Mink…not a species most people think about. When they do, it’s most often thoughts about mink coats or fur farming protests. While the mink industry has been on the decline in most regions, there is still a massive number of mink being farmed for fur internationally. Some of these farms are very large. Lots of animals in close contact with each other and with contact with people leads to risk of passing pathogens such as SARS-CoV-2 back and forth.

Mink aren’t the only critters in that family that we’re concerned about. Any species from the mustelid family probably have similar risks and the domestic pet relative of mink is the ferret. Ferrets are niche pets but are far from rare, and many ferrets have close contact with their owners.

So, in terms of mustelids, we have widely different issues in how we manage and interact with them. Regardless, the net result is situations where there’s a good chance for respiratory virus transmission.

What’s the story with mink?

I think it’s fair to say this caught us off guard. No one was talking about mink or risks to/from mink farms at the start. We were thinking about risks from various species (well…some of us were. Lots of people tried to ignore animal risks from what was clearly an animal-origin virus, but that’s a rant for another day).

While we may not have paid attention to mink, SARS-CoV-2 did. Mink are highly susceptible to this virus and there have been widespread outbreaks in mink farms, first identified in the Netherlands but subsequently in in multiple countries (including Canada and the US).

What happens when SARS-CoV-2 makes its way (from people) onto a mink farm is variable and there are lots of knowledge gaps. Some affected farms have had few health issues while others have reported widespread illness and deaths amongst mink, especially pregnant mink. The virus seems to have persisted on some farms, at least for a while, with little apparent disease, while on others more classic signs of a severe disease outbreak have been encountered. Why? We’re not sure. This virus clearly can cause disease in mink, but it doesn’t always. There may be a predilection for severe disease in pregnant mink (as with people) but there are still lots of things we don’t understand.

Can mink infect people?

While, for most species, I say “we don’t know and it would be hard to figure out”, we know that SARS-CoV-2 can be transmitted from mink back to people. This is detectable in mink because of the nature of spread and sampling. If mink and people on a farm all had positive tests, you couldn’t necessarily determine whether some of the people were infected by mink or whether the people all infected each other. However, looking at viral sequences and timing of infection, we can determine more on mink farms. Little, mainly innocuous, changes in the virus commonly occur during replication. Those little changes create a signature that can let us track the virus better. You can see the virus evolve on a farm, where the initial infections are a strain that’s present in people outside the farm (since that’s where it came from), but where the strain changes a bit as it passes through mink. If that slightly modified strain then pops up in people on the farm, it’s strongly suggestive that the virus was spread from mink back into people.

Are mink farms a reservoir for the virus?

That’s a big question and a big concern. “Reservoir” can be considered a few different ways. The main concern is whether the virus can spread on a farm for prolonged periods of time, creating an ongoing source of exposure and new variants.

Can the virus spread from farms to the community?

  • Yes. That’s been shown. It’s rare in the grand scheme of human COVID-19 but it has happened.

Can the virus spread from mink to wildlife or other animals?

  • This is an area of concern. When SARS-CoV-2 is present on a farm, there could be exposure of wildlife through contact with feces (which falls through cages and accumulates under them) or from aerosol exposure. This would include farm animals (farm dogs, barn cats) as well as a range of wildlife that may come and go. Transmission to farm dogs and cats has been identified.
  • Also, infected wild mink were found around an infected farm in the US. These were presumably mink that had escaped at some point from a farm, but regardless, it shows that the virus can make its way off the farm. This has also been seen in Spain, where infected feral American mink were found. Since that species isn’t the native European species, it’s safe to say those mink (or their ancestors) were escapees. Where it goes from there is a good question. If might just burn out but if it’s able to continue to find susceptible hosts (e.g. wild mink, certain mouse species, white tailed deer), it’s possible mink farms could be a source of broader spread, bridging human SARS-CoV-2 with wildlife.

Can the virus be sustained on a farm for longterm exposure?

  • A big factor that might influence risks from mink farms is whether there is longterm, sustained transmission. If the virus enters a farm, burns through it quickly and is eliminated naturally or via a cull, that’s one thing. There’s lots of transmission but over a short period of time. If SARS-CoV-2 can enter a farm and continue to spread over months to years through continuing to find new susceptible mink to infect (or re-infect), the risks probably increase substantially. We don’t know how much of a risk that it, but there is some risk as the virus has seemed to have maintained itself on some farms for prolonged periods of time. There’s limited info on that because often infected farms are culled or longterm testing isn’t done (or reported), but it’s something we need to watch.

Are mink a source of new variants?

