The life cycle of the malarial parasite

Written by Dr Charlie Easmon, specialist adviser in travel medicine

What is a malarial parasite?

The malarial parasite, Plasmodium, is a very small, single-cell blood organism, or 'protozoan'. It lives as a parasite in other organisms, namely man and mosquito. The parasite is the cause of the tropical disease malaria.

The Plasmodium parasite is dependent on a single species of mosquito, Anopheles, which is the only species capable of serving as host for it.

What are the different stages of the parasite's life cycle?

This small single-cell organism has three to four different forms. Each form is specialised in living in a certain place.

The gametocyte is the form that infects the mosquito and reproduces itself, as if it were both sexes. When the mosquito has sucked blood containing gametocytes, these pass into the salivary glands of the mosquito, where they develop into a new form, the sporozoite. The infection can then move on.

The sporozoite can be passed on to man when the mosquito bites, injecting its saliva into the tiny blood vessels. The sporozoite travels with the blood to the liver and enters the liver cells. In the liver some of the sporozoites divide (tachysporozoites) and become thousands of merozoites.

The merozoites are released from the liver to the blood where they are taken up by the red blood corpuscles. Some of these turn into ring-formed trophozoites, which split again to form schizonts.

Schizonts burst the red blood corpuscles at a certain moment, releasing the merozoites. This release coincides with the violent rises in temperature during the attacks seen in malaria.

The trophozoites that are left over during division can, in the course of the next day, develop into the sexual form, the gametocyte, which can be taken up by a blood-sucking mosquito and start another cycle.

The incubation period (time from infection to development of the disease) is usually about 10 to 15 days. This period can be much longer depending on whether any antimalarial medication has been taken.

Plasmodium ovale and Plasmodium vivax can produce a dormant form, a hypnozoite, which can cause relapses of the disease months and even years after the original disease (relapsing malaria), because it is dormant in the liver cells. This is why it is very important after these infections to be treated with primaquine to kill the liver stages. (Primaquine cannot be used by people with a condition called G6PD-deficiency.)

Where does malaria occur?

Malaria occurs where the Anopheles mosquito occurs. The parasite cannot be passed on by any other species of mosquito.

Plasmodium falciparum is by far the most widespread malaria in Africa, and at the same time the cause of malignant malaria. It also occurs in specific areas of Asia and South America.

P. Vivax is most common in South America and Asia.

Remember, it is always best to try to prevent malaria.

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