Entomology, chemical ecology, evidence-based environmentalism and science in general. I like big bugs and I cannot lie.

Wednesday 25 August 2010

Dangers at dusk: is it time to move beyond bednets?

I originally wrote this for The Guardian's "Write the World" International development journalism competition. You can see the winning pieces here, some of which are excellent. I'd particularly recommend "Aid: Dead or alive?" for a more balanced discusion of the benefits and pitfalls of western aid than you usually find in the papers.

Cattle amble lazily back to their night time pastures as the sun sets over the tiny Gambian village of Wellingara. In dusty compound courtyards women stoke the fires that will cook the evening meal of rice and oily stews, while in the streets their husbands wait for their dinner, brewing tooth-achingly sweet attaya tea over charcoal braziers. As the coals smoulder and light-hearted banter or serious matters of village politics fill the evening air you could be forgiven for thinking this was one of the most tranquil places on earth. But all is not as peaceful as it appears.

“You can tell the people who are not from around here because they fight themselves” laughs Tumani, a fieldworker for The Gambia’s Medical Research Council, as he demonstrates how foreigners slap their faces and arms when they feel mosquitoes landing. Locals enjoying the relative cool of the evening know instead to brush the pests from exposed skin, their hands in continuous, fluid motion. Whatever strategies are employed against it though, early evening mosquito biting before people go to bed is a problem which mosquito nets are unable to tackle.

Uptake of bednets in The Gambia has been extraordinarily successful, with almost three quarters of households estimated to own at least one. Although malaria-focussed health education campaigns have doubtless increased their use, like the elaborately carved hardwood beds bought for newlyweds bednets have become something of a status symbol with perceived benefits that go far beyond malaria control. They catch detritus falling from traditional thatched roofs, provide a measure of privacy in a country where large extended families typically share a compound and are appreciated as interior decoration. In the local markets gaudily coloured nets swing in the breeze, adorned with lacy ruffles like some bizarre cross between a jelly fish and a wedding dress. While there are of course caveats – many of the locally produced nets are untreated with insecticides that protect people sleeping against the nets from bites, and the youngest children most vulnerable to malaria may not be the ones sleeping under the nets – with malaria infections and deaths in decline in The Gambia, this tiny country provides an encouraging example of what could be achieved by widespread adoption of insecticide-treated bednets.

Unfortunately it seems that in some areas bednets are becoming victims of their own success. The most effective nets are those treated with pyrethroid insecticides, but in some parts of Africa populations of mosquitoes are evolving resistance to pyrethroids and as treated nets become more common resistance will offer a greater survival advantage and so is likely to spread through the population.

Concerns are also emerging that malarial mosquitoes may be changing their behaviour to bite earlier in the evening before people are protected by nets. To complicate matters humans are also changing their behaviour; particularly in urban areas where development may bring electric light and flickering televisions beaming Brazilian telenovelas or Kung Fu movies to rapt audiences, people are staying up later after sundown and remaining exposed to mosquitoes for longer. Taking into consideration the fact that malaria is not the only disease transmitted by mosquitoes - the species that transmits yellow fever, for example, is active at dusk – the importance of preventing early evening biting becomes increasingly apparent.

Large-scale mosquito control programmes, such as those treating water sources where mosquitoes breed with chemical or biological insecticides, have had impressive results in some areas but require a great deal of political will, organisation and stability to scale up and so may not be appropriate everywhere. Instead the use of repellents, chemicals that smell unpleasant to mosquitoes, is being suggested as an approach that can be targeted at the household level as bednets can.

Tourists and travellers visiting malaria-endemic areas have long protected themselves with DEET, the plastic-melting personal repellent that can be rubbed on skin, and with the development of gentler personal repellents such as those derived from lemon eucalyptus oil there has been interest in extending the benefits to the local population. This work is in its early stages, but results from work in Bolivia on the effects of personal repellents in addition to nets are encouraging. However, as campaigns to promote handwashing with soap in Africa have shown, encouraging a change in habits is difficult and attempting to foster a culture of repellent use from scratch where none existed before would be challenging to say the least.

Another approach is the use of spatial repellents, odorous chemicals which disperse and so could make a wide area, for example the veranda of a house, unattractive to mosquitoes. Here signs are perhaps more encouraging; families in The Gambia already regularly burn mosquito coils or local herbs to deter evening biting insects, and as our knowledge of mosquito behaviour and biology increases we will be increasingly well placed to evaluate the effectiveness of these particular blends and to design new mixtures of odours and methods of delivering them.

Of course the use of repellents may simply provide a different pressure for mosquito populations to evolve their way around. Peaceful as the streets of Wellingara may appear as the tropical sun slips below the horizon, in the ongoing war between humans and mosquitoes they are in fact a battle ground.

Saturday 7 August 2010


I isolate the individual odours attractive to my flies from a complicated blend using a process called chromatography. Although this is done at Rothamsted with a fancy piece of equipment the basic principle is very simple: you have two different substances or "phases" which have different affinities for the different components of the blend you're trying to separate. The mobile phase, a gas or liquid, is passed over the stationary phase which could be a polymer or wax. Things with a greater affinity for the mobile phase will spend a greater proportion of the time in it, so will come out of the system faster than things with a greater affinity for the stationary phase. Use a long enough system and different constituents of a mixture will reliably come out separately, one after another.

The random chemistry lesson comes courtesy of a brilliant demonstration of the principles of chromatography that I saw at Gatwick airport on the way home. A mixed crowd of Gambians and British people got off the plane and started walking through the terminal building. We all walked together until we reached the first moving walkway, which interestingly the British people got on but the Gambians didn't. Three walkways later, the airport-sized chromatography system had acheived perfect of a mixed group of human beings on the basis of nationality with the Brits emerging 50 metres ahead of the Gambians.

So now I'm home, with my bloke and my family and my knitting and my tea and my 24 hour amenities and I'd be lying if I said it was anything other than a huge relief. Apologies to everyone I haven't been in touch with yet, I've needed a little time to recover but I plan to get in touch with all of you very soon. And everyone I have managed to speak to, even if it's just on facebook, thanks for the lovely welcome.

I think Hounslow Borough Council also wanted to welcome me back, I got home to find this poster on the bus stop on our street!