THE IMPACT OF PERMETHRIN-IMPREGNATED BEDNETS ON MALARIA VECTORS OF THE KENYAN COAST

The effects of introducing permethrin-impregnated bednets on local pop ulations of the malaria vector mosquitoes Anopheles funestus and the A n.gambiae complex was monitored during a randomized controlled trial a t Kilifi on the Kenyan coast. Pyrethrum spray collections inside 762 h ouseholds were conducted between May 1994 and April 1995 after the int roduction of bednets in half of the study area. All-night human bait c ollections were performed in two zones (one control and one interventi on) for two nights each month during the same period. PCR identificati ons of An.gambiae sensu lato showed that proportions of sibling specie s were An. gambiae sensu stricto > An.merus > An.arabiensis. Indoor-re sting densities of An.gambiae s.l. and the proportion of engorged fema les decreased significantly in intervention zones as compared to contr ol zones. However, the human blood index and Plasmodium falciparum spo rozoite rate remained unaffected. Also vector parous rates were unalte red by the intervention, implying that survival rates of malaria vecto rs were not affected. The human-biting density of An.gambiae s.l., the predominant vector, was consistently higher in the intervention zone compared to the control zone, but showed 8% reduction compared to pre- intervention biting rates - versus. 94% increase in the control zone. Bioassay, susceptibility and high-performance liquid chromatography re sults all indicated that the permethrin content applied to the nets wa s sufficient to maintain high mortality of susceptible vectors through out the trial. Increased rates of early outdoor-biting, as opposed to indoor-biting later during the night, were behavioural or vector compo sition changes associated with this intervention, which would require further monitoring during control programmes employing insecticide-tre ated bednets.