The Plasmodium (Mosquito) Life Cycle
There are two major phases in the life cycle of Plasmodium species namely; the sexual and asexual phases. The sexual phase takes place in the parasite’s primary host and transmission vector. On the other hand, the asexual phase occurs in humans (the secondary host). A mosquito becomes infected when it takes a blood meal containing parasite gametocyte from an infected human. Once ingested, the gametocyte taken up in the blood will further differentiate into male and female gametes and then fuse in the mosquito gut. This is the sexual phase. The fused gamete eventually produces an ookinate that penetrates the gut lining and produces an oocyte in the gut wall.
When the oocyte ruptures, it releases sporozoites that migrate through the mosquito’s body to the salivary glands, where they are then ready to infect a new human host. This type of transmission is occasionally referred to as anterior station transfer (Talman et al., 2004). In the asexual phase, the sporozoites in the salivary gland of the mosquito are injected into human blood stream when the vector obtains another human blood meal. From the blood stream, they rapidly migrate to hepatic parenchyma cells, and cannot be detected in the blood after 30 minutes. In the liver, they develop and multiply into tissue schizonts. The length of this asymptomatic (exoerythrocytic) developmental stage varies between the different species of Plasmodia. For instance in P. vivax and P. falciparum, it lasts 10-14 days while for P. malariae it lasts 18 days to about 6 weeks (Adepoju-Bello and Ogbeche, 2003).
The next stage involves the rupturing of tissue schizonts and the subsequent release into the blood streams of thousands merozoites. This will invade circulating red blood cell, where they undergo further asexual development and multiplication. This stage of the infection is termed the erythrocytic stage. The infected erythrocytes burst at intervals, releasing further merozoites into the blood. These continue the cycle by invading other red blood cells. Thus, the febrile malaria attack is as a result of the periodic rupturing of infected erythrocytes. The length of the asexual cycle in the erythrocytic stage also varies between different species. It is about 48 hours in P.falciparum, P. vivax and P.ovale, but up to 72 hours in P.malariae (Adepoju-Bello and Ogbeche, 2003).
In some infected erythrocytes, gametocytes also develop which when injected by a female mosquito obtaining a blood meal, initiate the sexual stage of the Plasmodia life cycle in the insect (Bruce-Chwatt, 1962).