Molecular epidemiology for vector research on leishmaniasis

Abstract

Leishmaniasis is a protozoan disease caused by the genus Leishmania transmitted by female phlebotomine sand flies. Surveillance of the prevalence of Leishmania and responsive vector species in endemic and surrounding areas is important for predicting the risk and expansion of the disease. Molecular biological methods are now widely applied to epidemiological studies of infectious diseases including leishmaniasis. These techniques are used to detect natural infections of sand fly vectors with Leishmania protozoa and are becoming powerful tools due to their sensitivity and specificity. Recently, genetic analyses have been performed on sand fly species and genotyping using PCR-RFLP has been applied to the sand fly taxonomy. In addition, a molecular mass screening method has been established that enables both sand fly species and natural leishmanial infections to be identified simultaneously in hundreds of sand flies with limited effort. This paper reviews recent advances in the study of sand flies, vectors of leishmaniasis, using molecular biological approaches.

Keywords: Leishmania; epidemiology; mass screening; sand fly.

Figure 1.

Schematic life cycle of Leishmania parasites. 1. The sand fly ingests amastigotes during blood feeding. 2. Amastigotes transform into promastigotes. 3. Promastigotes colonize and multiply at the hindgut and midgut of the sand fly. 4. Infective promastigotes (metacyclics) migrate to the anterior part of the gut. 5. The infective stage of promastigotes are transmitted to a mammalian host by the bites of the sand fly. 6. Promastigotes invade host neutrophils. 7. Macrophages are infected by promastigotes directly or through the phagocytosis of infected neutrophils, or infected silently by promastigotes released from apoptotic neutrophils. 8. Promastigotes transform into amastigotes. 9. Amastigotes multiply in infected cells by binary fission. Note: Most Leishmania species are maintained by an animal-to-animal transmission cycle and humans are considered an accidental host (arrow). However, anthroponotic transmission without animal reservoirs is also reported in some Leishmania species (dashed arrow).

Figure 2.

A blood-sucking phlebotomine sand fly.

Figure 3.

A molecular mass screening method for the detection of Leishmania within individual sand flies and identification of both sand fly and Leishmania species.