Plasmodium ovale

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Plasmodium ovale
Plasmodium ovale trophozoite, Giemsa stain.
Scientific classification
Kingdom: Protista Phylum: Apicomplexa Class: Aconoidasida Order: Haemosporida Family: Plasmodiidae Genus: Plasmodium Species: P. ovale
Binomial name
Plasmodium ovale Stephens 1922

Plasmodium ovale 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.

Contents 1 Health 1.1 Epidemiology 1.2 Diagnosis 1.3 Treatment 2 Biology 2.1 Life Cycle 2.1.1 Human Infection 2.1.2 Liver Stage 2.1.3 Erythrocytic Cycle 2.1.4 Sexual Stage 2.1.5 Mosquito Stage 3 References

[edit] Health

[edit] Epidemiology

P. ovale is very limited in its range. It is endemic mainly to West Africa, the Philippines, eastern Indonesia, and Papua New Guinea.^[1]

[edit] Diagnosis

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 schizonts 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.

[edit] Treatment

Standard treatment is concurrent treatment with chloroquine and primaquine. The combination atovaquone-proguanil may be used in those patients who are unable to take chloroquine for whatever reason.^[2]

[edit] Biology

[edit] Life Cycle

[edit] Human Infection

[edit] Liver Stage

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 are formed.

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".^[3]

[edit] Erythrocytic Cycle

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. ^[5]

[edit] Sexual Stage

[edit] Mosquito Stage

[edit] References