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

Abstract

BackgroundIndoor 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.AimsThis study evaluated routine IRS practises and insecticide quality control in a typical endemic community in the Bolivian Chaco.MethodsAlpha-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 (IQK™). 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 IQK™ 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.ConclusionsSuboptimal 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 IQK™ 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.

Improving houses in the Bolivian Chaco increases effectiveness of residual insecticide spraying against infestation with Triatoma infestans, vector of Chagas disease

“…Recent T. cruzi transmission is present, based on paediatric infections detected during this project [10]. Triatoma infestans is the predominant vector [14,23]. Prior IRS records were unavailable, but reports suggest that blanket spraying was conducted in 2000 and 2003, followed by focal spraying of infested houses from 2005 to 2009 and irregular spraying between 2009 and 2011 [9,23].…”

“…Routine spray (RS). Indoor residual spraying was conducted by local health workers following routine SEDES practices [14]. Interior and exterior walls of houses and outbuildings were sprayed with alpha-cypermethrin 20% [SC] (Alphamost â -Hockley International Ltd., Manchester, UK) aiming for a delivery concentration of 50 mg active ingredient (a.i.…”

“…During IRS-1, insecticide concentrations were quantified as reported elsewhere [14]. Following WHO protocols [29], 8-9 filter papers were fixed on three inside walls prior to spraying, yielding 206, 108 and 157 papers from 25 RS, 12 HI and 19 AS houses, respectively.…”

“…)/m 2 . One house in the insecticide practices study [14] was excluded from this study, thus estimates are recalculated here.…”

“…However, reinfestation is rapid [11]. Low IRS effectiveness has been attributed to vulnerable house construction, suboptimal spray practices, short insecticide residuality and reduced vector susceptibility [11][12][13][14]. House improvement is particularly relevant against T. infestans, which establishes breeding colonies inside wall crevices [15,16].…”