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DPDx

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/malaria/.

Malaria

[Plasmodium falciparum] [Plasmodium knowlesi] [Plasmodium malariae] [Plasmodium ovale] [Plasmodium vivax]

Gametocyte of P. falciparum in a thin blood smear. Also seen in this image are ring-form trophozoites and an RBC exhibiting basophilic stippling (upper left).

Gametocyte of P. falciparum in a thin blood smear. Also seen in this image are ring-form trophozoites and an RBC exhibiting basophilic stippling (upper left).


Trophozoite of P. ovale in a thin blood smear.  Note the fimbriation and Schüffner's dots.

Trophozoite of P. ovale in a thin blood smear. Note the fimbriation and Schüffner's dots.


Adults of Anopheles freeborni.

Adult of Anopheles freeborni.

Causal Agent

Blood parasites of the genus Plasmodium. There are approximately 156 named species of Plasmodium which infect various species of vertebrates. Four species are considered true parasites of humans, as they utilize humans almost exclusively as a natural intermediate host: P. falciparum, P. vivax, P. ovale and P. malariae. However, there are periodic reports of simian malaria parasites being found in humans, most reports implicating P. knowlesi. At the time of this writing, it has not been determined if P. knowlesi is being naturally transmitted from human to human via the mosquito, without the natural intermediate host (macaque monkeys, genus Macaca). Therefore, P. knowlesi is still considered a zoonotic malaria.

Life Cycle

Life cycle of Plasmodium

The malaria parasite life cycle involves two hosts. During a blood meal, a malaria-infected female Anopheles mosquito inoculates sporozoites into the human host The number 1. Sporozoites infect liver cells The number 2 and mature into schizonts The number 3, which rupture and release merozoites The number 4. (Of note, in P. vivax and P. ovale a dormant stage [hypnozoites] can persist in the liver and cause relapses by invading the bloodstream weeks, or even years later.) After this initial replication in the liver (exo-erythrocytic schizogony The letter A), the parasites undergo asexual multiplication in the erythrocytes (erythrocytic schizogony The letter B). Merozoites infect red blood cells The number 5. The ring stage trophozoites mature into schizonts, which rupture releasing merozoites The number 6. Some parasites differentiate into sexual erythrocytic stages (gametocytes) The number 7. Blood stage parasites are responsible for the clinical manifestations of the disease.

The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested by an Anopheles mosquito during a blood meal The number 8. The parasites' multiplication in the mosquito is known as the sporogonic cycle The letter C. While in the mosquito's stomach, the microgametes penetrate the macrogametes generating zygotes The number 9. The zygotes in turn become motile and elongated (ookinetes) The number 10 which invade the midgut wall of the mosquito where they develop into oocysts The number 11. The oocysts grow, rupture, and release sporozoites The number 12, which make their way to the mosquito's salivary glands. Inoculation of the sporozoites into a new human host perpetuates the malaria life cycle The number 1.

Geographic Distribution

Malaria generally occurs in areas where environmental conditions allow parasite multiplication in the vector.  Malaria today is usually restricted to tropical and subtropical areas and altitudes below 1,500 m., although in the past malaria was endemic in much of North America, Europe and even parts of northern Asia, and today is still present on the Korean peninsula. However, this present distribution could be affected by climatic changes and population movements. Plasmodium falciparum is the predominant species in the world. P. vivax and P. ovale are traditionally thought to occupy complementary niches, with P. ovale predominating in Sub-Saharan Africa and P. vivax in the other areas; but their geographical ranges do overlap. These two species are not always distinguishable on the basis of morphologic characteristics alone, and the use of molecular tools will help clarify their diagnosis and exact distribution. P. malariae has wide global distribution, being found in South America, Asia, and Africa, but it is less frequent than P. falciparum in terms of association with cases of infection. P. knowlesi is found in southeast Asia.

More on: Malaria Risk Information and Prophylaxis by Country

Clinical Presentation

The symptoms of uncomplicated malaria can be rather non-specific and the diagnosis can be missed if health providers are not alert to the possibility of this disease. Since untreated malaria can progress to severe forms that may be rapidly (<24 hours) fatal, malaria should always be considered in patients who have a history of exposure (mostly: past travel or residence in disease-endemic areas). The most frequent symptoms include fever and chills, which can be accompanied by headache, myalgias, arthralgias, weakness, vomiting, and diarrhea. Other clinical features include splenomegaly, anemia, thrombocytopenia, hypoglycemia, pulmonary or renal dysfunction, and neurologic changes. The clinical presentation can vary substantially depending on the infecting species, the level of parasitemia, and the immune status of the patient. Infections caused by P. falciparum are the most likely to progress to severe, potentially fatal forms with central nervous system involvement (cerebral malaria), acute renal failure, severe anemia, or acute respiratory distress syndrome. Other species can also have severe manifestations. Complications of P. vivax malaria include splenomegaly (with, rarely, splenic rupture), and those of P. malariae include nephrotic syndrome.

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  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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