How Long Does it Take for Malaria to Affect the Body?

QUESTION

How long does malaria take to actually affect you?

ANSWER

When you are bitten by a mosquito that is infected with the parasites that cause malaria, some of the parasites enter your blood stream in the mosquito’s saliva. After that, it will take at least one week, and usually between two and four weeks, before you start to feel the symptoms of the disease. This is because the parasite first goes to the liver, where it infects liver cells and undergoes replication. These cells mature into a form called merozoites, which then re-enter the blood stream, and start to infect red blood cells. The stage in the liver is not symptomatic for the patient, and is known as the pre-patent stage; once the merozoites start infecting and killing red blood cells, the patient will begin to feel sick, and the infection is said to have become patent.

Feeling Ill After Malaria Infection

QUESTION

How soon after infection, will a person usually begin to feel ill?

ANSWER

There is a latent period, during which time the malaria parasites (which had entered the bloodstream via the bite of an infected Anopheles mosquito) migrate to the liver and undergo replication. The sporozoites (the life stage that enters the blood from the mosquito) differentiate in many merozoites, which are contained within a schizont in the liver cells. When the schizont ruptures, the merozoites are released and re-enter the blood, where they set about infecting red blood cells. Once in the red blood cells, the infected person will begin to experience symptoms—this is usually between 6-16 days after the initial infection, during which time the parasite is reproducing in the liver.

Examination of Malaria Parasite

QUESTION

How can you examine malaria parasites?

ANSWER

Malaria parasites are usually examined under a microscope using a peripheral blood smear method (also called a blood film). Thick blood smears, which use a large unsmeared drop of blood, are sensitive since a large number of red blood cells can be examined, though the parasites, if present, are difficult to distinguish morphologically.

For species-level identification of malaria parasites, a thin blood film is more commonly used, whereby a small volume of blood is smeared thinly across the slide and then stained, usually with Romanowsky stain, in order to see the detailed structures which differentiate the different species of malaria. It is crucial to make the blood films soon after the blood sample has been taken, and to store the blood in an appropriate anti-coagulate.

Characters to look out for include the presence of Maurer’s dots on the surface of red blood cells infected with Plasmodium falciparum. You may also see multi-infected red blood cells with this species, and it is rare to see mature trophozoites or schizonts with this parasite since when this stage is reached the red blood cells are usually sequestered deep within major organs and so are not readily present in the peripheral blood.

These parasites have crescent-shaped gametocytes. Plasmodium vivax, on the other hand, enlarges red blood cells that it infects and seems to show a preference for immature red blood cells. The presence of Schüffner’s dots is also characteristic – these looks like specks or granules on the cell surface of the infected red blood cell. P. ovale is very similar to P. vivax, in that it also enlarges the red blood cells and can have Schüffner’s dots, but fewer merozoites tend to be present per cell and infected red blood cells tend to look elongated. P. malariae does not alter the size or shape of the red blood cell it infects and tends to form rosette-like patterns of 8-10 merozoites. Later on in maturation, its trophozoites may form characteristic band-like patterns across the cell.

Schizont

QUESTION

What is a schizont?

ANSWER

A schizont is a malaria parasite which has matured and contains many merozoites, which are the parasite stage that infects red blood cells.

Schizonts can be produces during two separate phases of the life cycle within the human host: first in the hepatocytic cells in the liver (when sporozoites mature) during the exo-erythrocytic cycle and then within the red blood cells during the erythrocytic cycle (when trophozoites mature and divide).

When malaria parasites do not immediately mature into schizonts in the liver (as can be the case with Plasmodium vivax and P. ovale infections), the parasite instead becomes a hypnozoite, which can lay dormant in the liver for many weeks or even months (or, in rare case, years), and produce relapse of infection at a much later date.

Pathophysiology of Malaria

QUESTION

What is the pathophysiology of malaria?

ANSWER

Malaria causes disease through a number of pathways, which depend to a certain extent on the species. Malaria is caused by a single-celled parasite of the genus Plasmodium; there are five species which infect humans, being Plasmodium falciparum, P. vivax, P. ovale, P. malariae and P. knowlesi.

All these species are introduced into the human blood stream through the bite of an infected mosquito; the life stage of malaria at this point is called a “sporozoite”, and they pass first to the liver, where they undergo an initial stage of replication (called “exo-erythrocytic replication”), before passing back into the blood and invading red blood cells (called “erythrocytes”, hence this is the “erythrocytic” part of the cycle). The malaria parasites that invade red blood cells are known as merozoites, and within the cell they replicate again, bursting out once they have completed a set number of divisions. It is this periodic rupturing of the red blood cells that causes most of the symptoms associated with malaria, as the host’s immune system responds to the waste products produced by the malaria parasites and the debris from the destroyed red blood cells. Different species of malaria rupture the red blood cells at different intervals, which leads to the diagnostic cycles of fever which characterise malaria; P. vivax, for example, tends to produce cycles of fever every two days, whereas P. malaria produces fever every three.

In addition, Plasmodium falciparum produces unique pathological effects, due to its manipulation of the host’s physiology. When it infects red blood cells, it makes them stick to the walls of tiny blood vessels deep within major organs, such as the kidneys, lungs, heart and brain. This is called “sequestration”, and results in reduced blood flow to these organs, causing the severe clinical symptoms associated with this infection, such as cerebral malaria.

More details on the exact biochemical mechanisms for sequestration and its effect on the pathology of the infection can be found on the Tulane University website.