Five human volunteers were given weekly suppressive treatment with chloroquine (300 mg base) and then challenged with sporozoite induced Chesson strain vivax malaria. Therapy was continued for 4 to 6 weeks after exposure. Three non-medicated volunteers served as controls. Sera from each of the volunteers were subjected to tests for antibodies by the fluorescent antibody technique and for the determination of chloroquine levels.
All medicated volunteers maintained a chloroquine level of over 10 micrograms per liter during the period of therapy, with one exception. In this volunteer, on day 15 after exposure to malaria and 7 days after the previous dose of drug, the chloroquine level fell to 6 micrograms per liter.
Antibodies were first present in the controls and in 4 of the medicated volunteers 2 to 9 days after the onset of patent parasitemia. In the medicated volunteer whose chloroquine level fell below 10 µg/liter, antibodies developed while the patient was still receiving chloroquine.
It was suggested that the early appearance of antibodies in the one medicated volunteer was due to a transient subpatent parasitemia during a period of low serum-chloroquine level. It was concluded that circulating antibodies to erythrocytic parasites were not produced as a result of exoerythrocytic infection.
Present address: Department of Medicine, University Hospital, Syracuse 10, New York.
Laboratory of Parasite Chemotherapy, NIAID, National Institutes of Health, Bethesda, Maryland.
Medical Officer-in-Charge, Malaria Project, LPC, NIAID, NIH, U. S. Penitentiary, Atlanta, Georgia.