Mink are not a species most people think about. When they do, they typically think of mink coats or fur farming protests. While the mink industry has been on the decline in most regions for many years, there is still a massive number of mink being farmed for fur internationally. Some of these farms are very large, which makes for lots of animals in close contact with each other, and in contact with their human caretakers, which leads to the risk of pathogens such as SARS-CoV-2 passing back and forth between them all.

Mink aren’t the only critters in the mustelid family that are a concern when it comes to SARS-CoV-2. Any species from the mustelid family probably has similar susceptibility to the virus.  The domestic pet relative of mink is the ferret. Ferrets are “niche” pets but they’re far from rare, and many ferrets have very close contact with their owners.

So, we have widely different issues in how we manage and interact with different mustelids, whether on a farm, in the home or in the wild. Regardless, the net result is situations where there’s a good chance for respiratory virus transmission.

What’s the story with mink and SARS2CoV-2?

There’s been a lot of attention paid to mink during the COVID-19 pandemic, even since the original review I posted last October.  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 of the outbreak, even among those of us who were thinking about risks from other species (lots of people tried to ignore animal risks altogether from what was clearly an animal-origin virus, but that’s a rant for another day).

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

What happens when SARS-CoV-2 makes its way (from people) onto a mink farm varies, and there are still lots of knowledge gaps. Some affected farms have had few health issues, while others have reported widespread illness and deaths amongst mink, especially older or pregnant mink. The virus seems to have persisted on some farms, at least for a while, with little apparent disease, while on others it seemed to burn through the population and then disappear like a more classic respiratory outbreak. 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 mink of a certain age, or in pregnant mink (as with people), but there are still lots of things we don’t understand.

Can mink infect people with SARS-CoV-2?

For most species, I say “we don’t know if they can infect people and it would be hard to figure out”, we know that SARS-CoV-2 can definitely be transmitted from mink back to people, because of the nature of spread and sampling that has been done on mink farms and the people who work on them. 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, viral sequencing and the timing of infections on some mink farms has provided more information than we can usually get. Tiny, mainly innocuous changes in the virus commonly occur during replication, and those changes create a kind of “signature” that can help us track the virus better. Using sequencing, you can track how the virus evolves on a farm, where the initial infections are a strain that’s present in people in the community (since that’s where the virus usually comes from), and then the strain changes a bit as it’s transmitted over and over between 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 SARS-CoV-2 virus?

That’s an important 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 to people (and possibly wildlife) on the farm, including new variants of the virus.

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

  • When SARS-CoV-2 is present on a farm, there could be exposure of a range of wildlife that may come and go from the property through contact with mink feces (which fall through the animals’ cages and accumulate under them) or from aerosol exposure (e.g. virus in dust particles within the animal sheds). There’s also the potential for exposure of farm animals (farm dogs, barn cats). Transmission to farm dogs and cats has been identified.
  • Infected “feral” mink were found around an infected farm in the US; these were presumably mink that had escaped at some point from a nearby farm, but it shows another way the virus can make its way off the farm.  This has also been seen in Spain, where infected feral American mink were found (and at some distance from the closest mink farm…). Since American mink are not native to Europe, it’s safe to say those mink (or their ancestors) were escapees at some point. Where the virus could go from there is a good question. It might just burn out in the wildlife population (since wild mink are primarily solitary creatures), 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 SARS-CoV-2 virus be sustained on a mink farm long term?

  • A big factor that might influence the risk from mink farms is whether there is long term, sustained transmission of the virus within the farmed animal population. If the virus enters a farm, burns through the population quickly, and is eliminated (either naturally or through culling of infected animals), then there’s lots of transmission but over a very short period of time. If SARS-CoV-2 enters a farm and continues to spread over months (or even) years by continuing to find new susceptible mink in the population to infect (or re-infect), the risk probably increases substantially. We don’t know how much of a risk it is, but we know there’s some risk because the virus already seems to have maintained itself on certain farms for a long period of time (months). We still have limited information about the long term outcomes, because many infected farms were depopulated, and on those that weren’t long term testing wasn’t necessarily done (or reported).

Are mink a source of new variants of SARS-CoV-2?

