Detection of Plasmodium falciparum sporozoites in naturally infected anopheline species using a fluorescein-labelled DNA probe

A dot blot hybridization method was developed to detect Plasmodium falciparum sporozoites in naturally infected mosquitoes. A fluorescein-labelled oligomer was used as a probe. Initial non-specific hybridization was found to correlate with the presence of blood in the mosquitoes. This was eliminated by allowing digestion of the engorged blood by keeping the mosquitoes in cages for 48 h before processing. The limit of detection of the hybridization assay was estimated to be about 500 sporozoites. The assay was evaluated on 198 indoor resting blood fed female Anopheles gambiae s.l mosquitoes collected from three malaria hypo- and meso-endemic areas in Ethiopia. An application of the polymerase chain reaction (PCR) amplifying a fragment of the K1-14 gene of P. falciparum was used as a reference method. P. falciparum sporozoites were detected in four specimens (2%) by hybridization assay and by PCR alike. The results of this study indicate that the hybridization method can be potentially valuable in large scale epidemiological studies for detection of P. falciparum sporozoites in naturally infected anopheline species.