A new strategy for treating nets. Part I: formulation and dosage

The conventional dosages of pyrethroid insecticides on mosquito nets assume that nets will be retreated at 6-12 month intervals. However, dosage shoul d be related to washing of nets; if nets are only washed once or twice a ye ar, their dosage requirements will be different to those which are washed f ortnightly. A 'low-dose, frequent-wash' retreatment system might be technic ally more appropriate and more affordable where nets are washed frequently, as they are in Dar es Salaam. Moreover, for use as a domestic insecticide, water-based formulations of pyrethroid are preferable to the more commonly used emulsifiable concentrates (ECs). This paper reports laboratory evalua tions of three formulations (ECs, Flowable, CS) of three pyrethroids (delta methrin, lambdacyhalothrin, permethrin). insecticidal activity was tested u sing serial bioassays at a range of dosages using Anopheles gambiae. The wa ter-based formulations were no less effective than the ECs, even at the low est dosages. Nets treated with 3 mg/m(2) and then repeatedly washed and ret reated after each wash with either 3 mg/m(2) or 1 mg/m(2) were subjected to gas chromatography analysis. This showed that the amounts of pyrethroid in the nets accumulated rapidly over the first few wash-retreatment cycles an d then remained fairly stable over subsequent cycles. These nets gave consi stently high bioassay mortalities throughout the experiment, while the mort ality declined rapidly after several washes with the nets that were treated at 3 mg/m(2) but not retreated. Experimental huts were used to compare: th e effectiveness of these 2 net retreatment regimes and nets which were not retreated. Ail nets caused high mortality rates amongst Anopheles females, but had negligible effects on culicines; either in killing them or in preve nting feeding. Therefore use of a high 'loading' dose for initial treatment with lower 'maintenance' doses for retreatment may be preferable to ensure that net users promptly perceive the benefits of the insecticide against c ulicines.