Asexual Reproduction of Plasmodium Knowlesi in Rhesus Monkeys

Abstract

Approx. 60% of 18 rhesus monkeys with blood-induced infections (P. knowlesi) had progressively acute fatal infections, and the remainder had intermittently fatal ones. Larger numbers of uninucleated and multinucleated stages disappeared from the blood during the first few segmentations of some patent infections than of others. The asexual cycle was essentially 24 hrs. long. Maximal segmentation occurred around noon early in each infection over a period of 9 mos. Although it was often gradually delayed and prolonged from 1 to several hrs, it was occasionally accelerated several hrs. Segmenters formed 6-16 merozoites (mean 10-11) early in each infection and 3-14 merozoites later (mean 6-8, occasionally 4.5). The lengthening of the asexual cycle and decrease in the merozoite mean per segmenter indicate that the rate of reproduction was reduced 50-75% (rarely to 40%) of its initial value. The over-all death of parasites per segmentation was practically nil at the beginning of patent infections but increased to approximately 90% at the end of progressively acute infections. It was generally slightly higher in intermittently acute infections except for transient intermissions when it was practically nil or low. The gradual and progressive lengthening and derangement of the asexual cycle, lowering of the merozoite mean, occurrence of degenerate-appearing crisis forms and increase in death rates of parasites indicate that some immunity is acquired by rhesus monkeys against P. knowlesi infections,even progressively acute ones.