Image caption A coloured scanning electron micrograph of a black fly

Africa is beginning to win its battle against the parasitic river blindness infection, thanks in no small part to the painstaking work of villagers who catch the flies responsible for the disease.

The Kou Valley in Burkina Faso is one of the country's most fertile areas; a land of lush paddy fields, undulating hills and a meandering river.

But for decades it stood empty - its inhabitants too wary to cultivate it.

What kept them away, and many more from river basins across Africa, is river blindness (onchocerciasis), a disease transmitted by the black fly, a tiny insect that breeds in fast-flowing waters.

A Burkinabe legend claimed if you lived too close to the Kou river it would eat your eyes.

Following widespread prevention efforts since the 1970s, the disease is mostly a thing of the past in Burkina Faso.

Villagers have reclaimed their land and few remember people going blind from the disease.

River blindness is now slated for global elimination by 2025.

Image caption Black fly larvae, like these clinging to a plant pulled from the water, thrive in fertile areas like rivers Image caption Villagers like Yacouba work 12-hour shifts, using themselves as bait, to gather flies Image caption Samples are sent to a specialist lab, where the flies are dried, frozen, ground and blended for testing Image caption Flies are examined at MDSC lab in Ouagadougou to monitor whether the disease is present previous slide next slide

Efforts are shifting from prevention to surveillance because, although the disease has gone in some countries, the flies have not and the risk of recrudescence remains.

In Bodadougou, a picturesque village on the edge of the Comoue river, I met villagers Yacouba and Harouna and Koala Lassane, an entomologist at the University of Ouagadougou, Burkina Faso's capital.

We are going fly-hunting.

As we walk past mango trees and fields of millet, Koala explains that the reason why surveillance focuses on flies is because the disease can be hard to detect in humans in its early stages. In black flies, it shows up systematically.

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Villages like Bodadougou collect the insects regularly and send them on for testing. The capture method is unusual: the flies must be alive and whole so they are caught by hand using humans as bait.

Koala is responsible for training, supervising and paying local fly-catchers.

We arrived on the banks of the Comoue and the spot is idyllic - green, sunny, with the gentle noise of running water, basically the perfect breeding site for black flies.

"Does the bite hurt?" I half-jokingly asked Koala.

"You will soon find out," he deadpanned.

Yacouba and Harouna wandered off to look for the perfect fly-catching spots.

They brought small wooden stools with them and soon settled down. I sat with Koala, swatting nervously.

I watched Yacouba roll up his trouser legs and unpack a load of plastic tubes in front of him.

He picked one, opened it and waited, scrutinising his legs.

As the flies start landing, Yacouba scooped them in the tube. He must be quick: flies usually bite within 10 seconds of skin contact and despite their tiny size - less than a millimetre - they are ferocious.

"At the height of fly season, you can only use one leg, sometimes even just one foot," says Koala.

Fly-catchers usually work 12-hour shifts, alternating catching and sample preparation.

Image caption The lab requires 9,000 flies per site, prepared in samples of 300

MDSC, the lab testing the flies in Ouagadougou, is one of just three labs in the world capable of doing this work. It requires 9,000 flies per site, prepared in samples of 300.

Once at the lab, the flies are dried, frozen, decapitated, counted by hand, ground and blended for testing.

How dangerous is the testing? The fly-catchers do get bitten - as does anyone who lives near breeding sites such as fast-flowing rivers. All fly-catchers receive a preventive treatment in the form of a pill but to develop the disease, an individual would have to be bitten repeatedly by infected flies, which is highly unlikely in countries like Burkina Faso that are at surveillance stage (which is when they test flies): you would get bitten, but only one in a few thousand flies is likely to be infected. In countries where the disease is still prevalent, in central Africa for instance, mass drug administration programmes are still under way to curb the disease.

It is a meticulous, painstaking job, but the results are crucial to establish whether river blindness is being kept at bay.

Dr Laurent Toe, the lab's director, seemed unfazed by the task as he rattled through the numbers: "9,000 flies per site, 10 sites per country, seven countries for the time being, that's... 630,000 flies per year. Sometimes, it goes up to a million."

As more countries in Africa progress towards elimination, MDSC knows there are hundreds of thousands more flies in the pipeline.

Dr Toe said they are looking into ways to mechanise the counting process. They are also working on traps because of the ethical concerns of using fly-catchers like Yacouba and Harouna.

"We are villagers so the money helps us in the lean season. But what we really want is to get rid of river blindness. For good," says Yacouba.

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