Biology of the Simian Malarias of Southeast Asia. I. Host Cell Preferences of Young Trophozoites of Four Species of Plasmodium

Abstract

tendency of four species of simian malaria to invade reticulocytes and mature erythrocytes has been studied. Plasmodium knowlesi in Macaca mulatta shows no preference for a particular cell type, but the same parasite in Macaca irus selectively invades reticulocytes. Four strains of Plasmodium cynomolgi preferentially invade reticulocytes, but the degree of invasion varies with the strain. Plasmodium coatneyi and Plasmodium inui selectively invade mature red blood cells. Possible relationships between the type of host cell invaded by the malaria parasite and virulence and periodicity are discussed. taxonomic value of this characteristic in the separation of species and even strains of the same species of Plasmodium is emphasized. selective invasion of young red blood cells by malaria parasites has attracted the attention of investigators in the past. Craik, as early as 1920, reported a greater frequency of P. vivax in young than in mature red blood cells. Roberts (1922) suggested that Parasites may select the less resistant erythrocytes which are apt to be the younger ones. In 1934 Eaton, presumably working with P. vivax, was so impressed with the high frequency of invasion of young red cells by the malaria parasite that he proposed the following hypothesis: The red cell is susceptible to infection with malaria only when it is in the reticulocyte stage. Schizonts which do not infect promptly are phagocytized. Anything that increases the actual number of reticulocytes favors the progress of the disease. It was inevitable that such a sweeping proposal would be subject for further inquiry. Shushan (1937) confirmed Eaton's findings with P. vivax but concluded that P. falciparum would grow with equal facility in both mature and immature erythrocytes. Hegner (1938) reported that P. malariae and P. knowlesi invaded mature red cells while P. falciparum would invade reticulocytes but at a much lower level than that seen in P. vivax. This investigator believed that the ability of P. falciparum to invade any red blood cell accounted for its ability to develop overwhelming parasitemias. However, Malamos (1937) found no marked preferences for reticulocytes in P. knowlesi, Received for publication 2 September 1965. * Cytology Section, LPC, NIAID, NIH, P.O. Box 190, Chamblee, Georgia. P. vivax, or P. falciparum. In our laboratory in Kuala Lumpur, Malaysia, we maintained a large number of strains and species of simian malaria covering a broad range of virulence, periodicity, and physiology. This situation presented the opportunity to investigate further this long dormant aspect of the biology of malaria, and the results of these studies are the subject of this report. MATERIALS AND METHODS parasites used in these studies are native to Asia. Plasmodium knowlesi, P. coatneyi, and all of the strains of P. cynomolgi, except the Cambodian strain, were isolated in Malaya from monkeys or wild-caught mosquitoes. Cambodian strain of P. cynomolgi and the Taiwan strain of P. inui were isolated from monkeys trapped in the countries indicated. All preparations were made from pure infections in rhesus monkeys; P. knowlesi was also studied in Macaca irus. Approximately 5 mm3 of blood were obtained from the ear of the animal in a calibrated pipette and mixed with an equal part of a 1% solution of Brilliant Cresyl Blue in physiological saline to which a small amount of 2% sodium citrate had been added. After allowing the mixture to stand for 5 to 7 min, a thin film was prepared, allowed to air dry, and then fixed with methyl alcohol. Fixed preparations were stained with giemsa at pH 7.2. following information was obtained from each slide: the number of red blood cells parasitized with ring-stage trophozoites per 1,000 total red cells, the number of reticulocytes per 1,000 total red blood cells, the percentage of reticulocytes parasitized, and the total parasite count per mm3 of blood (from Earle Perez preparations). From the number of red blood cells parasitized with ringstage parasites, an expected percentage of parasitized reticulocytes could be estimated. That is, if reticulocytes were invaded by chance alone, they