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New mosquito net technology to combat insecticide resistance

A new type of mosquito net which combines two classes of insecticide has been shown to kill resistant mosquitoes which survive exposure to standard pyrethroid treated nets.

The authors of the study, which is published in PLOS ONE, describe the findings as a breakthrough in the development of new types of long lasting insecticide treated nets (LLIN) which can meet the challenge of malaria control where standard nets show signs of failing due to insecticide resistance.

The research was conducted by the London School of Hygiene & Tropical Medicine, who worked in partnership with the chemical company BASF SE and the Innovative Vector Control Consortium to develop and evaluate the new type of net, Interceptor® G2. The net combines two insecticides: chlorfenapyr and the pyrethroid alpha-cypermethrin in a long-lasting wash resistant formulation. The study evaluated the net in laboratory tests and in experimental hut trials in Benin.

Long-lasting insecticide treated nets (LLIN) that kill the Anopheline mosquitoes which transmit malaria are the most widely used method to prevent the disease. Over half the population of sub-Saharan Africa now sleep under LLIN and this has helped to reduce malaria cases by a third over the last 15 years.

Until now, nets have been wholly dependent on pyrethroids as the only class of insecticide that is safe to use on LLIN. Resistance to the pyrethoids in malarial mosquitoes has become common across Africa in recent years and threatens further progress unless new types of insecticide which are safe to use on LLIN can be developed.

Professor Mark Rowland, Research Coordinator and study author, said: "Our results are extremely encouraging and represent an important breakthrough in long lasting net development. We found that in field trials under household conditions the new net killed over 70% of pyrethoid-resistant Anopheles gambiae mosquitoes that entered the houses to feed, whereas the standard pyrethroid net killed only 20%. The mosquito mortality rates achieved by the new net are similar to those seen with the standard pyrethroid net 10 years ago, when mosquitoes were fully susceptible to pyrethroids before resistance began to take hold.

"Very importantly, the long lasting formulation retained insecticidal activity on the net after 20 washes in soap solution, which meets the WHO benchmark for long lasting insecticide treated nets."

The problem of insecticide resistance was anticipated more than 13 years ago when the School's Prof Rowland and Dr Raphael N'Guessan began discussions with BASF about repurposing an insecticide which previously had been used to control termites and domestic insect pests.

Prof Rowland said: "We knew that selection of resistance to pyrethroids in malarial mosquitoes was only a question of time. The challenge was to identify a new insecticide from another class that combined long-lasting activity against insects, low water solubility, no cross resistance to public health insecticide and a toxicological profile which would make it safe to use on mosquito nets. Chlorfenapyr, a pyrrole insecticide, seemed to have those rare attributes.

"A few years later, the School and BASF had demonstrated chlorfenapyr's potential against mosquitoes in the laboratory, subjected it to WHO toxicological risk assessment, and conducted the first experimental hut trials, a small-scale trial under house-like conditions in West and East Africa".

In 2011 BASF entered into partnership with the Innovative Vector Control Consortium (IVCC) together with the School to develop an effective long-lasting insecticidal net which would combine chlorfenapyr and the pyrethroid alpha-cypermethrin. This was the Interceptor® G2 net that was evaluated in the new study.

Dr Raphael N'Guessan, the scientist leading the trials in West Africa said: "Unlike standard public health insecticide which are neurotoxic, chlorfenapyr disrupts cellular respiratory pathways and is most toxic to mosquitoes which are active at night when they make contact with the net. This also means that insecticide resistance based on target site insensitivity in the insect nervous system shows no cross resistance to chlorfenapyr.

"Africa has become a net using culture. Insecticide treated nets are the most important tool we have to prevent malaria. This new technology demonstrates that insecticide treated nets will continue to be an essential weapon in the fight against malaria in the future despite pyrethoid resistance."

The study publication marks the first of several trials of Interceptor® G2 carried out by the School and the IVCC in African trial sites in Benin, Ivory Coast, Burkina Faso and Tanzania. BASF plans to submit the evidence on Interceptor® G2 to the WHO Pesticide Evaluation Scheme for interim recommendation next year.

The development and evaluation of Interceptor® G2 is the result of partnership between the manufacturer BASF SE in Germany, the London School of Hygiene & Tropical Medicine, and the Innovative Vector Control Consortium - the organisation funded by the Bill & Melinda Gates Foundation to support the development of new vector control tools to combat malaria.

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Image: Leila Abdala, 30, with her baby Kairat, 5 months, lie under a bed net at their home in Mtwara, Tanzania. © 2016 Riccardo Gangale/VectorWorks, Courtesy of Photoshare

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