Some microbial eukaryotes are pathogens that cause serious diseases in humans and other vertebrates. The best-known pathogenic protists are members of the genus Plasmodium, a highly specialized group of apicomplexans that spend part of their life cycle in mosquitoes of the genus Anopheles and part as parasites in human red blood cells, where they are the cause of malaria. In terms of the number of people affected, malaria is one of the world's three most serious infectious diseases: it infects over 350 million people, and kills over 1 million people, each year. On average, about two people die from malaria every minute of every day—most of them in sub-Saharan Africa, although malaria occurs in more than 100 countries. This tutorial describes the life cycle of the malarial parasite.
The parasite Plasmodium falciparum is responsible for most of the world's malaria cases. It has evolved in concert with its human and mosquito hosts and displays a complex life cycle that alternates between the two. An infected female mosquito transfers the parasite to a human when she takes a blood meal. The parasite, in the form of sporozoites, is injected with her saliva, which contains an anticoagulant that promotes a steady blood flow.
The circulatory system delivers the sporozoites to their target, the liver. A sporozoite enters a liver cell and begins to feed and grow. During the process, the parasite's nucleus divides many times, and then the nuclei bud off to produce thousands of new Plasmodium cells, now in a form called merozoites. About a week later, the merozoites burst out of liver cells and enter the bloodstream.
In the bloodstream, the merozoites infect red blood cells, where they again reproduce and burst out of the infected cells. The resulting merozoites can infect other red blood cells, yielding a cycle of infection and eruption. Malaria is characterized by repeated bouts of fever, each of which correlates with the synchronous release of merozoites from a population of red blood cells.
Some of the infecting merozoites develop into sexual forms, called gametocytes, which are either male or female. In the life cycle of Plasmodium falciparum, gametocytes become large oval structures within red blood cells. The gametocytes do not harm the human host, but when a mosquito takes the gametocytes into its gut during a blood meal, these cells initiate the next stage of the Plasmodium life cycle.
In the mosquito's gut, the gametocytes escape from the red blood cells and develop into male and female sex cells, called gametes. A male and a female gamete fuse to produce the first cell of the next generation, the zygote.
The zygote matures into a motile cell, called an ookinete, and migrates to the wall of the mosquito's gut, where it develops into an oocyst. Inside the oocyst, cell division occurs to produce many new cells, again in the form of sporozoites. The oocyst ruptures and releases the sporozoites, which migrate to the salivary glands. From the salivary glands, the sporozoites can infect the next human host and repeat the Plasmodium life cycle.
Mosquitoes of the genus Anopheles transmit Plasmodium to humans. In the mosquito, Plasmodium is an extracellular parasite, and in the human host, it is an intracellular parasite. As part of the life cycle in a human, the parasites multiply inside red blood cells, which then burst, releasing new swarms of parasites, which can infect more red blood cells. These episodes of bursting red blood cells coincide with the primary symptoms of malaria, which include fever, shivering, vomiting, joint pains, and convulsions.
The complex Plasmodium life cycle is best broken by the removal of stagnant water, in which mosquitoes breed. Using insecticides to reduce the Anopheles population can also be effective, but the benefits must be weighed against the ecological, economic, and health risks posed by the insecticides themselves.