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A new species of malaria parasite, Plasmodium youngi, is described from the Malayan whitehanded gibbon, Hylobates lar lar. The parasite is tertian in periodicity and produces quite severe infections in the type host with peak parasite densities higher than 100,000 per cmm. The young trophozoites are characterized by the frequent occurrence of double or multiple chromatin. Ameboidicity is great and “tenue” forms are seen frequently. Stippling of the Schüffner type is produced but there is no erythrocyte enlargement at any stage. In the older trophozoites and the younger schizonts there is frequently an accessory chromatin granule at the border of the parasite or extruded into the host erythrocyte. The mature schizont has 12 to 20 (average 14) merozoites which are randomly arranged. Pigment consists of dark, yellowish-brown, discrete granules and is moderate in amount.
Two other species of gibbon were found to be susceptible but the rhesus monkey was not. Anopheles maculatus is somewhat susceptible to infection to the oocyst stage, but sporozoites failed to develop in the mosquitoes studied.
Far East Research Project, Laboratory of Parasite Chemotherapy, U. S. Public Health Service (NIAID, NIH), Kuala Lumpur, Malaya.
Institute for Medical Research, Kuala Lumpur, Malaya.
University of California I. C. M. R. T. Project, Institute for Medical Research, Kuala Lumpur, Malaya.