In the first 3 parts of this review update that I posted last month, I dismissed the potential for dogs, cats and pigs to be significant sources of new SARS-CoV-2 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-to-mink and mink-to-human, and they are raised in large enough groups that there can be widespread and sustained transmission. Since variants emerge due to random mutations, and the likelihood of that is dependent on lots of virus replication, and more transmission leads to more replication, variant emergence is definitely a concern on large mink farms.

“Mink strains” of SARS-CoV-2 have been identified. Whether that’s because the virus adapted to be better able to infect mink or the changes were purely random (i.e. conferring no specific advantage to the new strain in terms of infecting more mink) isn’t clear. However, the new strains provide a way to help track virus transmission in some situations. In early outbreaks, there was concern about a mink variant that was identified in the Netherlands. There was also concern that mink strains with a common mutation (Y453F) that spread from mink farms into the general human population in Denmark might be less responsive to antibody-based treatments used in people  (these are important therapies for high-risk people with early infection). However, there was no evidence that these mink strains would compromise vaccine efficacy, and fortunately they didn’t end up being a significant problem as they weren’t any worse than “regular” strains in people in terms of disease. 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 that pose more risk to people, as well as new variants that would pose less risk to people. We can’t really predict what will happen, or when.

Realistically, the biggest risk of variant emergence still lies in the human population, since we still have rampant human-to-human transmission of SARS-CoV-2 internationally. But mink are a potential source, and all it takes is one event with the right (or wrong) mutation 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 to SARS-CoV-2?

Whether ferrets are “as susceptible” to the virus is hard to say, since they haven’t been directly compared. However, ferrets are clearly susceptible and are able to effectively transmit the virus to other ferrets. We’ve seen this in multiple experimental studies where ferrets were infected, got sick and were able to transmit the virus 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 of ferrets.  Despite the small number of reports, there have been documented infections in pet ferrets (e.g. Giner et al. 2021, Gortazar et al. 2021, Racnik et al. 2021) As with dogs and cats, infection in ferrets is likely under-diagnosed, and may actually be a common event that occurs under the radar in households where people have COVID-19. I assume the odds are 50:50 or greater than a ferret from a household with active COVID-19 in a person is, was or will become infected, if it has close and/or regular contact with infected people.

The health impact of SARS-CoV-2 infection on pet ferrets hasn’t been well described. Some get sick, but it’s mainly been mild disease, which fits with the findings of experimental studies as well. Some report infections with limited or no obvious signs of disease (e.g. Shi et al. 2020,  Schlottau et al. 2020,  Kim et al. 2020). However, more serious disease, sometimes requiring euthanasia, has been reported. That might be related to the dose of virus, as high doses were 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 ferrets with pre-existing health problems.

Can ferrets infect people with SARS-CoV-2?

We don’t know, but they probably could. Since ferrets are susceptible and can infect other ferrets, and we know that mink can infect people, it makes sense that ferrets could also infect people. However, the true risk to ferret 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 first be exposed to a person with SARS-CoV-2 infection. This would almost always be their owner. In that situation, the owner poses greater risk to other people in the household than the ferret does. The main risk to others is if the ferret leaves the household (e.g. if it needs to be taken to a veterinary clinic for an exam) during the period when the household members are infected.

What are the recommendations when it comes to mink, ferrets and SARS-CoV-2?

Anyone with COVID-19 should absolutely not go anywhere near a mink farm. Period.  That’s the big one. If we’re going to continue to farm mink for fur, there needs to be a strong focus on biosecurity and surveillance for this virus. Surveillance is an issue because of cost (i.e. who pays?) and the general lack of desire among many parties involved to really know what’s going on (especially if the mink don’t look sick).

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

  • 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 and animals.
  • If your ferret has been exposed to someone with COVID-19 and is sick, let your veterinarian know, to help determine whether it might be infected with SARS-CoV-2 (do that by phone, at least initially, rather than showing up to the veterinary clinic directly with your ferret).

Next up for a review update… horses.

What we know about SARS-CoV-2 in pigs hasn’t changed a lot since the first version of this post. It’s still a fairly “good news” situation, but one that could also use some more investigation.

