Unveiling the magnitude of submicroscopic parasitemia and gametocytaemia in different malaria endemic areas of Mainland Tanzania

Abstract

Background: The prevalence of Plasmodium falciparum in community samples is one of the cornerstones for describing malaria transmission. Prevalence measurements greatly depend on the diagnostic techniques applied and can be biased and inaccurate when parasite densities are low since parasites often remain undetected by the routinely used diagnostic tool, light microscopy. Highly sensitive molecular tools can detect submicroscopic infections and can serve to evaluate the performance of diagnostic tools used in field surveys. The study investigated local differences in the prevalence of submicroscopic infections and gametocyte carriage at different sites of varying endemicity. It investigated age trends in the submicroscopic carriage of asexual parasites and gametocytes from Tanzanian communities.
Methods: A community survey involved 1820 individuals from four regions of varying endemicity. Finger prick blood was collected by light microscopy, mRDT, and 18S rRNA qPCR for parasite detection. Gametocytes were detected by both light microscopy and qRT-PCR targeting transcripts of the gametocyte-specific expressed marker pfs25. Submicroscopic infections were those positive by qPCR but not by light microscopy.
Results: P. falciparum prevalence by qPCR varied from 37% at the site of high endemicity to 0.6% at the site of low endemicity. Of these, 52.7% (174/330) of qPCR-positive samples were submicroscopic infections. The submicroscopic carriage did not show a clear relationship with endemicity patterns and was 73% (11/15) in low, 63% (5/8), in moderate, and 51.1% (156/305) in high endemic sites. Molecular-determined gametocyte prevalence at each location closely followed the asexual parasite prevalence. But most (96%; 277/288) of gametocyte carriers were submicroscopic.
Conclusion: Molecular parasite detection revealed a high prevalence of submicroscopic carriage in Tanzania, particularly among adults. Submicroscopic infections were recorded in all endemic settings but were more prevalent in the high-endemic sites, where nearly half of the infections were not detected by routine light microscopy. Such data on submicroscopic parasitemia and gametocytemia from multiple sites within a country can help better assess the human infective reservoir for onward transmission and the relative contribution of asymptomatic populations to the ongoing malaria transmission.