblockingthereleaseoftheviruses

fromthehostcellsandtheirfurtherdisseminationinthebody.

Bunyaviruses

&Thebunyavirusandphlebovirusspeciesaretransmittedbyarthropods.

Theycausebenign,febrileinfections,morerarelyinfectionsoftheCNSand

hemorrhagicfever.Allbunyavirusesfeatureasingle-strandedantisenseRNA

genomewiththreesegments.Certaintypesofhantavirusesarethepatho-

gensresponsiblefor“hemorrhagicfeverwithrenalsyndrome”(HFRS),other

typescausethe“hantaviruspulmonarysyndrome”(HPS).Thehantavirus

speciesaretransmittedfrommousespeciestohumansaerogenically.

Diagnosis:serological.

Prevention:exposureprophylaxis. &

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Kayser, Medical Microbiology © 2005 Thieme