Larval habitats of the Anopheles farauti and Anopheles lungae complexes in the Solomon Islands

Abstract

BackgroundThere is an urgent need for vector control tools to supplement long-lasting insecticidal nets (LLINs) and indoor residual spraying; particularly in the Solomon Islands where the primary vector, Anopheles farauti, is highly anthropophagic and feeds mainly outdoors and early in the evening. Currently, the only supplementary tool recommended by the World Health Organization is larval source management (LSM). The feasibility and potential effectiveness of LSM requires information on the distribution of anophelines, the productivity of larval habitats and the potential impacts of larval control on adult fitness.MethodsThe distribution of anophelines in Central and Western Provinces in the Solomon Islands was mapped from cross-sectional larval habitat surveys. The composition and micro-distribution of larval instars within a large permanent river-mouth lagoon was examined with a longitudinal survey. Density-dependent regulation of An. farauti larvae was investigated by longitudinally following the development and survival of different densities of first instars in floating cages in a river-mouth lagoon.ResultsFive anopheline species were molecularly identified from a range of fresh and brackish water habitats: An. farauti s.s., An. hinesorum, An. lungae, An. nataliae and An. solomonis. The most common habitats used by the primary malaria vector, An. farauti, were coastal lagoons and swamps. In the detailed study of lagoon micro-productivity, An. farauti was non-uniformly distributed with highest densities found at collections sites most proximal and distal to the mouth of the lagoon. The survival of An. farauti larvae was more than twofold lower when larvae were held at the highest experimental density (1 larva per 3.8 cm2) when compared with the lowest density (1 larva per 38 cm2).ConclusionsThe only documented major malaria vector collected in larval surveys in both Central and Western Provinces was An. farauti. Lagoons and swamps, the most common, largest and (potentially) most productive larval sites of this malaria vector, were “few, fixed and findable” and theoretically, therefore, amenable to successful LSM. However, the immense scale and complexity of these ecosystems in which An. farauti larvae are found raises questions regarding the ability to effectively control the larvae, as incomplete larviciding could trigger density dependent effects resulting in increased larval survivorship. While LSM has the potential to significantly contribute to malaria control of this early and outdoor biting vector, more information on the distribution of larvae within these extensive habitats is required to maximize the effectiveness of LSM.

Mapping a Plasmodium transmission spatial suitability index in Solomon Islands: a malaria monitoring and control tool

“…The study by Russell et al [ 9 ], which sampled larvae in Central Province and Western Province in Solomon Islands from December 2011 to December 2012 was better adapted to validation of the TSI model, as it reduced the false-absence bias by repeated monthly sampling over 10 consecutive days in five sites in the north side of a lagoon and also monitored larval density [ 26 ].…”

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Fig. 6 Anopheles farauti presence and absence comparison TSI means test for Central Province and Western Province, from Russell et al [ 9 ], and for Guadalcanal, from Beebe et al [ 15 ]. Mean and confidence intervals (95%) are in black, boxplot are in colour, dots are representing TSI values for each sample, note that these values have been jittered (random noise added around the integer values to avoid overplotting)

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“… Based on a total of 52 collections sites in Guadalcanal Province from Beebe et al [ 15 ] for the year 1997 and of a total of 84 collection sites in Central Province and Western Province (CP and WP) from Russell et al [ 9 ] …”

“…More recently, in Central and Western provinces, a large larval study showed that most common habitats of An. farauti larvae were coastal lagoons and swamps [ 9 ]. The most productive sites were few, fixed and findable making them amenable for larval source management.…”

Malaria early warning tool: linking inter-annual climate and malaria variability in northern Guadalcanal, Solomon Islands

“…Preliminary research and discussions with local control authorities suggest areas of northern Makira and Central Province of the Solomon Islands possibly exhibit similar habitat characteristics to northern Guadalcanal. Entomological studies recently conducted in Central Province have investigated coastal larval habitats where water flow into the ocean was blocked by a sand mouth [ 53 ]. This deserves further investigation.…”