Parasite - Life Cycle

Life Cycle Of Malaria Parasite

Comprises

 

  • The exogenous sexual phase (sporogony) in female anophelene mosquitoes, and

  • An endogenous asexual phase (schizogony) in human host.

Malaria Parasite in Mosquito

  • When a female anophele mosquito bites a human host harbouring maria parasite, it ingests malaria parasites along with the blood meal.

  • The asexual forms are digested, while the mature forms called gametocytes undergo further development.

  • In the male gamatocytes the nucleus divides into 4-8 nuclei, each forming thread like structures called microgametes.

  • Female gamatocytes undergo maturation process and form macrogametes.

  • In the mosquito stomach, microgamete fuses with the macrogamete (fertilization) resulting in a product called a zygote.

  • Within 18-24 hrs, zygote develops into a long mobile worm like form called the Ookinete. (18-24u)

  • The Ookinete passes between epithelial cells to the outer surface of the mosquito stomach and becomes rounded up into a small sphere called an Oocyst.


  • The Oocyst increases in size and the nucleus divides repeatedly to form (40 to 80u) Sporoblast.

  • The divided nuclei of sporoblast form elongated sporozoites (10-15u) and are released into the body cavity.

  • These sporozoites migrate to the salivary gland of the mosquito and are now ready for transmission to human host.

  • When the mosquito feeds on blood sporozoites are released into the bloodstream of the human host.

Malaria Parasite in Human Host

Endogenous asexual phase includes

  • Exo-erythrocytic schizogony or tissure phase (sporozoites in the parenchyma cells of the liver.)

  • Erythrocytic schizogony (Development of sporozoites in the red corpuscles in the blood)

Exo-erythrocytic schizogony or tissure phase.

  • Sporozoites inoculated by a mosquito into the human host circulate for about half an hour and many are destroyed by the phagocytes.

  • Some sporozoites enter the parenchymal cells of the liver and undergo a process of development known as Pre-erythrocytic schizogony.

  • During pre-erthrocytic schizogony (In P.falciparam and P. malariae,) sporozoites develop into schizonts.

  • In P.ovale and P.vivax, within 40-48 hrs, after an infective bite, sporozoites shrunk and form narrow cytoplasmic rim structures called hypnozoites

  • Hypnozoites remain dormant in hepatocytes and then grow into schizonts.(Delayed Schizogony)

  • After about 6 to 16 days from the time of infection, mature schizonts (30 to 70u) enlarge and burst to release thousansands of merozoites(1 um) into the blood.

  • Merozoites re-enter fresh liver cells and repeat the process of schizogony, thus causing relapses of infection during the secondary exo erythrocytic schizogony.

Erythrocytic schizogony

  • Merozoites released from the tissue schizont enter red blood cells in five stages.
    • Initial recognition and attachment

    • Formation of juction

    • Creation of vacuole memberane continous with the red cell memberane

    • Entry into the vacoule through the moving junction

    • And Sealing of the erythrocytes after entry.

  • After entering the eyrthocytes merozoites assume rounded form.

  • These youngest stage of parasites in red blood cells are rounded bodies with annular appearance called as ring forms.

  • The ring forms grow into irregular shapes called trophozoites.

  • The parasite lives on cytoplasm of the RBC, absorbing haemoglobin and leaves a product of digestion a pigment called haemozoin(combination of haematin with protein)

  • After a period period of growth, trophozoites divide asexually forming schizonts by a process known as erythrocytic schizogony.

  • Mature schizonts divide into small round forms (merozoites)

  • When the process of schizogony is completed the red blood cell bursts, releasing the merozoites into the blood stream.

  • Merozoites invade fresh erythrocytes and repeat the erythrocytic schizogony cycle.

  • After several rounds of erythrocytic schizogony, some merocytes develop into sexual forms, the gamaetocytes.

  • The number of gametocytes increase with increasing number of erythrocytic schizogon

Last Updated on : 08/01/2014