Life Cycle of Plasmodium

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Chapter: Medicinal Chemistry : Antimalarials

The different stages of the reproductive cycle of the malarial parasite and the drugs acting at different stages of this cycle.


LIFE CYCLE OF PLASMODIUM

The different stages of the reproductive cycle (Fig. 9.1) of the malarial parasite and the drugs acting at different stages of this cycle are given below:

·Stage-I: No drug is effective in this stage.

·Stage-II: Primaquine and pyrimethamine can block at this stage.

·Stage-III: Primaquine can only prevent because fever occurs at this stage.

·Stage-IV: Chloroquine, amodiaquine, santoquine, proguanil.

·Stage-V: Primaquine only.


Two important phases of the parasite life cycle are the following:

1.           Asexual cycle—occurs in the infected host.

2.           Sexual cycle—occurs in the mosquito.

After the insect bite, the parasite forms rapidly. They leave the circulation and localize in the hepatocytes whereby they transform, multiply, and develop into tissue schizonts. The primary asymptomatic tissue stage lasts for 15 days and the tissue schizonts rupture, each releasing thousands of merozites. The released merozites invade more erythrocytes to continue the cycle’s synchronous rupture of erythrocytes to continue the cycle. Synchronous rupture of erythrocytes and release of merozytes into the circulation leads to febrile pattern attacks on day 1 and 3; hence, the designation is ‘tertian malaria’. 

Some erythrocyte parasites differentiate into several forms known as gametophytes. After infecting human blood, female mosquito ingests them. Then the exflagellation of male gametocyte is followed by the male gametogenesis and the fertilization of the female gametocytes in the insect’s guts. The resulting zygote, which develops as an oocyte in the gut wall, eventually gives rise to infective sporozoite, which invades the salivary glands of the mosquito. The insect then can infect another human by taking a blood meal.

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