In the first 3 parts of this review, I’ve dismissed the potential for dogs, cats and pigs to be significant sources of new variants because of reasons like poor susceptibility (pigs), minimal virus shedding (pigs, dogs) or lack of enough animals in close contact for sustained transmission within the animal population (dogs, cats).

Unfortunately, mink create the perfect storm for new variant emergence. They are a highly susceptible species that can effectively transmit the virus mink-mink and mink-human, and they are raised in large enough groups that there can be widespread and sustained transmission. Since variants emerge as random mutations and the likelihood of that is dependent on lots of transmission, variant emergence is a concern on large mink farms.

“Mink strains” of the virus have been identified. Whether that’s because they change to be better able to infect mink or a random change isn’t clear. However, it provides a way to track certain strains. With early outbreaks, there was concern about a mink variant that was identified in the Netherlands. There was concern that a common mutation associated with mink strains (Y453F) that spread from mink farms into the general human population in Denmark might be less responsive to antibody-based treatments (important therapies for high risk people with early infection). However, there was no evidence that they would compromise vaccines or be more dangerous in people, and that variant hasn’t ended up being a major issue. Fortunately, it didn’t end up being a significant problem as this variant wasn’t any worse than ‘regular’ strains. In fact, there’s some (albeit pretty weak) evidence that mink-derived variants might be less virulent in people. I think we have to assume both things could happen…..mink could be the source of new variants of concern as well as new variants that would pose less risk to people. We can’t really predict what will happen.

Realistically, our biggest risk of variant emergence still lies in the human population, since we still have rampant transmission internationally. But, mink are a potential source, and all it takes is one event to cause a problem. Further, as we (eventually) control this virus in the human population, animal reservoirs will become more important as the relative risk from them will increase if true reservoirs are being created through infection of different wild and domestic animal populations.

How about ferrets? Are they as susceptible as mink?

Whether they are “as susceptible” is hard to say since that’s not been compared. However, they are clearly susceptible and are able to effectively transmit the virus to other ferrets. We’ve seen this from multiple experimental studies where ferrets could be infected, could get sick and could pass it ferret-to-ferret.

I was a bit surprised that we didn’t see reports of naturally infected pet ferrets early in the pandemic. That was likely because of limited numbers of ferrets and limited testing. In our surveillance, we only got to test a handful.

Despite the small number of reports, there have been documented infections in pet ferrets.(e.g. Giner al at 2021, Gortazar et al 2021, Racnik et al 2021) As with dogs and cats, it’s likely under-diagnosed and maybe a common event that occurs under the radar in households where people have COVID-19. I’d go on the assumption odds are 50:50 or greater than a ferret from a household with active COVID-19 is, was or will become infected, if it has close and regular contact with people.

The health impact on pet ferrets hasn’t been well described. Some have been sick, but it’s mainly been mild disease. That fits with experimental studies. Some report infections with limited or no obvious signs of disease (e.g. Shi et al,  Schlottau et al.,  Kim et al). However, more serious disease, sometimes requiring euthanasia, has been reported. That might be related to the dose of virus, with high doses used in the experimental study where more serious disease was encountered. The overall health risk to pet ferrets is probably low, but we can’t rule out the potential for severe disease, particularly in older or pregnant ferrets, or ferret with pre-existing health problems.

Can ferrets infect people?
We don’t know. Probably. If ferrets are susceptible (which they are) and mink can infect people (they can), it makes sense that ferrets could. However, the true risk to owners needs to be considered.

Being able to infect a person is one thing. Actually being an important source of infection is another. To pose a risk, ferrets have to have been first exposed to a person with SARS-CoV-2 infection. This would almost always be from an owner. In that type of situation, that owner poses greater risk to other people in the household than the ferret does. The main risk comes in if the ferret leaves the household (e.g. vet visit) during the period that people are infected.

What are the recommendations?

Anyone with COVID-19 should absolutely not go near a mink farm. That’s the big one. If we’re going to continue to farm mink for fur, there needs to be a focus on biosecurity and surveillance. Surveillance is an issue because of cost (who pays?) and the general lack of desire among many parties to know.

In terms of ferrets, the same general approach that we recommend for dogs and cats would apply:

  • If you have COVID-19, try to limit or avoid contact with your ferret
  • If your ferret has been exposed to someone with COVID-19, keep it away from other people or animals.
  • If your ferret has been exposed to someone with COVID-19 and is sick, let your vet know, to determine whether it might be infected. (Do that by phone, at least initially, rather than showing up to the vet clinic with your ferret).

 

Next up…horses