Page 48 - Medical Parasitology_ A Textbook ( PDFDrive )

40 6 Malaria Parasites and Babesia

Types of Malarial Parasites and Their Morphology

1. Plasmodium falciparum (Malignant tertian)
This is the most pathogenic of all the plasmodia and unless treated, is often
fatal. The species is responsible for almost all deaths caused by malaria. They
infect both young and old erythrocytes. The early ring form is fine, measuring 1/6
the size of red blood cell (RBC). Rings are often seen attached along the margin
of the red cell (appliqué or accole form). Double chromatins are common.
Multiple rings may be seen within a single erythrocyte (Fig. 6.1a). Late trophozo-
ites and schizonts are not usually seen in the peripheral blood. These stages are
sequestrated in the internal capillaries. The mature schizont has 8–24 (usually 16)
merozoites. The erythrocytic schizogony takes about 36–48 h. The infected eryth-
rocytes are not enlarged. A good staining will show coarse dots which are called
Maurer’s clefts. The mature gametocytes are crescent or banana shaped (Fig. 6.1b).
2. Plasmodium vivax (Benign tertian)
Merozoites of P. vivax prefer reticulocytes. All erythrocytic stages can be seen
in peripheral smears. The infected erythrocytes are enlarged and with good stain-
ing will show granules known as Schuffner’s dots in the cytoplasm of the
RBC. The ring is about 1/3 the size of RBC (Fig. 6.1c). The ring develops rapidly
into trophozoite which is amoeboid shape and accumulates malarial pigment.
There are about 12–24 (usually 18) merozoites per schizont (Fig. 6.1d). The
erythrocytic schizogony takes 48 h. Both male and female gametocytes are large,
filling almost the enlarged RBC (Fig. 6.1e).
3. Plasmodium malariae (Quartan malaria)
Plasmodium malariae prefers older erythrocytes. The ring forms resemble
those of P. vivax but the cytoplasm is thicker. The infected erythrocytes may be
of the normal size or slightly smaller. Fine stippling, called Ziemann’s stippling
may be seen with special stains. The trophozoites stretch across the diameter of
the erythrocyte and is seen as a band form (Fig. 6.1f). The mature schizont has
an average of 8 merozoites, which usually present as a rosette appearance
(Fig. 6.1g). Erythrocytic schizogony takes 72 h. Both male and female gameto-
cytes occupy nearly the entire RBC (Fig. 6.1h).
4. Plasmodium knowlesi (Quotidian malaria)
The ring stage resembles P. falciparum. Accole form, double chromatins and
multiple infections are common in an infected RBC (Fig. 6.1i). The infected
RBC is not enlarged. The trophozoite stage has a band form resembling P.
malariae. The mature schizont has an average of 10 merozoites, with a maxi-
mum of 16 (Fig. 6.1j). Pigment collects into 1 or more yellowish-black masses
and eventually into a single mass in the mature schizont. The erythrocytic schi-
zogony takes 24 h. Both male and female gametocytes occupy nearly the entire
RBC (Fig. 6.1i).
5. Plasmodium ovale (Tertian malaria)
It is the rarest of all plasmodia infecting humans. The trophozoites resemble
those in P. vivax, but are usually more compact, with less amoeboid appearance
(Fig. 6.1k). Schuffner’s dots are present in the cytoplasm of the infected RBC