Plasmodium ovale [16]
Causative agent of benign tertian fever (fever on the first day, remission
of fever on the second, return of fever on the third). P. ovale infections
generally follow a benign course, though rare complications may arise due
to spleen rupture. Like P. vivax, it may cause relapses due to the presence of
liver hypnozoites. The degree of parasitemia is usually lower than in P. vivax
infections.
This species was correctly described in 1922 by Stephens, who named the
species itself after the shape of the parasitized red blood cells. The direct
examination of blood sample parasitized by P. ovale will reveal no ovalization
or fimbriation: these transformations are artificially induced at the time of
the smear, and only if the red blood cells are arranged in a single layer. The
same sample smeared to varying thicknesses may in fact result in stained
films containing very different percentages of forms displaying ovalization
and/or fimbriation. A greater number of these “deformed” red blood cells
will be found in the thinner areas of the film, which also dry more quickly
than the thicker areas.
In any case, though ovalization and fimbriation are purely an artifact, they
constitute the best diagnostic tool for species identification (Plate 9).
Parasitized red blood cells
They are generally young red blood cells (reticulocytes) [46].
Size: slightly increased, but to a lesser degree than in P. vivax infections [83].
Shape: apart from the characteristic oval shape with possible fimbriation
induced by the diagnostic procedure, one may observe deformed shapes as
in P. vivax or a perfectly round shape.
Schüffner’s stippling: present earlier in young trophozoites compared to P.
vivax: it appears as early as 12 hours after the merozoite’s penetration of
the red blood cell. In order for it to be visualized, the stain must be diluted
with water at pH 7.2-7.4; any color differences are linked to changes in pH
(more alkaline).
Polyparasitism: not very frequent.
Young trophozoite
Signet ring, similar to that of the other plasmodium species; if no other
stages of the parasite are present, in order to rule out other species infection,
it will be necessary to take a second sample 5-6 hours later, by which time
Shüffner’s stippling will have appeared.