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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)
Page last modified : April 23, 2004
Content source: Division of Parasitic Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)
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