STABLE PATTERNS OF ALLELIC DIVERSITY AT THE MEROZOITE SURFACE PROTEIN-1 LOCUS OF PLASMODIUM-FALCIPARUM IN CLINICAL ISOLATES FROM SOUTHERN VIETNAM

The extent of allelic diversity at the Merozoite Surface Protein-1 loc us of Plasmodium falciparum (PfMSP-1) was examined in isolates collect ed from symptomatic patients living in a mesoendemic area in southern Vietnam. The variable blocks 2, 4 and 10 were typed by polymerase chai n reaction and 24 PfMSP-1 gene types were defined as unique combinatio ns of allelic types detected in each variable block. Nineteen PfMSP-1 gene types were identified and 182 parasite populations were fully typ ed among 102 isolates. Forty-eight (47%) patients harbored more than o ne typed parasite population, and one patient had at least eight genet ically distinct subpopulations. As previously shown in the same endemi c area, recombination between blocks 4 and 10 was significantly less f requent than expected from random assortment of allelic types. The dis tribution of PfMSP-1 gene types, however, did not differ significantly from that observed in isolates collected in the same area 17-24 mo be fore the present study. Furthermore, the prevalence of the most common gene types and the average number of different gene types harbored by the same host did not decrease with age. This argues against the prom inence of frequency-dependent immune selection of PfMSP-1 polymorphism s in this parasite population.