Medical Microbiology

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