Malaria Parasite Life Cycle (عملي مايكرو, ملزمة ٣, د. لمى)

Malaria is caused by a parasite that spreads to humans through the bites of infected female Anopheles mosquitoes. The Plasmodium parasites that infect people with malaria cannot survive outside of their hosts: humans and Anopheles mosquitoes. -There are more than 150 blood parasites of the genus Pla...

Malaria is caused by a parasite that spreads to humans through the bites of infected female Anopheles mosquitoes. The Plasmodium parasites that infect people with malaria cannot survive outside of their hosts: humans and Anopheles mosquitoes. -There are more than 150 blood parasites of the genus Plasmodium. However, only five species of malaria parasites cause significant disease in humans: -Plasmodium falciparum – the most prevalent, causes the majority of severe malaria cases and deaths. -Plasmodium vivax – the main cause of relapsing malaria, with a blood and a liver infection causing acute and ongoing symptoms. -Less prevalent: Plasmodium ovale, Plasmodium malariae and Plasmodium knowlesi. Severe malaria is usually caused by a P. falciparum infection, and typically occurs when treatment of uncomplicated malaria is inefficacious, delayed or absent. Pregnant women in their second and third trimesters are among the most at-risk of severe malaria, as are children aged five and under, who account for around 75% of malaria-related deaths annually. Though P. vivax and P. knowlesi infections can also result in severe malaria, this occurrence is less frequent. The life cycle (Figure 1) is almost the same for all the five species that infect humans and follows three stages: (I) ifection of a human with sporozoites (II) asexual reproduction (III) sexual reproduction The two first stages take place exclusively into the human body, while the third one starts in the human body and is completed into the mosquito organism. 1Plasmodium sporozoites are the infective stage of malaria parasites that infect humans. The sporozoites residing in the salivary glands of female Anopheles mosquitoes are transmitted to humans via mosquito bites during blood feeding. Sporozoites are formed within oocysts, which are lodged between epithelial cells and the basal lamina in the mosquito midgut. 2- hypnozoit It is worth mentioning that, concerning P. vivax and P. ovale, sporozoites may not follow the reproduction step and stay dormant (hypnozoites) in the liver; they may be activated after a long time leading to relapses entering the blood stream (as merozoites) after weeks, months or even years. The exoerythrocytic phase is not pathogenic and does not produce symptoms or signs of the disease. Its duration is not the same for all parasite species. 3-Schizonts Schizonts are rarely seen in peripheral blood of Plasmodium falciparum infections, except in severe cases. When seen, schizonts contain anywhere from 8-24 merozoites. A mature schizont usually fills about 2/3 of the infected RBC. Schizont stage of Plasmodium occurs in human liver cells and erythrocytes. Schizont divides asexually by schizogony to form daughter cells in protozoans. Sporozoites enter the bloodstream of humans through the bite of an infected female Anopheles mosquito. Sporozoites travel to the hepatic cells through the bloodstream. In the liver cells (hepatic cells) they undergo one asexual replication cycle that results in a liver schizont containing thousands of merozoites. This is called exo-erythrocytic schizogony. Merozoite released from the liver cells penetrates erythrocytes. Inside the erythrocytes, these merozoites enlarge and are termed trophozoites. The trophozoites undergo asexual reproduction to produce schizont (erythrocytic schizogony, which eventually divides to produced merozoites and are released upon erythrocytes lysis. 4-The trophozoite (ring stage) stage of development is critical for the parasite to undergo morphological changes, grow in size, and remodel the host red blood cell (RBC) to suit its development and release of new infectious merozoite forms into circulation. The ring stage trophozoites mature into schizonts, which rupture releasing merozoites. Some parasites differentiate into sexual erythrocytic stages (gametocytes). Blood stage parasites are responsible for the clinical manifestations of the disease. 5-Gametocytes form mostly during each of the asexual erythrocytic cycles, whereas in some species such as the human parasite P. vivax gametocytes can also be derived from the merozoites emerging from the pre-erythrocytic schizonts. It is the infective stage to mesquite. Gametocytes will form zygote in the mosquito midgut. 6-ookinete The Plasmodium ookinete is the developmental stage of the malaria parasite that invades the mosquito midgut. It is formed from zygotes. The ookinete faces two physical barriers in the midgut which it must traverse to become an oocyst: the chitin- and protein-containing peritrophic matrix; and the midgut epithelial cell. 7-ocyst development is the longest stage of the Plasmodium life cycle and for this reason it is becoming considered an attractive target for new anti-malarial strategies.