Observations are reported on a group of nearly 100 patients with sporozoite-inoculated Plasmodium vivax (Chesson strain) treated with chloroquine. Included was the treatment of 83 primary attacks and 101 relapses. Two treatment regimens were used: 1.5 grams base orally or 0.4 gram base intramuscularly. The primary attacks responded rapidly to treatment with either regimen. All cases followed sufficiently long were found to relapse after treatment. Of these, 45 were treated with 1.5 grams and 8 with 0.4 gram; there was no appreciable difference between the two where the treatment-to-relapse interval was concerned. The severity or duration of the primary attack prior to treatment could not be related to the treatment-to-relapse interval, nor could the number of infective bites used to induce the infection.
Relapses were treated as they occurred with one of the two regimens and were followed to further relapse or to conclusion in a number of cases. Treatment-to-relapse intervals were about the same after each attack; there was no difference in intervals after 1.5 or 0.4 grams dosages of drug. Twenty-five of 30 first relapses showed renewed activity after treatment, as did 17 of 19 second relapses, 7 of 10 third relapses, and two of four fourth relapses. One of the fifth relapses showed renewed activity.
Immunes inoculated with sporozoites usually experienced a short clinical course and, after treatment, a single asymptomatic parasite relapse.
Relapses occurred later and less frequently after treatment of extended clinical attacks than was the case in those attacks terminated prior to extensive clinical malaria, as reported by others in experimental work in volunteers.
It is concluded that in the Chesson strain of P. vivax extreme variability in patterns of infection and relapse is the rule in cases of the type considered.