Currently, the only method for identifying infective hosts with
Leishmania infantum to the vector
Lutzomyia longipalpis
is xenodiagnosis. More recently, quantitative polymerase chain reaction (qPCR) has been used to model
human reservoir competence by assuming that detection of parasite DNA indicates the presence of viable parasites
for infecting vectors. Since this assumption has not been proven, this study aimed to verify this hypothesis. The concentration
of amastigotes in the peripheral blood of 30 patients with kala-azar was microscopically verified by leukoconcentration
and was compared to qPCR estimates. Parasites were identified in 4.8 mL of peripheral blood from
67% of the patients, at a very low concentration (average 0.3 parasites/mL). However, qPCR showed 93% sensitivity
and the estimated parasitaemia was over a thousand times greater, both in blood and plasma, with higher levels
in plasma than in blood. Furthermore, the microscopic count of circulating parasites and the qPCR parasitaemia
estimates were not mathematically compatible with the published proportions of infected sandflies in xenodiagnostic
studies. These findings suggest that qPCR does not measure the concentration of circulating parasites, but rather
measures DNA from other sites, and that blood might not be the main source of infection for vectors.
