AGE-DEPENDENT CARRIAGE OF MULTIPLE PLASMODIUM-FALCIPARUM MEROZOITE SURFACE ANTIGEN-2 ALLELES IN ASYMPTOMATIC MALARIA INFECTIONS

Genetic diversity of the merozoite surface antigen-2 gene of the human malaria parasite Plasmodium falciparum has been analyzed in a Senegal ese village where malaria is holoendemic. A cross-sectional survey of 65 residents was performed in 1992 during the high transmission season . Plasmodium falciparum was detected both by microscopy (77% positive samples) and DNA amplification using a single (29% or 38% positive sam ples, depending on the primers used) or nested polymerase chain reacti on (PCR) (78% positive samples). The overlap between the positive nest ed PCR and microscopic examination was not complete. The PCR fragments were analyzed for size polymorphism on agarose gels, and were subsequ ently assigned to the major allelic families 3D7 or FC27 by hybridizat ion with family-specific probes. Both allelic families were found, wit h a slightly higher prevalence for FC27. Chimeric alleles that failed to hybridize under stringent conditions to the reference probes were a lso observed. Some were typed using a novel PCR approach, using hybrid pairs of primers, consisting of a family-specific sense oligonucleoti de combined with an antisense oligonucleotide specific for the other f amily. Combining typing techniques, 82% of the positive PCR results yi elded more than one band. Both the overall number of fragments and the number of allelic types per carrier were markedly reduced around the age of 15 years. The number of DNA fragments decreased abruptly from a n average of four per carrier before the age of 15 years to an average of two in individuals more than 15 years of age. Similarly, the numbe r of individuals carrying more than one allelic type decreased with ag e, with a cutoff at the age of 15 years. This parallels the observed d ecrease in prevalence and parasite density in this village. There was, however, no age-dependent carriage of any particular allele, with the various alleles being detected in all age groups. The results, theref ore, indicate that acquiring anti-parasite immunity not only results i n decreasing parasite load, but also in decreasing the complexity of t he infections.