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Dr James Logan and Professor John Pickett FRS - Rothamsted Research
Professor Jenny Mordue - University of Aberdeen

Scientists at Rothamsted Research have developed a new theory to explain why some people are more attractive to mosquitoes and midges than others. 'It is estimated that over 1.2 million people die from malaria, which is carried by mosquitoes, each year,' says James Logan of Rothamsted Research. 'So this is clearly a major international health problem.'

Previously it was thought that people who were unattractive to biting insects lacked certain attractive chemicals within their body odour. Rothamsted's research has turned this on its head. 'What is really happening is that these people produce chemicals that mask the attractive odours,' explains James. Their research started in cattle where they found 'unattractive' cattle produced repellent or 'host-masking' chemicals. The team applied this theory to humans measuring levels of chemicals produced by 'attractive' or 'unattractive' individuals. 'The problem is that the human body produces somewhere in the region of 3 to 400 different volatile chemicals,' explains James. 'So the tricky bit is identifying which chemicals the mosquito or midge responds to.'

The key to Rothamsted's success has been the use of a technique known as gas chromatography-electroantennography (GC-EAG). James explains, 'The gas chromatography allows us to separate out the 'eau de human' into its individual chemical components. The EAG part allows us to simultaneously record the response of the mosquito's antennae its nose if you like to these individual chemicals.'

So why is it that some people naturally produce protective chemicals and others don't? The picture is far from clear. One theory is that it is a signal for the insect. Unattractive chemicals might be a sign that the person they are about to feed on is unsuitable due to stress caused by illness or disease. A further theory relates to exposure, with increased exposure to biting insects leading to the development of protection, something that has been demonstrated in plants.