398 7 GeneralVirology
Clinicalsymptomsintheseinfectionsfirstariseinthetargetorganssuchas
theCNS(polioviruses,echoviruses)ormusculature(coxsackieviruses).
Anothermodeofviraldisseminationinthemacroorganismisneurogenic
spreadalongthenervetracts,fromtheportalofentrytotheCNS(rabies),
orintheoppositedirectionfromtheganglionswherethevirusespersist
inalatentstatetothetargetorgan(herpessimplex).
OrganInfections,Organotropism
Whetheragivencelltypecanbeinfectedbyagivenviralspeciesatalldependson
thepresenceofcertainreceptorsonthecellsurface(p. 38 4).Thismechanism
explainswhyorganotropismisobservedinviruses.However,thetropismisonly
apparent;itismoreaccuratetospeakofsusceptibleandresistantcells(andhence
organs).Anotherobservationisthatcellsgrowninthelaboratoryincellculturescan
completelychangetheirsensitivityorresistancetocertainviralspeciescompared
withtheirorganoforigin.
Courseofinfection.Theorgandamagecausedbyvirusesismainlyofade-
generativenature.Inflammatoryreactionsaresecondaryprocesses.These-
verityoftheclinicalsymptomsdependsprimarilyontheextentofvirus-in-
duced(orimmunological,seebelow)celldamage.Thismeansmostofthe
viralprogenyareproducedpriortotheoccurrenceofclinicalsymptoms,
withconsequencesforepidemiologyandantiviraltherapies(p. 40 4).It
alsomeansthatinfectionscangounnoticedifcelldestructionisinsignificant
orlackingentirely.Insuchcases,thetermsinapparent,silent,orsubclinical
infectionareused,incontrasttoapparentviralinfectionswithclinicalsymp-
toms.Virusreplicationandreleasedotakeplaceininapparentinfections,as
opposedtolatentinfections(p. 39 4),inwhichnoviralparticlesareproduced.
Immunologicalprocessescanalsoinfluencethecourseofviralinfections,
wherebytheinfectioncanbesubduedorhealed(p. 40 1ff.).Ontheother
hand,theinfectionmayalsobeexacerbated,eitherbecauseimmunecom-
plexesareformedwithvirusesorviralcomponents(nephritis)orbecause
theimmunesystemrecognizesanddestroysvirus-infectedcells.Thisispos-
sibleifviralantigensareintegratedinthecellmembraneandthusexpressed
onthecellsurface.Theseprocessesbecomepathologicallysignificantincases
inwhichthevirusesthemselvescauselittleornocelldestruction(p.393).
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Kayser, Medical Microbiology © 2005 Thieme