The minimal infective dose of Plasmodium knowlesi for rhesus monkeys was found in this study to be between 1 and 10 parasites when injected intraperitoneally. As the dose of parasites is increased, the length of time prior to the appearance of circulating parasites is decreased. However, the severity of the infection once it is established is independent of the initial dose of parasites.

In passive protection experiments a quantitative relationship was demonstrated between the number of parasites in the inoculum and the effective amount of immune serum given at the time of infection and in equal doses daily for 9 days thereafter. The smaller the inoculum, the less the quantity of immune serum required to prevent the death of the animal.

When relatively large amounts of immune sera and small numbers of parasites were used in the protection experiment, infection was prevented.

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