The current study also observed that Olyset generally retained more insecticide when it was dried in direct sunlight or hanging under the shade compared to other nets. The high insecticide retention by OlysetR netting recorded in this study and its unavailability on the surface to cause corresponding mosquito mortality suggests that this requirement is still very important. This observation is supported by other studies which have documented that heat accelerates the rate of migration of permethrin molecules in the fibers, thereby increasing the bioavailability of insecticide on the net surface, especially if they are polyethylene based. In one such study OlysetR and PermaNetR were equally exposed to a controlled temperature of 30 and 60°C for 4 hrs after repeated washing, OlysetR, regenerated at 60°C to achieve >90% of mosquitoes exposed and not PermaNetR. The current study also observed that even, though PermaNet retained a higher amount of insecticide when dried hanging under the shade compared to other drying regimens, the differences were not statistically significant. The same trend was observed on TNT. In contrast, there were differences in insecticide loss when BSF was air-dried hanging under the shade compared when it was dried hanging in direct sunlight. The findings of the current study, concurs with the results of a similar study conducted in Iran. In the Iran study, Kayedi and others showed that drying PermaNet in direct sunlight for extended period of time of more than 3 hours was harmful. In the current study the four brands of LLINs used were left to dry for a specific period of 4 h.
The current study also compared the bio-efficacy of the above LLINs after repeated washing using WHOPES washing protocol and two local methods of hand rubbing and beating on rocks. The study found that in general nets that were washed using the WHOPES protocol remained effective longer than nets that were washed using local methods. When the differences were modeled by net type and washing method, it was observed that Olyset, PermaNet and BASF performed better when hand washed compared to machine wash. TNT and PermaNet performed better when washed by machine compared to washing on rocks. There was no difference between washing Olyset on rock and machine. The finding of the current study on some of the nets evaluated concurs with the results of a recent study by Sreehari. The study, compared the bio-efficacy of three LLINs: PermaNetR, OlysetR and K-O Tab 1, 2, 3R against An. culicifacies and An. stephensi after repeated washing using machine and hand wash and found that all the three LLINs that were evaluated, retained a significant amount of insecticide after repeated washing by both machine and hand. The study concluded that, nets washed by hand remained effective longer than nets washed by machine. In the current study it was observed that generally, PermaNetR retained its efficacy longer with successive washes using all the three washing methods compared to other three brands of LLINs, but hand washing performed better on this net than machine washing while washing on rocks performed worst. This finding concurs with other studies conducted elsewhere, for example, in Iran Kayedi et al after evaluating wash resistance of 3 brands after repeated washing found that PermaNetR was more effective. Elsewhere Gimnig et al also compared wash resistant and biological activity of five brands of long lasting insecticide treated nets; (PermaNetR, OlysetR, DawaR, InsectorR and AthanorR, with a conventionally treated net under laboratory, and found that PermaNetR was the most biologically effective while OlysetR was the most wash-resistant but biologically least biologically effective. The rest of the nets exhibited wide variation in insecticide retention and biological activity. There are no comparable results for the two newer net brands that were evaluated, (BASF and TNT). But their performances in the current study suggest that they are significantly different in efficacy and durability compared to Olyset and PermaNet. The findings that nets dried spread on the ground generally lost more insecticide and were less effective compared to other drying regimens has major implication on LLINs longterm usage in the rural villages because it is the most preferred drying method.
The results suggest that contact between wet net and ground accelerated the rate of insecticide loss from net surfaces. It is possible that insecticide molecules might be migrating from inside the netting to the ground or being denatured by exposure to sunlight. In the current study nets were left in place to dry for a defined period of four hours. In rural areas, it is a normal practice for washed nets to be left outdoor drying for an extended period, sometimes for the whole day. This means that LLINs used in the rural villages might be loosing insecticide at a faster rate depending on the washing frequency and subsequent drying using this regimen. It is not possible to standardise washing procedures in the field, but there is need to educate rural people on the need to adopt gentle washing on LLINs. The washing of LLINs by beating on hard surfaces such as rocks is a commonly used procedure in the rural areas. This method has not been widely evaluated and there are no comparable studies. The use of this washing procedure is commonly applied on white nettings that have accumulated a lot of dirt. The use of alternative colours for LLINs can indirectly minimise the use of this abrasive method. There is need therefore, to conduct more research and document the effect of this washing method on the long-term use of LLINs. The newer LLINs; BASFR and TNTR, whose protective performance was found to be better than OlysetR and lower than PermaNet R have a greater potential of becoming important tools in malaria vector control programmes. These products have not yet undergone extensive evaluation, hence the need for more studies on these products.
Conclusion
The current study has demonstrated that LLINs that were washed and dried hanging on line under the shade generally, retained more insecticide and remained effective longer than those that were dried using other regimens. The rate of insecticide loss and subsequent reduction in efficacy was also dependant on the washing method used. Nets washed by beating on rocks lost insecticide faster and were least effective compared to nets washed by hand rubbing and washing by machine. The local method of washing LLINs by beating on rocks and air-drying by spreading on the ground in direct sunlight as commonly practiced local villages should be discouraged.