Nature's mosquito repellent



People who escape the unwanted attentions of mosquitoes have been branded 'unattractive' by UK researchers. These lucky few repel mosquitoes with a cocktail of compounds related to the stress response, says project leader John Pickett of Rothamsted Research, Hertfordshire.

'We're not saying that the people who we've got in our testing group are actually stressed,' Pickett told Chemistry World, 'but the compounds they produce are like the ones that are produced in stressful situations.'

The findings echo those previously recorded in cattle, says Pickett, who studied the animals on an EU-funded project. He found the cattle that attract fewer flies produce elevated levels of stress-related compounds, notably 1-octen-3-ol and 6-methyl-5-hepten-2-one. Mosquitoes probably avoid stressed animals because they are more likely to shake off pests, he says.

There are clear parallels between cattle and flies, and humans and mosquitoes, says Pickett. But the chemistry of the compounds found in human subjects will not be released until patents have been applied for and the work published.

The compounds in humans were collected by wrapping subjects in foil sleeping bags and withdrawing air from the bags through a porous polymer that absorbs volatiles. The compounds were analysed by coupled gas chromatography-electrophysiology and by the behavioural effects they had on yellow fever mosquitoes. Mosquitoes are strongly repelled by the compounds released by unattractive subjects, says Pickett, and the coupled GC-electrophysiology shows that unattractive subjects produce extra compounds, or more of certain compounds. 'Those are the [compounds] we identify and those are the ones we then test, in the wind tunnel, to show this masking or repellent effect,' he said.

The work intrigues mosquito expert Martin Geier at the University of Regensburg, Germany. Geier previously showed that lactic acid has some influence on attractiveness to yellow fever mosquitoes, but agrees many compounds are involved. '[Pickett's] findings are very interesting and I look forward to seeing the publication,' said Geier. 'To lower the attractiveness of a person is possible by changing the attractive blend,' he said, 'to repel mosquitoes from an attractive host is more challenging.'

He's right, says Pickett. 'If you could make people into the guys that we've got in our group who are unattractive, then you would achieve a great deal,' he said. 'They don't get bitten!'

Bea Perks