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.