Real-time PCR to assess the Leishmania load in Lutzomyia longipalpis sand flies: screening of target genes and assessment of quantitative methods

Visceral Leishmaniasis is an endemic disease in Brazil caused by Leishmania infantum chagasi and its main vector species is the sand fly Lutzomyia longipalpis. Epidemiological studies have used conventional PCR techniques to measure the rate of infection of sand flies collected in the field. However, real-time PCR can detect lower parasite burdens, reducing the number of false negatives and improving the quantification of Leishmania parasites in the sand fly. This study compared genes with various copy numbers to detect and quantify L. infantum chagasi in L. longipalpis specimens by real-time PCR. We mixed pools of 1, 10 and 30 male sand flies with various amounts of L. infantum chagasi, forming groups with 50, 500, 5000 and 50,000 Leishmania parasites. For the amplification of L. infantum chagasi DNA, primers targeting kDNA, polymerase α and the 18S ribosome subunit were employed. Parasites were measured by absolute and relative quantification. PCR detection using the amplification of kDNA exhibited the greatest sensitivity among the genes tested, showing the capacity to detect the DNA equivalent of 0.004 parasites. Additionally, the relative quantification using these primers was more accurate and precise. In general, the number of sand flies used for DNA extraction did not influence Leishmania quantification. However, for low-copy targets, such as the polymerase α gene, lower parasite numbers in the sample produced inaccurate quantifications. Thus, qPCR measurement of L. infantum chagasi in L. longipalpis was improved by targeting high copy-number genes; amplification of high copy-number targets increased the sensitivity, accuracy and precision of DNA-based parasite enumeration.