A new strategy for treating nets. Part 2: Users' perceptions of efficacy and washing practices and their implications for insecticide dosage

The conventional way to treat a mosquito net with pyrethroid insecticide is to apply a standard dosage every 6-12 months, and to avoid washing the net until just before retreatment. In some places, nets are normally washed mu ch more often than this, and it may then be more appropriate to apply small er amounts of insecticide after each wash. The choice of strategy must take into account not only biological effectiveness, but also users' perception s of this effectiveness and their net-washing habits. We used focus groups to compare users' responses to nets treated with different dosages and chem icals. One hundred current net users in urban Dares-Salaam were each given a net that had been pretreated either with permethrin (200 or 500 mg/m(2)), or with lambdacyhalothrin (3 or 15 mg/ml), or with water. Neither particip ants nor investigators knew which group had received which treatment. Focus group discussions were held after 2, 8 and 12 weeks. Participants greatly preferred treated nets. Low doses were perceived to be less effective, espe cially after 8 and 12 weeks. After 12 weeks most participants had washed th eir nets, despite requests to the contrary. Dirty nets were regarded as unh ealthy and socially unacceptable. Few participants experienced side-effects or expressed fears about the safety of treatment. We conclude that asking people to refrain from washing their nets is unrealistic. A 'low-dose frequ ent-treatment' strategy of insecticide application may be more appropriate in the long run. At first, however, low doses give perceptibly inferior pro tection. An initial high (loading) dose, followed by frequent lower (mainte nance) dosages, might solve this problem.