What Is The Life Cycle Of Malaria?
Malaria Life Cycle in Mosquito
A female Anopheles mosquito is a definitive host of Plasmodium in which sporogony, the sexual phase occurs inside the mosquito. It starts when the mosquito bites an infected person with malaria and in the process takes up gametocytes present in the blood stream of the infected individual. Gametogenesis takes place inside the gut of the mosquito, in which gametocytes mature and become female (macrogametes) and male (microgametes) gametes. The time that is needed for gametocytes to mature differs in each plasmodium species, it takes about 8 to 10 days for P. falciparum, 3 to 4 days for P. vivax and P. ovale and 6 to 8 days for P. malariae. The fusion of microgamete and macrogamete in the mosquito gut leads to the formation of zygote, which develops into ookinete that embeds in the midgut wall of the mosquito where it forms into oocyst. The growth and division of oocyst takes place that produces thousands of sporozoites.
This sporogenic phase completes in about 8 to 15 days. After the rupture of oocysts, the sporozoites are released into the body cavity of the mosquito from where they travel to the salivary glands of the mosquito. Thus, when the mosquito (saliva loaded with sporozoites) bites a human being, the sporozoites are transmitted to the human host causing malarial infection in them. This leads to the next cycle in the human host. The infected mosquito and the parasite are in a symbiotic relationship which promotes transmission of infection. It has been known that the survival rate of Plasmodium infected mosquito increases with increased blood feeding, primarily from an infected host.
Malaria Life Cycle in Humans
Humans are the intermediary host for Plasmodium, in which the asexual phase of the Plasmodium life cycle occurs. This phase starts when an infected Anopheles mosquito bites an individual and sporozoites are transmitted to the human host. Some of the sporozoites reach the liver within hours of transmission through blood stream where the pre-erythrocytic phase or exoerythrocytic schizogony begins. The sporozoites multiply and develop into schizonts and each schizont contains thousands of merozoites (more in P. falciparum). This is the end of pre-erythrocytic phase. This phase lasts for around 5 to 16 days depending on the species: this averages 5 to 7 days in P. falciparum, 6-8 in for P. vivax, 9 days in P. ovale, 14 to 16 days in P. malariae and 8 to 9 days in P. knowlesi. In P. vivax and P. ovale, some sporozoites may stay dormant in the liver for months and these are known as hynozoites. These hynozoites are responsible for relapses of malaria infection caused by them in later months when they become active and change to merozoites. The exoerythrocytic phase is an asymptomatic stage of malarial infection.
After the merozoites from liver enter the bloodstream they travel to red blood cells where the second phase occur known as the erythrocytic phase or erythrocytic schizogony. The malarial parasite grows inside the red blood cell and multiplies by forming intracellular rings and forms trophozoite. These trophozoites nurture and multiply to form erythrocytic schizont. The erythrocytic schizogony releases merozoites into the blood stream after the rupture of red blood cells.
The erythrocytic cycle is a repetitive phase and a single cycle of this lasts about 48 hours in P. falciparum, P. vivax and P. ovale, 72 hours in P. malariae and lasts 24 hours in P. knowlesi after which the cycle begins again. These periods coincide with paroxysm of symptoms such as high fever and chills and sweating. These merozoites that are released from ruptured red blood cells infect more red blood cells. Some of the asexual parasites do not undergo erythrocytic schizogony, but go through sexual stage forming male and female gametocytes. These gametocytes are non-pathogenic and help in transmission of infection from one intermediate host to another with the help of Anopheles mosquito where the sexual cycle takes place.
- World Health Organization (WHO) – Malaria: https://www.who.int/health-topics/malaria
- Centers for Disease Control and Prevention (CDC) – Malaria: https://www.cdc.gov/malaria/
- The American Journal of Tropical Medicine and Hygiene – Malaria Life Cycle: https://www.ajtmh.org/content/journals/10.4269/ajtmh.2005.73.484