The Ongoing Risk of Leishmania donovani Transmission in Visceral Leishmaniasis Endemic Districts: An Entomological Investigation in Eastern Nepal during the Elimination Era

Abstract

Abstract Background Visceral leishmaniasis (VL), a life-threatening neglected tropical disease, is targeted for elimination from Nepal by the year 2026. The national VL elimination programme is still confronted with many challenges including the increasingly widespread distribution of the disease over the country, local resurgence and the questionable efficacy of the key vector control activities. We assessed the status and risk of Leishmania donovani transmission based on entomological indicators including seasonality, natural Leishmania infection rate, and feeding behavior of vector sand flies, Phlebotomus argentipes in three VL endemic districts in two epidemiologically contrasting settings; each district with one village with and one without reported VL cases in recent years.Methods Adult sand flies were collected using CDC light traps and mouth aspirators in each village for 12 consecutive months from July 2017 to June 2018. Sand fly species level identification was based on morphological characters and verified by the DNA barcoding method. We assessed P. argentipes abundance and seasonality based on the light trap collections, considering it as the standard method of collection. Leishmania infection was assessed in gravid sand flies targeting the small-subunit ribosomal RNA gene of the parasite (SSU-rRNA) and further sequenced for species identification. A segment (~ 350 bp) of the vertebrate cytochrome b (cytb) gene was amplified from blood-fed P. argentipes from dwellings shared by both humans and cattle and sequenced to identify the preferred host.Results Vector abundance varied among districts and village types and peaks in sand fly density was observed in June-July and September-November. The estimated Leishmania infection rate in vector sand flies was 2.2% (1.1% — 3.7% at 95% credible interval) and 0.6% (0.2% — 1.3% at 95% credible interval) in VL and non-VL villages respectively. The common source of blood meal was humans, 52.7% in VL villages and 74.2% in non-VL villages, followed by cattle.Conclusion Our findings highlight the risk of ongoing L. donovani transmission in VL endemic districts even in villages not reporting the presence of the disease over the past many years and emphasize the remaining threats to the VL elimination programme in Nepal.