Antimalarial activity of mefloquine and qinghaosu

Resistance of Plasmodium falciparum malaria to sulfadoxine-pyrimethamine ('Fansidar') in a refugee camp in Thailand.

Neutral mutations may hitchhike to high frequency when they are situated close to sites under positive selection, generating local reductions in genetic diversity. This process is thought to be an important determinant of levels of genomic variation in natural populations. The size of genome regions affected by genetic hitchhiking is expected to be dependent on the strength of selection, but there is little empirical data supporting this prediction. Here, we compare microsatellite variation around two drug resistance genes (chloroquine resistance transporter ( pfcrt ), chromosome 7, and dihydrofolate reductase ( dhfr ), chromosome 4) in malaria parasite populations exposed to strong (Thailand) or weak selection (Laos) by anti-malarial drugs. In each population, we examined the point mutations underlying resistance and length variation at 22 (chromosome 4) or 25 (chromosome 7) microsatellite markers across these chromosomes. All parasites from Thailand carried the K76T mutation in pfcrt conferring resistance to chloroquine (CQ) and 2–4 mutations in dhfr conferring resistance to pyrimethamine. By contrast, we found both wild-type and resistant alleles at both genes in Laos. There were dramatic differences in the extent of hitchhiking in the two countries. The size of genome regions affected was smaller in Laos than in Thailand. We observed significant reduction in variation relative to sensitive parasites for 34–64 kb (2–4 cM) in Laos on chromosome 4, compared with 98–137 kb (6–8 cM) in Thailand. Similarly, on chromosome 7, we observed reduced variation for 34–69 kb (2–4 cM) around pfcrt in Laos, but for 195–268 kb (11–16 cM) in Thailand. Reduction in genetic variation was also less extreme in Laos than in Thailand. Most loci were monomorphic in a 12 kb region surrounding both genes on resistant chromosomes from Thailand, whereas in Laos, even loci immediately proximal to selective sites showed some variation on resistant chromosomes. Finally, linkage disequilibrium (LD) decayed more rapidly around resistant pfcrt and dhfr alleles from Laos than from Thailand. These results demonstrate that different realizations of the same selective sweeps may vary considerably in size and shape, in a manner broadly consistent with selection history. From a practical perspective, genomic regions containing resistance genes may be most effectively located by genome-wide association in populations exposed to strong drug selection. However, the lower levels of LD surrounding resistance alleles in populations under weak selection may simplify identification of functional mutations.

Selection strength and hitchhiking around two anti-malarial resistance genes

The emergence and spread of drug-resistant Plasmodium falciparum have been a major impediment for the control of malaria worldwide. Earlier studies have shown that similar to chloroquine (CQ) resistance, high levels of pyrimethamine resistance in P. falciparum originated independently 4 to 5 times globally, including one origin at the Thailand-Cambodia border. In this study we describe the origins and spread of sulfadoxine-resistance-conferring dihydropteroate synthase (dhps) alleles in Thailand. The dhps mutations and flanking microsatellite loci were genotyped for P. falciparum isolates collected from 11 Thai provinces along the Burma, Cambodia, and Malaysia borders. Results indicated that resistant dhps alleles were fixed in Thailand, predominantly being the SGEGA, AGEAA, and SGNGA triple mutants and the AGKAA double mutant (mutated codons are underlined). These alleles had different geographical distributions. The SGEGA alleles were found mostly at the Burma border, while the SGNGA alleles occurred mainly at the Cambodia border and nearby provinces. Microsatellite data suggested that there were two major genetic lineages of the triple mutants in Thailand, one common for SGEGA/SGNGA alleles and another one independent for AGEAA. Importantly, the newly reported SGNGA alleles possibly originated at the Thailand-Cambodia border. All parasites in the Yala province (Malaysia border) had AGKAA alleles with almost identical flanking microsatellites haplotypes. They were also identical at putatively neutral loci on chromosomes 2 and 3, suggesting a clonal nature of the parasite population in Yala. In summary, this study suggests multiple and independent origins of resistant dhps alleles in Thailand.

