Medical Microbiology

DNAViruses 427

Pathogens.Thevirusesofthepoxgrouparethelargestvirusesofall.At 230!

350 nmtheyarejustwithintheresolutionrangeoflightmicroscopes.They

haveacomplexstructure(Fig.8. 7 )andaretheonlyDNAvirusesthatreplicate

inadefinedareawithinthehost-cellcytoplasm,aso-called“virusfactory”

(Fig.8. 8 ).

Thediseasessmallpox(variolamajor)andthemilderformalastrim(var-

iolaminor)nownolongeroccurinthehumanpopulationthankstoaworld-

widevaccinationprogramduringthe 197 0s.Thelastpersoninfectedby

smallpoxwasregisteredinSomaliain 1977 anderadicationofthedisease

wasformallyproclaimedin 19 80.Sincethen,populationsofthevirus

havebeenpreservedintwospeciallaboratoriesonly.

TheFamilyPoxviridae

Thisvirusfamilycomprisesseveralgenera:

&theorthopoxvirusesincludethevariolaandalastrimviruses,thevacciniavirus

usedinsmallpoxvaccines,thecloselyrelated(butnotidentical)cowpoxvirusesas

wellasthemonkeypox,mousepox,andrabbitpoxviruses.

Othergenerawithhumanpathogenstrainsinclude:

&theparapoxvirusesincludingtheorfvirusandthemilker’snodulevirus(notto

beconfusedwithcowpox),transmittedtohumansbysheepandcows,respectively;

&themolluscipoxviruses,i.e.,themolluscumcontagiosumvirus.

VacciniaViruses

500 nm

Fig.8. 8 The vaccinia

virusesarethe dark,

electron-dense inclu-

sions readily visible

here.Theyreplicatein

adiscretecytoplasmic

region(TEM).

8

Kayser, Medical Microbiology © 2005 Thieme