Plasmodium, commonly known as the malaria parasite, is a large genus of parasitic protozoa. There are about 200 species. Infection with these protozoans is called malaria, a deadly disease widespread in the tropics. The parasite always has two hosts in its life cycle: a mosquito vector and a vertebrate host.
The life-cycle is very complex. There is a sequence of different stages in the vector and the host. These stages include:
- sporozoites which are injected by the mosquito vector into the host's blood;
- hypnozoites which may rest undetected in the liver for up to 30 years;
- merosomes and merozoites which infect the red blood cells (erythrocytes) of the blood;
- trophozoites which grow in the red cells, and schizonts which divide there, producing more merozoites which leave to infect more red cells;
- gametocytes, male and female sexual forms, which are taken up by other mosquitoes. In the mosquito's midgut, the gametocytes develop into
- gametes which fertilize each other to form motile
- zygotes which escape the gut, only to grow into
- new sporozoites which move to the mosquito's salivary glands, from where they are injected into the mosquito's next host, infecting it and restarting the cycle.
The genus Plasmodium was first described in 1885. At least ten species infect humans. Other species infect other animals, including birds, reptiles and rodents, while 29 species infect non-human primates. The Apicomplexa – the phylum to which Plasmodium belongs – are thought to have originated within the Dinoflagellates – a large group of photosynthetic protozoa.
The most common forms of human malaria are caused by Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae. P. falciparum malaria, common in sub-Saharan Africa, is especially dangerous.