No influence of age on infection complexity and allelic distribution in Plasmodium falciparum infections in Ndiop, a Senegalese village with seasonal, mesoendemic malaria.

ABSTRACT

We have shown previously that in Dielmo, a Senegalese village with intense perennial Plasmodium falciparum transmission, the infection complexity and the distribution of some allelic types harbored by asymptomatic carriers was age-dependent. We report here an investigation of these parameters in Ndiop, a village located 5 km from Dielmo, where malaria is mesoendemic and seasonal, and where immunity is acquired at a very low rate, as indicated by the lifelong distribution of P. falciparum clinical attacks. Blood was collected from 143 and 125 inhabitants, including 122 individuals sampled in both surveys, during two cross-sectional surveys at one-month intervals during the 1994 transmission season. Plasmodium falciparum parasites were genotyped for three polymorphic single copy genes. Genetic diversity was very large, with 17, 43, and nine distinct alleles detected for the merozoite surface protein-1 (MSP-1), MSP-2, and glutamate-rich protein loci, respectively. These figures, similar to those previously observed in Dielmo, indicate that the parasite genetic diversity is not directly related to the inoculation rate, at least in the range of transmission intensity studied here. The complexity of the asymptomatic infections (average number of distinct genotypes per isolate) was more than two-fold lower in Ndiop than in Dielmo and importantly, did not decrease with age. Likewise, the allele distribution was not influenced by age, contrasting with the observations made in Dielmo. This indicates that the number of parasite types per isolate and the influence of age on complexity and allele distribution depend on the level of endemicity, consistent with the interpretation that they reflect acquired anti-parasite immunity.