Maintenance of effective control of Simulium damnosum in the face of insecticide resistance.

The control of onchocerciasis through reduction of its vector Simulium damnosum depends on highly effective insecticide treatments directed against the larvae. As these treatments must be applied weekly over wide areas for many years, ideal conditions for the development of resistance have been created. Resistance to organophosphate compounds has occurred. To maintain effective vector control, an intense operational research effort has been necessary. Close and constant monitoring of susceptibility, combined with cytotaxonomic identification of the tested larvae, has revealed the pattern of spread of the resistance by migration and apparently by hybridization between populations. These studies have helped indicate when it might be possible to contain or eliminate a resistant population. Screening of all available appropriate compounds has led to the selection of replacement compounds in the microbial (Bacillus thuringiensis H14), pyrethroid (permethrin), and carbamate (carbosulfan) groups. Laboratory studies have revealed esterase and multi-function oxidase mechanisms for the existing organophosphate resistance. Cross-resistance tests have shown no cross-resistance to carbamate insecticides with these mechanisms, but negative correlations with most pyrethroids. These considerations, along with estimated risk of future resistance, effects on non-target organisms, and costs have led to a complex alternation pattern of insecticides, with Bacillus thuringiensis H14 being used in the dry season and a series of classical chemical compounds in the wet season. Through these measures, vector control remains a viable method of onchocerciasis control for the WHO Onchocerciasis Control Programme in West Africa.