Reduction of malaria vector mosquitoes in a large-scale intervention trial in rural Burkina Faso using Bti based larval source management

Abstract

Background: Malaria remains one of the most important causes of morbidity and death in sub-Saharan Africa. Along with early diagnosis and treatment of malaria cases and intermittent preventive treatment in pregnancy (IPTp), vector control is an important tool in the reduction of new cases. Alongside the use of long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS), targeting the vector larvae with biological larvicides, such as Bacillus thuringiensis israelensis (Bti) is gaining importance as a means of reducing the number of mosquito larvae before they emerge to their adult stage. This study presents data corroborating the entomological impact of such an intervention in a rural African environment.

Methods: The study extended over 2 years and researched the impact of biological larviciding with Bti on malaria mosquitoes that were caught indoors and outdoors of houses using light traps. The achieved reductions in female Anopheles mosquitoes were calculated for two different larviciding choices using a regression model.

Results: In villages that received selective treatment of the most productive breeding sites, the number of female Anopheles spp. dropped by 61% (95% CI 54-66%) compared to the pre-intervention period. In villages in which all breeding sites were treated, the number of female Anopheles spp. was reduced by 70% (95% CI 64-74%) compared to the pre-intervention period.

Conclusion: It was shown that malaria vector abundance can be dramatically reduced through larviciding of breeding habitats and that, in many geographical settings, they are a viable addition to current malaria control measures.

Keywords: Anopheles; Bacillus thuringiensis israelensis; Burkina Faso; Larval source management; Vector control.

Fig. 1

On the left: Spraying of mosquito breeding sites with Bti WG by trained members of the community using knapsack sprayers. On the right: CDC light trap being checked before installation within a rural village by an entomological technician

Fig. 2

Study villages are shown with blue dots; bars indicate mosquito density in villages where mosquito captures took place. Bars show the average numbers of female Anopheles mosquitoes captured per trap per night indoors and outdoors using CDC light traps in September and October 2013 and 2014. Colours indicate treatment choice (Green = full treatment, orange = selective treatment, red = untreated control group). Closed lines encompass clusters of villages receiving the same treatment. In 2014, 9 additional villages were added to the mosquito collections

Fig. 3

Point estimates of the regression model for the intervention year compared to the baseline year indicating the reduction in the count of indoor and outdoor female Anopheles mosquitoes per night per trap achieved through guided or full Bti treatment. The reference line represents the rate ratio value under the null hypothesis: i.e. the count of female Anopheles mosquitoes in the control areas receiving no Bti treatment are not different from the counts in areas receiving guided or full Bti treatment

Fig. 4

Average numbers of female Anopheles mosquitoes per trap per night caught a Indoors and b outdoors during the successive sampling rounds of the study period. The different Bti treatments are colour coded. The duration of performed larviciding is indicated with gray bars