Tracking Origins and Spread of Sulfadoxine-Resistant Plasmodium falciparum dhps Alleles in Thailand

The effect of pyrimethamine and the combination of pyrimethamine-sulfadoxine (Fansidar) upon the termination of the acute attack of vivax malaria was studied in Thailand. Pyrimethamine was found to be ineffective, providing clearance of parasitaemia in only two of six patients by the end of seven days following treatment. The combination, administered in a two-tablet single dose (sulfadoxine 1 gm, pyrimethamine 50 mg) eliminated parasitaemia in only six of ten patients within seven days. Three tablets (sulfadoxine 1 . 5 gm, pyrimethamine 75 mg) given to 11 patients, provided clearance of parasitaemia in all within seven days; however, mean parasite and fever clearance times in this group were prolonged at 90 and 50 hours respectively. Chloroquine remains the drug of choice for the termination of the acute attack of vivax malaria. Subsequent primaquine is necessary for the prevention of relapse.

Treatment of vivax malaria with sulfadoxine-pyrimethamine and with pyrimethamine alone.

The efficacies of combinations of sulfalene.trimethoprim (SF-T) and sulphormethoxine-pyrimethamine (S-P) against falciparum malaria in Thailand were assessed. ThirtY'one patients were given a single dose of SF-T (1 g SF and 0.5 g T) and 34 patients were given Sop (1 g Sand 0.05 g P). Radical cures were observed in 84% of the patients given SF-T and in 91 % of those given S-P Clearances of fever and asexual parasitemias were similar. Factors of previous malaria infections as indicated by both the patients' medical history and by the presence of malaria fluore3cent antibodies and prior antimalarial ihtake showed no apparent influence on treatment outcome. No significant toxic side reactions were observed in subjects administered either one of the combination treatments. A plan for the field use of the long-acting sulfonamide combinations is proposed. Chloroquine-resistant falciparum malaria was ii:::,t reported from Thailand in 1962.' Since that time. investigations performed in Thailand by D:--. Tranakchit Harinasuta,2 SEATO 1vledical Resea:ch LaboratorylS and Thailand Malaria Operationai Research Unit" have shown rates ranging iron 50% to 100%. Because of this problem of chloroquine resis~ (znce and in view of the fact that administration 01 quinine to large numbers of patients in the field is operationally impractical, the need in TnaiIand for a simple and effective regimen against falcipal.:"um malaria is most urgent. Presently, two sulfonamide combinations have the potentlal for meeting this need. These are: 1) sulfalene-trimethoprim (SF-T), and 2) sulphor. Accepted 28 September 1972 ... This study was supported by the Agency f01 International Development, U. S. Department of State, and the U. S Almy, Department of Defense. This is contribution No. 1047 from the Army Malaria Research Progum. t Formerly Chief, TMORU. Plcsent address' American Embassy (Central America .Malatia. Research Station), APO New YOlk 09889. ~Forrncrly EpidemIOlogist, TMORU. § U. S. Army Medical Component, SEATO II Trad Pro{rincial Hospital, Thailand. methoxine (Fanasil,1il Roche)-pyrimethamine (S·P). The purpose of this study ''las to assess these two combinations agains t falciparum malaria in adult Thai males, with respect to rapidity of action, cure rate, the effect of pree."'{isting malaria antibodies on cure rate, and possible side effects, especially side effects in individuals whose erythrOlytes are deficient in glucose-6-ph05phate dehy· drogenase (G-6-PD) activity. MATERIALS ANn :urETHODS The site selected for study was Trad Province, located in Southeast Thailand approximately 400 km from Bangkok (see Fig. 1). Previous investigations have shown that the majority of Plasmodimn jalciparum, strains from this area are chloroquine resistant In 1968, investigations of the chloroquine sensitivity of P jaidparum were conducted in an area 50 km north of Trad Hospital.' Treatment failures following administration of chloroquine were obsen'cd in 8 (80%) of 10 subjects who receIved a total dosage of 25 mg (base)/kg and in 19 (100%) of 19 additional subjects who received a single. dose at 10 mg (base)/kg. Just prior to the present study, Colwell and co-workers reported, from the same hospital, a chloroquine-resistance rate of 93% in

David M. Bear

Papers

A comparative evaluation of sulfalene-trimethoprim and sulphormethoxine-pyrimethamine against falciparum malaria in thailand