Earlier this month, the Danish government ordered the country's mink farms to cull more than 1 million of the animals following a series of Covid-19 outbreaks, and thousands of mink have died at farms in the US states of Utah and Wisconsin.

A total 3.1 million mink were farmed in the United States in 2018, according to animal welfare charity Humane Society International (HSI). Current advice from the US Department of Agriculture does not recommend culls of mink herds.

In July, Spanish authorities ordered a cull of nearly 100,000 of the animals for the same reason, and in May the Netherlands mandated coronavirus testing at mink farms after suspecting that one of the animals had passed the virus to a human.

The testing has led to the culling of an estimated 2.6 million mink in the Netherlands, according to HSI. While some mink died from coronavirus, most of the animals were culled due to concerns that they could spread the virus to humans.

Although fur farms are banned in many countries, millions of animals are killed every year for their pelts, which are used in clothing. HSI said that 60 million mink were farmed for fur around the world in 2018, with China accounting for 20.7 million of the total.

A recent study conducted in the Netherlands found "strong evidence" that at least two people from four mink farms in the country contracted coronavirus from the animals, and study co-author Marion Koopmans, a virologist at ErasmusMC in Rotterdam, said that her team's research has confirmed mink-to-human transmission.

Studies have shown that ferrets are susceptible to coronavirus, so researchers in the Netherlands decided to look into the taxonomically similar mink during a routine animal testing program, said Koopmans.

"Researchers found that mink do transmit Covid-19 to each other more easily than other animals, Koopmans said. "It is amazing how easily this virus spreads in mink," she said.

Long-term animal welfare issues give way to human concerns

Mink, which are closely related to weasels, otters and ferrets, appear to suffer similar Covid-19 symptoms to humans.

Difficulty breathing and crusting around the eyes are usually seen, but the virus progresses rapidly, and most infected mink are dead by the day after symptoms appear, according to Dean Taylor, state veterinarian for the US state of Utah.

Conditions at the farms mean the virus is able to rip through captive populations, said Jo Swabe, senior director of public affairs at HSI Europe. "The animals are being kept in small wire cages, there's just rows and rows and rows of them," she said. "The animals can't escape each other."

Mink are naturally solitary and semi-aquatic, and it's impossible to provide for their welfare needs on farms, according to HSI, which campaigns for the closure of fur farms on ethical grounds.

In the Netherlands, there is now ongoing transmission between mink farms, as well as evidence that the virus has been circulating for some time at some facilities, said Koopmans.

There are a number of hypotheses as to how transmission occurs, in