Chemosuppressive field trials in Thailand. II. The suppression of Plasmodium falciparum and Plasmodium vivax parasitemias by a diformyldapsone-pyrimethamine combination.

In an area of Thailand where chloroquine-resistant falciparum malaria is known to exist, a chemosuppressive field trial was undertaken to test the efficacy of the preparation diformyldapsone (DFD) combined with pyrimethamine (Py) in suppressing falciparum and vivax parasitemias. Six hundred and fifty-nine Thai villagers were randomly assigned to one of five treatment groups: DFD-Py; dapsone (DDS)-Py; DFD alone; Py alone; and placebo. Five hundred and ninety-three study subjects completed the 26-week trial. The combination DFD-Py given weekly was shown to be an effective chemosuppressive against both falciparum and vivax parasitmias, causing a more than fourfold reduction in falciparum and an approximately threefold reduction in vivax parasitemias; however, this combination was not more efficacious than DDS-Py for the chemosuppression of falciparum malaria. DFD alone was only moderately effective, while there was no difference in chemosuppression between Py alone and placebo.