The Female Anopheles Mosquito As The Malaria Vector
Life Cycle Of The Parasite
There are two stages- sexual cycle in the mosquito and the asexual cycle in man.
Sporogony (Sexual cycle): When the female anopheles mosquitoes take blood from infected persons, gametocytes are ingested. In the mosquito’s stomach, the gametocytes are released from the erythrocyte exflagellates into 6 to 8 flagella and each flagellum fertilizes a female gamete. A zygote is formed which developes into ookinete and oocyst in the stomach wall. About 10,000 sporozoites develop within each oocyst. A mosquito may harbour a few to more than a hundred oocysts at a time. Depending upon the temperature and humidity, the sporozoites develop fully and penetrate the stomach wall to reach different tissues of the mosquito, including the salivary glands. During subsequent blood meals, the sporozoites are passed into the victim. An infected mosquito continues to carry the infection for the duration of its lifespan (1 month).
The Life Cycle Of Plasmodium Vivax
Schizogony (asexual cycle): This occurs in man and it consists of the exo-erythrocytic and erythrocytic cycles.
Primary exo-erythrocytic cycle (Pre-erythrocytic cycle): After inoculation by the mosquito, the sporozoites circulate for a short while and within hours they enter the hepatocytes to develop and multiply within them. Merozoites come out of the liver cells after about a week and invade erythrocytes to initiate the erythrocytic cycle. The interval required for the sporozoites to develop in the liver cells and appear in erythrocytes is called the pre-patent period. Sporozoites do not cause any permanent damage to the liver. The exo-erythrocytic cycle causes local inflammatory changes but does not cause permanent damage. Only the erythrocytic cycle is harmful to the host.
Erythrocytic Cylce: The merozoite develops in the erythrocyte utilizing its hemoglobin for metabolism. It grows into a ring form and later becomes amoeboid- the trophozoite. Within 48 to 72 hours, the trophozoite reaches full growth and divides by binary fission to form a schizont, which containes several merozoites. The number of merozoites in the schizont differs in the different species. The erythrocyte ruptures liberating the merozoites into the plasma. Many of them are destroyed, but others attach to fresh erythrocytes and successively repeat the cycle of multiplication. The number of parasites progressively increases. When the parasite level reaches a threshold, clinical symptoms set in. Metabolis of hemoglobin by the growing parasite results in the formation of malarial pigment which is seen in the parasite erythrocyte. This pigment (hemozoin) is taken up by macrophages in the reticuloendothelial organs. In a primary infection, after a few erythrocytic cycles lasting for 2 to 3 weeks, some of the merozoites develop into the male and female gametocytes, which are the sexual forms destined to continue sporogony in the mosquito. Female gametocytes outnumber the male gametocytes. They continue further development only if ingested by the vectors. If not, they die within 2 to 3 weeks (up to 8 weeks for P. falciparum) and are disposed off by macrophages. Both erythrocytic and exo-erythrocytic schizogony may lead to the formation of gametocytes.
Secondary exo-erythrocytic Cycle: This stage is also seen in the lifestyle of P. vivax, P. malariae and P. Ovale, but so far, this has not been recognized in the case of P. falciparum. Some of the merozoites liberated from the pre-erythrocyte phase re-enter the hepatocytes and continue the developmental cycle, liberating merozoites repeatedly. These merozoites either invade erythrocytes or re-enter fresh hepatocytes. This stage is responsible for persistence of the infection and late relapses even after the erythrocytic stage of the parasites is cleared by schizonticidal drugs.
© 2014 Funom Theophilus Makama