Polyolefin copolymers as controlled release devices for insecticides and repellents

Abstract

Malaria is a parasitic disease confined mostly to tropical areas and transmitted by female anopheles mosquitoes. It results in approximately 250 million clinical cases and nearly a million deaths annually. Malaria is particularly prevalent in sub-Saharan Africa where it affects mostly pregnant women and children less than five years of age. The World Health Organisation (WHO) mainly recommends the use of long lasting insecticide treated nets (LLIN) and indoor residual spray (IRS) to control mosquitoes. These interventions have been very effective for the most part however they leave gaps that threaten the goal of eliminating malaria: LLIN are only effective when a person is sleeping under the net whilst a mosquito can still bite and infect people outdoors. LLIN also relies on pyrethroid insecticides and mosquitoes are steadily becoming resistant to this class of insecticide. IRS is only effective when using DDT, a persistent organic pollutant whose use in public health is very contentious. Unfortunately insecticide alternatives to DDT, e.g. pyrethroids, carbamates and organophosphates fail prematurely due to alkaline hydrolysis in the environment. Using these alternatives will require repeated applications throughout the malaria transmission season making IRS unaffordable to relatively poor African countries. Lastly, both IRS and LLIN target the malaria vector indoors whilst infection can actually happen outdoors. The work presented here sought to develop two cost effective and innovative ideas that may bridge the gap left by implementing current recommended vector control interventions. These ideas relied on the use of polymer matrices to stabilise and slowly release active agents used in malaria vector control thereby increasing their residual effectiveness. The first idea pursued the development of an insecticide treated wall lining (ITWL). It was envisaged that the lining may substitute the use of IRS and also complement the use of LLIN. This lining was produced using simple extrusion of 10wt.% and 18wt.% (alpahacypemethrin and delatamethrin loading respectively) polyethylene masterbatches with a 1:1 polymer blend of high density polyethylene (HDPE) and low density polyethylene (LDPE) to produce a Netlon® mesh. This mesh contained alphacypermethrin and deltamethrin in concentrations ranging from 0.29wt.% to 0.85wt.%. The mesh linings were evaluated for acceptability, durability and perceived effectiveness in field trials carried out in the Vhembe district of Limpopo province, north of South Africa. In these trials it was established that majority of the field trial participants perceived the Netlon® lining to be effective and user friendly. The linings were stable to environmental elements that persisted inside the dwellings where they were installed however there was some rodent damage observed. Standard bioassay tube tests indicated that these nets remained effective for at least 24 months in the field and 36 months after manufacture. The second idea sought to address the need for protection against mosquito bites outdoors. It entailed the use of polymer matrices to trap large amounts of a repellent and to release it slowly over an extended period of time. This could increase the residual effectiveness of volatile repellents. Possible future product concepts based on this idea include long-life mosquito repellent bracelets and low cost slip slops. The repellent N,N-Diethyl-meta-toluamide (DEET) was incorporated into the polymer via the technique of spinodal decomposition. It was found possible to trap up to 50wt.% in poly(ethylene-co-vinyl acetate) (EVA). Thermogravimetric analysis and oven mass loss studies showed that the filled EVA polymer matrix reduced the rate of release of the repellent. Laboratory repellency tests suggested that these bracelets may be effective in repelling mosquito bites for at least one month. This suggests that, in theory at least, low cost bracelets and slip slops can be designed that would last that long. Future products based on this idea could help reduce infections due to ankle biting by An. gambiae mosquitoes which are responsible for a significant number of malaria infections in Africa.