Influence of environmental factors on the abundance of Anopheles farauti larvae in large brackish water streams in Northern Guadalcanal, Solomon Islands

Abstract

Background: The main vector of malaria in Solomon Islands is Anopheles farauti, which has a mainly coastal distribution. In Northern Guadalcanal, Solomon Islands, high densities of An. farauti are supported by large brackish streams, which in the dry season are dammed by localized sand migration. The factors controlling the high larval productivity of these breeding sites have not been identified. Accordingly the influence of environmental factors on the presence and density of An. farauti larvae was assessed in three large naturally dammed streams.

Methods: Larval sites were mapped and anopheline larvae were collected monthly for 12 months (July 2007 to June 2008) from three streams using standard dippers. Larval collections were made from 10 locations spaced at 50 m intervals along the edge of each stream starting from the coast. At each collection point, floating filamentous algae, aquatic emergent plants, sun exposure, and salinity were measured. These environmental parameters along with rainfall were correlated with larval presence and density.

Results: The presence and abundance of An. farauti larvae varied between streams and was influenced by the month of collection, and distance from the ocean (p <0.001). Larvae were more frequently present and more abundant within 50 m of the ocean during the dry season when the streams were dammed. The presence and density of larvae were positively associated with aquatic emergent plants (presence: p = 0.049; density: p = 0.001). Although filamentous algae did not influence the presence of larvae, this factor did significantly influence the density of larvae (p < 0.001). Rainfall for the month prior to sampling was negatively associated with both larval presence and abundance (p < 0.001), as high rainfall flushed larvae from the streams. Salinity significantly influenced both the presence (p = 0.002) and density (p = 0.014) of larvae, with larvae being most present and abundant in brackish water at < 10‰ seawater.

Conclusion: This study has demonstrated that the presence and abundance An. farauti larvae are influenced by environmental factors within the large streams. Understanding these parameters will allow for targeted cost effective implementation of source reduction and larviciding to support the frontline malaria control measures i.e. indoor residual spraying (IRS) and distribution of long-lasting insecticidal nets (LLINs).

Figure 1

Map of the study sites. (A) Map of the Solomon Islands indicating the study area on Guadalcanal province. (B) North Guadalcanal showing the three study sites. (C, D, E) The relative locations of each of the dammed brackish water stream and surrounding villages.

Figure 2

Typical dammed brackish water stream. (A) Lateral view of the dammed stream separated from the sea by sand bar. (B) Showing dammed stream. (C) Floating filamentous algae on dammed brackish water stream margin as indicated by arrow.

Figure 3

A temporal and spatial comparison of mean larval mosquito density (A, C; mean ± SE) and the proportion of sites containing An. farauti larvae (B, D).

Figure 4

A temporal and spatial comparison of the environmental factors recorded the study streams: filamentous algae (A, E; mean ± SE), emergent aquatic plants (B, F; mean ± SE), salinity (C, G; mean ± SE) and rainfall (D; monthly total).

Figure 5

Correlations between larval An. farauti presence and the 6 environmental factors in the study streams: filamentous algae, emergent aquatic plants, current rainfall, rainfall lagged by one month, salinity and sun exposure. The factors with a pink top-panel were significantly associated with An. farauti presence (see Table 2).

Figure 6

Correlations between larval An. farauti density and the 6 environmental factors in dammed brackish water streams: filamentous algae, emergent aquatic plants, current rainfall, rainfall lagged by one month, salinity and sun exposure. The factors with a pink top-panel were significantly associated with An. farauti density (see Table 2).