Plasmodium
Authors: Bannister Lawrence H , Sherman Irwin W 
Published: December 2009

Plasmodium

is a genus of parasitic protozoa which infect erythrocytes of vertebrates and cause malaria. Their life cycle alternates between mosquito and vertebrate hosts. Parasites enter the bloodstream after a mosquito bite, and multiply sequentially within liver cells and erythrocytes before becoming male or female sexual forms. When ingested by a mosquito, these fuse, then the parasite multiplies again to form more invasive stages which are transmitted back in the insect's saliva to a vertebrate. All invasive stages have specialized secretory structures (apical organelles) typical of the protozoan subphylum Apicomplexa, enabling them to invade cells and tissues. Parasites exploit erythrocytes by ingesting haemoglobin and exporting molecules which change erythrocyte membrane properties. Five species infect humans, the most lethal being

Plasmodium falciparum

which can cause pathology and death by clogging blood vessels in brain, viscera and placenta. Many hundreds more species infect other mammals, birds and lizards.

Key Concepts:

Intracellular parasitism. Many apicomplexan protozoa invade and exploit the cells of their hosts, situated in a nutrient‐rich environment and protected from attack by their host's immune system.

Alternation of hosts. All parasites of the genus
Plasmodium
have two different types of host which they parasitize alternately, a vertebrate and a mosquito. This arrangement enables the parasite to multiply numbers greatly in the richly nutrient environment of the vertebrate host, and also to find new hosts via the mosquito vector.

The parasitophorous vacuole. Blood stage parasites are enclosed in a cavity within the erythrocyte (parasitophorous vacuole) lined by membrane (parasitophorous vacuole membrane) derived from the erythrocyte surface membrane. The parasite is therefore not in direct contact with the erythrocyte cytoplasm.

Life‐cycle stages. The structure, molecular composition and behaviour of
Plasmodium
parasites change constantly throughout its life cycle, as the expression patterns of its genes vary within both vertebrates and mosquito hosts, enabling parasites to exploit different aspects of the host's biology.

Intracellular invasion. Intracellular life requires the ability to selectively capture and enter host cells, seen in the invasion by merozoites into erythrocytes and of sporozoites into liver hepatocytes. In both cases this depends on the secretion of adhesive proteins and membrane‐altering substances by the parasite, and on its actin–myosin driven locomotion.

Host cell transformation. Intracellular parasites export various molecules into the host cell to transform its properties. In erythrocytes this increases permeability to nutrients and in some species causes the erythrocyte membrane to adhere to blood vessel walls. The parasite also consumes host cell cytoplasm.

Malaria. This is a generic name given to diseases in vertebrates caused by species of
Plasmodium
. They are typified in mammals by cyclic episodes of fever and may have many other complications including anaemia, spleen pathology, placental dysfunction, neurological complications and in severe malaria, coma and death.

Parasite evolution. Molecular studies indicate that the evolution of
Plasmodium
species is closely linked to the evolution of terrestrial vertebrates and has impacted significantly on the human genome.