Malawi changed its national policy for
malaria treatment in 1993, becoming the first country in Africa to replace
chloroquine by
sulfadoxine and
pyrimethamine combination (SP) as the first-line
drug for uncomplicated
malaria. Seven years after this change, we investigated the prevalence of
dihydropteroate synthase (dhps) and
dihydrofolate reductase (dhfr) mutations, known to be associated with decreased sensitivity to SP, in 173 asymptomatic Plasmodium falciparum
infections from Salima, Malawi. A high prevalence rate (78%) of parasites with triple Asn-108/Ile-51/Arg-59 dhfr and double Gly-437/Glu-540 dhps mutations was found. This 'quintuple mutant' is considered as a molecular marker for clinical failure of SP treatment of P.
falciparum malaria. A total of 11 different dhfr and dhps combinations were detected, 3 of which were not previously reported. Nineteen isolates contained the single Glu-540 mutant dhps, while no isolate contained the single Gly-437 mutant dhps, an unexpected finding since Gly-437 are mostly assumed to be one of the first mutations commonly selected under
sulfadoxine pressure. Two isolates contained the dhps single or double mutant coupled with dhfr wild-type. The high prevalence rates of the three dhfr mutations in our study were consistent with a previous survey in 1995 in Karonga, Malawi, whereas the prevalences of dhps mutations had increased, most probably as a result of the wide use of SP. A total of 52 P. falciparum isolates were also investigated for
pyrimethamine and
sulfadoxine/pyrimethamine activity against parasite growth according to WHO in vitro standard protocol. A
pyrimethamine resistant profile was found. When
pyrimethamine was combined with
sulfadoxine, the mean EC(50) value decreased to less than one tenth of the
pyrimethamine alone level. This synergistic activity may be explained by
sulfadoxine inhibition of dhps despite the double mutations in the dhps genes, which would interact with
pyrimethamine acting to block the remaining
folate despite dhfr mutations in the low
p-aminobenzoic acid and low
folic acid medium mixed with blood.