Insecticide-impregnated dog collars for the control of visceral leishmaniasis: evaluation of the susceptibility of field Lutzomyia longipalpis populations to deltamethrin.

Abstract

Background: Visceral leishmaniasis (VL) is a zoonotic disease caused by Leishmania infantum and transmitted by the sand fly Lutzomyia longipalpis. Dogs are the major domestic reservoir of L. infantum. To prevent the spread of the disease, dog collars impregnated with 4% deltamethrin have been effectively used in VL endemic areas. However, this approach may contribute to the emergence of insecticide resistance in sand flies. Therefore, it is important to characterize the susceptibility of different populations of Lu. longipalpis to deltamethrin in areas where insecticide-impregnated dog collars are used.

Methods: Six field sand fly populations from Brazil were exposed to deltamethrin in CDC bottle bioassays at the diagnostic doses (DD) of 21.9 μg/bottle and 30 μg/bottle. For the dose-response (DR) experiments, doses of 1, 3, 5, 7, 9 and 11 μg/bottle of deltamethrin were used to impregnate bottles; control group bottles were impregnated with acetone only. Each bottle contained an average of 20 sand flies, both male and female, and they were exposed to either deltamethrin or acetone for 60 min.

Results: Based on the DD of 21.9 μg/bottle, three populations were susceptible to deltamethrin. In contrast, two populations collected from the states of Ceará and Minas Gerais exhibited mortality rates of 94.9% and 95.7%, indicating possible resistance, and one population from the state of Ceará showed resistance, with a mortality rate of 87.1%. At the DD of 30 μg/bottle, two populations from the states of Ceará and Piauí showed possible resistance, while the other four populations were susceptible. The resistance ratio (RR₅₀) ranged from 2.27 to 0.54, and RR₉₅ ranged from 4.18 to 0.33, indicating a low resistance intensity.

Conclusions: This study established a DD for Lu. longipalpis using the CDC bottle bioassay. We found that Lu. longipalpis populations in three Brazilian states where insecticide-impregnated dog collars were used for VL control were susceptible to deltamethrin. However, one population in Ceará State was classified as resistant to deltamethrin. These results contribute to the current knowledge on sand fly resistance and surveillance, and highlight the need for a better understanding of the resistance mechanisms of Lu. longipalpis in areas where insecticide-impregnated dog collars have been widely used.