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Background  This is an investigation of anti-malarial molecular markers coupled with a therapeutic efficacy test of chloroquine (CQ) against falciparum malaria in an area of unstable malaria in Lahj Governorate, Yemen. The study was aimed at assessment of therapeutic response to CQ and elucidation of baseline information on molecular markers for  Plasmodium falciparum  resistance against CQ and sulphadoxine/pyrimethamine (SP).    Methods  Between 2002 and 2003 the field test was conducted according to the standard WHO protocol to evaluate the therapeutic efficacy of CQ in 124 patients with falciparum malaria in an endemic area in Lahj Governorate in Yemen. Blood samples collected during this study were analysed for  P. falciparum  chloroquine resistance transporter gene ( pfcrt )-76 polymorphisms, mutation  pfcrt- S163R and the antifolate resistance-associated mutations dihydrofolate reductase ( dhfr )-C59R and dihydropteroate synthase ( dhps )-K540E. Direct DNA sequencing of the  pfcrt  gene from three representative field samples was carried out after DNA amplification of the 13 exons of the  pfcrt  gene.    Results  Treatment failure was detected in 61% of the 122 cases that completed the 14-day follow-up. The prevalence of mutant  pfcrt  T76 was 98% in 112 amplified pre-treatment samples. The presence of  pfcrt  T76 was poorly predictive of  in vivo  CQ resistance (PPV = 61.8%, 95% CI = 52.7-70.9). The prevalence of  dhfr  Arg-59 mutation in 99 amplified samples was 5%, while the  dhps  Glu-540 was not detected in any of 119 amplified samples. Sequencing the  pfcrt  gene confirmed that Yemeni CQ resistant  P. falciparum  carry the old world (Asian and African) CQ resistant haplotype CVIETSESI at positions 72,73,74,75,76,220,271, 326 and 371.    Conclusion  This is the first study to report baseline information on the characteristics and implications of anti-malarial drug resistance markers in Yemen. It is also the first report of the haplotype associated with CQR  P. falciparum  parasites from Yemen. Mutant  pfcrt T76 is highly prevalent but it is a poor predictor of treatment failure in the study population. The prevalence of mutation  dhfr Arg59 is suggestive of emerging resistance to SP, which is currently a component of the recommended combination treatment of falciparum malaria in Yemen. More studies on these markers are recommended for surveillance of resistance in the study area.

Molecular markers of anti-malarial drug resistance in Lahj Governorate, Yemen: baseline data and implications