Limited geographical origin and global spread of sulfadoxine-resistant dhps alleles in Plasmodium falciparum populations.

BACKGROUND: Plasmodium falciparum malaria resistant to chloroquine and pyrimethamine originated in limited foci and migrated to Africa. It remains unresolved whether P. falciparum resistance to sulfadoxine, which is conferred by mutations in dihydropteroate synthase (DHPS), evolved following a similar pattern. METHODS: The dhps locus of 893 P. falciparum isolates from 12 countries in Asia, the Pacific Islands, Africa, and South America was sequenced. Haplotypes of 6 microsatellite loci flanking the dhps locus were determined to define the genetic relationships among sulfadoxine-resistant lineages. RESULTS: Six distinct sulfadoxine-resistant lineages were identified. Highly resistant lineages appear to have originated only in Southeast Asia and South America. Two resistant lineages found throughout Southeast Asia have been introduced to East Africa, where they appear to have spread. CONCLUSIONS: The infrequent selection of parasites highly resistant to sulfadoxine and the subsequent migration of resistant lineages from Asia to Africa are similar to the patterns observed in chloroquine and pyrimethamine resistance. These findings strongly suggest that the global migration of resistant parasites has played a decisive role in the establishment of drug-resistant P. falciparum parasites, and that similar patterns may be anticipated for the spread of artemisinin resistance.

Selection of antifolate-resistant Plasmodium falciparum by sulfadoxine-pyrimethamine treatment and infectivity to Anopheles mosquitoes.

Resistance-conferring mutations in dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) in Plasmodium falciparum are selected by treatment with sulfadoxine pyrimethamine (SP). We assessed the association between these mutations and transmission capacity of parasites to Anopheles mosquitoes on the Pacific coast of Colombia. Patients with uncomplicated P. falciparum malaria received SP treatment and were followed-up to compare the prevalence of DHFR and DHPS mutations before and after SP treatment. Membrane feeding assays were used to measure infectivity to mosquitoes of post-treatment gametocytes with and without these mutations. Per-protocol treatment efficacy was 95.0% (132 of 139). Gametocytes carrying resistance-conferring mutations were selected after SP treatment and were infective to mosquitoes. Seven days after treatment, infections with two DHFR mutations had a 10-fold higher probability of infecting mosquitoes than infections with no DHFR mutations (odds ratio = 10.23, P < 0.05). Low-level drug resistance mutations have the potential to enhance transmission of P. falciparum and spread resistant parasites.

Use of area under the curve to characterize transmission potential after antimalarial treatment.

To evaluate transmission potential of Plasmodium falciparum, we use the area under the curve (AUC) of gametocyte levels after treatment as an approach to combine their duration and magnitude. Analysis of determinants of AUC was based on two main exposures: parasite clearance time (PCT) and presence of dihydrofolate reductase and dihydropteroate synthase mutations associated with sulfadoxine-pyrimethamine (SP) resistance in vitro. Exposures were determined based on the first three days after treatment with SP of 96 individuals who had malaria, cleared parasitemia by days 1-3, and were followed-up for 21 days. Using regression methods, we characterized both the heterogeneity of the presence of gametocytes (AUC > 0) and the magnitude of the AUC among those with an AUC > 0. A PCT of two or three days was associated with a substantial and highly significant odds ratio for presence of gametocytes. Among those who developed gametocytes, if their clearance time was 3 days or if they had any mutations (1 or 2) the magnitude of gametocytemia was > or = 3-fold. Methods presented are applicable to both observational studies and clinical trials assessing the effect of therapies on transmission potential.

Genetics of drug-resistant Plasmodium falciparum malaria in the Venezuelan state of Bolivar.

The state of Bolivar in Venezuela experiences episodic outbreaks of multidrug-resistant Plasmodium falciparum malaria. We obtained P. falciparum-infected blood samples in Bolivar in 1998-2000, and performed molecular assays for mutations conferring resistance to the antifolate combination of sulfadoxine-pyrimethamine (SP) and to chloroquine. All infections carried the dihydrofolate reductase (dhfr) S108A and N51I mutations, and 45% of the infections had the dhfr C50R mutation, which has been implicated in mid-level resistance to SP. Two dihydropteroate synthase (dhps) mutations also involved in SP resistance, A581G and K540E, were detected in 90% and 67% of the samples, respectively. The dhfr 1164L mutation, which confers high-level resistance, was not identified. The P. falciparum chloroquine resistance transporter (pfcrt) K76T mutation, which is critical for chloroquine resistance, was found in 167 of 168 infections. Six dhfr/dhps allelotypes and four pfcrt-resistant alleles were observed. Their interrelationships suggest a semi-clonal propagation of P. falciparum malaria in Bolivar, and an invasion of multi-resistant pathogens from Brazil. Despite national restrictions on the use of SP and chloroquine, genotypic resistance to these therapies remains widespread in Bolivar.

Origin and dissemination of Plasmodium falciparum drug-resistance mutations in South America.

Multidrug resistance is a major obstacle to the control of Plasmodium falciparum malaria, and its origins and modes of dissemination are imperfectly understood. In this study, haplotyping and microsatellite analysis of malaria from 5 regions of the South American Amazon support the conclusion that the parasite mutations conferring mid- and high-level resistance to the antifolate combination sulfadoxine-pyrimethamine have a common origin. Parasites harboring these mutations are also found to share drug-resistance alleles that confer a unique chloroquine resistance phenotype and to be similar at loci not linked to drug resistance, although not genetically identical. Since the 1980s, multidrug-resistant P. falciparum has spread in a north-northwest manner across the continent, from an origin likely in the lower Amazon. This study highlights the importance of continent-wide malaria-control policies and suggests that the containment of resistance to the next generation of therapies may be feasible.

Determinants of treatment response to sulfadoxine-pyrimethamine and subsequent transmission potential in falciparum malaria.

Drug resistance is contributing to increasing mortality from malaria worldwide. For assessment of the role of resistance-conferring parasite mutations on treatment responses to sulfadoxine-pyrimethamine (SP) and transmission potential, 120 subjects with uncomplicated falciparum malaria from Buenaventura, Colombia, were treated with SP and followed for 21 days in the period February 1999 to May 2000. Exposures of interest were mutations in Plasmodium falciparum dihydrofolate reductase (DHFR) and dihydropteroate synthase that confer resistance to pyrimethamine and sulfadoxine, respectively. Although SP was highly efficacious (96.7%), the presence together of DHFR mutations at codons 108 and 51 was associated with longer parasite clearance time (relative hazard = 0.24, p = 0.019) more so than the 108 mutation alone (relative hazard = 0.45, p = 0.188). This association remained after controlling for potential confounders. Infections with these mutations were also associated with the presence of gametocytes, the sexual form of the parasite responsible for transmission, 14 and 21 days after treatment (p = 0.016 and p = 0.048, respectively). Higher gametocytemia is probably due to DHFR mutations prolonging parasite survival under drug pressure, resulting in longer parasite clearance time and allowing asexual parasites to differentiate into gametocytes. These results suggest that even when SP efficacy is high, DHFR mutations that are insufficient to cause therapeutic failure may nevertheless increase malaria transmission and promote the spread of drug resistance.