is a species of parasitic protozoa
that causes tertian malaria
in humans. It is closely related to Plasmodium falciparum
and Plasmodium vivax
, which are responsible for most malaria. It is rare compared to these two parasites, and substantially less dangerous than P. falciparum
is very limited in its range. It is endemic mainly to West Africa
, the Philippines
, eastern Indonesia
, and Papua New Guinea
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 on morphological grounds alone. There is no difference between the medical treatment of P. ovale
and P. vivax
, and therefore some laboratory diagnoses report "P. vivax/ovale
", which is perfectly acceptable as treatment for the two are very similar. Schüffner's dots
are seen on the surface of the parasitised red blood cell
, but these are larger and darker than in P. vivax
and are sometimes called "James's dots". About twenty percent of the parasitized cells are oval in shape (hence the species name) and some of the oval cells also have fimbriated edges (the so-called "comet cell"). The mature schizont
of P. ovale
never have more than twelve nuclei within them and this is the only reliable way of distinguishing between the two species.
P. vivax and P. ovale that has been sitting in EDTA for more than half-an-hour before the blood film is made will look very similar in appearance to P. malariae, which is an important reason to warn the laboratory immediately when the blood sample is drawn so they can process the sample as soon as it arrives.
Standard treatment is concurrent treatment with chloroquine
. The combination atovaquone-proguanil
may be used in those patients who are unable to take chloroquine
for whatever reason.
The P. ovale
sporozoite enters a hepatocyte and begins its exoerythrocytic schizogony stage. This is characterized by multiple rounds of nuclear division without cellular segmentation. After a certain number of nuclear divisions, the parasite cell will segment and merozoites
There are situations where some of the sporozoites do not immediately start to grow and divide after entering the hepatocyte, but remain in a dormant, hypnozoite stage for weeks or months. The duration of latency is variable from one hypnozoite to another and the factors that will eventually trigger growth are not known; this explains how a single infection can be responsible for a series of waves of parasitaemia or "relapses".
While similar to P. vivax, P. ovale is able to infect individuals who are negative for the Duffy blood group, which is the case for many residents of sub Saharan Africa. This explains the greater prevalence of P. ovale (rather than P. vivax) in most of Africa.