Plasmodium ovale
Encyclopedia
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.
P. ovale has recently been shown by genetic methods to consist of two species, P. ovale curtisi
and P. ovale wallikeri
.
, 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.
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 degrees F and 4.5 fever episodes of > or = 104 degrees F. Mean maximum parasite levels were 6,944/microl for sporozoite-induced infections and 7,310/microl for trophozoite-induced infections.
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.
and primaquine
. The combination atovaquone-proguanil may be used in those patients who are unable to take chloroquine
for whatever reason.
morphologically appears to be the closest relation to P.ovale. This has yet to be confirmed with DNA studies. 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.
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.
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
that causes tertian malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
in humans. It is closely related to Plasmodium falciparum
Plasmodium falciparum
Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. It is transmitted by the female Anopheles mosquito. Malaria caused by this species is the most dangerous form of malaria, with the highest rates of complications and mortality...
and Plasmodium vivax
Plasmodium vivax
Plasmodium vivax is a protozoal parasite and a human pathogen. The most frequent and widely distributed cause of recurring malaria, P. vivax is one of the four species of malarial parasite that commonly infect humans. It is less virulent than Plasmodium falciparum, which is the deadliest of the...
, which are responsible for most malaria. It is rare compared to these two parasites, and substantially less dangerous than P. falciparum.
P. ovale has recently been shown by genetic methods to consist of two species, P. ovale curtisi
Plasmodium ovale curtisi
Plasmodium ovale curtisi is a species of parasitic protozoa that causes tertian malaria in humans. It was described in 2010 when it was realised that Plasmodium ovale was in fact two distinct species - Plasmodium ovale curtisi and Plasmodium ovale wallikeri - which are morphologically...
and P. ovale wallikeri
Plasmodium ovale wallikeri
Plasmodium ovale wallikeri is a species of parasitic protozoa that causes tertian malaria in humans. It was described in 2010 when it was realised that Plasmodium ovale was in fact two distinct species - Plasmodium ovale curtisi and Plasmodium ovale wallikeri - which are morphologically...
.
Epidemiology
While it is frequently said that P. ovale is very limited in its range being limited to West AfricaWest Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, eastern Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
, and Papua New Guinea
Papua New Guinea
Papua New Guinea , officially the Independent State of Papua New Guinea, is a country in Oceania, occupying the eastern half of the island of New Guinea and numerous offshore islands...
., it has been reported from Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...
, Cambodia
Cambodia
Cambodia , officially known as the Kingdom of Cambodia, is a country located in the southern portion of the Indochina Peninsula in Southeast Asia...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
and Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...
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.
Clinical features
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 degrees F and 4.5 fever episodes of > or = 104 degrees F. Mean maximum parasite levels were 6,944/microl for sporozoite-induced infections and 7,310/microl for trophozoite-induced infections.
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 dotsSchüffner's dots
Schüffner's dots refers to a hematological finding that is associated with malaria, exclusively found in Plasmodium ovale and Plasmodium vivax....
are seen on the surface of the parasitised red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
, 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
Plasmodium vivax
Plasmodium vivax is a protozoal parasite and a human pathogen. The most frequent and widely distributed cause of recurring malaria, P. vivax is one of the four species of malarial parasite that commonly infect humans. It is less virulent than Plasmodium falciparum, which is the deadliest of the...
and P. ovale that has been sitting in EDTA
EDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...
for more than half-an-hour before the blood film is made will look very similar in appearance to P. malariae
Plasmodium malariae
Plasmodium malariae is a parasitic protozoa that causes malaria in humans. It is closely related to Plasmodium falciparum and Plasmodium vivax which are responsible for most malarial infection. While found worldwide, it is a so-called "benign malaria" and is not nearly as dangerous as that...
, 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.
Treatment
Standard treatment is concurrent treatment with chloroquineChloroquine
Chloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
and primaquine
Primaquine
Primaquine is a medication used in the treatment of malaria and Pneumocystis pneumonia. It is a member of the 8-aminoquinoline group of drugs that includes tafenoquine and pamaquine.-Radical cure:...
. The combination atovaquone-proguanil may be used in those patients who are unable to take chloroquine
Chloroquine
Chloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
for whatever reason.
