Life Cycle of Plasmodia (Malarial causative)

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Chapter: Pharmaceutical Microbiology : Protozoa

Plasmodia have a complex life cycle involving a number of life cycle stages and two hosts. The human infective stage comprises the sporozoites ( c.1–7 μm), which are produced by sexual reproduction in the midgut of the mosquito (vector) and migrate to its salivary gland.


LIFE CYCLE OF PLASMODIA (MALARIAL CAUSATIVE)

 

Plasmodia have a complex life cycle (Figure 6.1) involving a number of life cycle stages and two hosts. The human infective stage comprises the sporozoites ( c.1–7 μm), which are produced by sexual reproduction in the midgut of the mosquito (vector) and migrate to its salivary gland. When an infected Anopheles mosquito bites a human, sporozoites are injected into the bloodstream and are thought to enter liver parenchymal cells within 30 minutes of inoculation. In these cells the parasite differentiates into a spherical, multinucleate schizont which may contain 2000–40 000 uninucleate merozoites. This process of growth and development is termed exoerythrocytic schizogony. This exoerythrocytic phase usually takes between 5 and 21 days, depending on the species of Plasmodium; however, in P. vivax and P. ovale the maturation of schizonts may be delayed for up to 1–2 years. These ‘quiescent’ parasites are called hypnozoites. Clinical illness is caused by the erythrocytic stage of the parasite life cycle; no disease is associated with sporozoites, the developing liver stage of the parasite, the merozoites released from the liver, or gametocytes.

 

The common symptoms of malaria are due to the rupture of erythrocytes when erythrocytic schizonts mature (Figure 6.2a). This release of parasite material triggers a host immune response, which in turn induces the formation of inflammatory cytokines, reactive oxygen intermediates and other cellular products. These proinflammatory molecules play a prominent role in pathogenesis, and are probably responsible for the fever, chills, sweats, weakness and other systemic symptoms associated with malaria. In P. falciparum malaria, infected erythrocytes adhere to the endothelium of capillaries and postcapillary venules, leading to obstruction of the microcirculation and localized anoxia. The pathogenesis of anaemia appears to involve haemolysis or phagocytosis of parasitized erythrocytes and ineffective erythropoiesis.

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