What is Plasmodium falciparum?

Answer 1

Plasmodium is a genus of parasitic protozoa. Infection with these parasites is known as malaria. The genus Plasmodium was created in 1885 by Marchiafava and Celli. Currently over 200 species in this genus are recognized and new species continue to be described.[1] [2] Of the 200+ known species of Plasmodium, at least 10 species infect humans. Other species infect animals, including birds, reptiles and rodents. The parasite always has two hosts in its life cycle: a mosquito vector and a vertebrate host. The genus is currently (2006) in need of reorganization as it has been shown that parasites belonging to the genera Haemocystis and Hepatocystis appear to be closely related to Plasmodium. It is likely that other species such as Haemoproteus meleagridis will be included in this genus once it is revised. Life Cycle Mosquitoes of the genera Culex, Anopheles, Culiceta, Mansonia and Aedes may act as vectors. The currently known vectors for human malaria (> 100 species) all belong to the genus Anopheles. Bird malaria is commonly carried by species belonging to the genus Culex. Only female mosquitoes bite. Aside from blood both sexes live on nectar, but one or more blood meals are needed by the female for egg laying as the protein content of nectar is very low. The life cycle of Plasmodium was discovered by Ross who worked with species from the genus Culex. The life cycle of Plasmodium is complex. Sporozoites from the saliva of a biting female mosquito are transmitted to either the blood or the lymphatic system [3] of the recipient. The sporozoites then migrate to the liver and invade hepatocytes. This latent or dormant stage of the Plasmodium sporozoite in the liver is called the hypnozoite. The development from the hepatic stages to the erythrocytic stages has until very recently been obscure. In 2006[4] it was shown that the parasite buds off the hepatocytes in merosomes containing hundreds or thousands of merozoites. These merosomes have been subsequently shown[5] to lodge in the pulmonary capilaries and to slowly disintegrate there over 48-72 hours releasing merozoites. Erythrocyte invasion is enhanced when blood flow is slow and the cells are tightly packed: both of these conditions are found in the alveolar capilaries. Within the erythrocytes the merozoite grow first to a ring-shaped form and then 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. The parasite feeds by ingesting haemoglobin and other materials from red blood cells and serum. The feeding process damages the erythrocytes. Details of this process have not been studied in species other than Plasmodium falciparum so generalisations may be premature at this time. At the molecular level a set of enzymes known as plasmepsins which are aspartic acid proteases are used to degrade hemoglobin. The parasite digests 70-80% of the erythrocyte's haemoglobin [6] but utilises only ~15% in de novo protein synthesis [7] The excess amino acids are exported from the infected erythorocyte by new transport pathways created by the parasite. [8] The reason proposed for this apparently excessive digestion of haemoglobin is the colloid-osmotic hypothesis [9] which suggests that the digestion of haemoglobin increases the osmotic pressure within the infected erythrocyte leading to its premature rupture and subsequent death of the parasite. To avoid this fate much of the haemoglobin is digested and exported from the erythrocyte. This hypothesis has been experimentally confirmed. [10] 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 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 and subcutaneous tissue of the next host the mosquito bites. The majority appear to be injected into the subcutaneous tissue from which they migrate into the capillaries. A proportion are ingested by macrophages and still others are taken up by the lymphatic system where they are presumably destroyed. The sporozoites which successfully enter the blood stream move to the liver where they begin the cycle again. The pattern of alternation of sexual and asexual reproduction which may seem confusing at first is a very common pattern in parasitic species. The evolutionary advantages of this type of life cycle were recognised by Mendel. Under favourable conditions asexual reproduction is superior to sexual as the parent is well adapted to its environment and its descendents share these genes. Transferring to a new host or in times of stress, sexual reproduction is generally superior as this produces a shuffling of genes which on average at a population level will produce individuals better adapted to the new environment. Reactivation of the hypnozoites has been reported for up to 30 years after the initial infection in humans. The factors precipating this reactivation are not known. In the species Plasmodium malariae, Plasmodium ovale and Plasmodium vivax hypnozoites have been shown to occur. Reactivation was not thought to occur in infections with Plasmodium falciparum but there are been two reports to date suggesting that this may occur (see below) . It is not known if hypnozoite reactivaction may occur with any of the remaining species that infect humans but this is presumed to be the case. A report of recurrence of P. falciparum in a patient with sickle cell anaemia has been published [11] but this needs confirmation as hypnozoites are not known to occur in P. falciparum infections. A second report of P. falciparum malaria eight years after leaving an endemic area has also been published.[12] While this is consistent with the existence of a hypnozoite stage additional confirmation seems desirable.

Answer 2

Plasmodium is a genus of parasitic protists. Infection by these organisms is known as malaria.

Answer 3

The protozoan Plasmodium falciparum causes malaria in humans.

Answer 4

Apicomplexa

Answer 5

Apicomplexa

Answer 6

Alveolata