Chloroquine sensitivity of Plasmodium vivax in Thailand

Chloroquine has been the standard treatment for Plasmodium vivax malaria fo r more than 40 years in most regions of the world. Recently, however, chlor oquine-resistant P. vivax has been reported from Oceania, several parts of Asia, and South America. In order to assess the situation in Thailand, 886 patients with vivax malaria who were admitted to the Bangkok Hospital for T ropical Diseases from 1992 to 1997 were followed prospectively. Most of the patients had been infected on the western border of Thailand and were expe riencing their first malarial infection when admitted. All received oral ch loroquine (approximately 25 mg base/kg body weight, administered over 3 day s) and then were randomized to receive primaquine (15 mg daily for 14 days) or no further treatment. All the patients were initially responsive to chl oroquine, clearing their parasitaemias within 7 days, and there were no sig nificant differences in the clinical or parasitological responses between t hose treated with primaquine and those given no further treatment. Plasmodi um vivax parasitaemias re-appeared within 28 days of chloroquine treatment in just four patients. In each of these four cases, re-treatment with the s ame regimen of chloroquine resulted in eradication of the parasitaemia, wit h no further appearance of parasitaemia during the next, 28-day, follow-up period. These data indicate that virtually ail acute (i.e, blood-stage) P. vivax infections acquired in Thailand can still be successfully treated wit h chloroquine.