The need for larval source management accompanying urban development projects in malaria endemic areas: a case study on Bioko Island

Abstract

Background: In 2017, several new housing districts were constructed on Bioko Island, Equatorial Guinea. This case study assessed the impact construction projects had on mosquito larval habitats and the effectiveness of larval source management in reducing malaria vector density within the surrounding area.

Methods: Anopheline larval presence was assessed at 11 new construction sites by the proportion of larval habitats containing Anopheline pupae and late instar larval stages. Bacillus thuringiensis israelensis (Bti) larvicide was applied weekly to nine locations for 30 weeks, while two locations received no larvicide and acted as controls. Adult mosquito density was monitored via human landing collections in adjacent communities of six construction sites, including the two control sites.

Results: The sites that received Bti had significantly lower observation rates of both pupae (3.2% vs. 18.0%; p < 0.001) and late instar Anopheles spp. mosquitoes (14.1 vs. 43.6%; p < 0.001) compared to the two untreated sites. Anopheles spp. accounted for 67% of mosquitoes collected with human landing collections and were captured at significantly lower levels in communities adjacent to treated construction sites compared to untreated sites (p < 0.001), with an estimated 38% reduction in human biting rate (IRR: 0.62, 95% CI IRR: 0.55, 0.69). Seven months after the start of the study, untreated sites were treated due to ethical concerns given results from treatment sties, necessitating immediate Bti application. The following week, the number of habitats, the proportion of larval sites with Anopheles spp. pupae, late instars, and adult biting rates in adjacent communities to these sites all decreased to comparable levels across all sites.

Conclusion: Findings suggest larval source management represents an effective intervention to suppress mosquito populations during infrastructure development. Incorporating larval source management into ongoing and planned construction initiatives represents an opportunity to fine tune vector control in response to anthropogenetic changes. Ideally, this should become standard practice in malaria-endemic regions in order to reduce viable mosquito habitats that are common by-products of construction.

Keywords: Anopheles; Construction; Larval habitats; Larval source management; Malaria.

Fig. 1

Map of Bioko Island, Equatorial Guinea, with study locations. A map of Bioko Island, Equatorial Guinea, is presented with the geographic relation to the region of West Central Africa and the African continent, along with the capital city of Malabo, major roadways, and administrative districts. The monitored construction sites are also presented, with red circles representing control sites (no larvicide) and blue circles representing intervention sites (larvicide applied)

Fig. 2

Total Number of Larval Habitats Identified and Rainfall. The number of potential larval habitats identified at the construction sites are presented with respect to study week (May 1, 2017, is week one), with 95% confidence intervals depicted in gray, and the total weekly rainfall in inches

Fig. 3

The proportion of habitats with pupae and larvae for Anopheles spp. mosquitoes. The proportion of potential habitats that contained mosquito pupae or late instar larvae are presented with respect to study week (May 1, 2017, is week one) for untreated (red, left side) and LSM treated (blue, right side) construction sites. Both the actual average proportions (black dots) and smoothed average (dotted line) are shown, along with a 95% confidence interval for the smoothed average (gray shading). The vertical purple dash line indicates when the untreated sites were treated as a public health precaution due to increased adult mosquito activity within the surrounding area