Plamodium Life Cycle


Plasmodium life cycle
35Kb
, (from the Atlas of Tropical Parasites)

Giemsa-stained bloodsmear with P.falciparum bloodstages
(Courtesy TDR of WHO)

 

 

Life cycle of P. falciparum. The sporozoites discharged from the salivary glands of the mosquito develop in the liver cells (the hepatocytes) first as schizonts which transform and discharge merozoites into the bloodstream. The liver stage is also called the exoerythocytic (or pre-erythrocytic) stage of infection. Merozoites infect the red blood cells where they develop into trophozoites, schizonts and merozoites which are capable of reinfecting other erythrocytes. This stage of the infection is also called the erythrocytic cycle. The destruction of the red blood cells and the release of the parasites' waste products produce the episodic chills and fever that characterize the disease. Some merozoites from the blood transform into gametocytes which, when taken by the mosquito, initiate sexual development in the midgut, involving ookinetes and oocysts.

For this reason the human population serves as a reservoir for the infection of mosquitoes and completion of the transmission cycle is assured. Sporozoites then infect the salivary glands of the mosquito. In P.vivax infections which are characterized by relapses, a dormant stage, called the hypnozoite, remains in the liver. From this stage relapsing infections may occurr at a later stage. P. falciparum infection relapses do not occur. It is, therefore, assumed that the sporozoites of this species develop uniformly producing pre-erythocytic schizonts at the same time and these schizonts, once formed, discharge all the merozoites simultaneously; do not remain dormant as in P. vivax.

While all four species have a haemolytic component ie. when a new brood of parasites break out of the red blood cell this is usually of little consequence. The exception is falciparum malaria where the parasites multiply very rapidly and may occupy 30% or more of the red blood cells causing a very significant level of haemolysis. One reason for this is that P.falciparum invades red cells of all ages whereas P.vivax and P.ovale prefer younger red cells, while P.malariae seeks mature red cells.




The stages of malaria tropica or falciparum malaria encountered in the human

 Exo-erythrocytic stages

  • sporozoites
    enters hepatocytes of liver
  • schizonts
    dividing forms of liver stages
  • merozoites
    are released by hepatocytes
  • hypnozoites
    resting liver stages

 Morphology of the malaria parasite Plasmodium falciparum inside the infected erythrocyte: 1, early throphozoite; 2, early throphozoite (double infection); 3, early throphozoite double chromatin with a few Maurer's dots; 4, late throphozoite with Maurer's dots and crenated red cell; 5, Mature schizont with merozoites and clumped pigment; 6, macrogametocyte with bluish cytoplasm and compact chromatin; 7, microgametocyte with pinkish cytoplasm and dispersed chromatin.

Blood stages

  • merozoites
    enter erythrocytes
  • trophozoites
    feeding and growing stages in red cells
  • schizonts
    multinuclear stages in red cells
  • gametocytes
    sexual blood stages



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Last updated: 4 December 1997

created by :Fred Opperdoes