It started in October 2022, when several dead mink were discovered on a farm in Galicia in northwestern Spain. Veterinarians initially blamed the coronavirus. But tests revealed that the highly pathogenic avian flu virus H5N1 was the culprit.
To stop the spread of the dangerous pathogen, more than 50,000 mink on the farm were killed.
While farm workers themselves were not infected, the case remains a cause of concern for scientists.
Mink farm outbreak 'incredibly concerning'
The spread of the virus from birds to other species is nothing new. The pathogen that causes bird flu, or avian influenza, has been found in raccoons, foxes and seals, though these remain isolated cases.
While there have been some cases of H5N1 infecting humans, the World Health Organization (WHO) says there's no evidence of human-to-human transmission so far.
When the disease has spread to humans and other mammals, it has been via direct contact with excrement from infected birds or their carcasses, according to Timm Harder, an avian influenza expert at the Friedrich Loeffler Institute's diagnostic virology department in Germany.
But the mink outbreak appears to be a rare case where mammals are transmitting the disease to each other rather than through direct contact with an infected bird. This is something "new," said Harder.
Part of the problem is that mink are intensively farmed. They're kept in high numbers in confined spaces which means infection spreads rapidly in the highly susceptible mammals, said Harder.
Harder added that researchers have identified several pathogen mutations in the mink, one of which allows "the virus to better reproduce in mammals."
Scientists are worried that the virus, which has led to the deaths of tens of millions of birds globally, could spread to more mink farms and become "more transmissible."
"This is incredibly concerning," said Tom Peacock, a virologist at Imperial College London, in an interview with the scientific journal, Science. "This is a clear mechanism for a H5 pandemic to start."
Could avian influenza trigger a human pandemic?
Of the 868 known cases of H5N1 infection in humans worldwide between January 2003 and November 2022, 457 were fatal, according to the WHO.
However, because there's no sustained human-to-human transmission, the risk of human infection from avian flu is low, said the WHO.
Some highly pathogenic avian influenza viruses have an increased potential to cause new zoonotic diseases — which are transmissible from animals to humans and vice versa.
While Timm Harder said there are "numerous hurdles for a more extensive adaptation to humans," he added the mutations seen in the virus that infected mink must be further studied and evaluated.
How a harmless virus became dangerous
Waterfowl have long played host to influenza viruses, but these early strains were low in pathogenicity, said Wolfgang Fiedler, an ornithologist at the Max Planck Institute of Animal Behavior. The viruses weren't too contagious or damaging.
But when these viruses that were harmless to wild birds spread to factory poultry farms — where thousands of animals were crammed tight — the disease spready rapidly and the virus could mutate, Fiedler explained.
The result was the highly contagious virus strains H5N1 and H5N8, which likely originated on poultry farms in East Asia, according to the Scientific Task Force on Avian Influenza and Wild Birds established by the UN.
Farmed ducks likely become infected from wild birds. Ducks are "kept together with pigs, for example," which aided the mutation process, noted Fiedler. Such animal husbandry methods "make a virus like this insanely happy."
In fact, outbreaks of these highly pathogenic strains are typically associated with "intensive domestic poultry production and associated trade and marketing systems ... via contaminated poultry, poultry products and inanimate objects," according to the UN's bird flu task force.
The highly contagious H5N1 and H5N8 virus strains were in turn transmitted to wild birds via infected farmed birds, explained virologist Timm Harder. The viruses can then be transmitted over great distances during bird migrations.
How much damage has the bird flu outbreak caused?
The ongoing avian flu outbreak is considered the largest observed in Europe to date, according to the European Food Safety Authority (EFSA), an EU agency.
Between October 2021 and September 2022, 50 million farm birds had to be culled in 37 countries.
More than 3,800 highly pathogenic bird flu cases were counted in wild birds. The number of unreported cases is probably much higher, say experts.
Until recently, bird flu mainly occurred in fall and winter.
"Now the virus is also circulating in wild birds during the summer months," Timm Harder confirmed, noting that the animals breed closely in large colonies in the warmer months, providing ideal conditions for the spread of the virus.
The avian influenza wave also reached South America for the first time in the fall. Peru, Venezuela, Ecuador and Colombia were among the countries affected. In Honduras, more than 240 dead pelicans were found in one week alone.
Harder said he is concerned that the virus could spread from South America to Antarctica and endanger penguin populations. Apart from Antarctica, only Australia has escaped the bird flu wave.
Despite the acute outbreak among birds, Harder sees one ray of hope in that the broad spread of the virus could promote immunity in wild birds. Antibodies have already been found in live animals.
This article was adapted from German by Stuart Braun.