Changes in vector species composition and current vector biology and behaviour will favour malaria elimination in Santa Isabel Province, Solomon Islands

Abstract

Background: In 2009, Santa Isabel Province in the Solomon Islands embarked on a malaria elimination programme. However, very little is known in the Province about the anopheline fauna, which species are vectors, their bionomics and how they may respond to intensified intervention measures. The purpose of this study was to provide baseline data on the malaria vectors and to ascertain the possibility of successfully eliminating malaria using the existing conventional vector control measures, such as indoor residual spraying (IRS) and long-lasting insecticidal nets (LLIN).

Methods: Entomological surveys were undertaken during October 2009. To determine species composition and distribution larval surveys were conducted across on the whole island. For malaria transmission studies, adult anophelines were sampled using human landing catches from two villages - one coastal and one inland.

Results: Five Anopheles species were found on Santa Isabel: Anopheles farauti, Anopheles hinesorum, Anopheles lungae, Anopheles solomonis, and Anopheles nataliae. Anopheles hinesorum was the most widespread species. Anopheles farauti was abundant, but found only on the coast. Anopheles punctulatus and Anopheles koliensis were not found. Anopheles farauti was the only species found biting in the coastal village, it was incriminated as a vector in this study; it fed early in the night but equally so indoors and outdoors, and had a low survival rate. Anopheles solomonis was the main species biting humans in the inland village, it was extremely exophagic, with low survival rates, and readily fed on pigs.

Conclusion: The disappearance of the two major vectors, An. punctulatus and An. koliensis, from Santa Isabel and the predominance of An. hinesorum, a non-vector species may facilitate malaria elimination measures. Anopheles farauti was identified as the main coastal vector with An. solomonis as a possible inland vector. The behaviour of An. solomonis is novel as it has not been previously found biting humans in any numbers. Both species appear to be short-lived, a characteristic that will limit their transmission potential. The early night feeding behaviour and a degree of outdoor biting seen in An. farauti and particularly in An. solomonis will require that their response to IRS and LLIN be closely monitored. In coastal villages, where large, favourable breeding sites allow for high numbers of An. farauti may require the addition of larval control to achieve elimination.

Figure 1

Internal transcribed spacer 2 amplification products from: Anopheles farauti (f1), Anopheles hinesorum (f2), Anopheles nataliae (nat), Anopheles lungae (lun), and Anopheles solomonis (sol), digested with Msp I and run on a 3% agarose gel with a 100 base pair ladder.

Figure 2

Speciation and distribution of anopheline fauna on Santa Isabel based on larval surveys conducted in October 2009. Insert is the map of the Southwest Pacific.

Figure 3

Typical transient ground pools, in this case wheel ruts - small, turbid, with clay substrate, and devoid of any flora or fauna. In the past these sites were used exclusively by Anopheles punctulatus, on Santa Isabel they are now being used exclusively by Anopheles hinesorum.