Abstract
BackgroundIn Brazil, members of the sand fly species complex Lutzomyia longipalpis transmit Leishmania infantum, a protist parasite that causes visceral leishmaniasis. Male Lu. longipalpis produce a sex pheromone that is attractive to both females and males. During a cluster randomised trial, to determine the combined effect of synthetic sex-aggregation pheromone and insecticide on Le. infantum transmission Lu. longipalpis had been continuously exposed to insecticide for 30 months. The objective of this study was to determine the effect of continuous exposure to the insecticides used in the trial on the susceptibility of Lu. longipalpis population.MethodsDuring the trial the sand flies had been exposed to either lambda-cyhalothrin [pheromone + residual insecticide spray (PI)], deltamethrin [dog collars (DC)] or no insecticide [control (C)], for 30 months (November 2012 to April 2015). The insecticide treatment regime was kept in place for an additional 12 months (May 2015-April 2016) during this susceptibility study. Sand flies collected from the field were exposed to WHO insecticide-impregnated papers cyhalothrin (0.05%), deltamethrin (0.5%) and control (silicone oil) in a modified WHO insecticide exposure trial to determine their susceptibility.ResultsWe collected 788 Lu. longipalpis using CDC-light traps in 31 municipalities across the three trial arms. Probit analysis showed that the knockdown times (KDTs) of Lu. longipalpis collected from the lambda-cyhalothrin exposed PI-arm [KDT50: 31.1 min, confidence interval (CI): 29.6–32.6 and KDT90: 44.2 min, CI: 42.1–46.7] were longer than the KDTs from the non-insecticide-treated C-arm (KDT50: 26.3 min, CI: 25.1–27.6 and KDT90: 38.2, CI: 36.5–40.2) (no-overlapping 95% CIs). KDTs of Lu. longipalpis collected from the deltamethrin exposed DC-arm had similar values (KDT50: 13.7 min, CI: 10.1–16.2 and KDT90: 26.7 min, CI: 21.8–30.6) to those for the C-arm (KDT50: 13.5 min; CI: 12.2–14.8 and KDT90: 23.2 min, CI: 21.4–25.4) (overlapping CIs). The wild-caught unexposed Lu. longipalpis (C-arm), took approximately twice as long to knock down as laboratory-colonised specimens for both insecticides.ConclusionsOur study reveals slight changes in KDT, in sand flies after prolonged exposure to lambda-cyhalothrin in the presence of pheromone. These changes are not considered to have reached the reference levels indicative of resistance in sand flies suggesting that pheromone and insecticide treatment at the level indicated in this study do not constitute a significant risk of increased insecticide resistance. Prolonged exposure to deltamethrin in dog collars did not result in changes to KDT.
Highlights
In Brazil, members of the sand fly species complex Lutzomyia longipalpis transmit Leishmania infantum, a protist parasite that causes visceral leishmaniasis
Lutzomyia longipalpis collected in the pheromone insecticide arm (PI-arm) had higher KDT90 values: 44.2 min (CI: 42.1–46.7) compared to 43.1 min (CI: 41.7–44.6) for the dog-collar arm (DC-arm) and 38.2 min Results Morphological examination of the genitalia confirmed that all the specimens were either Lu. longipalpis (98.1%) or Brumptomyia spp. (1.9%)
Lutzomyia longipalpis collected from the dog collars (DC)-arm, had similar KDT50 values (13.7 min, confidence interval (CI): 10.1–16.2) to those collected in the pheromone + insecticide (PI)-arm (12.8 min, CI: 10.4–15.0) and the control arm (C-arm) (13.5 min, CI: 12.2–14.8)
Summary
In Brazil, members of the sand fly species complex Lutzomyia longipalpis transmit Leishmania infantum, a protist parasite that causes visceral leishmaniasis. During a cluster randomised trial, to determine the combined effect of synthetic sexaggregation pheromone and insecticide on Le. infantum transmission Lu. longipalpis had been continuously exposed to insecticide for 30 months. The objective of this study was to determine the effect of continuous exposure to the insecticides used in the trial on the susceptibility of Lu. longipalpis population. Lutzomyia longipalpis (Diptera: Psychodidae) is the most important vector of Leishmania infantum, the protist parasite that causes zoonotic visceral leishmaniasis (VL) in the Americas. Over the past few years Lu. longipalpis has expanded its distribution across the state with a consequent increase in the number of municipalities reporting canine and human transmission [3,4,5]. IRS is assumed to help reduce VL burden; there is little direct empirical evidence for this [8], as it depends on reduced biting behaviour of the vector [9] and compliant human behavioural response to insecticide-based protective measures [10]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call