RESUMEN
The sand fly Lutzomyia longipalpis is the main vector of Leishmania infantum in Brazil. Synthetic male-produced sex/aggregation pheromone co-located with micro-encapsulated λ-cyhalothrin in chicken sheds can significantly reduce canine infection and sand fly densities in a lure-and-kill strategy. In this study, we determined if insecticide-impregnated netting (IN) could replace insecticide residual spraying (IRS). We compared numbers of Lu. longipalpis attracted and killed in experimental and real chicken sheds baited with pheromone and treated with a 1 m2 area of either insecticide spray or netting. First, we compared both treatments in experimental sheds to control mortality established from light trap captures. We then compared the long-term killing effect of insecticide spray and netting, without renewal, in experimental sheds over a period of 16 weeks. Finally, a longitudinal intervention study in real chicken sheds compared the numbers and proportions of Lu. longipalpis collected and killed before and after application of both treatments. In Experiment 1, a higher proportion of males and females captured in IRS- and IN-treated sheds were dead at 24 h compared to controls (P < 0.05). No difference was found in the proportion of females killed in sheds treated with IN or IRS (P = 0.15). A slightly higher proportion of males were killed by IRS (100%) compared to IN (98.6%; P < 0.05). In Experiment 2, IN- and IRS-treated traps were equally effective at killing females (P = 0.21) and males (P = 0.08). However, IRS killed a significantly higher proportion of females and males after 8 (P < 0.05) and 16 (P < 0.05) weeks. In Experiment 3, there was no significant difference between treatments in the proportion of females killed before (P = 0.88) or after (P = 0.29) or males killed before (P = 0.76) or after (P = 0.73) intervention. Overall, initially the IN was as effective as IRS at killing female and male Lu. longipalpis in both experimental and real chicken sheds. However, the relative lethal effect of the IN deteriorated over time when stored under prevailing environmental conditions.
RESUMEN
BACKGROUND: In 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. METHODS: During 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. RESULTS: We 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. CONCLUSIONS: Our 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.