Are pigs susceptible to SARS-CoV-2?

Kind of, but not really.  There are somewhat conflicting experimental data, but the debate is really whether pigs have very little vs no susceptibility to the virus. There is obviously a difference between “no” and “yes, but only a little,” but from a big picture standpoint, we don’t currently have evidence that there are issues for pig health, pigs as a source of infection in people, or pigs as a potential reservoir for emergence of new virus variants.

Why did we talk a lot about pigs initially?

At the start of the pandemic, we were worried about the potential for this virus to infect pigs because of their susceptibility to the original SARS virus, and because it was predicted that they would be quite susceptible based on their ACE2 receptor. ACE2 is the receptor that the virus uses to enter the cells of the host. If the virus can’t enter cells, it can’t infect them. Different animals have slightly different ACE2 receptors on their cells, and the pig ACE2 receptor is quite similar to the human ACE2 receptor, suggesting there could be similar susceptibility in both species. Looking at ACE2 receptors has been interesting, but we’ve seen that it doesn’t always link up with what actually happens, as was the case in pigs.

Trying to grow the virus in cell lines from an animal species can provide some additional information about potential susceptibility. In one study, the SARS-CoV-2 virus was grown in 2/3 pig cell types, but did not damage those cells. In another study, the virus grew in the pig cells and caused some cell damage. Similar results were reported in another cell line study.

These all contributed to the concerns about the susceptibility of pigs; however, there are limitations to what in vitro studies can tell us. To get the real story, we need to look at actual pigs. So far, all the information we have about the virus is live pigs is from a couple of experimental studies.

So, are pigs susceptible to SARS-CoV-2?

In one study,  5 pigs were experimentally infected and mixed with 3 other pigs. In another study, 9 pigs were infected and then mixed with 3 other pigs. A third study infected 9 pigs and added 6 uninfected pigs.

  • Nothing remarkable happened in any of these studies. None of the pigs got sick, and all samples collected were negative for the virus. Antibodies against the virus weren’t found in any of the pigs. This all indicated that the pigs were not infected, and there was a big sigh of relief as it appeared that concerns about pigs were unnecessary.

In yet another study, pigs were exposed to the virus via the nose, the trachea and by injection. All the pigs stayed healthy and the virus wasn’t detected in any samples from the pigs, but antibodies against the virus were found in pigs that were injected with the virus. That shows the body responded to the virus, but since it was injected, it’s not really relevant to the natural situation.

However, leave it to Canadians to be disruptive – another experimental study in pigs changed the story a little bit.  It didn’t raise major concerns, but it suggested things are not quite as clear cut as we’d hoped.

  • In the Canadian study, 16 pigs were exposed to a higher dose of the virus, and nothing remarkable happened. Some developed mild discharge from the eyes for a few days. One had a slight cough and was mildly depressed for a few days.
  • Low levels of virus were detected by PCR in respiratory samples from two of the sixteen pigs, but live virus could not be isolated.
  • The virus was isolated from a lymph node of one pig, and antibodies were detected in the blood of two pigs, supporting some level of true infection.
  • Two pigs were added to the exposed pigs 10 days after inoculation, and they didn’t become infected.
  • So, this study showed some degree of susceptibility in pigs, but with infrequent mild disease and no evidence that pigs are infected to the degree that they would be able to pass on the virus to another animal (or person).

Another similar study involving inoculation of pigs with SARS-CoV-2 by different routes (blood, trachea, nose) also found none of the pigs got sick. Viral RNA was detected from oral, nasal or rectal swabs by PCR in some inoculated pigs, but virus wasn’t isolated and transmission to in-contact pigs wasn’t observed. These two studies are still consistent with a “don’t worry” narrative – if there was human-to-pig transmission of SARS-CoV-2, the pig would not likely get sick and would not likely be able infect other pigs or people.

Have any pigs outside of a lab been infected with SARS-CoV-2?

There are no reports of any naturally infected pigs, but I’m also not aware of any actual testing of pigs on farms. (“We don’t think there’s anything to investigate” is often stated with an unspoken “we don’t really want to know.”)

