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Saving Lives, Buying Time: Economics of Malaria Drugs in an Age of Resistance
quitoes. Knock-down resistance (kdr) is a separate resistance phenotype linked to a point mutation in sodium channels targeted by both pyrethroids and DDT. Although prevalent in A. gambiae in West Africa, kdr has not impaired ITN efficacy in the region (Sina and Aultman, 2001; Hemingway and Bates, 2003). In southern Africa, in contrast, the local vector A. funestus has acquired metabolic resistance to pyrethroids, rendering ITNs ineffective (Chandre et al., 1999; Brooke et al., 2001). A looming concern for the future is that A. gambiae in equatorial Africa will acquire the same metabolic resistance to pyrethroids seen in A. funestus in southern Africa. Currently, metabolic resistance to pyrethroids in A. gambiae is limited to focal areas of West Africa and Kenya (Ranson et al., 2002).
In coming years, strategies to decrease insecticide resistance may include rotations, mosaics, and mixtures of agricultural and environmental insecticides guided by mathematical models (Tabashnik, 1989). Until now, little field-testing of models has been conducted; however, with new biochemical and molecular field tools, large-scale trials of resistance management are feasible. Treating ITNs with two insecticides with differing mechanisms of action is another approach that may be implemented in the near future. In West Africa, bi-treated nets pairing pyrethroids with carbosulfan (a carbamate insecticide), or chlorpyrifos-methyl (an organophosphate insecticide) are currently under evaluation (Muller et al., 2002).
INSECTICIDE-TREATED BEDNETS AND INDOOR RESIDUAL SPRAYING
History of ITNs
More than two thousand years before Ronald Ross and Giovanni Battista Grassi showed that mosquitoes transmit malaria, human beings used nets to fend off night-biting insects. Mosquito nets appear in historical records from the Middle East to West Africa to Papua New Guinea (Lindsay and Gibson, 1988). The Greek writer Herodotus (484 - ?425 BC) described how Egyptians living in marshy lowlands protected themselves with fishing nets.
Every man there has a net which he uses in the daytime for fishing, but at night he finds another use for it: he drapes it over the bed … and then crawls in under and goes to sleep. Mosquitoes can bite through any cover or linen blanket … but they do not even try to bite through the net.