Malaria

Malaria (Italian: "bad air"; formerly called ague or marsh fever in English) is an infectious disease which in humans causes about 350-500 million infections and approximately 1.3 million deaths annually, mainly in the tropics. Sub-Saharan Africa accounts for 85% of these fatalities.{{mn|OnePointThree|1}}

Diagnosis

The gold standard for the diagnosis of malaria is microscopic examination of blood films. Two sorts of blood films are traditionally used: 1. thin films, and 2. thick films. Thin films are similar to usual blood films and allow the microscopist to speciate malaria. Thick films allow the microscopist to screen a larger volume of blood, so as to pick up low levels of infection.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In areas where microscopy is not available, there are antigen detection tests that require only a drop of blood (Pattanasin S, 2003). OptiMAL-IT® will reliably detect falciparum down to 0.01% parasitaemia and non-falciparum down to 0.1%. Paracheck-Pf® will detect parasitaemias down to 0.002% but will not distinguish between falciparum and non-falciparum malaria. An experienced microscopist can detect parasite levels down to as low as 0.000001%

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Microscopic features

Diagnosis must be based on the features of a number of parasites. It is not possible to give a species one the basis of a single parasite. The early trophozoite ("ring form") of all

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

four species looks identical and species identification should therefore NEVER be given

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

on the basis of a single ring form.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;P. falciparum

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:Only early trophozoites and gametocytes are seen in the peripheral blood. It is unusual to see mature trophozoites or schizonts in peripheral blood smears as these are usually sequestered in the tissues. The parasitised erythrocytes are not enlarged and it is common to see cells with more than one parasite within them (multiply parasitised erythrocytes). Occasionally, faint comma shaped red dots are seen on the red cell surface called "Maurer's dots". P. falciparum will parasitise erythrocytes of any age and up to 50% of all circulating erythrocytes may be parasitised. The banana-shaped gametocytes of P. falciparum are diagnostic of that species.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;P. vivax

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:The parasitised erythrocyte is up to twice as large as a normal erythrocyte and fine pink "Schüffner's dots" are seen on the surface of the parasitised cell; the parasite within it is often wildly irregular in shape (described as "amoeboid"). Schizonts of P. vivax have up to 20 merozoites within them. It is rare to see cells with more than one parasite within them. Merozoites will only attach to reticulocytes (immature erythrocytes) and therefore it is unusual to see more than 3% of all circulating erythrocytes parasitised.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;P. ovale

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:The microscopic appearance of P. ovale is very similar to that of P. vivax and if there are only a small number of parasites seen, it may be impossible to distinguish the two species. There is no difference between the medical treatment of P. ovale and P. vivax, and therefore the laboratory may report "P. vivax/ovale", and this is perfectly acceptable. Schüffner's dots are seen on the surface of the parasitised erythrocyte, but these are larger and darker than in P. vivax and are sometimes called "James's dots". The about 20% of the parasitised eythrocytes are oval in shape (hence the species name) and some of the oval erythrocytes also have fimbriated edges (the so-called "comet cell"). The matures schizonts of "P. ovale" never have more than 12 nuclei within them and this is the only reliable way of distinguishing between the two species.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

;P. malariae

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:The parasitised erythrocyte is never enlarged and may even appear smaller than that of a normal erythrocytes. The cytoplasm is not decolourised and no dots are visible on the cell surface. The food vacuole is small and the parasite is compact. Multiply parasitised erythrocytes are rare. Band forms (where the parasite forms a thick band across the width of the infected erythrocyte) are characteristic of this species. Large grains of malarial pigment are often seen in these parasites: more so than any other Plasmodium species.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~