Distribution of Simian Malaria Parasites in Various Hosts

Abstract

On the basis of periodicity and morphology the simian malaria parasites are considered to fall into six distinct groups. In line with the evolution of these parasites each is briefly considered with reference to a diagramatic primate tree, and to a corresponding diagram, showing the degrees of susceptibility of these primates to the parasites of malaria. The term "malaria parasites" is applied either in a narrow sense to species of Plasmodium sensu stricto or to all pigmented parasites of erythrocytes. In the former the parasite is defined as undergoing two types of schizogony in the vertebrate host: the first in tissue cells and the second, accompanied by gametogony, in the blood with the production of "malaria pigment," while sporogony occurs in an insect, usually a mosquito. In the wider sense, haemoproteids are also included under the name of "malaria parasites," and these are defined as parasites which undergo schizogony in the tissues, but not in the blood where (pigmented) gametogony alone occurs, and which undergo sporogony in an insect other than a mosquito. Some authorities still retain mammalian parasites of the latter type in the genus Plasmodium, e.g., P. kochi or P. brodeni, while the presence of pigmented parasites in blood films of wild animals inevitably raises the question as to whether they are true Plasmodium or belong to the Haemoproteidae, and it is frequently difficult to be certain of the diagnosis, particularly in mixed infections. The organisms are therefore often referred to as "malaria parasites," and it is for these reasons that I propose in the present paper to include the two genera, Plasmodium and Hepatocystis, Received for publication 8 October 1963. * Presented at the XVI International Congress of Zoology, Washington, D. C., on 21 August 1963 and sponsored by the American Society of Parasitologists and the British Society of Parasitology. in my survey. Moreover, their respective species must have a close phylogenetic relationship and it is useful to consider them together. Having defined "malaria parasites," the next question is the delimitation of the word "simian." The term simian, in contrast to primate malaria, excludes man at the top and the tarsioids, lemuroids, etc., at the bottom of the scale, and parasitologically such a separation is convenient because, on the one hand, the malaria parasites of lemurs are highly specialized and appear to belong to the rather heterogeneous group found in small mammals, while on the other, the human parasites again exhibit distinctive features-they are limited, for instance, to species within the genus Plasmodium. Hepatocystis is thus absent in man and also in the higher apes (chimpanzee and gorilla) which have parasites almost indistinguishable from the human ones, including the most highly evolved species in the subgenus Laverania. There is evidence today that the chimpanzee and gorilla are closer to man than the other so-called higher apes-the orangutan and still more the gibbon-and for this reason the present survey will omit the chimpanzee and gorilla from detailed consideration while including those of the gibbon and orangutan. For tracing the phylogeny, however, both lemuroid and human parasites must be included. The distribution of species of simian malaria parasites could be discussed in at least two ways: (1) the distribution of similar types of