Field effectiveness of microbial larvicides on mosquito larvae in malaria areas of Botswana and Zimbabwe

Abstract

BackgroundThe successful control of malaria vectors requires the control of both the larval and adult stages. The adult control methods through indoor residual spraying (IRS) and use of long-lasting insecticidal nets (LLINs) continue to be widely used with some high measure of success. Larval control methods are also being used by a number of National Malaria Control Programmes (NMCPs) with limited understanding of its contribution. Larval control might be needed in some areas to move from malaria control to elimination. This experimental study was conducted to assess the field effectiveness of winter larviciding on the larval stages of the mosquito in Botswana and Zimbabwe.MethodsTwo villages were selected in each of the two countries, one as an intervention and the other as the control. Water bodies in the intervention villages were treated using the commercial product VectoBac® WG (Valent BioSciences Corporation, IL, USA) containing the active ingredient Bacillus thuringiensis var. israelensis (Bti), a WHO recommended bio-larvicide, applied at a rate of 300 g per hectare. Random-effects Poisson regression was employed during data analysis to compare intervention with control sites with respect to larval counts.ResultsThe average marginal effect of larviciding on the mosquito larvae taking interaction with time (period) into account, was −1.94 (95% CI −2.42 to −1.46) with incidence rate ratio of 0.14, thus an 86% larval reduction attributable to the intervention for both countries combined. There was a 92% and 65% effect for Botswana and Zimbabwe respectively. The effect on the early larval and late stages was 77% (P < 0.001) and 91% (P < 0.001), respectively. Overall, intervention larval sampling points had five more larvae than the control at baseline and 26 less after 16 weeks. The effect on the different species also showed similar trends.Discussion/conclusionLarval control using Bti showed a high effect on the population of the mosquito larvae. The reduction of the early and late larval stages can lead to reduced adult mosquito emergence and low adult mosquito densities. Larviciding can be used to control mosquito vector population by suppressing the larval stages thereby reducing adult emergence and malaria risk.

Bacterial larvicides used for malaria vector control in sub-Saharan Africa: review of their effectiveness and operational feasibility

“…For publications involving multi-country studies, each country was counted towards the total, e.g. Kenya and Tanzania [94] and Botswana and Zimbabwe [79]. Note : Zaire: now The Democratic Republic of the Congo…”

“…funestus Not specified[85]Bobirwa, Botswana; Buhera, ZimbabweRuralJun 2015–Sep 2015; Aug 2012–Oct 2013 An. arabiensis Riverbed pools/drains, hoof prints[79, 81]Anseba, Gash-Barka, Debub and North Red Sea zones, EritreaRuralNot stated An. arabiensis Pools, canals, swamps, dams, stream bed pools, wells, ponds[88, 89]Chikhwawa, MalawiRuralMay 2016–April 2018Not statedNot stated[90]Nouna, Burkina FasoRural2013–2015 An.…”

Efficacy and persistence of long-lasting microbial larvicides against malaria vectors in western Kenya highlands

“…israelensis (Bti) and Bacillus sphaericus (Bs) can target both insecticide resistant and outdoor biting malaria vectors [ 15 – 17 ]. Several studies in Africa have demonstrated the effectiveness of these bacterial larvicides in granular, powder and tablet formulations in reducing the density of malaria vectors [ 15 , 18 – 27 ] and malaria transmission [ 23 , 28 ]. However, these formulations are of short duration of activity and hence require regular applications which are associated with high operational costs as compared to the longer-lasting chemical-based interventions; hence, their use has not been widely embraced by the national malaria control programs in sub-Saharan Africa [ 29 , 30 ].…”

Vector control for malaria elimination in Botswana: progress, gaps and opportunities

“…IRS, which is implemented in six malaria-endemic districts in Botswana is the main vector control intervention and is supplemented by LLINs (first introduced in 2010 through mass distribution) and larviciding with Bacillus thuringiensis serovar israelensis [ 11 , 29 ] (Table 1 ). Since 2013, larviciding is occasionally implemented in 3 districts whose water bodies are amenable for the intervention [ 6 , 29 ] and has been shown to reduce larval densities which could reduce malaria risk due to low adult emergence [ 20 , 44 ].…”

“…LSM is a population suppression technique that can be further classified into (i) habitat modification, (ii) habitat manipulation, (iii) biological control, and (iv) larviciding [16]. Larviciding with microbial larvicides has been shown to reduce vector populations in various settings [17][18][19][20][21]. This is achieved by killing mosquito larvae and pupae and/or getting rid of breeding sites, thereby reducing adult density and possibly the number of infective bites per person per year [22].…”