Impregnating hessian strips with the volatile pyrethroid transfluthrin prevents outdoor exposure to vectors of malaria and lymphatic filariasis in urban Dar es Salaam, Tanzania

Background: Semi-field trials using laboratory-reared Anopheles arabiensis have shown that, delivering the volatile pyrethroid transfluthrin by absorption into hessian strips, consistently provided > 99 % human protective efficacy against bites for 6 months without retreating. Here the impact of this approach upon human exposure to wild populations of vectors for both malaria and filariasis under full field conditions is assessed for the first time. Methods: Transfluthrin-treated and untreated strips were placed around human volunteers conducting human landing catch in an outdoor environment in urban Dar es Salaam, where much human exposure to malaria and filariasis transmission occurs outdoors. The experiment was replicated 9 times at 16 outdoor catching stations in 4 distinct locations over 72 working nights between May and August 2012. Results: Overall, the treated hessian strips conferred 99 % protection against An. gambiae (1 bite versus 159) and 92 % protection against Culex spp. (1478 bites versus 18,602). No decline in efficacy over the course of the study could be detected for the very sparse populations of An. gambiae (P = 0.32) and only a slow efficacy decline was observed for Culex spp. (P < 0.001), with protection remaining satisfactory over 3 months after strip treatment. Diversion of mosquitoes to unprotected humans in nearby houses was neither detected for An. gambiae (P = 0.152) nor for Culex spp. (Relative rate, [95 % CI] = 1.03, [0.95, 1.11], P = 0.499). Conclusion: While this study raises more questions than it answers, the presented evidence of high protection over long periods suggest this technology may have potential for preventing outdoor transmission of malaria, lymphatic filariasis and other vector-borne pathogens.