Field data are always useful because experimental studies don’t tell the full story. So, some data about pigs exposed to infected farmers would be useful to have, to round out the story. There have to have been large numbers of pigs exposed to infected people, especially on some large farms in areas where COVID-19 has run rampant. The fact that we haven’t heard rumblings of problems is good. However, without formal surveillance, it only tells us we don’t have evidence of a significant pig health issue. We can’t rule out the potential that pigs get infected but don’t get sick. That’s why we really should have more active surveillance, looking at pigs that have potentially been exposed.

What’s the recommendation when it comes to SARS-CoV-2 and pigs?

The same as for other animal species. If we keep infected people away from animals, we don’t need to worry about human-to-animal transmission, or any subsequent animal health or animal-to-human transmission issues. While the odds of someone infecting a pig are very low, it’s best to avoid exposing pigs to infected people whenever possible. That may not be an option on small farms run by one person or a family, but the more we can keep infected people away from animals (of all kinds), the better.

What about new variants of SARS-CoV-2 in pigs?

That’s the wild card for all our animal discussions. Experimental studies were done early in the pandemic and used the original strain of the virus. The SARS-CoV-2 strains we’re seeing now are quite different, at least in humans. Odds are low that delta or other variants would be much more able to infect pigs, but we simply don’t know. As we see new variants, we need to realize that what we know from earlier work isn’t necessarily still the case. It’s another reason ongoing surveillance would be good, but I won’t hold my breath on that.

There have been calls to develop antibiotics that are just for use in animals, the idea being to have separate antibiotics for animals and people, so that antimicrobial resistance that develops as a result of antibiotic use in animals won’t impact people.

Makes sense, right?

It does, at least at first glance. However, “makes sense” and “will work” aren’t the same.  The concept is sound, but the reality is  different.

When an antibiotic is used in animals, there’s a chance of resistance developing in any of the bacteria carried by that animal, and that includes resistance in bacteria that cause disease in people. Transfer between different kinds of bacteria of genes that cause resistance is also a concern, and can also result in resistance in bacteria that infect people. Antibiotic residues can also contaminate the environment, selecting for more antibiotic resistance, and antimicrobial resistant bacteria themselves can can end up in the environment, potentially exposing animals and people. These are just a few examples of why antimicrobial resistance is such a complex ecological problem.

Let’s say a new antibiotic class is developed that it can kill a range of bacteria of concern in animals, and resistance to this antibiotic doesn’t also confer resistance to other antibiotics.

That would be great.  But, what would we want to do with a drug that kills clinically relevant bacteria and doesn’t create cross-resistance in bacteria that cause disease in people?

  • We’d use it in people, not animals! We need new antibiotic drug classes in people, and since we’re typically targeting the same kinds of bugs in people and animals, any new drug class that works for animals is probably going to be of interest and use in people.

Ultimately, unless we have a drug class that can’t be used in humans (e.g. too toxic), I can’t see us ever having animal-only antibiotics.  That doesn’t mean we shouldn’t think about it, though. We need new drug classes, and maybe some would be used more in animals or in people, which would help reduce some of the issues around emergence of resistance. But animal-specific antibiotics aren’t likely, apart from a small number of drugs that target a really narrow range of bacteria that are only relevant in animals.

Where does this leave us?

Antimicrobial stewardship – by everyone – is the key.

  • There’s still lots that we can do to reduce and improve antimicrobial use, in both animals and people. Importantly, we can do these things today, and for less than the hundreds of millions of dollars required to get a new human antimicrobial drug to market.

A key aspect of stewardship that gets overlooked frequently is improving health systems.

  • Money invested in improving human and animal health systems will reduce the need for antibiotics and improve how we use them. It won’t fix the antimicrobial resistance problem (no one thing will), but it is among the most effective things we can do, and one that has many benefits beyond just antimicrobial resistance.

There’s not as much to update about SARS-CoV-2 in dogs as there was in cats. We have more numbers than we did before, but the overall issues in dogs and our understanding of them haven’t really changed.

Spoiler alert: dogs and owners can both relax, as the risks are very limited.

Are dogs susceptible to the SARS-CoV-2 virus?

