The Lifecycle of the Malaria Parasite - Mark Lertiendumrong Malaria is an infectious and parasitic disease carried by mosquitos of the Anopheles genus as vectors. What causes the infection is a Protozoan, a single-celled eukaryote of the Protoctista kingdom, of the genus Plasmodium. Although there are around 156 species of the parasite, only 4 have humans as their exclusive natural host: P. vivax, P. ovale, P. malariae and P. falciparum with P. falciparum accounting for nearly 90% of all cases. When a person gets bit by an infected mosquito, it injects its saliva through its hypodermic needle-like mouthpiece into a small blood vessel. Its saliva contains anticoagulants to allow it to draw blood without the blood clotting and simultaneously, the parasites which are in the saliva are released in the form of sporozoites into the bloodstream. In the span of a few minutes, the sporozoites enter liver cells and start to replicate rapidly. It is the fastest known replication of a eukaryote with 1 becoming 10000 in 48 hours. This exoerythrocytic stage takes 10 days for P. falciparum and is completely silent. After this rapid replication, the liver cells burst and releases all the parasites now in the form of merozoites into the bloodstream. They invade the red blood cells and through a process of asexual reproduction called schizogony, multiply from 1 to 16 to 32 within a single red blood cell in 3 to 4 days cycles. There are many intermediate forms such as the trophozoite form and the schizont form which the process is named after. The parasite breaks down the haemoglobin to use as amino acids until only the heme is left. The residual heme is toxic to the parasite and so to detoxify it, the parasite creates haemozoin, an inorganic crystal. This erythrocytic stage is where symptoms start showing with the distinct fever and chills caused by the immune response to the haemozoin released into the blood when the infected red blood cell bursts. Furthermore, as the infected red blood cells lose its biconcave shape and become a lumpy shape, it attaches itself onto the extracellular matrix of its surroundings, including non-infected red blood cells. This is to stop it from reaching the spleen where disfigured red blood cells are destroyed. As a result of this attachment, blood clots can form and a severe case would be in the brain, cerebral malaria, which is the largest cause of malaria-related death. The new merozoites repeat the cycle resulting in an increase of parasitaemia, the number of parasites in the blood, by 10 or even 20 times in 1 cycle. Even though there are a large number of merozoites in the blood, only 1 to 5% will undergo its sexual stages and differentiate to become male and female gametocytes which are haploid. When a mosquito takes a blood meal, the gametocytes will then infect the mosquito. This is the end of the parasite’s journey in human hosts. Soon after the gametocytes infect the mosquito, the male gametocytes will undergo exflagellation, which includes 3 rounds of mitotic division, creating 8 flagella which separate to become individuals described as microgametes like a sperm cell. The female gametocytes, on the other hand, will only form 1 macrogamete, an egg, each. A microgamete will fertilise a macrogamete, producing a diploid zygote which soon transforms into a motile form, an ookinete. It will then enter the midgut cells of the mosquito and transform into an oocyst. This whole process from fertilisation until becoming an oocyst takes around 24 hours. During this time, the parasite suffers massive losses in terms of numbers, from perhaps thousands in the initial blood meal to only a few becoming oocysts. This is due to the
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