Identifying the most productive breeding sites for malaria mosquitoes in The Gambia

Ulrike Fillinger,Emiel Van Loon,Silas Majambere,Heleen Sombroek,Willem Takken,Steven W Lindsay

doi:10.1186/1475-2875-8-62

Abstract

BackgroundIdeally larval control activities should be targeted at sites that generate the most adult vectors, thereby reducing operational costs. Despite the plethora of potential mosquito breeding sites found in the floodplains of the Gambia River, about 150 km from its mouth, during the rainy season, only a small proportion are colonized by anophelines on any day. This study aimed to determine the characteristics of larval habitats most frequently and most densely populated by anopheline larvae and to estimate the numbers of adults produced in different habitats.MethodsA case-control design was used to identify characteristics of sites with or without mosquitoes. Sites were surveyed for their physical water properties and invertebrate fauna. The characteristics of 83 sites with anopheline larvae (cases) and 75 sites without (controls) were collected between June and November 2005. Weekly adult productivity was estimated with emergence traps in water-bodies commonly containing larvae.ResultsThe presence of anopheline larvae was associated with high invertebrate diversity (Odds Ratio, OR 11.69, 95% CI 5.61–24.34, p < 0.001), the presence of emergent vegetation (OR 2.83, 95% CI 1.35–5.95, p = 0.006), and algae (at borderline significance; OR 1.87, 95% CI 0.96–3.618, p = 0.065). The density of larvae was reduced in sites that were larger than 100 m in perimeter (OR 0.151; 95% CI 0.060–0.381, p < 0.001), where water was tidal (OR 0.232; 95% CI 0.101–0.533, p = 0.001), vegetation shaded over 25% of the habitat (OR 0.352; 95% CI 0.136–0.911, p = 0.031) and water conductivity was above 2,000 μS/cm (OR 0.458; 95% CI 0.220–0.990, p = 0.048). Pools produced the highest numbers of Anopheles gambiae adults compared with rice fields, floodwater areas close to the edge of the floodplain or close to the river, and stream fringes. Pools were characterized by high water temperature and turbidity, low conductivity, increased presence of algae, and absence of tidal water.ConclusionThere are few breeding sites that produce a high number of adult vectors in the middle reaches of the river in The Gambia, whereas those with low productivity are larger in area and can be found throughout the rainy season. Even though risk factors could be identified for the presence and density of larvae and productivity of habitats, the results indicate that anti-larval interventions in this area of The Gambia cannot be targeted in space or time during the rainy season.

Highlights

Larval control activities should be targeted at sites that generate the most adult vectors, thereby reducing operational costs
More recently larval control has been shown to be highly effective at reducing malaria transmission in Eritrea [8,9], Kenya [10,11] and Tanzania [12,13]
In The Gambia, the landscape is dominated by the river and its floodplains dividing the small country into the river's north and south bank

Summary

Introduction

Larval control activities should be targeted at sites that generate the most adult vectors, thereby reducing operational costs. More recently larval control has been shown to be highly effective at reducing malaria transmission in Eritrea [8,9], Kenya [10,11] and Tanzania [12,13]. These studies were all carried out in sites with well-defined breeding sites that were not too extensive. The work presented here is part of a series of studies recently undertaken in the The Gambia to determine whether larviciding with microbials [14] can reduce malaria transmission in an area of extensive seasonal flooding and to design rational application strategies for operational implementation. Malaria prevalence in children ranges between 5–90% [16,17] and the average entomological inoculation rate (EIR) between 0–150 infective bites per person per year [18]

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