Long-lasting insecticide-treated nets (LLINs) are currently preferred to conventionally insecticide-treated nets (ITNs) for use in malaria control programmes. Although LLINs offer an alternative solution to regular net re-treatment, their actual wash durability under field conditions is not known. For example the frequency of washing, the washing methods, and drying regimens that are used in the field are some of the factors which are likely to affect their efficacy and durability. The standard WHOPES recommended washing procedure is not applicable under field conditions and might be underestimating the abrasive washing practices that are used in the field and, therefore, overestimating the biological durability of LLINs in many countries.
The manufacturers of LLINs, suggest that these kind of products do not require any further retreatment throughout their lifetime, because they are treated with special binders which are wash durable. Several studies undertaken under specific environmental conditions have since confirmed that LLINs offer longer time protection than conventional ITNs. The challenge is now to understand how these products will perform under various climatic conditions in the tropics where they are expected to be used. Past studies have demonstrated different wash resistance of different brands of LLINs from one region to the other and even between laboratories. Due to these differences, WHOPES has provided guidelines on standardized procedures for washing and drying of LLINs for the purposes of comparison of similar products between different laboratories. However, it may not be possible to standardize washing methods in every region or country due to local economics and general practices. It is, therefore, necessary to undertake studies like this, so as to understand and document how different washing and drying regimens can affect the performance of these new innovations and give guidelines on the best washing and drying procedures.
LLINs have been promoted for use in reducing human-vector contact for a long time. However, there is limited information on the effect of different drying regimens and washing methods and how they affect the efficacy of current brands of LLINs, especially under tropical conditions. A recent study in Iran showed that the effect of exposing PermaNet for a short period to direct sunlight during drying was much smaller, and that drying nets for long hours in the sun is not harmful. It has also been demonstrated that direct sunlight is harmful to pyrethroid-based insecticide because uv rays break down pyrethrin molecules thereby rendering the insecticide ineffective. However, the role played by sunlight in enhancing or rendering the pytherins ineffective still remains, controversial. Two studies conducted in India, and, showed that exposing Olyset to sunlight for a few hours enhances efficacy. On the 4th update report on LLINs, WHO after reviewing several studies on efficacy of Olyset, recommended placing the nets in polythene bags and exposing them to sunlight for a few hours to enhance heat assisted insecticide migration to the surface at the same time preventing the effect of UV light after washing.
It has also been 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 laboratory study Olyset and PermaNet were equally exposed to a controlled temperature of 60ºC for 4 hrs after repeated washing, only Olyset, regenerated.