Research
Influence of environmental factors on the abundance of Anopheles farauti larvae in large brackish water streams in Northern Guadalcanal, Solomon Islands
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* Corresponding author: Cheng-Chen Chen mosquito@ym.edu.tw
1 Institute of Tropical Medicine, National Yang-Ming University, No. 155, Sec.2, Li-Nong Street, Taipei 112, Taiwan
2 National Vector Borne Disease Control Programme, Ministry of Health, Honiara, Solomon Islands
3 Malaria, Other Vector-Borne and Parasitic Diseases, Regional Office for the Western Pacific, World Health Organization, San Lazaro Hospital Compound, Manila, Philippines
4 Pacific malaria Initiative Support Center, Australian Center for Tropical and International Health, School of Population Health, The University of Queensland, Herston, 4006, Australia
5 Australian Army Malaria Institute, Gallipoli Barracks, Enoggera, 4052, Australia
6 Research Centre for Biodiversity, Academia Sinica, 128 Sec 2, Academia Road, Taipei 115, Taiwan
Malaria Journal 2011, 10:262 doi:10.1186/1475-2875-10-262
Published: 13 September 2011Abstract
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).