Yes, but… not very… maybe.  It depends on what you mean by “susceptible.”

Nice and clear, eh?

There’s a difference between getting infected and getting sick. Yes, dogs can clearly be infected. However, they don’t seem to be as susceptible as cats, and it’s debatable whether they get sick from the infection… I’m still a bit on the fence about that (more below).

How often do dogs get infected?

This is where we’ve gotten the most information recently.  Studies that have looked at dogs living with COVID-19-infected people have generally identified impressive rates of human-to-dog transmission. In these studies, researchers either look for evidence of the virus itself in the dogs (usually using a PCR test) or they look for antibodies against the virus in dogs’ blood. The problem with PCR testing is that there’s a very narrow window of virus shedding in this species, so it’s easy to miss the window (in which case the test comes back negative even though the dog was infected) based on sampling logistics and timing. With good antibody tests, we can get a good idea of whether dogs were infected in the past (although there are potential issues there too) because the antibodies hang around for a lot longer.

Early in the pandemic, the virus was identified by PCR in dogs from infected households, setting the scene for further studies. The best initial effort was from Hong Kong, where they identified the SARS-CoV-2 virus in nasal, oral and rectal swabs from 2/15 dogs that were quarantined because their owner was infected. Neither dog had signs of infection, both developed antibodies to the virus, and gene sequencing showed that the viruses in the dogs were the same as the viruses in their respective owners. Subsequent work has found similarly, fairly low rates of PCR-positivity among dogs with household exposure to infected people (e.g. Hamer 2021).

Serological studies looking for antibodies against SARS-CoV-2 in dogs have shown that transmission is actually much more common than this, with rates of up to 46% in dogs from infected households (e.g. Stevanovic 2021,  Hamer 2021). The results from our (hopefully soon to be submitted) Canadian study were similar, with about 43% of dogs with household exposure testing positive for antibodies to the virus.

Some studies have tested blood from undefined populations of dogs, for example by testing leftover samples of blood submitted by veterinarians to diagnostic labs. These studies tell us very little, because there’s no accompanying info about the dogs, particularly whether dogs were exposed to anyone with COVID-19. Not surprisingly, low rates of antibody detection (0.2-3.4%) have been found in these stuies (e.g. Ito et al. 2021,  Patterson 2020, Smith 2021). Whether these positives represent infected dogs from households with infected people vs false positive results isn’t clear.

Do dogs get sick when they are infected?

That’s still unclear. In small experimental studies, dogs could be infected but didn’t show any signs of disease (e.g. Shi et al. 2020, Bosco-Lauth et al. 2020). Field studies are harder to evaluate because there’s nothing specific about the clinical signs we’d expect to see from SARS-CoV-2 infection in a dog (e.g. lethargy, coughing, sneezing, decreased appetite and other flu-like signs). These non-specific signs can be caused by lots of diseases in dogs, so if we find a dog that has evidence of previous SARS-CoV-2infection (i.e. antibodies) and it was reported to have been sick, it’s hard to say whether it was sick because of SARS-CoV-2 or whether it was sick because of something else and had an incidental SARS-CoV-2 infection. Larger and better designed studies are needed to figure that out.

In our preliminary work, we found an association between antibodies in dogs and the owner reporting that the dog was acting sick around the same time the owner was infected. That is to say, this finding was significantly more common in dogs that had antibodies than in those that didn’t, suggesting infection with SARS-CoV-2 could have caused (or contributed)  to illness in (at least some of) the dogs. However, the signs that were reported were very mild and often vague (e.g. “the dog was a little quieter”), so while it might suggest that dogs can get sick, it would seem any illness is generally really mild.

Serious disease from SARS-CoV-2 has been identified in cats, and there has been some chatter about a very small number of dogs getting really sick or dying. However, when you consider the massive number of people that have been infected and the apparent high rate of transmission to dogs, if there was a true serious disease issue, I think we’d see more evidence of it by now. Also, we’re going to find incidental infection in some dogs with serious diseases or that die for other reasons, just because of the large number of infected dogs.

My take-home message on this at this point – with the variants that are currently circulating – is that SARS-CoV-2 poses very little health risk to dogs. More work on that is underway, though.

