Stinky socks help replace human bait in surveys for river blindness

New research shows that it’s possible to end the practice of using people as human bait to catch and test the black flies that spread river blindness (onchocerciasis). The study by international non‐profit Sightsavers in partnership with the Global Institute for Disease Elimination (GLIDE) and ministries of health in Ghana, Cote d’Ivoire, Malawi and Mozambique, was presented at the American Society of Tropical Medicine and Hygiene (ASTMH) on 13 November 2025.

Every year, countries across Africa collect many thousands of black flies to monitor for the presence of river blindness, a parasitic infection that causes severe and painful itching, and in the worst cases, irreversible blindness. Until now, “human landing catch,” where volunteers sit by rivers to lure and catch the flies on their skin, has been the only method recommended by the World Health Organization (WHO) for collecting black flies so that they can be tested for the disease.

Innovative traps offer ethical alternative

The research tested 16 different variations of insect traps (called Esperanza window traps) at black fly breeding sites in Ghana, Cote d’Ivoire, Malawi and Mozambique. Traps used carbon dioxide to mimic human breath alongside color variations, different orientations, worn socks (black flies are drawn to the smell of sweat) and even a fake stinky foot scent. Although the optimum trap design varied from location to location, the researchers found that seven pairs of traps, easy to set up and requiring minimal maintenance, were as effective as one fly catcher.

In Ghana and Malawi, where fly numbers were high, traps using yeast‐generated CO₂ with worn socks performed as well as traps with synthetic CO₂ and scent. In Côte d’Ivoire and Mozambique, where overall fly numbers were lower, the traps with yeast‐generated CO₂ and worn socks performed best.

Reducing risks for volunteers and communities

“Using people as bait, even if they’ve willingly volunteered, raises big ethical questions,” says research co‐lead Louise Hamill, Sightsavers Director for Onchocerciasis. “We clearly need a better solution, and with the help of some dirty socks, our team has found one.”

“Flycatchers sit for hours at a time with their skin exposed, braced to collect any black fly that lands on them. They are given precautionary treatment and try to catch the insects before they bite, but it’s hard work and I know from experience how painful the bites can be. Sitting by water all day also puts them at risk of tsetse flies’ and mosquitoes’ bites, which can lead to other diseases, as well as snake bites.”

“An additional benefit is that when set up at high density, the traps reduce black fly populations at breeding sites by catching the females looking for a bloodmeal before they’re able to lay their eggs. This has the potential to offer communities long‐term protection from river blindness.”

The burden and impact of river blindness

River blindness is caused by a parasitic worm (Onchocerca volvulus), which is spread by bites from infected black flies. According to the WHO, at least 250 million people in 29 countries are at risk of contracting the disease. Adult worms, up to 30 cm long, can live for 15 years inside their human host, producing millions of baby worms, called microfilariae. These migrate to the skin and eyes, causing symptoms like severe and painful itching and chronic inflammation. If the microfilariae migrate to a person’s eyes, they cause lesions that lead to permanent sight loss.

The socioeconomic impact of onchocerciasis is profound. It can significantly impair infected people’s ability to work and perform daily activities, leading to loss of income. The stigma associated with the visible manifestations of the disease contributes to social exclusion, and early exposure to onchocerciasis in childhood has been associated with a particular type of epilepsy, known as onchocerciasis‐associated epilepsy.

There is no vaccine or preventative medicine for river blindness, but the drug ivermectin kills the microfilariae, and when given out across entire communities, helps to prevent transmission. In January 2025, the WHO confirmed that Niger had become the first country in Africa to eliminate the disease.

Provided by
Royal Commonwealth Society for the Blind