Point mutations in the dihydrofolate reductase and dihydropteroate synthase genes of Plasmodium falciparum and resistance to sulfadoxine-pyrimethamine in Sri Lanka.

Sulfadoxine-pyrimethamine (SP) is the second-line treatment for Plasmodium falciparum malaria in Sri Lanka. Resistance to SP is caused by point mutations in the dihydrofolate reductase (Pf-dhfr) and dihydropteroate synthase (Pf-dhps) genes of P. falciparum. We determined the genotype of Pf-dhfr and Pf-dhps and the clinical response to SP in 30 field isolates of P. falciparum from Sri Lanka. All patients treated with SP had an adequate clinical response. Eighty-five percent (23 of 27) of pure field isolates carried parasites with double mutant alleles of Pf-dhfr (C59R + S108N) and showed about 200-fold higher levels of resistance to pyrimethamine than the wild type in a yeast system. None of the isolates had either known or novel mutations at other positions in the dhfr domain. In contrast, 67% (20 of 30) of the isolates carried parasites that were wild type for Pf-dhps. In Sri Lanka, detection of the triple mutant allele of Pf-dhfr will require tracking mutations at codon 51.