RESUMEN
Polymerase chain reaction (PCR) is both a sensitive means of detecting malaria parasitaemia, and a simple tool for identifying genetic differences in parasites infecting human subjects. We compared PCR to microscopy in detection of Plasmodium falciparum infection in peripheral, placental and cord blood samples collected from 131 pregnant Malawian women and their infants in 1997-99. Infections detected by species-specific PCR were genotyped at the merozoite surface protein 1 and 2 loci, and minimum numbers of infecting genotypes determined. PCR was of similar sensitivity to microscopy in detecting peripheral and placental infection, and placental blood PCR was 100% specific compared to placental histology. Cord blood parasitaemia was more frequently detected by PCR than microscopy, 20% versus 6%. Genotype numbers in peripheral blood (mean 2.36; range 1-5), placental blood (mean 2.41; range 1-6) and cord (mean 2.14; range 1-4) were similar. The frequency of detection of each allelic family did not differ between sites. Genotypes from different sites in each patient were compared. In 69% of women, genotypes were detected in peripheral blood and not placenta, or vice versa, suggesting possible differential sequestration of different parasite populations. Cord blood genotypes were usually a subset of those in peripheral and placental blood, but, in some cases, genotypes were found in cord blood that were absent from the mother. Transplacental infection before term, and clearance of maternal infection, is postulated.