A strain of malaria parasite was recovered from a man who had served as an Army officer in the West Pacific during World War II. This strain, termed the Donaldson strain, was studied in about 125 neurosyphilitics undergoing therapeutic malaria. Both sporozoite induced and blood induced infections were included, the former composing about one-third of the cases. Similar studies were done on two strains of P. vivax for comparison of strain characteristics.
In sporozoite induced Donaldson strain the mean prepatent period was 15.3 days and the incubation period 17.7 days. The strain was characterized by a short, mild primary clinical attack, with an average of only 8.1 paroxysms per attack and a mean maximum parasitemia of 8,314 per cmm. Termination of the clinical attack was almost invariably spontaneous. The mean paroxysmal interval, indicative of the periodicity of the strain, was 49.5 hours. The strain was characterized by a tendency to produce multiple parasite broods results in frequent quotidian periodicity.
The infection was very sensitive to antimalarial drugs and easily eradicated with small amounts of quinine or sodium bismuth thioglycollate, as well as with standard dosages (1.5 g.) of chloroquine.
Untreated parasitemias persisted for considerable periods of time, with an average total duration of 54.5 days. Symptomatic relapses occurred in only two cases, and these were following very early termination of the primary attack with chloroquine. The clinical relapses were mild and terminated spontaneously after only a few febrile episodes.
The Donaldson strain is equally infective to Negroes and whites. One Donaldson strain infection seems to confer immunity to reinfection with the same strain, but not to the other species of Plasmodium. Previous infections with P. vivax, P. falciparum, or P. malariae do not significantly affect subsequent Donaldson strain infections.
The characteristics of the Donaldson strain are compared with those of the Chesson and St. Elizabeth vivax strains and with P. ovale. It appears that the Donaldson strain resembles P. ovale more than it does P. vivax.