Plasmodium ovale is one of the two rare species of human malaria parasites, usually present in fewer than 5% of people in any region where it is present. Although probably native to West Africa, P. ovale has now spread into several parts of Southeastern Asia and Papua New Guinea. P. ovale is commonly called tertian malaria because it produces fever cycles every two days (note that the Romans had no zero, thus the term) and very often is difficult to diagnose in a bloodsmear from Plasmodium vivax. P. ovale can relapse for several years as well, via dormant stages in the liver known as hypnozoites. Like the other mammalian Plasmodium species, P. ovale is vectored by Anopheles mosquitoes. The taxonomy of P. ovale has been somewhat troublesome as this parasite seems to be genetically quite different from other species. Some studies have placed it as closely related to Hepatocystis parasites in bats and baboons, but a recent paper reported parasites in wild chimpanzees that were closely related to P. ovale.
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