Plasmodium is a genus, belonging to the parasitic group of protozoa. At present, more than 200 species of this genus are identified, of which about 10 species infect humans. The deadliest species of all is Plasmodium falciparum, which causes maximum health complications and mortality in humans.
Acute plasmodium infections, if left untreated, can progress to life-threatening conditions, whereas chronic infections can lead to severe anemia. It also infects animals like rodents, monkeys as well as birds. Plasmodium requires two organisms to complete its life cycle, namely, a mosquito vector and a vertebrate host.
Life Cycle of Plasmodium
Extensive studies have been conducted on P. falciparum, as this protozoan causes the deadly malaria disease in humans. The life cycle of this protozoan is highly complex, and also, it undergoes subsequent changes during the transmission. It resides in the salivary gland of female anopheles mosquito in the form of sporozoites.
In total, there are 68 species of anopheles that can transmit malaria. For better understanding about P. falciparum life cycle, refer to the following information.
Sporozoites and the Liver Stage
When a female anopheles mosquito carrying plasmodium bites humans during a blood meal, it transmits the sporozoites into the human body. On an average, a single bite by the infected mosquito contains more than 100 sporozoites. The plasmodium sporozoites enter the blood circulatory system and infect the liver cells within 30 minutes.
Merozoites and the Erythrocytic Stage
In the liver cell or hepatocyte, the sporozoites undergo asexual multiplication called schizogony to give rise to merozoites. Within a span of two weeks, thousands of merozoites are formed.
After maturation, the merozoites (1.5 μm length, 1 μm diameter) leave the liver schizont cells and each of them is dispersed into red blood cells (RBCs). This stage of infection is called erythrocytic (RBC) stage, which lasts for about 48 hours.
Schizont and the Erythrocytic Stage
The merozoites differentiate further in the cytoplasm of RBC to form an enlarged, round-shaped trophozoite. Similar to the previous liver stage, the trophozoite undergoes schizogony, in which DNA replication takes place. This particular stage is referred to as erythrocytic schizont.
Inside the schizont, cellular differentiation continues and gives rise to about 12 - 16 merozoites. This particular stage continues for about 72 hours.
Rupturing of the Infected RBCs
In this stage, the merozoites are released through bursting of the infected RBCs. It is observed that some of the released merozoites further invade RBCs.
Nutrition of the plasmodium parasites in the erythrocytic stage is derived from the digestion of hemoglobin. As expected, the infected RBCs are circulated to various body organs such as brain, heart, and liver. The presence of plasmodium infected RBCs in the brain results in cerebral malaria.
Formation of Male and Female Gametes
In the erythrocytic stage, some of the parasitic merozoites differentiate to male and female gametocytes. In the next bite by a female anopheles mosquito, the plasmodium gametocytes are taken up by the mosquito.
Both gametocytes undergo gametogenesis in the body of the mosquito to form male and female gametes. The male gamete further divides in the midgut, resulting in 8 flagellated microgametes.
Fertilization and Zygote Formation
The microgamete then fertilizes with the female gamete (macrogamete) to produce a zygote. The zygotic stage develops to form ookinete. The ookinete passes through the epithelium of the midgut and forms an oocyst on the exterior wall of the midgut.
This oocyst matures to form an enlarged structure, after which several nuclear divisions take place. Oocyst ruptures and releases hundreds of sporozoites. Eventually, these sporozoites migrate to the salivary gland of the female anopheles mosquito. This way, the life cycle of P. falciparum is completed.
As per researches, clinical manifestation of malaria symptoms is related to rupturing of the infected erythrocytes. Overall, the development and differentiation of plasmodium parasite inside the human body are highly synchronized. In fact, nearly all the parasites enter a specific stage of infection at the same time.