Investigation of the sequence profile of the Plasmodium falciparum 18SrRNA diagnostic target in isolates from naturally infected children with uncomplicated malaria

Abstract

The gold standard for malaria diagnosis is evidence of parasitological confirmation but the traditional method by light microscopy and the routinely used rapid diagnostic tests (RDTs) have limitations. Molecular assays are known to have higher sensitivity and specificity but there are indications that they may also be compromised by genetic variability of the target sequence. The aim of this study therefore, was to evaluate the DNA sequence profile of the diagnostic target of the P. falciparum 18S rRNA PCR assay in field isolates from North-Central Nigeria. Blood samples were collected from 324 children presenting with acute febrile illness suspected to be uncomplicated malaria. Light microscopy and 18S rRNA PCR assay were employed to determine the presence of P. falciparum parasites. Sequence profile of the diagnostic target was evaluated by Sanger sequencing of the PCR products on ABI PRISM® 3100 DNA sequencer (PE Applied Biosystems). Of the 324 children enrolled into this study, 134 (41.4%) were positive for P. falciparum by microscopy while 218 (67.3%) were positive by PCR. The sensitivity of microscopy was 61.47%(95% CI= 57.88% - 69.64%) using the PCR assay as reference standard. The degree of agreement between microscopy and PCR as measured by Cohen's kappa was  moderate (κ = 0.502, 95% CI = 0.463 - 0.715).Sequence analysis showed that the DNA target of the P. falciparum 18S rRNA PCR from the field isolates were highly conserved. Only one A>T single nucleotide polymorphism was found within the target sequence  among the isolates in this study. This study showed that the DNA target sequence of the18S rRNA PCR assay is highly conserved in field isolates in the study region suggesting little or no impact of selective pressure acting on the locus and has implications for the enhanced sensitivity of the molecular assay.