In-vivo sensitivity of Plasmodium vivax isolates from Rond nia (western Amazon region, Brazil) to regimens including chloroquine and primaquine.

Seventy-nine adults with Plasmodium vivax malaria, from the Porto Velho area of Rond nia (western Amazon region, Brazil), gave informed consent to participate in a blind, clinical study of two regimens of treatment with chloroquine (CQ) and primaquine. The effectiveness of the 'classical' regimen (CQ for 3 days, followed by primaquine for 14 days) was compared with that of a 'short' regimen in which the two drugs were given simultaneously for 5 days. There were no cases of recrudescence indicative of CQ resistance (i.e. within 30 days of the first treatment dose) among the 73 patients who each completed a full, supervised course of treatment. However, 10 cases of apparent relapse were observed (all > 60 days after first treatment dose), representing 6.5% (2/31) of the patients who completed 60 days of follow-up after the classical treatment and 26.7% (8/30) of the short-regimen patients who completed the same period of follow-up. PCR-based comparison of parasitic DNA collected pre- and post-treatment was successful for eight of the 10 cases of apparent relapse and indicated that two such cases, both given the short regimen of treatment, were, in fact, probable cases of re-infection rather than of relapse. The results indicate that the classical schedule of treatment with chloroquine and primaquine was more effective at preventing relapses than the short regimen. However, since prolonged treatment with primaquine often produces side-effects that are severe enough to reduce compliance, the short schedule could be a useful alternative for malaria control in endemic areas of the Amazon region.