Plasmodium
From Wikipedia, the free encyclopedia
- A plasmodium[[1]] is also the macroscopic form of the unusual protozoa known as slime moulds.
Plasmodium is a genus of parasitic protozoa. Infection with this genus is known as malaria. The parasite always has two hosts in its life cycle: a mosquito vector and a vertebrate host. At least ten species infect humans. Other species infect other animals, including birds, reptiles and rodents.
There are over 175 species recognised in this genus. The genus is currently (2006) in need of reorganisation as it has been shown that other parasites belonging to the genera Haemocystis and Hepatocystis appear to be closely related to this genus. It is likely that other species such as Haemoproteus meleagridis will be included in this genus once it is revised.
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[edit] Discovery of the life cycle
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.
- Plasmodium relictum is probably responsible for more avian extinctions than any other protist.
The species of Plasmodium that parasitise humans include
- 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)
- Plasmodium knowlesi
- Plasmodium semiovale
- Plasmodium simium
The first four listed here are the most common species that infect humans. With the use of the polymerase chain reaction additional species have been and are still being identified that infect humans.
The life cycle of Plasmodium is very complex. Sporozoites from the saliva of a biting female mosquito are transmitted to either the blood or the lymphatic system<ref name="Lymph">[http://www.hhmi.org/news/menard20060122.html HHMI Staff (22 January 2006) "Malaria Parasites Develop in Lymph Nodes" HHMI News Howard Hughes Medical Institute</ref> of the recipient. The sporozoites migrate to the liver and invade hepatocytes. The so-called latent or dormant stage of the Plasmodium sporozoite in the liver is called the hypnozoite. From the 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.
[edit] Notes
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[edit] References
- Short, H. E. (1951) Life-cycle of the mammalian malaria parasite" British Medical Bulletin 8(1): pp. 7-9, (PMID 14944807);
- Baldacci, Patricia and Ménard, Robert (Oct. 2004) "The elusive malaria sporozoite in the mammalian host" Molecular Microbiology 54(2): pp. 298-306, (AN 14621725);
- Bledsoe, G. H. (December 2005) "Malaria primer for clinicians in the United States" Southern Medical Journal 98(12): pp. 1197-204 (PMID 16440920);
[edit] External links
- Life cycle of plasmodium (Flash animations)de:Plasmodien
es:Plasmodium fr:Plasmodium it:Plasmodium lt:Plazmodis pl:Zarodziec pt:Plasmodium sv:Plasmodium zh:瘧原蟲
