Sensitivity of Plasmodium falciparum isolates to chloroquine in Kisumu and Malindi, Kenya.

The chloroquine sensitivity of Plasmodium falciparum isolates from infected persons living in Kisumu and Malindi, Kenya, was determined in vivo and vitro. There was no evidence of chloroquine resistance in 217 patients with P. faliparum infections who underwent standard W.H.O. 7-day in vivo tests. In 71 extended 35-day in vivo tests parasitemia recurred in 14 patients on days 21, 28, or 35. Parasites isolated from these 14 persons during the following period were tested in vitro. Eight tests were successful and showed the isolates to be chloroquine sensitive in vitro, suggesting that the recurrence of parasitemia resulted from reinfection rather than resistance. Macro in vitro tests were done on an additional 67 infected persons, 11 of whom also had sensitive 7-day in vivo tests. Chloroquine resistance was not demonstrated in vitro. In Malindi 100% of isolates were inhibited by a chloroquine concentration of less than or equal to 0.75 nmol/ml blood and 80% by less than or equal to 0.5 nmol as compared with 69% and 27.3% respectively of those from Kisumu. These data from individuals living in malarious areas of Kenya contrast with continuing reports of proven chloroquine-resistant P. falciparum malaria in non-immune visitors who acquired their infections in Kenya.