Medical Microbiology

RNAviruses 439

Isolatedcasesandminoroutbreaksofenteritisaretypicallyattributedtoun-

specifiedviralinfections.Besidesunidentifiedbacterialinfections,theviral

pathogensthatcancausesuchinfectionsincludeadenovirus,rotavirus,astro-

virus,andcalicivirus,wherebythetaxonomyofthelattertwohavenotbeen

confirmed.

Astroviruses

Pathogen.Theastrovirusis 28 – 30 nminsizeandowesitsnametoitsstarlike

appearance.ItcontainssenseRNAwithapproximately 7500 nucleotidesand

appearstohaveareplicationstrategysimilartothatofthepicornaviruses.

Pathogenesisandclinicalpicture.Astrovirusesthatareanimalandhuman

pathogensareassociatedwithepisodesofdiarrheathatnearlyalwaysruna

harmlesscourse.Theetiologicalroleoftheseviruseshasstillnotbeenclar-

ified.Astrovirusesappeartopossessonlyalowlevelofpathogenicity.It

shouldbementionedatthispointthattheroleofvirusesinenteritisisfre-

quentlyexaggerated.

Diagnosis.Detectionbymeansofelectronmicroscopy.

Epidemiology.Astrovirusesoccurworldwide.Theytendtoinfectyoungchil-

drenandolderpersonsweakenedbyotherdiseases.

Caliciviruses

Pathogen.Calicivirusesare 30 – 35 nm,possessonlyonecapsidproteinanda

polyadenylated, 750 0-nucleotideRNAwithaVPgatthe 50 end.Thesurfaceof

theviruseshasacharacteristicstructurewithsmall,regular,calyxlikecon-

cavitiesthatgivethecapsidtheformofaStarofDavid.

Calicivirusesareclassifiedbasedongenomicsimilaritiesaseitherhuman

caliciviruses(HuCV)or“small,round-structuredviruses,”SRSV.Thisdesigna-

tionstemsfromtheirinitialidentificationundertheelectronmicroscopeas

“small,round,virusparticles.”TheSRSVaregroupedintwosubtypes,IandII.

TypeIincludestheNorwalkvirusandanumberofsimilarvirusesnamedfor

theirgeographicvenues,somewithantigenicitydifferingfromtheNorwalk

type.

Clinicalpicture.Calicivirusescauseenteritis.Togetherwithrotaviruses

(p.456)andadenoviruses(p. 41 6),theyarethemostfrequentviralenteritis

pathogensinchildren,oftencausingminorepidemicsduringthewinter

months(“wintervomitingdisease”).

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Kayser, Medical Microbiology © 2005 Thieme