Genetic evidence for Plasmodium falciparum resistance to chloroquine and pyrimethamine in Indochina and the Western Pacific between 1984 and 1998.

Plasmodium falciparum resistance to chloroquine and pyrimethamine is widely distributed in malaria-endemic areas. The origin and geographic spread of this drug resistance have been inferred mainly from records of clinical resistance (treatment failure). Identification of the Plasmodium falciparum chloroqunie resistance transporter (pfcrt) gene and the dihydrofolate reductase (dhfr) gene as target genes of chloroquine and pyrimethamine, respectively, has made it possible to trace the history of genetic resistance to these two drugs. However, evidence for genetic resistance has been limited because of scarcity of archival specimens. We examined genotypes of pfcrt and dhfr in Indochina (Thailand, Myanmar, and Laos) and the Western Pacific (the Philippines, Indonesia, and Papua New Guinea) between 1984 and 1998 by testing samples obtained from malaria cases imported to Japan. Results show that 96% (28 of 29) and 77% (20 of 26) of samples had resistant genotypes of pfcrt and dhfr, respectively, substantiating the inferred history of clinical resistance in these geographic areas during this period.