Bed nets and the fight against malaria

Tuesday September 26, 2017

By Philip Bejon, Bob Snow, Charles Mbogo

Bed nets have probably saved the lives of tens of thousands of Kenyans over the last decade, and likely hundreds of thousands of people worldwide. KEMRI has played a key role in studies leading to the introduction of bed nets treated with insecticides, and bed nets are now a central tool in controlling malaria and likely to remain so for the foreseeable future. We describe the history of bed nets, why we think they are so important, the need for insecticides, and what the future might hold.

Bed nets use is described in early history to prevent all manner of insect bites in records from Ptolemaic Egypt, pre-colonial India and imperial China. Oral histories suggest long-standing use in parts of Africa to prevent insects biting at night, and bed nets were used specifically in malaria control programmes in the first half of the 20th century in the US and in Europe. Scientific evidence of the benefits of bed nets in Africa was not provided until the late 1980s in The Gambia. Scientists noted that children using bed nets were less likely to have swollen spleens, suggesting that they had been protected from malaria. They also began to investigate the new approach of dipping nets in insecticides to enhance the killing of mosquitoes and prevent mosquitoes entering houses. In the mid-1980s experiments were conducted on this new type of bed net. The first scientific trial evidence was provided by allocating children to receive (or not receive) treated bed nets at random, and then comparing children with and without nets over time. These first scientific trials were led by Bob Snow, a scientist now based with KEMRI-Wellcome in Nairobi, and confirmed the benefits of treated nets in preventing malaria.

Since then the scientific evidence on the benefits of bed nets has become very detailed and replicated in many countries in Africa. The evidence clearly shows that insecticides doubled the effect of bed nets, and therefore the major large scientific trials focused on insecticide treated nets. Key major trials of insecticide treated bednets were conducted in The Gambia, Ghana and Kenya (in Kilifi on the Coast and in Asembo in Western Kenya, led by KEMRI Wellcome and CDC KEMRI collaborations) sponsored by the World Health Organization. These studies showed beyond doubt that insecticide treated bed nets reduced rates of malaria, and also reduced rates of anaemia, fever, hospital admission, low birth weight babies and the rates of childhood death. These trials established insecticide treated bed nets as one of the most important tools for ensuring child survival described in decades. These trials also showed that if a whole community uses treated nets the benefits are multiplied, as large numbers of malaria-transmitting mosquitoes are killed.

The last of these critical trials was conducted in 2003, but two problems prevented the realization of public health benefits; the bed nets needed to be retreated with insecticides every 6 months to retain their effect, and hardly anyone that needed a bed net had access to one. Work by KEMRI scientists showed that in 2004 only 5% of the Kenyan population had access to a bed net. Tactics such as social marketing and subsidies were tried, and despite substantial investment in these tactics access to bed nets did not really improve, especially among the poorest groups in the population. Since the poor are at highest risk of malaria, and find treatment the hardest to access, it was especially critical to boost bed net use among this group.

From 2006 onwards an emphasis was placed on free mass distributions, linked to vaccine catch-up campaigns in Kenya using funds from the Global Fund. These catch-up campaigns where nets were distributed free of charge produced substantial and rapid increases in access to bed nets, and were effective in reaching the poor. Throughout the history of bed net delivery, the KEMRI team carefully studied the populations protected by nets and monitored effectiveness of bed nets in saving lives. Work published by the KEMRI team in 2007 influenced global policy on the value of mass catch-up campaigns, which are common now across Africa.

Another advance was the production of long-lasting insecticide-treated nets or LLINs. The insecticide treatment for regular nets lasts only for 6 months maximum, and for much less if the nets are washed regularly. The LLINs contain insecticide bound into the fibres of the net material, and the insecticide lasts much longer, even if the net is washed. This makes the LLINs more user-friendly and more reliable.

The choice of insecticide for LLINs is limited. The World Health Organization recommends pyrethroids. Pyrethroids are industrially made chemicals that resemble pyrethrins, the natural product of chrysanthemum flowers. Pyrethrins cannot currently be made at large scale, and biodegrade rapidly and can be broken down by sunlight. For these reasons, LLIN net manufacturers currently use pyrethroids such as deltamethrin or permethrin. These insecticides are safe to humans, and have even been used in shampoos for treating lice or clothing to prevent bites by sandflies. The concentrations in LLINs are very low, the release rate is slow, and toxicity to humans has not been reported. However, there are environmental concerns as the pyrethroids are toxic to fish and insects including bees, and so old bed nets should be disposed of carefully.

What to do with old LLIN nets is likely to become a concern, since mass distributions leave many old nets with holes that are no longer useful for sleeping under. KEMRI scientists studying this have found many creative uses including window screens, well coverings, chicken coops, seedlings and vegetable coverings and even traditional clothing such as mahando. In Ghana old bed nets are reprocessed into floor tiles. It is usually old and torn nets rather than newly acquired nets that are repurposed. When we see bed nets being used creatively, we should not jump to judgements that undermine free distributions as we may be seeing old nets being repurposed rather than the misuse of new nets.

Another concern is insecticide resistance. Mosquitoes that used to die rapidly when exposed to insecticide are becoming resistant and can survive for longer. In theory, high levels of insecticide resistance among mosquitoes could make the insecticide ineffective, and reduce the effect of LLINs to being similar to ordinary untreated bed nets. However, ongoing monitoring of LLINs by KEMRI demonstrates that they continue to be effective under current conditions in Kenya and elsewhere in Africa and so their use in malaria control can still be supported. We must continue monitoring for the future, and we should also consider using alternative insecticides. Dual-insecticide treated nets are appearing on the market which may be an insurance policy against future resistance. Research on these newer nets is ongoing.

We can conclude that LLIN bed nets are an essential and life-saving part of malaria control work, and we should have no hesitation in supporting their use and using LLIN nets ourselves. It is important that further work is done on new insecticides for bed nets, but we should remember that many Kenyans are alive today because they (or their parents) used bed nets, and for the present we need to use the current stock of bed nets to save lives.