Comparative infectivity of knobless and knobby clones ofPlasmodium falciparum in splenectomized and intactAotus trivirgatus monkeys

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Abstract
In two experiments, two knobless (K−) and two knob-producing (K+) clone-cultures ofPlasmodium falciparum, FCR-3/Gambia strain, were injected into fourAotus trivirgatus monkeys. The parasitemia in the K−-infected splenectomized (S−) monkey rose to a peak of 2.1% on the 16th day, while it reached only 0.7% at the same time in the K+-infected S− animal. Passage from these animals (karyotype VI) into two intact (S+), naive monkeys of karotype III resulted in very light infections somewhat higher with K+ than with K−. This experiment was repeated with two different clones in two other S− monkeys of karyotype III. Again, the parasitemia of the K+ infected monkey was appreciably below that of the K− monkey. Transfer of parasites into S+ animals of karyotype II resulted in very light infection and, as before, the K+ did somewhat better. About 2 months after its initial infection, the K+-infected S− animal from the second experiment came down with a recurrent malaria infection. Electron-microscopic observations on blood from this monkey revealed that the previously K+ parasites had become knobless (K−). Transfer of this material into an S+, naive monkey, again, gave a barely detectable infection. After splenectomy a recrudescence occurred. The results strongly indicate that K− clones ofP. falciparum are more infectious to S− Aotus monkeys than K+ clones, whereas in S+ monkeys the situation is reversed.