Evolutionary relatedness of some primate models of Plasmodium

Primate--and, specifically, monkey--malaria infections are commonly usedfor understanding the pathology of and immune response to the human diseasebecause they are thought to resemble most closely the host-parasiterelationship found in humans. Plasmodium cynomolgi is used extensively as amodel for the human parasite, P. vivax, and P. knowlesi is used primarilyas a model for the development of erythrocytic-stage vaccines. Both ofthese simian parasites can naturally infect man, resulting in mildlysymptomatic episodes of the disease. The phylogenetic relationship betweenthese two simian parasites and previously characterized Plasmodium species,including P. vivax, was examined by comparison of the asexually expressedsmall- subunit ribosomal RNA genes. Our analysis confirmed that P. vivax ismost closely related to P. cynomolgi and that it remains an appropriatemodel of the human pathogen. Furthermore, with P. knowlesi and P. fragile,these two species form a group of closely related species, distant fromother Plasmodium species. What is considered to be the most ancient of thehuman malaria pathogens, P. malariae, was also included in the analysis anddoes not group at all with other simian or human parasites.