Malaria Home > Biology >

Malaria Parasites

Malaria parasites are micro-organisms that belong to the genus Plasmodium. There are more than 100 species of Plasmodium, which can infect many animal species such as reptiles, birds, and various mammals. Only four species of Plasmodium infect humans in nature. (There are some other species which can, exceptionally or under experimental conditions, infect humans.)

The four species infecting humans are:

Plasmodium falciparum, which is found worldwide in tropical and subtropical areas. It is the only species that can cause severe, potentially fatal malaria. It is estimated that every year 700,000 to 2.7 million people are killed by P. falciparum, especially in Africa where this species predominates. Plasmodium falciparum can cause severe malaria because it multiples rapidly in the blood, and can thus cause severe blood loss (anemia). In addition, the infected parasites can clog small blood vessels. When this occurs in the brain, cerebral malaria results, a complication that can be fatal.

P. vivax, which is found mostly in Asia, Latin America, and in some parts of Africa. Because of the population densities especially in Asia it is probably the most prevalent human malaria parasite. While P. vivax only exceptionally causes death (most often due to rupture of an enlarged spleen), it can cause symptoms that are incapacitating. Thus, P. vivax contributes substantially to the disease burden (morbidity) of malaria, with a resulting social and economic impact. Plasmodium vivax (as well as P. ovale) has dormant liver stages ("hypnozoites") that can activate and invade the blood ("relapse") several months or years after the infecting mosquito bite.

The two other species are less frequently encountered:

Plasmodium ovale is found mostly in Africa (especially West Africa) and the islands of the western Pacific. It is biologically and morphologically very similar to P. vivax. However, differently from P. vivax, it can infect individuals who are negative for the Duffy blood group, which is the case for many residents of sub Saharan Africa. This explains the greater prevalence of P. ovale (rather than P. vivax ) in most of Africa.

P. malariae, found worldwide, is the only human malaria parasite species that has a quartan cycle (three-day cycle). (The three other species have a tertian, two-day cycle). Plasmodium malariae causes a long-lasting, chronic infection that in some cases can last a lifetime. In some patients P. malariae can cause serious complications such as the nephrotic syndrome.

See Also

Disease caused by human malaria parasites.

Comparison of the human malaria parasites (with microscopy) (DPDx site)