Insecticide-impregnated mosquito nets

'Olyset' mosquito net impregnated with insecticide, approved for use by the World Health Organisation (WHO).

Credits:Science Museum London

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A simple net has long been the standard physical barrier against the mosquito menace. The Greek historian Herodotus (c. 484-425 BCE) observed mosquito nets in use in Egypt. Whereas silk was used in Ancient China, nets woven from the fibrous plant ramie - known as ‘China grass’ - would probably have been more accessible. An essential purchase for Victorian colonialists and explorers, by the following century the mosquito net had become a key element in the global fight against malaria. Today, nets impregnated with insecticide are being distributed across Africa in an attempt to reduce malaria infection rates.

The first serious attempts to extend the function of nets beyond simple barriers occurred during the Second World War when they were treated with DDT. When DDT fell out of favour alternative insecticides were sought. The development of synthetic chemicals known as pyrethroids in the 1970s provided a safer option. Effective against mosquitoes but much less harmful to the environment, nets soaked in pyrethroids such as permethrin were effective mosquito killers as well as a reliable barrier.

Permethrin naturally breaks down quite rapidly in the open air, so several soakings were needed each year to keep nets viable. Fortunately, a technological solution to this limitation has been introduced in the form of long-lasting insecticidal nets (LLINs). Here, permethrin is incorporated into the net’s polyethylene fibres during manufacture. It gradually diffuses out onto the surface of the net to replace the permethrin removed by washing and general wear and tear. This new generation of mosquito nets is at the forefront of the ongoing global ‘Roll Back Malaria’ initiative.