sion 2000 (P.malariae)to 30000 (P.falciparum)merozoitesareproduced.
Thisdevelopmenttakessix(P.falciparum) to 15 (P.malariae)days.Shortly
thereafter,thetissueschizontsreleasethemerozoites,whichtheninfect
erythrocytes(seebelow).IninfectionswithP.vivaxandP.ovale,sporozoites
developintotissueschizontsasdescribedabove,butsomeremaindormant
asso-calledhypnozoites,whichmaydevelopintoschizontsfollowingactiva-
tionaftermonthsoryears.Merozoitesreleasedfromtheseschizontsthen
infecterythrocytes,causingrelapsesofthedisease(seep. 52 7).
&Erythrocyticdevelopment.Themerozoitesproducedintheliverarere-
leasedintothebloodstreamwheretheyinfecterythrocytes,inwhichthey
reproduceasexually.Themerozoitesaresmall,ovoidformsabout 1 .5lm
inlengththatattachtoreceptormoleculesontheerythrocytesurface.These
receptorsarespecies-specific,whichexplainswhycertainPlasmodiumspe-
ciesprefercertaincelltypes:P.malariaeinfectsmainlyoldererythrocytes,
P.vivaxandP.ovalepreferreticulocytes,andP.falciparuminfectsyounger
andoldererythrocytes.Followingreceptorattachment,merozoitespenetrate
intotheerythrocyte,wheretheyareenclosedinaparasitophorousvacuole.
APlasmodiumthathasrecentlyinfectedanerythrocyte(< 12 hours)appears
ring-shapedwithathincytoplasmicriminaGiemsa-stainedbloodsmear.
Alsovisibleareacentralfoodvacuoleandthedark-stainednucleuslocated
attheperipheryoftheparasite.ThisstageisverysimilarinallfourPlasmo-
diumspecies(Fig.9. 18 ,p.524).Theringformsdevelopintoschizonts,which
feedonglucoseandhemoglobin.Thelatterisbrokendowntoabrownish-
blackpigment(hemozoin)—aftertheaminoacidsusedbytheplasmodiaare
splitoff—anddepositedintheparasite’sfoodvacuoleas“malariapigment.”
Theschizontundergoesmultipledivisionstoproducemerozoites,indifferent
numbersdependingonthePlasmodiumspecies(6–36).Themerozoitesenter522 9 ProtozoaFig.9. 17 a Inhumans: 1 sporozoitefrominfectedAnophelesmosquito; 2 devel-
opmentintheliver;2aprimarytissueschizontsandschizogonyinhepatocytes
(allPlasmodiumspecies);2bhypnozoitesanddelayedschizogonyinhepatocytes
(P.vivaxandP.ovaleonly); 3 furtherschizogenicdevelopmentinerythrocytes;
3adevelopmentofsexuallydifferentiatedplasmodia(femalemacrogametocytes
andmalemicrogametocytes).
bintheAnophelesmosquito: 4 macrogametocytesandmicrogametocytestaken
upbybloodsuckingmosquito; 5 fertilizationofmacrogametes(round)bymicro-
gametes(long); 6 fertilizedmacrogamete(ookinete)inintestinalwallofmosquito;
7 oocystwithsporozoitesinintestinalwall; 8 infectivesporozoitesinsalivarygland
(accordingtoPetersW.ChemotherapyandDrugResistanceinMalaria.Vol. 1 ,Lon-
don:AcademicPress; 1987 :16). "9
Kayser, Medical Microbiology © 2005 Thieme