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| Plasmodium ovale | ||||||||||||||
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 Plasmodium ovale trophozoite, Giemsa stain.
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| 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 |
Health
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
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.
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
Biology
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".3
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
Sexual Stage
Mosquito Stage
References
- ^ Baird KJ and Hoffman SL (2004). "Primaquine Therapy for Malaria" (dead link). Clin Infect Dis 39: 1336–1345. doi:, http://www.journals.uchicago.edu/CID/journal/issues/v39n9/33149/33149.html.
- ^ Radloff PD, Philipps J, Hutchinson D, Kremsner PG (1996). "Atovaquone plus proguanil is an effective treatment for Plasmodium ovale and P. malariae malaria". Trans R Soc Trop Med Hyg 90 (6): 682. doi:. PMID 9015517.
- ^ "Malaria eModule - Exo-Erythrocytic Stages".
- ^ Bozdech, Zbynek (August 18, 2003). "The Transcriptome of the Intraerythrocytic Developmental Cycle of Plasmodium falciparum". PLoS Biology 1 (1), http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0000005.
- ^ "Biology: Malaria (CDC malaria)".
Wikipedia content modification information:
- This page was last modified on 22 October 2008, at 00:30.
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