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DIFFERENTIAL SLIDES LEGEND
CYCLE 43 SLIDE 1 Plasmodium ovale
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 subspecies, P. ovale curtisi and P. ovale wallikeri.
History This species was first described by Stephens in a patient from East Africa in 1922.
Epidemiology 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 prevalence 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.
Clinical features The pre-patent 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
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
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P.O. Box 131375, Bryanston, 2074
Ground Floor, Block 5 Bryanston Gate, 170 Curzon Road Bryanston, Johannesburg, South Africa
804 Flatrock, Buiten Street, Cape Town, 8001
www.thistle.co.za Tel: +27 (011) 463 3260 Fax: +27 (011) 463 3036 Fax to Email: + 27 (0) 86-557-2232 e-mail : [email protected] 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.
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.
P. ovale gametocytes are round to oval with large nuclei P. ovale schizonts have 6 to 14 merozoites, clustered and may almost fill the red blood cells. Pigment is brown around a mass of dark-brown pigment. and more coarse in comparison to P. vivax.
In P. ovale infections, red blood cells can be normal or slightly enlarged (up to 1 1/4×) in size, may be round to oval, and are sometimes fimbriated. Under optimal conditions, Schüffner's dots may be seen in Giemsa stained slides.
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.
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P.O. Box 131375, Bryanston, 2074
Ground Floor, Block 5 Bryanston Gate, 170 Curzon Road Bryanston, Johannesburg, South Africa
804 Flatrock, Buiten Street, Cape Town, 8001
www.thistle.co.za Tel: +27 (011) 463 3260 Fax: +27 (011) 463 3036 Fax to Email: + 27 (0) 86-557-2232 e-mail : [email protected]
Life Cycle Human Infection 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".
References 1. http://www.dpd.cdc.gov/dpdx/HTML/PDF_Files/Povale_benchaidV2.pdf 2. http://en.wikipedia.org/wiki/Plasmodium_ovale
Questions 1. Discuss the clinical features of Plasmodium ovale. 2. Discuss the lab findings in a patient diagnosed Plasmodium ovale. 3. Discuss the life cycle of Plasmodium ovale.
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