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The ecology of primate malaria was investigated in an area of West Malaysia where a person had been naturally infected with Plasmodium knowlesi. Anopheles balabacensis introlatus and Anopheles leucosphyrus (both proved vectors of simian malaria) were trapped in the jungle with man as bait at ground level and monkey as bait in the forest canopy. Neither mosquito was found to invade a nearby village. Anopheles maculatus was found in both village and jungle traps but in such small numbers as to make it an unlikely liaison between human and simian malarias. Sporozoite-infected salivary glands were recorded from both A. b. introlatus and A. leucosphyrus, but no infections resulted when these sporozoites were inoculated into malaria-free rhesus monkeys. Malaria infections, which were believed to be nonprimate in origin, were recorded from Anopheles umbrosus-group mosquitoes. Malaria parasites were observed in 2.5% of 1,117 persons examined from villages, mostly adjacent to the forest study area. However, 14% of this group had fluorescent-antibody titers of 1:20 or higher, indicating recent experience with malaria. Whole, heparinized blood from the same 1,117 persons produced no infections when inoculated into malaria-free rhesus monkeys. Plasmodium knowlesi and Plasmodium inui were identified from indigenous populations of Macaca irus. Leaf monkeys (Presbytis sp.) and gibbons (Hylobates sp.) were also present in the study area but were not examined. It is concluded that man can become naturally infected with simian malaria in this environment only when he becomes involved in the normal mosquito-monkey cycles in the jungle.
Present address: Central America Malaria Research Station, San Salvador, El Salvador, USAID, APO New York, N. Y. 09889.