Indoor residual spraying practises against Triatoma infestans in the Bolivian Chaco: contributing factors to suboptimal insecticide delivery to treated households.

Background: Indoor Residual Spraying (IRS) of insecticides is a key method to reduce transmission by Triatoma infestans, vector of Chagas disease in a large part of South America. However, the successes of IRS in the Gran Chaco region straddling Bolivia, Argentina and Paraguay, have not equalled those in other Southern Cone countries. Aims: This study evaluated routine IRS practises and insecticide quality control in a typical endemic community in the Bolivian Chaco. Methods: Alpha-cypermethrin active ingredient (a.i.) concentrations captured onto filter papers fitted to sprayed wall surfaces, and in prepared spray tank solutions, were measured using an adapted Insecticide Quantification Kit (IQKTM). The results were analysed by negative binomial GLM regression in relation to the time (minutes) spray teams spent treating houses, spray rates (surface area to spray [m2/minute]), householder compliance to empty houses for IRS delivery, and the visual presence/absence of filter papers. The IQKTM assays developed for these samples were validated against HPLC quantification methods. Results: Substantial variations in the delivered a.i. concentrations were observed; only 10.4% (50/480) of filter papers, and 8.8% (5/57) of houses received the target concentration. The delivered concentrations were not related to those in the matched spray tank solutions. The sedimentation of a.i. in the surface solution of prepared spray tanks was rapid, resulting in a 29% loss of a.i. content within 5 minutes, and 48.5% after 15 minutes. The delivered concentrations were positively associated with the time spent spraying the house, and inversely related to the spray rate, but showed weak correlations in both cases. The influence of householder compliance on spray rates were significant, though associated differences in delivered concentrations were not detected. No differences were observed in spray rates between houses fitted with filter papers and houses without. Conclusions: Suboptimal delivery of IRS is partially attributed to the insecticide physical characteristics, and the need for revision of insecticide delivery methods, which includes training of IRS teams and community education to encourage compliance. The IQKTM is a necessary field-friendly tool to improve IRS quality, and to facilitate health worker training and decision making by the Chagas disease vector control managers.