Molecular analysis of strains of Plasmodium vivax from paired primary and relapse infections.

Relapse infections are an important obstacle to the successful treatment and control of Plasmodium vivax malaria, but the molecular basis of relapse remains poorly understood. To provide insight into the molecular mechanisms of relapse, paired primary and relapse isolates of P. vivax were subjected to single-strand conformational polymorphism (SSCP) and sequence analysis of the circumsporozoite and merozoite surface protein 1 genes. All unrelated isolates were unique by both SSCP and sequence analysis, but 5 of 6 relapse isolates were identical to or were clones of their matched primary isolates. These results indicate that most relapses are caused by the same parasite populations that circulate during the primary infection and do not arise from a genetically distinct subpopulation.