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Pathogenesis 397

festation.Immunologicalresponsescancontributetoeliminationofthe

virusesbydestroyingtheinfectedcells,butthesameresponsemayalso

exacerbatethecourseofthedisease. &

Transmission.Virusescanbetransmittedhorizontally(withinagroupof

individuals(Table 7. 3 )orvertically(frommothertooffspring).Verticalinfec-

tioniseithertransovarialorbyinfectionofthevirusinutero(ascendingor

diaplacental).Connatalinfectionisthetermusedwhenoffspringareborn

infected.

Portalofentry.Themostimportantportalsofentryforvirusesarethe

mucosaoftherespiratoryandgastrointestinaltracts. Intactepidermis

presentsabarriertoviruses,whichcan,however,beovercomethrough

microtraumata(nearlyalwayspresent)ormechanical inoculation(e.g.,

bloodsuckingarthropods).

Viraldisseminationintheorganism.Therearetwoformsofinfection:

&Localinfection.Inthisformofinfection,thevirusesspreadonlyfromcell

tocell.Theinfectionandmanifestdiseasearethusrestrictedtothetissuesin

theimmediatevicinityoftheportalofentry.Example:rhinovirusesthatre-

produceonlyinthecellsoftheupperrespiratorytract.

&Generalizedinfection.Inthistype,thevirusesusuallyreplicatetosome

extentattheportalofentryandarethendisseminatedviathelymphductsor

bloodstreamandreachtheirtargetorganeitherdirectlyorafterinfectinga

furtherorgan.Whenthetargetorganisreached,viralreplicationandthere-

sultingcelldestructionbecomesowidespreadthatclinicalsymptomsdevel-

op.Examplesofsuchinfectioncoursesareseenwithenterovirusesthat

replicatemainlyintheintestinalepithelium,butcausenosymptomsthere.

Table 7. 3 HorizontalTransmissionofPathogenicViruses

Modeoftransmission Examples

Directtransmission

• fecal-oral(smearinfection)


• aerogenic(dropletinfection)


• intimatecontact(mucosa)

Enteroviruses

Influenzaviruses

Herpessimplexvirus

Indirecttransmission

• alimentary


• arthropodvectors


• parenteral

HepatitisAvirus

Yellowfevervirus

HepatitisBvirus

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Kayser, Medical Microbiology © 2005 Thieme