Phylogenetics
Among the species infecting the great apes, Plasmodium schwetziPlasmodium schwetzi
Plasmodium schwetzi is a parasite of the genus Plasmodium subgenus Plasmodium.Like all Plasmodium species P. schwetzi has both vertebrate and insect hosts. The vertebrate hosts for this parasite are mammals.- Description :P...
morphologically appears to be the closest relation to P.ovale. This has yet to be confirmed with DNA studies. The original species has been shown to be two morphologically identical forms - Plasmodium ovale curtisi
Plasmodium ovale curtisi
Plasmodium ovale curtisi is a species of parasitic protozoa that causes tertian malaria in humans. It was described in 2010 when it was realised that Plasmodium ovale was in fact two distinct species - Plasmodium ovale curtisi and Plasmodium ovale wallikeri - which are morphologically...
and Plasmodium ovale wallikeri
Plasmodium ovale wallikeri
Plasmodium ovale wallikeri is a species of parasitic protozoa that causes tertian malaria in humans. It was described in 2010 when it was realised that Plasmodium ovale was in fact two distinct species - Plasmodium ovale curtisi and Plasmodium ovale wallikeri - which are morphologically...
- which can be differentiated only by genetic means. Both species have been identified in Ghana
Ghana
Ghana , officially the Republic of Ghana, is a country located in West Africa. It is bordered by Côte d'Ivoire to the west, Burkina Faso to the north, Togo to the east, and the Gulf of Guinea to the south...
, Myanmar
Myanmar
Burma , officially the Republic of the Union of Myanmar , is a country in Southeast Asia. Burma is bordered by China on the northeast, Laos on the east, Thailand on the southeast, Bangladesh on the west, India on the northwest, the Bay of Bengal to the southwest, and the Andaman Sea on the south....
, Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
, São Tomé
São Tomé
-Transport:São Tomé is served by São Tomé International Airport with regular flights to Europe and other African Countries.-Climate:São Tomé features a tropical wet and dry climate with a relatively lengthy wet season and a short dry season. The wet season runs from October through May while the...
, Sierra Leone
Sierra Leone
Sierra Leone , officially the Republic of Sierra Leone, is a country in West Africa. It is bordered by Guinea to the north and east, Liberia to the southeast, and the Atlantic Ocean to the west and southwest. Sierra Leone covers a total area of and has an estimated population between 5.4 and 6.4...
and Uganda
Uganda
Uganda , officially the Republic of Uganda, is a landlocked country in East Africa. Uganda is also known as the "Pearl of Africa". It is bordered on the east by Kenya, on the north by South Sudan, on the west by the Democratic Republic of the Congo, on the southwest by Rwanda, and on the south by...
. The separation of the lineages is estimated to have occurred between 1.0 and 3.5 million years ago in hominid hosts.
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".
Erythrocytic Cycle
Vectors
- Anopheles albimanusAnopheles albimanusAnopheles albimanus is a species of mosquito.-Habitat:Larvae of this species live in a wide range of permanent freshwater habitats. They are, however, salt tolerant. They live in sites that contain abundant amounts of floating, emergent vegetation. The larvae can also live in floating algae and scum...
- Anopheles atroparvus
- Anopheles dirusAnopheles dirusAnopheles dirus is a vector of malaria in Asian forested zones.-Taxonomy:It is often seen as a species complex including at least seven closely related and efficient forest-based malaria vectors in Asia. Hence, its geographical distribution is overlapping with areas of high malaria prevalence rates...
- Anopheles farauti
- Anopheles freeborni
- Anopheles gambiaeAnopheles gambiaeAnopheles gambiae is a complex of at least seven morphologically distinguishable species of mosquitoes in the genus Anopheles. This complex was recognised in the 1960s and includes the most important vectors of malaria in sub-Saharan Africa and the most efficient malaria vectors known.This species...
- Anopheles maculatus
- Anopheles quadrimaculatus
- Anopheles stephensiAnopheles stephensiAnopheles albimanus is a species of mosquito....
- Anopheles subpictusAnopheles subpictusAnopheles subpictus is a species complex of four species of mosquito belonging to the genus Anopheles.-Subgenus classification:...
External links
- Malaria - TDR: For research on diseases of poverty