It comes after a “very rare” case of bird flu was detected in a person in the UK, and the public was urged not to touch sick or dead birds.
Animal welfare campaigners said the disposal, although necessary, involved leaving possibly infected birds in the open air, risking viruses spreading to the surrounding area.
The footage, taken three days before Christmas at a farm supplying UK supermarkets, shows workers in full hazmat suits tipping wheelbarrows of hens into a mechanical loader, which then dumps the carcasses into a skip.
The Open Cages animal welfare organisation, which obtained the footage, said dead and possibly infected birds were left in the open air while the workers went on their lunch breaks.
“Discharge and fluids from the birds can be seen in the machinery as the dust enters the open air. Bird viruses can be present in air samples up to 110 metres from infected farms,” said Open Cages chief executive Connor Jackson.
A 2018 study found traces of an avian virus in air samples 50-110 metres from the infected farm even after the birds had been removed.
“Bird flu was once a very rare disease among chickens, but today outbreaks occur every year: this footage helps explain why,” said Mr Jackson.
“When you take tens of thousands of chronically stressed animals and cram them into an indoor facility you create an ideal environment for disease.
“The more the animals suffer, the more stressed they become and the less their immune systems can cope.”
Open Cages is calling on the government to end factory farming, which it says creates the perfect conditions for diseases to spread “like wildfire” and then mutate.
A 2020 report warned of the “catastrophic” danger for the poultry industry to spawn a new pandemic, which the authors claimed could kill millions of people.
The authors warned of mutations that could transfer the virus to a human host via contact with a “vast amount” of biological matter in chicken farming. The 1918 flu pandemic, which killed 50 million people, was thought to have originated in birds.
“These images should shock us more than they do,” Mr Jackson added. “Across Britain, two million intelligent, sensitive, sentient individuals have been disposed of like numbers on a spreadsheet. It’s an absolutely horrendous sight.
“Two million animals have been gassed to death and we’re treating it as normal.
“It shouldn’t be normal: bird flu was once a very rare disease.”
Bird-to-human transmission of bird flu – or avian flu – has only occurred a small number of times in the UK, with the risk described as “low”.
The H5N1 strain of bird flu - thought to be that circulating this winter among birds - has so far killed 134 people out of 239 infected, according to the World Health Organisation. This is a mortality rate of over 50 per cent in humans, Mr Jackson pointed out.
“A bird flu virus only needs to mutate once to infect a worker or nearby resident, and then we have another pandemic on our hands,” he said.
“But this time, it will be more catastrophic. Many virologists and disease experts say intensive farming is the most likely cause of the next pandemic.”
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