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Plasmodium falciparum is one of four distinct species of the malaria parasite that affect humans, of which two predominate as threats to public health. Plasmodium falciparum is found globally but is commonest in Africa. This gives rise to acute infections that may rapidly become life-threatening. Chronic infections also cause debilitating anaemia.
Plasmodium vivax, which ranges widely throughout Asia, Africa, the Middle East, Oceania and the Americas (and is resurgent in Eastern Europe), can cause recurring and debilitating infections, but rarely kills. A minority of malaria cases are caused by Plasmodium ovale and Plasmodium malariae.
Background: Four species of Plasmodium
When an female mosquito infected with Plasmodium falciparum feeds on a human, parasites in the 'sporozoite' stage enter the bloodstream, are carried to the liver and infect liver cells. Here, they develop over seven days into a large 'hepatic schizont' which contains about 30 000 tiny invasive parasites called merozoites.
The liver cell ruptures and merozoites are released into the blood where they invade red blood cells. About 20 minutues after invading, the merozoite turns into a form called a 'feeding trophozoite'. This eats the contents of the cell, and breaks down haemoglobin to amino acids and haem.
Later, the trophozoite divides several times to produce 12-32 merozoites, which fill the red blood cell as an 'erythrocytic shizont'. The red cell ruptures and merozoites are released into the blood stream, where they repeat this multiplication step by invading uninfected red blood cells. This is called the 'blood cycle', and greatly amplifies the infection.
Some merozoites do not develop into trophozoites, but instead form sexual stage gametocytes. Gametocyte development takes 10-12 days for Plasmodium falciparum. Early stage gametocytes lurk in organs such as brain and bone marrow, while late stage gametocytes circulate in the blood and are taken up by a mosquito during a blood meal.
The fever associated with untreated Plasmodium falciparum infection classically occurs every 48 hours when large numbers of infected red blood cells rupture at the same time, releasing pyrogens (fever-causing chemicals) into the blood. Red blood cell rupture also contributes to anaemia associated with malaria.
At the peak of infection, a person will carry up to 2 million parasites per microlitre of blood. Red blood cells infected with trophozoite stage Plasmodium falciparum sequester in capillaries of the brain. This contributes to symptoms of cerebral malaria.
Background: Severe malaria
Infected cells can remain hidden from the immune system and evade treatment, and re-emerge to replicate (recrudesce) years later, causing another bout of clinical malaria.
Image credit: D Greenwood