Evaluation of current and potential Triatoma infestans vector control practices in the Bolivian Chaco

ABSTRACT

This study aims to evaluate the current vector control against Triatoma infestans in a hyperendemic community in the Gran Chaco, and to pilot experiments towards its improvement. Methods: Four studies were conducted (i) Assessment of Indoor Residual Spraying (IRS); evaluating householder compliance to empty their houses, the IRS application routine, and insecticide delivered dose; (ii) Mark-released recapture (MRR) to investigate the accuracy of methods to estimate infestation density, conducted in experimental huts; (iii) Evaluation of an educational intervention to improve Chagas disease Knowledge, Attitudes and Practices (KAP); (iv) Pilot study to test efficacy of housing improvement as a complement to vector control, comparing vector infestation and density in 3 treatment groups House improvement + IRS (HI), Assisted IRS (AS), and Routine IRS (RS). Results: (i) Only 26% (15/58) of notified householders prepared their house to receive IRS; 19% (11/57) of houses presented median insecticide concentration at target dose; median insecticide concentrations were positively related to time spent spraying the house. (ii) Use of flushing-agent did not improve triatomine capture in artificial huts; increasing effort to 1 person/hour improved estimates of infestation in adobe huts; (iii) Baseline knowledge was considered deficient in disease transmission and moderate in vector ecology and biology. The educational intervention resulted in a moderated improvement of questionnaire scores for transmission and disease (d=0.63) and vector biology and ecology (d=0.70). However, the overall impact in scores improvement was considered large (Cohen d=0.88); (iv) In longitudinal analysis, all treatments presented reduction in triatomine density from baseline to a mean of 10 months post-treatment (GLM neg binomial b=-0.006; z=-4.36; p<0.001). However, HI treatment presented the highest reduction. In the same period, infestation rates decreased from 58% to 5.3% in HI; from 45% to 20% in RS; and from 74% to 23% in AS. Assessment conducted in mean 4 months post- 2 nd IRS application, registered infestation rates of 0%; 18% and 20%, respectively. Conclusions and future work (i) Quality of IRS application is hampered by the lack of health worker training, particularly in insecticide mix preparation and spray rate, which contributes to the low delivery dose. Health worker continuous training and IRS monitoring are key elements to minimize technical gaps during insecticide application, and the calibration of a field kit to assess the insecticide delivered dose in situ is a promising tool to improve the quality application. Community is not engaged in vector control activities, and educational programs to promote awareness regarding the relevance of community participation is highly recommended. (ii) TMC performance differed according to wall materials, and the combination of increased TMC to 1 person/60 min, for 2 days of searches, and application of MRR Lincon-Petersen model provided better estimates of the infestation. Overcoming technical issues regarding sample size and study design, future experiments to improve surveillance should be focused in monitoring re-infestation, using artificial huts to investigating pull-push effects, testing the imoact of combined vector control strategies in the triatomine behaviour and biology; (iii) An educational intervention has a potential to improve students' knowledge, and long-term educational projects managed by school community are recommended to develop entomological surveillance strategies according to each community reality. The adoption of different pedagogical approaches, as theatre and music, for example, are highly recommended to present information available in the educational materials in order to sensitive the community. The involvement of different actors, as doctors, nurses, and health worker, in educational activities is also recommended, as they also play an important role in information dissemination in the community; (iv) HI resulted in a greater reduction in triatomine density compared to AS and RS groups. However, the mean costs associated to housing improvement was about $662 USD per house, and the Bolivian municipalities cannot afford this strategy. Still, our results suggest that only IRS application in non-improved houses, conducted twice in a year, is only able to reduce infestation rates between 18%-20%. Nevertheless, as triatomines can survive hiding in house contents, the design of integrated vector control measures is recommended for Bolivian Chaco communities, including the impact of animal management in peridomicile in human's exposure to vector.