Plasmodium Life Cycle

Plasmodium is a genus, belonging to the parasitic group of protozoa. At present, more than 200 species of this genus are identified, out of which about 10 species infect humans. Among several species, Plasmodium falciparum is reported to cause 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 hosts to complete its life cycle, namely, a mosquito vector and a vertebrate host. Extensive studies have been conducted on P. falciparum, as this protozoa causes malaria. Let's discuss in brief about the life cycle of P. falciparum.

Plasmodium Life Cycle

The life cycle of P. falciparum is highly complex and undergoes subsequent changes during the transmission. This plasmodium species inhabits the salivary gland of female anopheles mosquito in the form of sporozoites. In total, there are 68 species of anopheles that can transmit malaria. When a female anopheles mosquito with plasmodium bites humans, it transmits the sporozoites into the human body. On an average, a bite of the infected mosquito contains more than 100 sporozoites.

The plasmodium sporozoites enter the blood circulatory system and infect the liver cells within 30 minutes. Thus, plasmodium enters the liver stage. In the liver cells or hepatocytes, the sporozoites transform into a feeding stage called trophozoite that undergoes asexual multiplication (schizogony). Within a span of two weeks, the trophozoite gives rise to thousands of merozoites. After maturation, the merozoites (1.5 μm length, 1 μm diameter) leave the liver cells and each of them is dispersed into red blood cells (RBCs). This stage of plasmodium infection is called erythrocytic (RBC) stage, which lasts for about 48 hours.

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 schizont. Schizont continues the cellular differentiation and gives rise to about 12-16 merozoites. During this stage, the merozoites are released through bursting of the infected RBC. The released merozoites then invade other RBCs. The nutrition of plasmodium parasites in the erythrocytic stage is derived from the digestion of hemoglobin.

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. In the erythrocytic stage, it is observed that some of the parasitic merozoites differentiate to male and female gametocytes. In the next bite by a female anopheles mosquito, the plasmodium gametophytes are taken up by the mosquito. Both gametophytes undergo gametogenesis in the body of the mosquito to form male and female gametes. The male gamete divides in the midgut, resulting in 8 flagellated microgametes. The microgamete then fertilizes with the female gamete (macrogamete) resulting into a zygote.

The zygotic stage of plasmodium develops to form ookinete. The ookinete passes through the epithelium of the midgut and forms an oocyst on the exterior wall of the midgut. The 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. The development and differentiation of plasmodium parasite inside the human body is highly synchronized. In fact, nearly all the parasites enter a specific stage of infection at the same time.