A plasmodium is also the macroscopic form of the unusual protozoa known as slime moulds.

Plasmodium is a genus of parasitic protozoa, four species of which cause malaria in humans. Other species infect other animals, including birds, reptiles and rodents. In 1898 Ronald Ross demonstrated the existence of Plasmodium in the stomach of the Anopheles mosquito. For this discovery he won the Nobel Prize in 1902. However some credit must also be given to the Italian professor, Giovanni Battista Grassi, who showed that human malaria could only be transmitted by Anopheles mosquitoes.

The four species of Plasmodium that attack humans are

• Plasmodium falciparum (the cause of malignant tertian malaria)
• Plasmodium vivax (the most frequent cause of benign tertian malaria)
• Plasmodium ovale (the other, less frequent, cause of benign tertian malaria)
• Plasmodium malariae (the cause of benign quartan malaria)

The life cycle of Plasmodium is very complex. Sporozoites are injected by a biting female mosquito, and migrate to the liver, where the parasite invades hepatocytes. The parasite replicates into thousands of merozoites, which then invade red blood cells. Here the parasite grows from a ring-shaped form to a larger trophozoite form. In the schizont stage, the parasite divides several times to produce new merozoites, which leave the red blood cells and travel within the bloodstream to invade new red blood cells. Most merozoites continue this replicative cycle, but some merozoites differentiate into male or female sexual forms (gametocytes) (also in the blood), which are taken up by the female Anopheles mosquito. In the mosquito's midgut, the gametocytes develop into gametes and fertilize each other, forming motile zygotes called ookinetes. The ookinetes penetrate and escape the midgut, then embed themselves onto the exterior of the gut membrane. Here they divide many times to produce large numbers of tiny elongated sporozoites. These sporozoites migrate to the salivary glands of the mosquito where they are injected into the blood of the next host the mosquito bites. The sporozoites move to the liver where they repeat the cycle.

On a molecular level, the parasite damages red blood cells using plasmepsin enzymes. Plasmepsins are aspartic acid proteases which degrade hemoglobin.

Apicomplexa | Malaria

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