Malaria parasites of primates are clustered together based on certain biologic and morphological characteristics that assist in their identification and their selection as models for research. Plasmodium ovale is a relapsing malaria parasite with a latent liver stage that often persists for many months; all stages of the asexual cycle of the parasite are present in the peripheral circulation; the asexual parasite count rarely reaches high density, indicating restriction to specific population of erythrocytes; the course of parasitemia is short compared to other human-infecting malaria parasites; unlike P. vivax, host susceptibility is not controlled by the absence of the Duffy gene blood group; geographically restricted to sub-Saharan Africa and islands of the western Pacific; infectious to anopheline mosquitoes outside its geographic distribution, and thus the reasons for geographic isolation are not due to vector incompetence; and it is the only human-infecting malaria parasite that has not infected (experimentally) New World monkeys.
Biologically, P. ovale has latent liver stages and is thus classified as one of the relapsing malaria parasites. These include the primate-infecting malaria species P. vivax, P. cynomolgi, P. fieldi, and P. simiovale. Infected erythrocytes of these species all exhibit Schüffner's dots. However, other primate-infecting species such as P. simium and P. gonderi, which also exhibit Schüffner's dots, have not been shown to have latent liver forms. Of the Old World monkey malaria parasites, the ones that appear to be most similar biologically and morphologically to P. ovale are P. fieldi from Malaysia and P. simiovale from Sri Lanka. There is also morphological similarity in the blood stages between the chimpanzee parasite P. schwetzi and P. ovale. However, the sporogonic stages of these two species are markedly different in size and rate of development (24).
A phylogenetic analysis of all the primate malaria parasites tested, based on the gene encoding the cytochrome b protein from the mitochondrial genome, indicated that they formed a monophyletic group with the exception of P. falciparum and P. reichenowi. There is no molecular evidence suggesting that it is closely related to any other of the primate malaria parasites that have been examined so far. Plasmodium ovale appears to represent an independent colonization of humans by malaria parasites (33).
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