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The mechanisms producing the genetic polymorphism at Plasmodium falciparum merozoite surface antigen-1 locus (pfmsp1) include the insertion and deletion of the different type of dimorphic Block 2 9-nucleotide repeat units as well as the intragenic recombination. To study relative occurrence frequencies of these two distinct mechanisms, we have developed a sensitive PCR strategy to identify both 5' recombinant types and the number of Block 2 repeats from the same sample. This method can specifically detect the target 5' recombinant type (Blocks 2-6) at the sensitivity of 1-4 copies of the pfmsp1. Applying the new method to field isolates from the Solomon Islands enabled us to identify six different 5' recombinant types and variation in Block 2 repeat number in three of them, thus distinguishing 10 different alleles. Distribution of these alleles in local three villages in the study area suggests that frequencies of variation in the number of Block 2 9-bp repeats and recombination events within Blocks 2-6 are mutually independent and the frequency of repeat variation is relatively high as compared to that of recombination events at the pfmsp1 locus in P. falciparum populations from the Solomon Islands.

We studied the polymorphism of msp-1, which encodes a major surface protein on the merozoite, isolated from blood samples from western Thailand in 1999. Our study area was a low-transmission area for malaria, where mefloquine has been used as an antimalarial drug since 1994. Forty-nine patients were confirmed to have contracted falciparum malaria twice within 24 weeks. The number of detected haplotypes in 49 patients was 89 at the first diagnosis and 68 at the second diagnosis. The mean number of haplotypes per patient significantly decreased from 1.82 to 1.39 but the frequency distributions of msp-1 haplotypes did not change significantly with the use of mefloquine. Our study strongly suggests that the antigenic diversity of Plasmodium falciparum is retained during mefloquine therapy in low-transmission areas.