Biased distribution of msp1 and msp2 allelic variants in Plasmodium falciparum populations in Thailand

Plasmodium falciparum isolates were obtained from Thai patients attending a malaria clinic on the Thai-Kampuchean border over 4 cross-sectional survey s carried out at S-monthly intervals. The genetic structure of the parasite populations was determined by nested polymerase chain reaction (PCR) ampli fication of polymorphic regions of 3 P. falciparum antigen genes: msp1, msp 2 and glurp. Although a high degree of diversity characterized these isolat es, the overall population structure of the parasites associated with paten t malaria infections was observed to remain relatively stable over time. Th e highest degree of polymorphism was observed with msp2, and the mean numbe r of lines per infection (multiplicity of infection) calculated with this m arker was higher than that obtained using msp1 or glurp alone, or combined. Infections with greater than or equal to 2 parasite lines were seen in 76% of the samples, and were proportionally more numerous at the start and end of the rainy season. Two interesting exceptions to the random distribution were observed and involved 2 allelic variants which in one case were found dissociated (msp1 MAD20-family) and in the other were associated (msp2 FC2 7-family). The epidemiological significance of these types of data is discu ssed.