Medical Microbiology

evenepidemicproportionsinrecentyears.Theriskofinfectionfortravelers

stayingbrieflyinendemicareasislow,butinfectionsdooccuronaregular

basis.

Lifecycle.T.gambienseandT.rhodesienseparasitizeextracellularintheblood

plasmaorinotherbodyfluidsofvertebrates(Fig.9. 5 ).Thetrypomastigote

formsarepleomorphicinhumanblood(Fig.9. 6 ):withincreasingparasitemia

theytransformtoslender, 25 – 40 lm-longformswiththeflagellartipextend-

ingbeyondtheanteriorendwhichreproducebylongitudinalbinaryfission.

Withdecreasingparasitemia,theyappearasshort,“stumpy”approximately

1 2–2 5 lmlongformswithoutafreeflagellarend.Theseformsdonotdivide

inbloodbutareinfectiveforGlossina(tsetseflies).Underalightmicroscope

inaGiemsa-stainedbloodsmearthetrypanosomespresentasspindlyorgan-

ismswithacentralnucleus,akinetoplastattheposteriorend(bothstained

violet)andanundulatingmembrane(Fig.9. 5 ).Thecellsurfaceoftheblood-

streamformsiscoveredwithauniformlayer(about 10 – 15 nmthick)ofa

specificglycoprotein,whichcanbereplacedbyanotherglycoprotein.These

glycoproteinsaredenominatedasvariantspecificsurfaceantigens(VSSA),

theexpressionofwhichiscodedbyabout 1000 genes;theyformthebasis

oftheorganisms’antigenvariation(seebelow).

ThetrypanosomestakenupbyGlossina(tsetseflies)whentheysuck

bloodfromaninfectedhostgothroughacomplexdevelopmentalandre-

productivecycleintheinsectslasting 15 – 35 days(Fig.9. 6 ).Theresulting

(metacyclic)stagescanthenbeinoculatedintotheskinofahostwiththe

fly’ssaliva.InfectedGlossinacantransmitthetrypanosomesthroughouttheir

entirelifespan(uptosixmonths).

486 9 Protozoa

Trypanosomabruceirhodesiense
Fig.9. 5 Giemsastainingof
9 abloodsmearpreparation.

Kayser, Medical Microbiology © 2005 Thieme