In vitro sensitivity of southern African reference isolates of Plasmodium falciparum to chloroquine and pyrimethamine.

The in vitro sensitivity to chloroquine and pyrimethamine of 19 culture-adapted southern African reference isolates of Plasmodium falciparum was determined using a 48-hour assay. Four isolates collected in KwaZulu, Natal, were sensitive to chloroquine, and one of these was sensitive to the drug in vivo. Eight isolates from KwaZulu or Mozambique were resistant to chloroquine in vitro. Six of these isolates were chloroquine-resistant in varying degrees in vivo. Four of five isolates from north-eastern Transvaal and two clinically chloroquine-resistant Malawian isolates were resistant to chloroquine in vitro. A wide range of pyrimethamine susceptibilities was detected (0.01 mumol/l to greater than 3.0 mumol/l), although most isolates were inhibited at 0.1 mumol/l, indicating a low level of resistance. These results confirm the presence of both chloroquine and pyrimethamine resistance in the endemic areas of South Africa. This has serious implications for the prophylaxis and treatment of P. falciparum malaria in South Africa.