Molecular markers for chloroquine-resistant Plasmodium falciparum malaria in Thailand and Laos

Chloroquine-resistant Plasmodium falciparum is well documented in Thailand. Laos, however, continues to use chloroquine (CQ) as the first-line therapy for the treatment of P. falciparum malaria. The objective of the present s tudy was to determine the prevalence, in these two areas, of the cg2, pfmdr 1 and pfcrt allelic types that have previously been associated with CQ resi stance. Isolates of P. falciparum were collected from participants in ongoi ng treatment studies conducted in Thailand (near the Thai-Cambodian border) and in Laos (Vang Vieng district). The pfmdr1 and pfcrt alleles were chara cterized by PCR-RFLP and mutations in cg2 were characterized by PCR and sin gle-stranded-conformation-polymorphism (SSCP) electrophoresis. Eight (32%) of the 25 Laotian isolates but only one (4%) of the 25 Thai isolates were f ound to contain the pfmdr1 mutation N86Y (P = 0.02). In contrast, the cg2 p olymorphisms previously associated with CQ resistance were present in only 10 of the isolates from Laos but 24 of those from Thailand (40% v. 96%; P<0 .001). All the samples from both countries contained the pfcrt K76T mutant allele reported to confer resistance to CQ. The results may indicate that d rug pressure for the maintenance of the pfmdr1 and cg2 alleles varies in in tensity in the Thai and Laotian study areas, probably reflecting difference s in the national malaria-treatment policies of Thailand and Laos.