Submicroscopic and multiple plasmodium falciparum infections in pregnant Sudanese women

Control of malaria during pregnancy remains a major public health challenge in developing countries. Microscopic parasite detection represents a pivotal step in malaria control, while modern molecular techniques are deemed to improve detection rates markedly.

This study aimed to investigate the frequency of submicroscopic and multiple Plasmodium falciparum (P. falciparum) infections during pregnancy, using the P. falciparum merozoite surface protein1 (MSP-1) gene as a polymorphic marker.

The study was a cross-sectional, analytical study that was conducted at Omdurman Maternity Hospital, Sudan, between July 2003 and December 2004. Following informed consent, 836 pregnant women between the ages of 16-47 years with different gestational ages were enrolled in the study. Thin and thick blood films were stained with Giemsa and examined by experienced microscopists. Parasite DNA was extracted using Chelex method. Nested polymerase chain reaction (PCR) assays specific for P. falciparum were carried out to detect infections below the threshold of microscopy and to genotype different strains in the samples using merozoite surface protein-1.

More than a quarter of the study participants (219/836; 26.2%) were smear-positive for malaria infection. The results of the PCR-based assays showed that 41.8 % (257/617) of the smear-negative women were PCR positive and therefore had submicroscopic infections. The mean number of genetically different P. falciparum parasites detected was 2.7 (range 1-9). The multiplicity of infection identified by at least two alleles of MSP-1 was significantly higher among paucigravidae (45.6%) compared to multigravidae (28.9%), with mean number of alleles of 2.4 and 1.9, respectively (p=0.009). This likely indicates the gradual acquisition of immunity.

Conventional microscopy underestimates the actual extent of malaria infections during pregnancy in endemic regions. Multiplicity of infection may be an important factor in the gradual acquisition of strain-specific immunity.