Comparison of Parasite Burden Using Real-Time Polymerase Chain Reaction Assay and Limiting Dilution Assay in Leishmania major Infected Mouse.

Background: Limiting dilution assay is considered as the gold standard method for quantifying the number of parasites in the animal model of Leishmania infection. Nowadays, real-time PCR is being increasingly applied to quantify infectious agents. In the present study, a real-time PCR assay was developed to estimate parasite burdens in lymph nodes of Leishmania major infected BALB/C mice. Enumeration of parasites was also performed by limiting dilution assay and compared with the results of real-time PCR based quantification.

Methods: The SYBR Green based real-time PCR assay was performed to amplify a 75 bp fragment of superoxide dismutase B1 gene in the lymph nodes of L. major infected BALB/C mice 8 weeks post infection. Mice were infected subcutaneously at the base of their tail with 2 × 10(5) L. major promastigotes in the stationary phase of growth. To compare parasite burdens obtained by real-time PCR assay with those of limiting dilution assay, twelve 8-fold serial dilutions of the lymph node homogenates were prepared in the Schneider medium and incubated at 26°C. After 7 days, wells containing motile parasites were identified by direct observation under an inverted light microscope and the total number of parasites was estimated using the ELIDA software.

Results: Spearman's correlation coefficient of the parasite burdens between real-time PCR and limiting dilution assay was 0.72 (P value = 0.008).

Conclusion: Real-time PCR assay is an appropriate replacement to existing limiting dilution assay in quantifying parasite burden in the experimental model of Leishmania infection.