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Plasmodium fragile continues to be investigated because of its biologic similarities to the human malaria parasite, Plasmodium falciparum. Two strains of P. fragile are available for study; one strain is able to infect mosquitoes, whereas the other strain is transmissible only by blood inoculation. The Sri Lanka strain of P. fragile was transmitted to Macaca mulatta, Macaca fascicularis, Aotus lemurinus griseimembra, Aotus nancymaae, Aotus vociferans, and Saimiri boliviensis monkeys via sporozoites that developed to maturity only in Anopheles dirus mosquitoes. The prepatent periods ranged from 12 to 35 days for macaques and from 15 to 30 days for New World monkeys after intravenous injection of sporozoites. Eight rhesus monkeys were infected with the Nilgiri strain and followed for 482 days. Parasitemia in 6 animals persisted at relatively high density through the period of observation. Erythrocyte, hematocrit, and hemoglobin values reached their lowest levels 3 wk after infection and slowly recovered; however, the values did not approach preinfection levels as long as parasitemia persisted in the monkeys. The mean corpuscular volume and corpuscular hemoglobin concentration reached their peak and lowest values, respectively, at day 38 and then returned to the preinfection level. The mean corpuscular hemoglobin value decreased to its lowest level at day 87 and then returned to preinfection level.

A strain of Plasmodium vivax from India was adapted to develop in splenectomized Saimiri boliviensis, Aotus lemurinus griseimembra, A vociferans, A. nancymai, A. azarae boliviensis, hybrid Aotus monkeys, and splenectomized chimpanzees. Infections were induced via the inoculation of sporozoites dissected from the salivary glands of Anopheles stephensi and An. dirus mosquitoes to 12 Aotus and 8 Saimiri monkeys; transmission via the bites of infected An. stephensi was made to 1 Aotus monkey and 1 chimpanzee. The intravenous passage of infected erythrocytes was made to 9 Aotus monkeys and 4 chimpanzees. Gametocytes in 13 Aotus monkeys and 4 chimpanzees were infectious to mosquitoes. Infection rates were markedly higher in mosquitoes fed on chimpanzees. PCR studies on 10 monkeys injected with sporozoites revealed the presence of parasites before their detection by microscopic examination. The India VII strain of P. vivax develops in Aotus and Saimiri monkeys and chimpanzees following the injection of parasitized erythrocytes, or sporozoites, or both. The transmission rate via sporozoites to New World monkeys of approximately 50% may be too low for the testing of sporozoite vaccines or drugs directed against the exoerythrocytic stages. However, the strain is highly infectious to commonly available laboratory-maintained anopheline mosquitoes. Mosquito infection is especially high when feedings are made with gametocytes from splenectomized chimpanzees.