Mink farming poses risks for future viral pandemics

Mink are highly susceptible to infection with several viruses that also infect humans. In late 2020, government agencies and academics in Europe and North America repeatedly documented that farmed mink had become infected with SARS-CoV-2, the causative agent of COVID-19 (1, 2). Evidence of mink-adapted viruses spilling back into local communities further demonstrated the poor biosecurity guidelines and practices in the industry (1, 2). With this in mind, some countries—for example, the Netherlands—shut down mink production altogether. Fortunately, the mink-adapted variants of 2020 were not fitter than viruses circulating at the time in humans (3) and, hence, did not spread widely. However, many countries continued to farm mink during the pandemic, and other nations that had stopped have now resumed their activity—for example, Denmark.

SARS-CoV-2 viruses have continued to circulate in farmed mink (4, 5). They frequently go undetected because the animals often show few clinical signs. Importantly, whereas successive waves of new variants have displaced the previous variant from human circulation, this is not the case in some other animals. Many of the strains of SARS-CoV-2 recently present in mink are now no longer found in humans. The establishment of animal reservoirs for viruses that evolve on a separate trajectory from variants in humans sets a potential time bomb for re-emergence of the virus in humans—especially as immunity wanes in the older population and unexposed younger people make up a larger proportion of the population. This is the scenario that led to the emergence of pandemic H1N1 influenza virus from pigs in 2009. SARS-CoV-2 has also been seeded in a wild animal reservoir, the white-tailed deer. Researchers discovered that lineages last found in humans in 2020 were still present in deer in North America well into 2022 (6), again posing a risk over time for reintroduction of antigenically distant variants back into humans.

Although it’s very hard to control these events in wild animals, researchers have suggested that a recently isolated, highly divergent lineage of SARS-CoV-2 found in deer might have infected these animals via farmed mink as an intermediate host (7). And some researchers have speculated that farming mink, either through escape or poor biosecurity, could result in the infection of wild populations of mink or other related mustelids (4). Taken together, these findings further implicate farmed mink as potential intermediate hosts that could pose an ongoing zoonotic or spillback risk.

Mink and ferrets are both mustelids and known to be susceptible to influenza viruses (8). Ferrets are the gold standard experimental model for human airborne transmissibility of influenza viruses because they express receptors for influenza virus with a similar distribution to humans, a pattern also described for mink (8). Avian influenza viruses circulating in mink have frequently gained typical mammalian adaptations similar to those seen during human infections, particularly in the virus polymerase (9, 10). These could act as enablers for further changes in the surface proteins that are the ultimate prerequisite for the generation of the next influenza pandemic. As recently as October 2022, there were disturbing reports of sustained transmission of the panzootic, highly pathogenic avian influenza (H5N1) at a Spanish mink farm (11). At least one mammalian adaptation in the virus polymerase emerged in the mink during this outbreak; in all likelihood, we narrowly escaped a larger disaster, as the incident appears to have been contained.

Any situation in which an RNA virus is allowed to transmit among multiple densely housed animals may lead to the evolution of virus with altered phenotypes, including those with enhanced pandemic potential. Under many current regulatory authorities, an equivalent laboratory “experiment” of this nature would be classed as “gain of function” research and therefore prohibited. Even if it were approved, regulations would require that it be performed under high-containment conditions following an extensive risk–benefit assessment. Current practice in mink farms opens up the possibility for similar adaptation to occur in a completely unregulated and uncontrolled manner.

Furthermore, as they are susceptible to both human and avian influenza viruses (3, 8, 10, 12), mink could also serve as mixing vessels for reassortment, a known pathway to pandemic emergence. A hypothetical route to co-infection is easy to envisage: Mink infected with avian influenza virus—which they acquired from being fed dead poultry or via exposure to wild birds—could become directly co-infected with human seasonal influenza transmitted to them from infected handlers. The last three influenza pandemics arose after the mixing of influenza genes among avian and human-origin strains, which are often thought to occur in a “mixing vessel” species. Swine are often cited as the vessel because pigs are susceptible to many avian and human strains. Pigs, however, are somewhat refractory to H5N1 infections, including the currently panzootic clade 2.3.4.4b viruses (13). Mink clearly are not. Mink farms could provide ample opportunity for the reassortment between H5 subtype avian influenza viruses and human-adapted strains.

Fur farming is banned across many European countries and North American states or territories. Several other regions have set dates for phasing it out. These bans have historically been a response to ethical concerns about the treatment of these animals. We strongly urge governments to also consider the mounting evidence suggesting that fur farming, particularly mink, be eliminated in the interest of pandemic preparedness. Fur farming should be in the same category of high-risk practices as the bushmeat trade and live animal markets. These activities all increase the likelihood of future pandemics. At the very least, biosecurity practices and active surveillance at fur farms must be reviewed, greatly enhanced, and closely enforced.