Entomological evaluation of three brands of manufactured insecticidal nets and of nets conventionally treated with deltamethrin, after repeated washing

The present study evaluated the efficacy and wash resistance of three types of commercial, deltamethrin-treated nets (PermaNetTM, YorkoolTM and A-Z nets) — that their manufacturers claimed to be long-lasting insecticidal nets (LLIN) — and those of nets conventionally treated with deltamethrin, at either 23–27 mg/m2 (using one K-O Tab® tablet/net) or 46–54 mg/m2 (using two K-O Tab tablets/net). The nets were tested unwashed or after being washed, by hand or machine, five or 15 times. After each wash, the nets were dried vertically on a line, in the shade, for 8 h. Insecticidal activity was assessed, using two types of bio-assay and wild-caught female Anopheles stephensi, as mean median knock-down times and as mortality 24 h after a 3-min exposure.For each type of nets tested, the insecticidal activity of the unwashed nets was greater than that of the nets washed five washes, and that of the nets washed five times was greater than that of the nets washed 15 times, with the latter decline considerably greater than the former. The pattern of decrease, however, differed with net type. The PermaNet and A-Z nets generally had the highest insecticidal activity, and, overall, the Yorkool nets did not perform significantly better than the conventional nets treated with 23–27 mg deltamethrin/m2.Although washing with detergents can clearly remove insecticide from the fibres of all types of treated nets, the consequent loss of activity varies with net type. After 15 washes, the PermaNet nets showed higher insecticidal activity than any other net type but there is still scope for the manufacturer of the PermaNet nets to improve wash resistance.