It is driven by an increase in China's production of ejiao -- a product marketed as a health supplement that uses collagen from donkey skins -- which is a $6.8-billion industry, according to China-based research firm Qianzhan.

China, whose donkey population has plummeted from 11 million in 1992 to 1.5 million in 2023, has turned to Africa to meet its demand.

With donkey populations falling, the African Union issued a 15-year moratorium on donkey slaughter last year.

UK-based charity The Donkey Sanctuary said "the ejiao industry drives a massive global trade in donkey skins, much of it illegal".

It said around 5.9 million donkeys were killed worldwide last year.

The ejiao trade is expected to require at least 6.8 million donkey skins by 2027.

The rising value of donkeys means they have increasingly become targets for criminals.

"Traders exploit vulnerable people using large and sophisticated networks of agents to pressure donkey owners into selling their animals," the charity said.

"Illegal networks operate across the continent, often without consequence, stealing and slaughtering donkeys in the night," it added.

"Donkeys die in often unregulated, inhumane, and unsanitary conditions, and large numbers perish on their way to slaughterhouses," it continued.

The impact on humans is particularly felt by women and children, who rely on donkeys for farming activities such as ploughing and for carrying goods to market.

The criminal operations also generated health risks, The Donkey Sanctuary said.

"The transportation of untreated skins and improper disposal of donkey carcasses risk triggering the spread of infectious diseases and damaging local ecosystems."