Plasmodium falciparum

Plasmodium falciparum is a unicellular protozoan causing the most lethal type of malaria. It is an endoparasite. The female Anopheles mosquito acts as a vector and transmits the Plasmodium parasite to humans. Plasmodium falciparum accounts for half of the malaria cases.

Plasmodium falciparum was first identified by Alphonse Laveran in 1880 and he named it Oscillaria malariae. Later, William H. Welch gave the parasite its present name Plasmodium falciparum. It causes the most dangerous form of malaria. It is also found to be associated with the development of blood cancer, i.e. Burkitt’s lymphoma. It is a cancer of the lymphatic system.

Plasmodium falciparum completes its life cycle in humans and mosquitoes. It attains various forms during its life cycle. The sporozoites are the infective stage for humans and are transmitted from the salivary glands of the mosquito.

Classification and Characteristics

Plasmodium falciparum is a unicellular eukaryotic organism. It is heterotrophic and present as endoparasites in mosquitoes and human cells. It is placed under the kingdom, Protista. It is a protozoan. Plasmodium falciparum is a spore-producing organism, hence it was grouped under Sporozoa.

As per the new classification, Plasmodium is placed under the phylum Apicomplexa. The phylum Apicomplexa comprises earlier so-called sporozoans that are unicellular, non-motile and spore-forming parasites. They are mostly obligate endoparasites. All apicomplexans possess an apicoplast that is used to penetrate the host cells.

Other Plasmodium species causing malaria are P. vivax, P. ovale and P. malariae.

Plasmodium falciparum attains various forms and shapes during its life cycle. The important characteristics of Plasmodium falciparum are as follows:

• Sporozoite is the infective stage that enters the human body from the salivary glands of mosquitoes. It is around 10-15 µm long and spindle-shaped.
• The sporozoite enters the liver cell and develops into a schizont. It is ovoid in shape and measures 30-70 µm in diameter.
• The schizont releases multiple merozoites on rupture. They are 1-1.5 µm in diameter.
• Immature trophozoites are ring-shaped and are produced in the erythrocytes from merozoites released from liver schizont. They are about 1-2 µm in diameter.
• Mature trophozoites feed on host haemoglobin and form a pigment called haemozoin.
• The gametocytes are produced in the RBCs. The gametocytes of Plasmodium. falciparum are crescent-shaped. They are 8–12 μm long and 3–6 μm wide.
• A zygote is formed by the fertilisation of micro and mega gametocytes in the stomach of the female Anopheles mosquito.
• Ookinetes are elongated and the size ranges from 18-24 µm. They penetrate the midgut wall of the mosquito and transform into oocysts.
• Oocyst is round and can grow up to 80 µm in diameter.
• The apical complex comprises multiple organelles and is involved in the movement, adhesion and invasion of the host cell.
• The apicoplast is a non-photosynthetic plastid, which is a characteristic of all apicomplexans. It is involved in the synthesis of lipid and other compounds. Apicoplast produces isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) through the non-mevalonate pathway in the blood. These are isoprenoid precursors.
• It reproduces asexually in humans and sexually in mosquitoes.

Life Cycle

The life cycle of Plasmodium falciparum comprises sexual and asexual stages. The sexual phase of the life cycle occurs in the female Anopheles mosquito, which is the definitive host of the parasite. Asexual reproduction occurs in humans. Humans are the intermediate hosts of the parasite.

Asexual Stage in Humans

Asexual reproduction occurs in humans. The sporozoites first enter the liver cells from the blood and then the merozoites infect RBCs.

1. Exo-erythrocytic Schizogony in Liver Cells

It is the initial stage of multiplication occurring in the liver cells. The sporozoite from the salivary gland of the mosquito reaches the bloodstream of humans bitten by the infected mosquito. It then enters the hepatocytes and starts multiplying. It undergoes multiple rounds of cell division to produce a syncytial or multinucleate cell known as schizont. The schizont ruptures and releases numerous haploid daughter cells known as merozoites in the blood of the host.

2. Erythrocytic schizogony in Erythrocytes (RBCs)

Merozoites use apical complexes and invade erythrocytes or RBCs. The merozoites after entering the RBCs, lose apical complexes and develop into immature trophozoites, which are ring-shaped. The young sporozoite matures into a round, mature trophozoite. Trophozoites undergo schizogony and transform into schizonts. The cell undergoes multiple mitotic divisions and each schizont contains 16-18 merozoites. Schizonts rupture and release merozoites that invade fresh erythrocytes.

It takes around 48 hours to complete one cycle in erythrocytes, which is responsible for the characteristic relapse of fever and chills in falciparum malaria. It is due to the synchronous rupture of erythrocytes and the release of a toxic substance known as haemozoin. Haemozoin is derived from the digestion of haemoglobin.

The parasite can even alter the shape of erythrocytes. The sequestration of P. falciparum-infected RBCs in the brain may lead to cerebral malaria, which increases the risk of fatality.

Some cells in erythrocytes undergo gametocytogenesis and differentiate into male and female gametocytes. The gametocytes go into the gut of a female Anopheles mosquito with the blood meal of an infected person.

Sexual Stage in Female Anopheles

Sexual reproduction occurs in the female Anopheles mosquitoes. The mosquito takes up the gametocytes through the blood meal of the infected person. The multiplication in mosquitoes is called the sporogonic cycle. The male gametocytes multiply and produce flagellated microgametes. The flagellated microgamete penetrates the female gametocyte and produces a diploid zygote after fertilisation.

The zygote develops into a motile ookinete. Ookinetes penetrate the midgut epithelium and develop into immotile oocysts. The Oocyst undergoes multiple divisions to produce a syncytial cell containing multiple nuclei. The oocyst ruptures and releases sporozoites. Sporozoites enter the salivary gland of the mosquito and are stored there. They get transmitted to the human bloodstream when an infected female Anopheles mosquito bites a human.

This was all about the Plasmodium falciparum. Learn more about other related concepts for NEET, only at BYJU’S.

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