We have a lot of different concerns about SARS-CoV-2 on mink farms. Mink are really susceptible to this virus, and human-to-mink transmission isn’t uncommon, so if we continue to farm mink, we will continue to expose mink to SARS-CoV-2 as long as the virus is still circulating in people.

Two of the biggest concerns relate to the potential for infection of mink to make things worse in the “big picture:”

  1. Development of mutations of the virus in mink that lead to new variants of concern (VOCs) that spread back to people
  2. Spread of the virus to other animal species

For this post, I’ll focus on concern #2.

We want to keep COVID-19 a human-only disease as much as possible. If it gets established in animal populations, especially wildlife, that changes the game in terms of disease control. I doubt we’re going to eradicate this virus entirely regardless, but if it establishes animal reservoirs, there’s no chance whatsoever.

Furthermore, if SARS-CoV-2 gets established in animals (of any species), we may have to deal with greater risk of VOCs. VOCs are already worsening and dragging out this pandemic, and with more transmission (in animals or people), there will be more VOCs. Mutations are a normal event when viruses replicate. The more a virus spreads, the more mutations occur, and the greater the chance that a mutation that impacts transmissibility, virulence or vaccine evasion emerges.

For a virus to establish an animal reservoir that poses a risk to people, a few things need to happen:

  • Animals have to be exposed, directly or indirectly, to infected people.
  • The animal species has to be susceptible to the virus, and able to transmit it between individuals.
  • The species must live in large enough groups for sustained transmission within the population.
  • There has to be a mechanism to spread the virus back to people from the animals

That might seem like a tough list to fulfill, but it’s not impossible (and it’s what has happened with some other zoonotic viruses).

Therefore, we want to keep this virus away from animals, particularly wildlife.  This is where we come back to mink.

A recent paper in Transboundary and Emerging Diseases (van Aart et al. 2021) reports on surveillance for SARS-CoV-2 in dogs and cats on mink farms in the Netherlands, where SARS-CoV-2 was identified in over half of the mink farms in the country.

  • 13 farm dogs and 101 (mostly feral) cats (69 adults and 32 kittens) on 10 farms were tested.
  • SARS-CoV-2 was detected by PCR in 3 (4.4%) adult cats and 1 (7.7%) dog.
  • Since PCR testing just tells you a single point-in-time result, antibody testing was used to identify prior infection. Antibodies to the virus were found in 18% of adult cats, 0% of kittens and 2 (15%) dogs.
  • When PCR and serological test results are combined, 19.4% of adult cats and 15% of dogs had evidence of infection with SARS-CoV-2.

That’s pretty impressive.  There’s no guarantee that all the dogs and cats were exposed to the virus by mink vs an infected farm worker, but mink-to-cat transmission seems reasonable since these were mostly feral cats that would not have interacted closely with people. The source of infection of in the dogs is harder to tease out since they presumably had closer contact with farm personnel.

Interestingly, none of the nine domestic cats that were tested were positive, despite having infected owners. Infection was only found in the feral cats. Whether all the cats were infected by the mink or there was some subsequent cat-to-cat transmission (a likely scenario) is also impossible to discern.

Mink-to-cat infection would have been through indirect exposure, because the cats generally wouldn’t have direct contact with the mink in their cages.  This raises concerns that other species could have been similarly infected through indirect contact with the mink, including small mammals like mice, or from direct/indirect contact with an infected cat (e.g. mouse surviving an encounter with an infected cat).

These concerns are why there is wildlife surveillance conducted around many infected mink farms. That type of work has also found SARS-CoV-2 in “wild” (or more likely “previously escaped” free-ranging) mink.

What’s the relevance of all this?  It’s hard to say.

However, it supports plausible concerns about animal-to-animal transmission of SARS-CoV-2 and the need to reduce exposure to animals.  Vaccination of mink is a consideration, and I’ve written about that recently. Ultimately, the best way to reduce the risks associated with the virus in animals is to control this disease in people.

Overall, this shows the importance of the “One Health” approach and a need to be proactive to identify and hopefully prevent problems, rather than our typical reactionary approach whereby we wait for definitive proof of an animal-related issue before putting any real effort into addressing the risk.