Plasmodium vivax clinically resistant to chloroquine in Colombia

Chloroquine-resistant Plasmodium vivax has been the subject of numerous cas e reports and prospective studies from Oceania and Asia. In contrast, only case reports exist from the Americas. We performed a prospective study with 28-day follow-up of clinical responses to chloroquine in 2 P. vivax-endemi c regions of Colombia. Three (11%) of 27 patients failed to respond to trea tment with the standard regimen of chloroquine (1,500 mg of base over 3 day s). One patient demonstrated RI resistance on Day 26; one patient demonstra ted RI resistance due to recrudescence of blood stages on Day 11; and one p atient demonstrated RII resistance of blood stages by never displaying clea ring of peripheral parasitemia. All patients were successfully treated with primaquine, which has some blood stage efficacy against P. vii,ax, combine d with a second course of chloroquine. Clinical resistance of P. vivax to c hloroquine is present in Colombia and should be monitored in the Americas.