Life cycle of Parasite - Protozoan

The life cycle of the parasite in man begins when an infected mosquito, while biting and taking a blood meal, injects sporozoites into the human blood along with its salivary secretions. The sporozoites soon disappear from the circulation and enter the cells of the liver, where they undergo multiple divisions (schizogony). As they increase in number, the parasitised liver cell ruptures, liberating a large number of parasites. We call these pre-erythrocytic merozoites, because they have not yet invaded the red blood cells. The period when the parasite is in the liver is known as incubation period which may last for 6-15 days depending on the species of Plasmodium. These pre- erythrocytic merozoites initiate an asexual multiplication cycle in the red blood cells (RBC), the erythrocytic cycle. Inside the RBC, the parasite become amoeboid trophozoite feeding on hemoglobin. The end product of the trophozoite digestion is called hemozoin which accumulates in the host cell. This process of asexual reproduction in the RBC is known as schizogony, and a large number of organisms are produced from one individual. Schizogony tends to occur synchronously in a large number of erythrocytes and at the end of each cycle a large number of merozoites are liberated from the parasitised RBC.

When the RBCs burst they also release the metabolic products of the parasite and these foreign toxins cause the characteristic chills and fever of malaria. These merozoites enter fresh RBCs and repeat the multiplicatory cycle. Since the population of merozoites released from RBC is synchronized to some degree, therefore, the episodes of chill and fever have a periodicity characteristic of each species of Plasmodium. In P viva (benign tertian) malaria the episodes occur every 48 hrs; in p malariae (quartan) every 72 hrs; P ovale every 48 hours and P falciparum (malignant tertian) every 48 hours. After a few generations some of the merozoites do not undergo a schizogonous cycle upon entering a fresh RBC but develop into large gamete-forming uninucleate cells. This is the gametocytic stage of the parasite. Some of those are male or microgametocyte and some, female or macrogametocytes. These gametocytes will not develop further any more in the vertebrate host. Further development occurs only In the stomach of the mosquito. when an anopheline female mosquito bites an Infected person, it also ingests the gametocytes along with the blood meal. (Male mosquito feeds on plant sap and is not a blood feeder). In the stomach of the mosquito the process of gametogony gets completed with the formation of gametes. The two gamete types fuse to form a zygote.

The zygote becomes a motile ookinete. It penetrates the stomach wall and lodges itself under the outer limiting membrane forming and oocyst. The oocyst undergoes numerous cell divisions, grows in size and gives rise to many spindle-shaped cells, the sporozoites. When filled to its capacity, i.e, after 10-20 days of the blood meal, the oocyst bursts open and liberates these sporozoites into the haemocoel of the mosquito. The sporozoites reach the salivary glands of the mosquito. These are infectious to human host and are injected into the blood when the mosquito feeds on human blood. This is how the parasite completes its life cycle.