High Efficacy of Primaquine Treatment for Plasmodium vivax in Western Thailand.

Primaquine is the only licensed antimalarial drug that is capable of clearing dormant Plasmodium vivax liver stage parasites. To date, there is no clear evidence of resistance of the liver stage parasite against this drug, because of the difficulty in ascertaining the cause of recurrent infection. We followed 52 Thai P. vivax patients for 9 months after directly observed treatment of 15 mg primaquine daily for 14 days. Blood samples taken at 2-4 weekly intervals were assessed by microscopy and polymerase chain reaction (PCR) for the presence of parasites. Only four of 52 (7.7%) volunteers had recurrent P. vivax infections, all at least 8 weeks after treatment. This demonstrates that primaquine retains a high efficacy in this population. Although a risk of new infections could not be ruled out, parasite genotyping at two polymorphic markers suggested a high probability of late relapsing infections in these volunteers. Continued monitoring of primaquine efficacy in this region is advisable.