Sand fly synthetic sex-aggregation pheromone co-located with insecticide reduces canine Leishmania infantum infection incidence: a stratified cluster randomised trial
Authors: Courtenay Orin , Dilger Erin , Calvo-Bado Leo A. , Kravar-Garde Lidija , Carter Vicky , Bell Melissa J. , Alves Graziella B. , Goncalves Raquel , Makhdoomi Muhammad M. , González Mikel A. , Nunes Caris M. , Bray Daniel P. , Brazil Reginaldo P. , Hamilton James G. C. 
Published: July 2019

Objective

To evaluate the efficacy of a synthetic sex-aggregation pheromone of the sand fly vector Lu. longipalpis, co-located with residual insecticide, to reduce the infection incidence of Leishmania infantum in the canine reservoir, and to reduce sand fly vector abundance. To compare the outcomes to those resulting from fitting deltamethrin-impregnated collars to the canine reservoir.

Methods

A stratified cluster-randomised trial was designed to detect a 50% reduction in canine incident infection after 24 months in 42 recruited clusters, randomly assigned to one of three intervention arms (14 cluster each): pheromone + insecticide, insecticide-impregnated dog collars, or placebo control. Infection incidence was measured by seroconversion to anti-Leishmania antibody, Leishmania parasite detection and canine tissue parasite loads. Changes in relative Lu. longipalpis abundance within households were measured by setting three CDC light traps per household.

Results

A total 1,454 seronegative dogs were follow-up for a median 15.2 (95% C.I.s: 14.6, 16.2) months per cluster. The pheromone + insecticide intervention provided 13% (95% C.I. 0%, 44.0%) protection against anti-Leishmania antibody seroconversion, 52% (95% C.I. 6.2%, 74·9%) against parasite infection, reduced tissue parasite loads by 53% (95% C.I. 5.4%, 76.7%), and reduced household female sand fly abundance by 49% (95% C.I. 8.2%, 71.3%). Variation in the efficacy against seroconversion varied between trial strata. Equivalent protection attributed to the impregnated-collars were 36% (95% C.I. 14.4%, 51.8%), 23% (95% C.I. 0%, 57·5%), 48% (95% C.I. 0%, 73.4%) and 43% (95% C.I. 0%, 67.9%), respectively. Comparison of the outcomes of the two interventions showed no statistically consistent differences in their efficacies; however, the errors were broad for all outcomes. Reductions in sand fly numbers were predominant where insecticide was located (chicken and dog sleeping sites), with no evidence of insecticide-induced repellency onto humans or dogs.

Conclusion

The synthetic pheromone lure-and-kill approach provides protection particularly against L. infantum parasite transmission and sand fly vector abundance. The effect estimates are not dissimilar to those of the insecticide-impregnated collars, which are documented to reduce canine infection incidence, and human infection and clinical VL disease incidence, in different global regions. As a low-cost alternative or complimentary vector control tool, optimisation of best community deployment of the pheromone + insecticide are now underway.

Author summary

The sand fly vector of the intracellular parasite Leishmania infantum causing human and canine visceral leishmaniasis in the Americas is Lutzomyia longipalpis. Dogs are the proven reservoir. Vector control tools to reduce transmission suited to this predominantly exophilic vector are lacking. Insecticide-impregnated dog collars protect dogs against infectious bites from sand fly vectors, resulting in reductions of new infections in both dogs and humans. However, collars are costly particularly for endemic communities, and alternative approaches are needed. Recent bulk synthesis of a sex-aggregation pheromone produced by male Lu. longipalpis was shown to attract large numbers of conspecific females to lethal pyrethroid insecticides. This study, conducted in Brazil, evaluated the efficacy of this novel lure-and-kill approach to reduce seroconversion and infection incidence with L. infantum in the canine reservoir, in addition to measuring its impact on household abundance of Lu. longipalpis. Deployed in 14 stratified clusters, the outcomes were compared to those resulting attributed to the collars fitted to dogs in another 14 clusters; each intervention was compared to the 14 clusters that received placebo treatments. The beneficial effects of the lure-and-kill method were most noticeable on confirmed infection incidence and clinical parasite loads, and in reducing sand fly abundance. The overall effect of the two interventions were not statistically dissimilar, though the confidence intervals were broad. We conclude that the novel low-cost lure-and-kill approach should be added to the vector control toolbox against visceral leishmaniasis in the Americas.