Resistance to DDT was first noted just 10 years after its introduction (WHO, 1957). As DDT resistance spread, the faster-acting pyrethroids were introduced; this class of insecticides replaced DDT in many malaria control programs. As pyrethroid resistance started to develop, many control programs attempted to revert back to DDT. However, because these insecticide classes share a common target site (Soderlund and Bloomquist, 1989), the sodium channel and cross-resistance had developed to both insecticide classes in many locations (Martinez-Torres et al., 1998; Ranson et al., 2000). The spread of pyrethroid resistance may be critical for sustainability of ITNs, as this is currently the only insecticide group recommended for net impregnation. Figure 2-11 shows insecticide resistance in Africa between 1950 and 2006.

The economics of developing, registering, and marketing insecticides means that new insecticides are primarily developed for large agricultural markets. Some of these insecticides eventually cross over into the public health arena, which means that vectors breeding in agricultural areas may have previously been

FIGURE 2-11 Resistance in Africa, 1950-2006.

SOURCE: Reprinted from Coleman et al. (2006), with permission from the Entomological Society of America. Copyright 2006.

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