The changing pattern of Plasmodium falciparum susceptibility to chloroquine but not to mefloquine in a mesoendemic area of Somalia

Abstract

The response of Plasmodium falciparum to chloroquine and mefloquine was investigated in a mesoendemic area of Somalia from 1986 to 1989. Serial in vivo field tests for chloroquine sensitivity were performed and the sensitivity in vitro to chloroquine and mefloquine was evaluated using the standard WHO in vitro microtest. Chloroquine treatment in vivo (25 mg base/kg body weight) resulted in parasite clearance in all patients within 7 d (S/RI) in 1986, while 17%, 19% and 30% RII/RIII responses were found in 1987, 1988 and 1989 respectively. There was consistent increase of parasite clearance time of the S/RI cases in all years. The sensitivity study in vitro in 1986 showed a low degree of chloroquine resistance in 3 of 29 isolates tested and a mean 50% effective dose (EC₅₀) and EC₉₉ of 0·34 × 10⁻⁶m and 1·99 × 10⁻⁶m, respectively. In contrast, in 1989, 12 of the 19 isolates tested were resistant to chloroquine. The mean EC₅₀ and EC₉₉ values had increased to 0·78 × 10⁻⁶m and 7·50 × 10⁻⁶m respectively. The data in vivo and in vitro indicate a rapid increase of chloroquine resistance both in frequency and degree. All isolates tested in 1986 and 1989 were fully inhibited by mefloquine at 3·2 × 10⁻⁶m, suggesting full sensitivity. Thus, increased resistance of P. falciparum to chloroquine did not significantly influence the sensitivity pattern of mefloquine.