The life cycle of malaria
Let’s talk about what happens when someone is infected with malaria. Remember, malaria results from infection with one of the various species of the parasite Plasmodium. Malaria-causing Plasmodia are carried by a special type of mosquito, the anopheline mosquito.
Exposure to the malaria sporozoites through a mosquito's bite
To become infected, a person has to be bitten by a female mosquito that carries immature malaria cells called sporozoites. As the mosquito finishes feeding, she injects the crescent-shaped sporozoites into her victim’s bloodstream where they make their way to the liver.
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Replication and formation of merozoites inside the liver
As the blood carrying the sporozoites passes through the liver, specialized white blood cells—fixed-tissue macrophages called Kupffer cells—filter the foreign sporozoites from the blood. But, the sporozoites are able to escape destruction by the macrophages and migrate into the cells of the liver.
In the hepatocyte, the sporozoites are able to begin an asexual reproduction process called schizogony. This occurs when a sporozoite divides and fills the liver cell—now known as a liver schizont—with many smaller, identical copies of itself called merozoites. Eventually, the liver schizont ruptures and releases the merozoites into the bloodstream.
Invasion of red blood cells (RBCs) is essential for survival and continued reproduction of malaria parasites
Once in the bloodstream, the merozoites find and invade red blood cells (RBCs) where they have two fates: asexual reproduction or sexual reproduction.
Asexual reproduction of the malaria parasite
Inside the RBC, the merozoite will continue the process of schizogony creating an RBC schizont that eventually bursts, releasing more of these merozoites, which will find and invade other RBCs and the process will start all over again.
Sexual reproduction and the formation of malaria gametocytes
Some of those merozoites do not undergo schizogony in the RBC. Instead, they become sexual structures called gametocytes which hang around until a mosquito bites.
Formation of new malaria sporozoites in the mosquito vector (sporogony)
Then, when a mosquito bites the infected individual, it ingests the gametocytes. And, once taken in by a mosquito the female and male gametocytes form a kinetic egg called an ookinete. The egg eventually becomes an oocyst that releases sporozoites as it matures. And when the mosquito bites another person, these sporozoites are transferred, infecting them with malaria. And the cycle continues.
We know that can be a lot to remember, so we've created this handy summary of the key points:
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- Ashley, EA, Phyo, AP, and Woodrow, CJ. 2018. Malaria. Lancet. 391:1608–1621. PMID: 29631781
- Fairhurst, RM and Wellems, TE. 2014. “Malaria (Plasmodium Species)”. In: Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, edited by Bennett, JE, Dolin, R, Blaser, MJ. 8th edition. Philadelphia: Elsevier Saunders. (Fairhurst and Wellems 2014, 3070–3090)
- Phillips, MA, Burrows, JN, Manyando, C, et al. 2017. Malaria. Nat Rev Dis Primers. 3: 17050. PMID: 28770814
- World Health Organization. 2019. World malaria report 2019. World Health Organization. https://www.who.int