A real-time PCR assay to estimate Leishmania chagasi load in its natural sand fly vector Lutzomyia longipalpis

Leishmania chagasi, transmitted mainly by Lutzomyia longipalpis sand flies, causes visceral leishmaniasis and atypical cutaneous leishmaniasis in Latin America. Successful vector control depends upon determining vectorial capacity and understanding Leishmania transmission by sand flies. As microscopic detection of Leishmania in dissected sand fly guts is laborious and time-consuming, highly specific, sensitive, rapid and robust Leishmania PCR assays have attracted epidemiologists' attention. Real-time PCR is faster than qualitative PCR and yields quantitative data amenable to statistical analyses. A highly reproducible Leishmania DNA polymerase gene-based TaqMan real-time PCR assay was adapted to quantify Leishmania in sand flies, showing intra-assay and inter-assay coefficient variations lower than 1 and 1.7%, respectively, and sensitivity to 10 pg Leishmania DNA (∼120 parasites) in as much as 100 ng sand fly DNA. Data obtained for experimentally infected sand flies yielded parasite loads within the range of counts obtained by microscopy for the same sand fly cohort or that were around five times higher than microscopy counts, depending on the method used for data analysis. These results highlight the potential of quantitative PCR for Leishmania transmission studies, and the need to understand factors affecting its sensitivity and specificity.