Plasmodium vivax [40][49][54]
Causative agent of benign tertian fever (fever on the first day, remission of
fever on the second, return of fever on the third). Like P. ovale this species
will cause frequent relapses, due to the presence of liver hypnozoites, if
the infection is not correctly treated with a hepatic schizonticidal drug
(Primaquine). Two forms of this species are known and their period of
incubation differs:
- The classic form associated with warm or temperate climates, whose period
of incubation varies from 15 days to several weeks [19].
- A cold climate form with a period of incubation lasting up to 8-10 months,
reported in Russia and North Korea. This form of P. vivax (Nicolaev strain,
P. vivax hibernans) has adapted to the changing of seasons; i.e. during the
wintertime, when activity of the vector (mosquito) is suppressed, it remains
in the form of a liver hypnozoite (Plate 8).
Parasitized red blood cells
They are generally young red blood cells (reticulocytes) [46].
Size: increased; this is the species that can induce the biggest changes.
Shape: round or, more often, quadrangular, trapezoidal, polygonal.
Schüffner’s stippling: small red-pink granules evenly distributed throughout
the part of the red blood cell not occupied by the parasite, also present in
erythrocytes parasitized by P. ovale, where they appear earlier. Almost always
absent in the young ring forms, where they should not be confused with
basophilic granulations or nuclear residues of the reticulocytes. They are
easier to detect than the spots produced by P. ovale, provided that the stain
used has been diluted with water at an alkaline pH (7.2-7.4). Excessive
washing of the film will cause them to disappear; moreover, if the pH is
greater than 7.4, they tend to become very dark. If acidic water is used they
will not stain at all (Plate 3).
Polyparasitism: possible, but rare.