Better Bait

River blindness, characterised by itchy skin rashes as well as visual impairment, is caused by the parasitic worm Onchocerca volvulus, transmitted to humans through the bites of black flies. The disease is a major public health concern in tropical parts of Africa, and to monitor the parasite’s prevalence or study its transmission, scientists need to collect the flies. The photo shows some new apparatus being tested for that purpose. Sixteen versions of this trap were trialed across Ghana, Côte d’Ivoire, Malawi and Mozambique. They all provided two substances irresistible to the flies – carbon dioxide and the scent of human sweat (either organic or synthetic) – with those versions combining yeast-produced carbon dioxide with pre-worn socks being the most effective. Until now, the only recommended trapping technique was to use an actual person sitting by a river as bait. The yeast-sock combo is thus a cheap, convenient, not to mention safer option.

Written by Ruth Williams

Ruth Williams

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Ruth is a freelance science journalist based in the US. She's a regular correspondent for The Scientist, and her work has appeared in The Lancet, Nature, Scientific American Mind, BBC Focus and elsewhere. Twitter @rooph

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• Image by Louise Hamill, Sightsavers Director for Onchocerciasis
• Sightsavers in partnership with the Global Institute for Disease Elimination (GLIDE) and ministries of health in Ghana, Côte d’Ivoire, Malawi and Mozambique
• Image originally published with no restrictions – hence published here with as Creative Commons Attribution 4.0 International (CC BY 4.0)
• Presented at the American Society of Tropical Medicine & Hygiene