Feeling Ill After Malaria Infection

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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

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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

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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.

Malaria Schizonts

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QUESTION:

What is the difference between schizont of Plasmodium vivax and P. falciparum?

ANSWER:

P. falciparum schizonts tend to fill up to about two-thirds of the host red blood cell, and contain 8-24 merozoites (see image below for development of schizont). However, schizonts of P. falciparum are rarely seen in peripheral blood; instead, multiple, smaller rings are the usual diagnostic sign. Characteristic crescent-shaped gametocytes may also be observed, though usually later on in infection.

 

falciparum schizont CDC

The stages of maturation of a Plasmodium falciparum schizont. Image courtesy of CDC (www.dpd.cdc.gov)

P. vivax schizonts are large and fill up the entirety of the red blood cell with 12-24 merozoites, each containing visible chromatin and cytoplasm (see below). Their size and shape can differentiate them from the more compact P. ovale and P. malariae schizonts, though separating the former can sometimes be difficult.

vivax schizont CDC

The stages of maturation of a Plasmodium vivax schizont. Image courtesy of CDC (www.dpd.cdc.gov)