When the bioefficacy was also measured, using the standard WHO cone bioassay method, mortalities of An. gambiae also varied with the number of washes, net brand and washing method. When the residual insecticide contents were compared using statistical procedures, it was also found that, the four drying regimens and the three washing procedures were significantly different and each net brand responded different to each drying and washing method. Nets that were washed by beating on rocks lost insecticide more rapidly and were generally less effective compared to nets washed by hand rubbing and by machine. The finding of the current study suggests that the standard WHOPES washing protocol of using specific machines and soap might be underestimates the actual insecticide loss on nets and overestimating their efficacy and durability under real field conditions where abrasive washing procedures are more abrasive. For example, after the 20th wash, none of the nets achieved >80% mortality on the susceptible strain of An. Gambiae ss by all the three washing methods. There are very few studies that have evaluated the effect of different drying regimens on the above LLINs, two of which have been recently recommended as LLINs.
The current study attempted to do so. In some unrelated earlier studies, it had been shown that direct sunlight was harmful to pyrethroids based insecticides because UV rays breaks down pyrethrin molecules thereby rendering the insecticide infective. On the basis of this earlier evidence, WHO recommended placing OlysetR in sealed polythene bags and leaving them for a few hours in the sun after washing. The idea was to facilitate insecticide migration to the surface by heating the net at the same time protecting it from uv rays. This recommendation was arbitrary withdrawn because it was cumbersome and the manufacturers of OlysetR claimed that the incorporation of special binders, will, together with high temperatures in the tropics, facilitated automatic insecticide migration to the surface at the same time stabilize them against the effect of UV rays. In the current study it was observed that overall, nets that were dried hanging on line under the shade, generally retained more insecticide. Individually Olyset and BASF brands of LLINs retained more insecticide when dried under the shade. While, the insecticide loss by drying PermaNet and TNT using the four regimens were not statistically different It would have been expected that the trend in insecticide retention by BASF and PermaNet, both of which are polyester based would be the same while that one of TNT and Olyset would also be the same, but this was not observed in the current study.
The role played by sunlight in enhancing or rendering the pyrethrins ineffective still remains, controversial. Two studies conducted recently in India, showed that exposing OlysetR to sunlight for a few hours enhanced efficacy.