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

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.