Differences in drug response of Plasmodium falciparum within an area of the Amazon region.

Clinical characteristics of patients with falciparum malaria, as well as sensitivity of Plasmodium falciparum to chloroquine and mefloquine, were investigated in 2 distinct strata within the same geographical area of the Amazon Basin. One stratum was the population living along the road, the other that living along the river, both near Rio Branco, capital of Acre State, Brazil. The clinical features did not differ between the 2 strata. Full in vitro sensitivity of P. falciparum to mefloquine was observed in both areas. However, significant differences in chloroquine sensitivity were observed between the 2 strata. EC50 values for chloroquine were 0.8484 mumol/litre for parasite isolates from the road stratum (E) and 0.4638 mumol/litre for parasite isolates from the river stratum (R). EC90 values were 2.8095 and 1.2549 mumol/litre in strata 'E' and 'R', respectively. Continuous drug pressure over years in area 'E' and relatively low drug pressure in area 'R' were presumably responsible for these differences.