Long-term field performance of a polyester-based long-lasting insecticidal mosquito net in rural Uganda

Background: In order to evaluate whether criteria for LLIN field performance (phase III) set by the WHO Pesticide Evaluation Scheme are met, first and second generations of one of these products, PermaNet (R), a polyester net using the coating technology were tested. Methods: A randomized, double blinded study design was used comparing LLIN to conventionally treated nets and following LLIN for three years under regular household use in rural conditions. Primary outcome measures were deltamethrin residue and bioassay performance (60 minute knock-down and 24 hour mortality after a three minute exposure) using a strain of Anopheles gambiae s. s. sensitive to pyrethroid insecticides. Results: Baseline concentration of deltamethrin was within targets for all net types but was rapidly lost in conventionally treated nets and first generation PermaNet (R) with median of 0.7 and 2.5 mg/m(2) after six months respectively. In contrast, second generation PermaNet (R) retained insecticide well and had 41.5% of baseline dose after 36 months (28.7 mg/m(2)). Similarly, vector mortality and knockdown dropped to 18% and 70% respectively for first generation LLIN after six months but remained high (88.5% and 97.8% respectively) for second generation PermaNet (R) after 36 months of follow up at which time 90.0% of nets had either a knockdown rate >= 95% or mortality rate >= 80%. Conclusion: Second generation PermaNet r showed excellent results after three years of field use and fulfilled the WHOPES criteria for LLIN. Loss of insecticide on LLIN using coating technology under field conditions was far more influenced by factors associated with handling rather than washing.