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Duration of Plasmodium Knowlesi Infections in Man

The duration of infection of Plasmodium knowlesi malaria has been studied in 30 white and 12 Negro patients with general paresis, infected for therapeutic purposes. After these patients were inoculated, the appearance of parasites and the length of time they persisted in the peripheral blood stream were noted by microscopical examination of blood smears. After the disappearance of parasites from the blood stream as determined by this method, the infectivity of each patient's blood was tested in the highly susceptible rhesus monkey. When the blood was found to be noninfectious for monkeys, 18 white and 7 Negro patients were reinoculated with the homologous parasites as a test of their immunity. From this study the following observations were made:

1. In the 30 white patients parasites were first seen in blood smears as early as the 3d day and as late as the 18th day following inoculation and persisted from 3 to 14 days. Blood first proved to be noninfectious for rhesus monkeys as early as the 18th day and as late as the 131st day after the original inoculation. After their blood was no longer infectious for monkeys, 18 patients were reinoculated on 23 occasions, with time intervals of from 30 to 491 days following the original inoculation, and all were found to be immune.

2. Negro patients were less susceptible to P. knowlesi infections than white patients; the incubation period was longer (7 to 27 days); and the duration of microscopically visible parasites ranged from 2 to 11 days. Their blood was infectious for monkeys as late as the 36th day and noninfectious as early as the 28th day after inoculation. Seven patients whose blood was noninfectious for monkeys were reinoculated at intervals of from 28 to 107 days after the initial inoculation, and all were immune.

Received May 2, 1938.