Medical Microbiology

444 8 VirusesasHumanPathogen

cludesapronouncedviremia.Theillness,inthisstageoftennotrecognized

asaflavivirusinfection,maythenbeoveroritmayprogressafteroneto

threedaystoasecond,severeclinicalpicture:ahemorrhagicfeverwitha

highlethalityrateinvolvinghemorrhagesandintravasalcoagulation.In

Denguefever,thisformisbecomingmoreandmorefrequentandiscalled

Denguehemorrhagicfever(DHF)orDengueshocksyndrome(DSS)depend-

ingonthepredominantcharacteristics.

DengueHemorrhagicFever(DHF)orDengueShockSyndrome(DSS)

OnereasonforthemanifestationofmoresevereDenguecoursesistheincreasing

mobilityofthepopulace.Thisleadstothephenomenonofpeopleovercomingone

Dengueinfection,thentravelingtoanareawhereadifferentDengueserotypeis

endemic.Whathappensiftheyareinfectedagainisknownasan“antibody-depen-

dentenhancementofviralinfection”orADE,seep.399).Antibodiesfromthefirst

infectionattachtothevirusesofthefreshinfection,whichare,however,notneu-

tralized,butinsteadallowedentranceintocellsviaFcreceptors,resultinginDHF

andDSS.Childrenstillcarryingantibodiesfromtheirmotherduringthefirstyearof

lifecanalsoexperiencethesesevereinfectioncoursesduetothesamemechanism.

Diagnosis.Aflavivirusinfectionalwaysinvolvesviremia(transmissionby

bloodsuckingarthropods!).Thevirusescanbeisolatedfrombloodbyinocu-

latingcellculturesornewbornmice.Inautopsiesoffatalcasestheycanbe

isolatedfromlivertissue.Thevirusesarelabilebynatureandidentification

cantaketime,forwhichreasonthediagnosticfocusisonserology(titerrise

orIgMdetection).

Epidemiologyandprevention.Acycleofinfectioninvolvingavertebrate

host(mammals,birds)andatransmittingvector(bloodsuckingmosquitoes

andflies,ticks)hasdevelopedformostflavivirusinfections.Thecyclesare

efficientforthevirusandrelativelyharmlessforthehost.Thevertebrate

hostfrequentlyshowsfewsignsofdiseaseandrecoversfromtheinfection

afterabriefviremia.Duringthisperiod,thebloodsuckingvectorisinfected,

whichthereafterremainsalifelongsalivarysecretorandthusinfectious.In

ticks,transovariantransmissionofthevirusisalsopossible.Thehumanhost

isadeadendforthevirus,notanormalcomponentofthecycle.Exceptionsto

thisareDenguefeverandurbanyellowfever.

HumansaretheonlyknownmainhostsfortheDenguevirus.Thereare

twoformsofyellowfever:ruralorjungle(“sylvatic”)yellowfeverwitha

monkey-mosquito-monkey(sometimeshuman)cycleandurbanyellowfever

withhumansasthemainhostsandAedesmosquitoesasthetransmitting

vectors.Thisformisontheupswingduetoincreasingnumbersofbreeding

places(e.g.,emptytincansingarbagepiles)forthevector.Another“new”

(moreaccurately:revived)infectiousdiseaseistheWestNileviralinfection,

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Kayser, Medical Microbiology © 2005 Thieme