Medical Microbiology

Rickettsia,Coxiella,Orientia,andEhrlichia 331

Classification.ThebacteriaofthisgroupbelongtothefamiliesRickettsiaceae

(RickettsiaandOrientia),Coxelliaceae(Coxiella),andEhrlichiaceae(Ehrlichia,

Anaplasma,Neorickettsia).Someoftheseorganismscancausemild,self-

limitinginfectionsinhumans,othersseveredisease.Arthropodsarethe

transmittingvectorsinmanycases.

Morphologyandculture.Theseobligatecellparasitesarecoccoid,shortrods

measuring0.3–0.5lmthattakegramstainingweakly,butGiemsastaining

well.Theyreproducebyintracellular,transversefissiononly.Theycanbecul-

turedinhenembryoyolksacs,insuitableexperimentalanimals(mouse,rat,

guineapig)orincellcultures.

Pathogenesisandclinicalpictures.WiththeexceptionofC.burnetii,theor-

ganismsaretransmittedbyarthropods.Inmostcases,thearthropodsexcrete

themwiththeirfecesandtickstransmitthemwiththeirsalivawhilesucking

blood.Theorganismsinvadethehostorganismthroughskininjuries.C.bur-

netiiistransmittedexclusivelybyinhalationofdustcontainingthepatho-

gens.Onceinsidethebody,rickettsiaereproducemainlyinthevascularen-

dothelialcells.Thesecellsthendie,releasingincreasingnumbersoforgan-

ismsintothebloodstream.Numerousinflammatorylesionsarecausedlocally

aroundthedestroyedendothelia.Ehrlichiaereproduceinthemonocytesor

granulocytesofmembrane-enclosedcytoplasmicvacuoles.Thecharacteristic

morulaeclusterscompriseseveralsuchvacuolesstucktogether.

Table4. 13 summarizesanumberofcharacteristicsoftherickettsioses.

Diagnosis.Directdetectionandidentificationoftheseorganismsincellcul-

tures,embryonatedheneggs,orexperimentalanimalsisunreliableandis

alsonottoberecommendedduetotheriskoflaboratoryinfections.Special

laboratoriesusethepolymerasechainreactiontoidentifypathogen-specific

DNAsequences.However,themethodofchoiceiscurrentlystilltheantibody

assay,wherebytheimmunofluorescencetestisconsideredthegoldstandard

amongthevariousmethods.TheWeil-Felixagglutinationtest(p.295)isno

longerusedtodayduetolowsensitivityandspecificity.

Therapy.Tetracyclineslowerthefeverwithinonetotwodaysandarethe

antibioticsofchoice.

Epidemiologyandprevention.Theepidemicformoftyphus,andearlier

scourgeofeasternEuropeandRussiainparticular,hasnowdisappeared

fromEuropeandoccursonlyoccasionallyinotherpartsoftheworld.Murine

typhus,ontheotherhand,isstillawidespreaddiseaseinthetropicsand

subtropics.Spottedfevers(e.g.,RockyMountainspottedfever)occurwith

increasedfrequencyincertaingeographicregions,especiallyinthespring.

TsutsugamushifeveroccursonlyinJapanandSoutheastAsia.Theblood-

suckinglarvaeofvariousmitespeciestransmititspathogen.Qfeverepi-

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Kayser, Medical Microbiology © 2005 Thieme