Plasmodium vivax resistance to chloroquine?

Two soldiers continued weekly prophylaxis with 300 mg chloroquine base on their return to Australia from Papua New Guinea but were not protected against Plasmodium vivax malaria. Both had symptoms and parasitaemia although plasma concentrations of chloroquine were considerably higher than those regarded as adequate for suppression of vivax malaria. Parasitaemia did not clear after one of the patients was treated with 600 mg chloroquine base. The results suggest the emergence of strains of P vivax with a reduced susceptibility to chloroquine.