Medical Microbiology

428 8 VirusesasHumanPathogen

Pathogenesisandclinicalpicture.Variolavirusesaretransmittedaerogeni-

cally.Themucosaoftheupperrespiratorytractprovidestheportalofentry.

Fromthere,thepathogensenterthelymphoidorgansandfinallypenetrateto

theskin,wheretypicaleruptionsformand,unlikevaricellapustules,allde-

veloptogetherthroughthesamestages(Fig.8. 9 ).Themucosaeoftherespira-

toryandintestinaltractsarealsoaffected.Lethalityratesincasesofsmallpox

(variolamajor)wereashighas 40 %.Inalastrim(variolaminor)thelevelwas

2 %,wherebythecauseofdeathwasoftenabronchopneumonia.

Thevacciniavirusisadistinctviraltypeofunknownorigins,andnotan

attenuatedvariolavirus.Itwasformerlyusedasavaccinevirustoprotect

againstsmallpox.Thevaccinationcausedapustularexanthemaroundthe

vaccinationsite,usuallyaccompaniedbyfever.Encephalitis,thepathogenesis

ofwhichwasnevercompletelyclarified,wasafearedcomplication.Itisas-

sumedthatanautoimmunereactionwasthedecisivefactor.Othercomplica-

tionsincludegeneralizedvacciniainfectionandvaccinialkeratitis.Vaccinia

infectionsandtheircomplicationsdisappearedforthemostpartwhensmall-

poxwaseradicatedbytheWHOvaccinationcampaign.Vacciniavirusesare

stillfrequentlyusedasvectorsinmolecularbiologylaboratories.Theinher-

entpathogenicityofthevirusshouldofcoursebekeptinmindbyexperimen-

ters.

Infectionswithcowpox,orf,andmilker’snodulevirusesarerareandusual-

lyharmless.Thelesionsremainlocalizedontheskin(contactsite),accom-

paniedbyalocallymphadenitis.Thesearetypicaloccupationalinfections

(farmers,veterinarians).Themolluscumcontagiosumvirusisunusualin

thatin-vitroculturingofthevirushasnotsucceededtodate.Infections

withthisvirusdonotconferimmunity.Theinfectioncausesepidermal,be-

nigntumors(“molluscumcontagiosumwarts”).

Smallpox(Variola)

Fig.8. 9 Incontrasttothelesionsin

varicellainfections,allsmallpoxpustules

areatthesamedevelopmentalstage.

8

Kayser, Medical Microbiology © 2005 Thieme