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

Abstract

BackgroundIn 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).MethodsEntomological 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.ResultsFive 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.ConclusionThe 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.

A low technology emanator treated with the volatile pyrethroid transfluthrin confers long term protection against outdoor biting vectors of lymphatic filariasis, arboviruses and malaria

“…Across much of the tropics, two or more mosquito-borne pathogens are co-endemic and are often transmitted by the same vectors [1]. These tools are effective against anthropophilic (prefer feeding on humans), endophagic (feed indoors) and endophilic (rest indoors) mosquitoes [2,3] but are less effective against exophagic (feed outdoors) and exophilic (rest outdoors) mosquitoes [4]. A wide diversity of outdoor-biting mosquitoes mediate transmission of pathogens that cause filariasis, dengue, yellow fever and malaria, frustrating efforts to eliminate them with LLINs and IRS [5].…”

Why Use of Interventions Targeting Outdoor Biting Mosquitoes will be Necessary to Achieve Malaria Elimination

“…It has been shown both theoretically (Killeen and Smith, 2007; Killeen et al, 2007) and in the course of operational control (Hawley et al, 2003; Klinkenberg et al, 2010) that significant community-wide reductions in transmission can be obtained even when intervention coverage levels are modest (35–75%). However, it is unlikely that these packages of interventions of their own will be sufficient to achieve malaria elimination in the most endemic settings where transmission rates are extremely high (Gillies and Smith, 1960; White, 1969; Oyewole and Awolola, 2006; Bayoh et al, 2010; Van Bortel et al, 2010; Bugoro et al, 2011a; Reddy et al, 2011; Russell et al, 2011). Even should the considerable financial, logistic, and behavioral obstacles that currently limit attainment of 100% coverage be overcome (Vanden et al, 2010; Larson et al, 2012), the combined use of effective anti-malarial drugs and these vector control interventions are not predicted to be sufficient for elimination in these settings (Killeen et al, 2000; Griffin et al, 2010) Because they do not cover the full spectrum of all locations where mosquito exposure occurs, and even if only a small percentage of mosquitoes remain and bite outside, their existence could be enough to prevent the transition from very low to zero transmission.…”

“…Achieving this goal will require full understanding of where and when persons are most exposed to the bites of mosquito vectors in order to target interventions where they can achieve maximum impact. While elimination is possible in some settings with low malaria transmission intensity (WHO, 2009; Griffin et al, 2010), and where the dominant vectors exhibit the stereotypical behaviors of biting indoors and late at night where they can be targeted by LLIN and/or IRS (Mabaso et al, 2004; Sharp et al, 2007b; John et al, 2009; WHO, 2009), it is unlikely that these methods will be sufficient to push prevalence below the WHO-defined pre-elimination threshold (<1 case/1000 population/year) in areas of high transmission (Molineaux and Gramiccia, 1980; Kleinschmeidt et al, 2009; Russell et al, 2010) and where the majority of human exposure to transmitting mosquitoes occurs outside human dwellings to which most current interventions are restricted (Taylor, 1975; Pates and Curtis, 2005; Tirados et al, 2005; Oyewole and Awolola, 2006; Geissbühler et al, 2007; Griffin et al, 2010; Van Bortel et al, 2010; Bugoro et al, 2011a; Yohannes and Boelee, 2012). To date, probably the most comprehensive attempt to achieve local elimination within an endemic region of Africa was made in the Garki region of northern Nigeria in the 1970’s (Molineaux and Gramiccia, 1980; WHO, 2008b).…”

“…Even low levels of exophagy, exophily, or zoophagy (Table 1) may substantially attenuate the impact of LLIN and IRS because this allows mosquitoes to obtain blood while avoiding fatal contact with insecticides (Smith and Gillies, 1960; Taylor, 1975; Molineaux and Gramiccia, 1980; Pates and Curtis, 2005; Geissbühler et al, 2007; Killeen and Smith, 2007; Govella et al, 2010; Bugoro et al, 2011a). Furthermore, the deployment of insecticides inside houses is likely to place strong selection on even highly endophilic species to switch their behaviors, which could not only prevent the achievement of elimination but undermine the continued effectiveness of these interventions (White, 1974; Pates and Curtis, 2005; Van Bortel et al, 2010; Bugoro et al, 2011a).…”

Roles and challenges of construction firms and public health entomologists in ending indoor malaria transmission in African setting

“…complex [4]. The use of LLINs and IRS indoors has reduced vectors house entry behaviour and caused species shifts due to indoor insecticidal pressure [5, 6], shifts to early-evening or early-morning biting [7]; shifts to exophagy [6, 8]; shifts to zoophily [9] and shifts to exophily [10]. More efforts are needed to address restriction of vector house entry behaviour and zero indoor malaria transmission.…”