Species Shifts in the Anopheles gambiae Complex: Do LLINs Successfully Control Anopheles arabiensis?

Abstract

IntroductionHigh coverage of conventional and long-lasting insecticide treated nets (ITNs and LLINs) in parts of E Africa are associated with reductions in local malaria burdens. Shifts in malaria vector species ratio have coincided with the scale-up suggesting that some species are being controlled by ITNs/LLINs better than others.MethodsBetween 2005–2006 six experimental hut trials of ITNs and LLINs were conducted in parallel at two field stations in northeastern Tanzania; the first station was in Lower Moshi Rice Irrigation Zone, an area where An. arabiensis predominates, and the second was in coastal Muheza, where An. gambiae and An. funestus predominate. Five pyrethroids and one carbamate insecticide were evaluated on nets in terms of insecticide-induced mortality, blood-feeding inhibition and exiting rates.ResultsIn the experimental hut trials mortality of An. arabiensis was consistently lower than that of An. gambiae and An. funestus. The mortality rates in trials with pyrethroid-treated nets ranged from 25–52% for An. arabiensis, 63–88% for An. gambiae s.s. and 53–78% for An. funestus. All pyrethroid-treated nets provided considerable protection for the occupants, despite being deliberately holed, with blood-feeding inhibition (percentage reduction in biting rates) being consistent between species. Veranda exiting rates did not differ between species. Percentage mortality of mosquitoes tested in cone bioassays on netting was similar for An. gambiae and An. arabiensis.ConclusionsLLINs and ITNs treated with pyrethroids were more effective at killing An. gambiae and An. funestus than An. arabiensis. This could be a major contributing factor to the species shifts observed in East Africa following scale up of LLINs. With continued expansion of LLIN coverage in Africa An. arabiensis is likely to remain responsible for residual malaria transmission, and species shifts might be reported over larger areas. Supplementary control measures to LLINs may be necessary to control this vector species.

Malaria vector species composition and relative abundance in Mutare and Mutasa districts, Zimbabwe

“…gambiae and An. arabiensis populations (Kitau et al, 2012). A similar change in species composition and abundance was reported during the implementation of IRS in Zimbabwe; example being the observation of An.…”

“…In general, species replacement for various reasons, especially as a result of IRS and LLINs is not a new phenomenon (Kitau et al, 2012). Recent data from East Africa showed changes in sibling species following the scaling-up of ITNs/LLINs, with An.…”

A second chance to tackle African malaria vector mosquitoes that avoid houses and don't take drugs

“…1 Recent studies of residual transmission systems where highly endophagic mosquitoes have been effectively tackled by IRS or LLINs remind us that behavioral avoidance of indoor insecticide exposure, by such a naturally evasive species as An. arabiensis, 6,7,34,35 does not necessarily represent failure, or even waning impact, of these measures but rather their inherent limitations from the outset. 18,[31][32][33][36][37][38] Recent reports of surprising 39 and previously undescribed 40 primary vectors most probably does not reflect the emergence of new vector systems but rather that their role in transmission is now more obvious because they are no longer outnumbered by An.…”

Comprehensive Sterilization of Malaria Vectors Using Pyriproxyfen: A Step Closer to Malaria Elimination

“…New paradigms in vector control are in great demand, especially for vectors such as An. arabiensis 4,15 and other anophelines 16 that exhibit flexibility in feeding and resting indoors and outdoors and minimize their contact with conventional adulticides applied indoors. The findings reported here have limitations given that the experiments were conducted within an enclosed environment on insectary-reared mosquitoes that had never been subjected to insecticide pressure.…”