Search

Background: Intermittent preventive treatment in infants (IPTi) is the administration of sulfadoxine-pyrimethamine (SP) at 2, 3, and 9 months of age to prevent malaria. We investigated the influence of IPTi on drug resistance., Methods: Twenty-four areas were randomly assigned to receive or not receive IPTi. Blood collected during representative household surveys at baseline and 15 and 27 months after implementation was tested for SP and resistance markers., Results: The frequency of SP in blood was similar in the IPTi and comparison areas at baseline and at 15 months. dhfr and dhps mutations were also similar at baseline and then increased similarly in both arms after 15 months of SP-IPTi. First-line treatment was switched from SP to artemether-lumefantrine before the final survey, when SP positivity fell among infants in comparison areas but increased in IPTi areas. This was accompanied by an increase in dhfr but not dhps mutations in IPTi areas (P = .004 and P = .18, respectively)., Conclusions: IPTi did not increase drug pressure or the selection on dhfr and dhps mutants, when SP was the first-line malaria treatment. Introduction of artemether-lumefantrine was followed by an increase in dhfr mutations, consistent with weak selection attributable to SP-IPTi, but not by an increase in dhps mutations, suggesting a fitness cost of this mutation.

Background: It is argued that, the efficacy of anti-malarials could be prolonged through policy-mediated reductions in drug pressure, but gathering evidence of the relationship between policy, treatment practice, drug pressure and the evolution of resistance in the field is challenging. Mathematical models indicate that drug coverage is the primary determinant of drug pressure and the driving force behind the evolution of drug resistance. These models show that where the basis of resistance is multigenic, the effects of selection can be moderated by high recombination rates, which disrupt the associations between co-selected resistance genes., Methods: To test these predictions, dhfr and dhps frequency changes were measured during 2000-2001 while SP was the second-line treatment and contrasted these with changes during 2001-2002 when SP was used for first-line therapy. Annual cross sectional community surveys carried out before, during and after the policy switch in 2001 were used to collect samples. Genetic analysis of SP resistance genes was carried out on 4,950 Plasmodium falciparum infections and the selection pressure under the two policies compared., Results: The influence of policy on the parasite reservoir was profound. The frequency of dhfr and dhps resistance alleles did not change significantly while SP was the recommended second-line treatment, but highly significant changes occurred during the subsequent year after the switch to first line SP. The frequency of the triple mutant dhfr (N51I,C59R,S108N) allele (conferring pyrimethamine resistance) increased by 37% - 63% and the frequency of the double A437G, K540E mutant dhps allele (conferring sulphadoxine resistance) increased 200%-300%. A strong association between these unlinked alleles also emerged, confirming that they are co-selected by SP., Conclusion: The national policy change brought about a shift in treatment practice and the resulting increase in coverage had a substantial impact on drug pressure. The selection applied by first-line use is strong enough to overcome recombination pressure and create significant linkage disequilibrium between the unlinked genetic determinants of pyrimethamine and sulphadoxine resistance, showing that recombination is no barrier to the emergence of resistance to combination treatments when they are used as the first-line malaria therapy.

The antimalarial combination of sulfadoxine and pyrimethamine (SP) was introduced as first-line treatment for uncomplicated malaria in Tanzania during 2001 following 18 years of second-line use. The genetic determinants of in vitro resistance to the two drugs individually are shown to be point mutations at seven sites in the dihydrofolate reductase gene (dhfr) conferring resistance to pyrimethamine and five sites in the dihydropteroate synthase (dhps) gene conferring resistance to sulfadoxine. Different combinations of mutations within each gene confer differing degrees of insensitivity, but information about the frequency with which allelic haplotypes occur has been lacking because of the complicating effects of multiple infection. Here we used a novel high-throughput sequence-specific oligonucleotide probe-based approach to examine the present resistance status of three Plasmodium falciparum populations in northern Tanzania. By using surveys of asymptomatic infections and screening for the presence of all known point mutations in dhfr and dhps genes, we showed that just five dhfr and three dhps allelic haplotypes are present. High frequencies of both triple-mutant dhfr and double-mutant dhps mutant alleles were found in addition to significant interregional heterogeneity in allele frequency. In vivo studies have shown that the cooccurrence of three dhfr mutations and two dhps mutations in an infection prior to treatment is statistically predictive of treatment failure. We have combined data for both loci to determine the frequency of two-locus genotypes. The triple-dhfr/double-dhps genotype is present in all three regions with frequencies ranging between 30 and 63%, indicating that treatment failure rates are likely to be high.