Plasmodium ovale is one of the five described Plasmodium species that cause malaria in humans (with Plasmodium falciparum generally causing the most severe disease and most of the mortality from the disease).
Plasmodium ovale is generally thought of as having a relatively limited distribution, with endemic transmission traditionally described as being limited to areas of tropical Africa, New Guinea, the eastern parts of Indonesia, and the Philippines. However, P. ovale infections have also been reported from the Middle East, the Indian subcontinent, and parts of Southeast Asia. In West Africa (and to a lesser extent Central Africa), age specific prevalence (based on light microscopy, LM) of >10% has been observed. However, in most places where P. ovale is observed, it is relatively uncommon and its prevalence (as detected by LM) rarely exceeds 3–5%. As is the case for P. malariae and P. falciparum, P. ovale infections in West African populations tend to be most common in children under ten years of age. Little is known about the potential for interaction between P. ovale and other malaria infections. However, the fact that P. ovale has been found to be most prevalent in areas of West Africa where P. vivax is nearly absent because of the virtual absence of the Duffy blood-group-antigen, which P. vivax requires to invade red blood cells, might indicate a negative interaction between these two species. (Mueller et al. 2007 and references therein) Recent work indicates that there are likely two distinct Plasmodium species that have both been referred to as P. ovale (Sutherland et al. 2010; Oguike et al. 2011)
Like P. vivax, P. ovale has long been thought to have a dormant stage "hypnozoite" stage that can persist in the liver and cause relapses by invading the bloodstream weeks (or even years) later, but Richter et al. (2010) have questioned whether such a stage actually exists for P. ovale.
The human malaria parasite life cycle involves two hosts, a mosquito and a human. The life cycle is very complex, including both sexual and asexual phases (see life cycle diagram) and involves a stage in the liver as well as the blood stage, the latter being responsible for the clinical manifestations of the disease. (Centers for Disease Control Parasites and Health website)
Evolution and Systematics
Systematics and Taxonomy
There are more than 150 named species of Plasmodium that infect various species of vertebrates. Four species are well known as true parasites of humans, utilizing humans almost exclusively as a natural intermediate host: P. falciparum, P. vivax, P. ovale and P. malariae. In recent years it has become apparent that the simian malaria parasite P. knowlesi also regularly infects humans, as well as its natural monkey intermediate hosts. (Centers for Disease Control Parasites and Health website) Furthermore, it is now apparent that there are likely two distinct Plasmodium species that have both been referred to as P. ovale (Sutherland et al. 2010; Oguike et al. 2011).
Plasmodium ovale is a species of parasitic protozoa that causes tertian malaria in humans. It is one of several species of Plasmodium parasites that infect humans including Plasmodium falciparum and Plasmodium vivax which are responsible for most malarial infection. It is rare compared to these two parasites, and substantially less dangerous than P. falciparum.
This species was first described by Stephens in a patient from East Africa in 1922.
While it is frequently said that P. ovale is very limited in its range being limited to West Africa, the Philippines, eastern Indonesia, and Papua New Guinea, it has been reported from Bangladesh , Cambodia, India, Thailand and Vietnam
The reported prevalence is low (<5%) with the exception of West Africa, where prevalences above 10% have been observed.
The epidemiology of this parasite is in need of updating because the most recent global map of its distribution was produced in 1969.
It has been estimated that there are about 15 million cases of infection each year with this parasite.
The prepatent period in the human ranges from 12 to 20 days. Some forms in the liver have delayed development and relapse may occur after periods of up to 4 years after infection.
The developmental cycle in the blood lasts approximately 49 h. An examination of records from induced infections indicated that there were an average of 10.3 fever episodes of > or = 101 ºF (38,3 ºC) and 4.5 fever episodes of > or = 104 ºF (40,0 ºC). Mean maximum parasite levels were 6,944/microl for sporozoite-induced infections and 7,310/microl for trophozoite-induced infections.
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' dots or James' stippling. About twenty percent of the parasitised 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.
Molecular tests (tests that look for DNA material from P. ovale in blood) must take into account the fact that there are two subspecies of ovale and tests designed for one subspecies may not necessarily detect the other.
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.
The original species has been shown to be two morphologically identical forms - Plasmodium ovale curtisi and Plasmodium ovale wallikeri - which can be differentiated only by genetic means. Both species have been identified in Ghana, Myanmar, Nigeria, São Tomé, Sierra Leone and Uganda. The separation of the lineages is estimated to have occurred between 1.0 and 3.5 million years ago in hominid hosts. A second analysis suggests that these species separated (95% confidence interval 0.5 – 7.7 Mya).
The two species appear to differ in their biology with P. ovale wallikeri having a shorter latency period than P. ovale ovale.
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".
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. 
- Anopheles albimanus
- Anopheles atroparvus
- Anopheles dirus
- Anopheles farauti
- Anopheles freeborni
- Anopheles gambiae
- Anopheles maculatus
- Anopheles quadrimaculatus
- Anopheles stephensi
- Anopheles subpictus
Chimpanzees and Saimiri monkeys can be infected with this parasite.
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