Mosquito nets are an efficient preventive tool in global malaria control. Insecticide-treated nets (ITNs), knitted polyester nets that must be treated with insecticide and retreated every 6 months, are effective preventing malaria, but have several drawbacks. They require re-impregnation with insecticide on a regular basis to maintain their efficacy. This constraint requires that net users be educated about the importance of re-impregnation, and campaigns organized to ensure that this occurs. Carrying out these campaigns on a large-scale is expensive and logistically demanding, and current re-impregnation rates are poor (field studies show figures generally lower than 5%), particularly when ITN users are asked to pay for re-impregnation.
However, the development of technologies within the textile and the chemical industries permitting the durable impregnation of fiber with insecticide has led to several possibilities for making Long-Lasting Insecticidal Nets (LLIN), which can retain efficacy throughout the normal life span of the netting material itself (5 years for polyethylene, and 2-3 years for polyester nets).
The purchase of factory pre-treated nets other than LLINs is not recommended except for emergencies. It has recently been found that insecticide concentrations in these nets are extremely variable, frequently far below the target concentration. Such variation can be accepted in the case of emergencies, when rapid and effective treatment of nets on site is almost impossible to achieve. Under normal circumstances, preference should be given to LLINs that have been recommended by WHO or to non-treated nets bundled with an insecticide treatment kit. It is recommended that both net and insecticide should comply with WHO specifications.
There are various materials used to produce netting materials. Cotton is not recommended because it is less durable and has a lower quality/price ratio than synthetic yarns. Polyester (multi-filament) has been the most widely used material, especially in Africa. This material is widely available and relatively cheap. Nylon is not recommended. High-density polyethylene (mono-filament) is used for a type of LLIN and is a common netting material in India. Nets made of this material have been found stronger and more durable than 75 or 100 denier polyester nets. Other materials such as polypropylene are under development which might combine advantages of both polyester and polyethylene, especially for LLINs.
WHO publishes and disseminates regular updates on LLINs to inform buyers and users about new developments and the status of WHO recommendations(on www.who.int/rbm technical strategies, vector control, insecticide treated materials).
Cost effectiveness of LLINs
The investment cost of LLINs is higher than conventional treated nets. However, if the cost of re-treatment of the conventional net is taken into account, in order to provide the same level of protection as the LLIN, the average cost per year of the LLIN is lower (Figure 1).
Figure 1. Cost effectiveness of LLINs
Source: UNICEF Supply Division
Moreover, the price of LLINs should fall as supply volumes increase and new suppliers come on stream the cost of ITNs is unlikely to decrease further.