Can dogs infect other animals or people with SARS-CoV-2?

Probably not. Dogs are probably much lower risk than cats, and the even the risk from cats is still unclear. The fact that the virus has been isolated from dogs (e.g. Hamer 2021) is a concern, because if there was live virus in the dog’s nose, you have to assume there was some risk of exposure to individuals in-contact with that dog. Whether there was enough virus being shed to actually infect someone is completely unknown, and it’s probably exceptionally rare for a dog to be shedding enough virus to pose a risk. Experimentally, dog-to-dog transmission has not been seen. That’s not a guarantee that it can’t happen, since these experimental studies were conducted in an artificial environment with very small numbers of animals, but it provides more support of limited risk.

Overall, I’d say the risk of SARS-CoV-2 infection from dogs is very low. I don’t think we can say it’s zero (we can’t guarantee much with this virus), but I think it’s very unlikely that a dog would pose a realistic risk to a person or another animal.

That said, why chance it? If a dog is infected or at risk of being infected (e.g. living in a household with an infected person), it should be kept away from other people and dogs. Dogs interact nose-to-nose and nose-to-bum a lot, and have a lot of contact with their faces, so keeping exposed dogs under control and away from others is a reasonable precaution. We’ve also seen transmission of other respiratory viruses between neighbouring dogs through fence-line contact, so this should be avoided as well, just in case.

Could dogs be an important reservoir of the SARS-CoV-2 virus once it’s controlled in people?

No, dogs are not susceptible enough to the virus to serve as a reservoir. To be a reservoir, the virus would have to be able to keep spreading dog-to-dog. That’s not going to happen because of the low susceptibility and short shedding time in this species. You’d need a very large number of dogs in regular close contact to even begin to have a risk, and then only IF dogs were able to effectively transmit the virus.

What about variants of concern (VOCs) in dogs?

Variants of SARS-CoV-2 such as alpha have been reported in dogs (e.g. Barroso-Arevalo 2021 and Hamer 2021). That’s expected as different variants become dominant in people, because people are the source of infection in dogs. Unless a human variant has more or less affinity for dogs than the original strains (possible, but not very likely), we expect the strains infecting dogs to be a reflection of the strains infecting in humans. I assume that large numbers of dogs have been infected with the delta variant as it now dominates in people as well.

Could new variants of concern emerge in dogs?

Almost certainly not. Variants develop by chance during viral replication. The more transmission, the more replication, the greater the risk of a variant emerging through random mutation. Since dogs are not going to be involved in sustained transmission of the virus, there’s pretty much no chance we’d see a new variant emerge in dogs and spread back to people. Yes, it just takes one replication error and transmission event for a variant to emerge, but the odds of it happening from a human-to-dog transmission AND the dog then infecting a person are pretty much zero.

Could dogs be a bridge to infecting wildlife with SARS-CoV-2?

Probably not, or at least much less likely than cats. Their low susceptibility, short period of infection, limited (if any) infectivity to others, and limited direct contact with susceptible wildlife mean the odds of dogs being infected by their owners and then infecting wildlife are very low.

So, we shouldn’t worry about SARS-CoV-2 in dogs?

Worry, no.  But we still need to pay attention to it.

What should be done with dogs?

Do the same as for cats:

  • If you are infected, try to stay away from animals… all animals, human and otherwise.
  • If your dog has been exposed to a person with COVID-19, keep it inside and away from others.

The risks to and from dogs are exceptionally low, but precautions are common sense and easy… a few short term mild hassles for some peace of mind.

Ultimately, dogs are part of the family – so if your family is isolating, the dog should be included in that too.

Back in October/November 2020, I wrote a series of posts about what we know about SARS-CoV-2 and different animal species. It’s a dynamic field, so it’s about time I got around to updating them. We’ll start back at the beginning with one of the most susceptible domestic species: cats.  A lot of research about SARS-CoV-2 in cats in the last year has largely supported our initial observations and have helped refined what we know.

Are cats susceptible to the SARS-CoV-2 virus?

Yes, cats are clearly susceptible. No change here. We’ve known that for a while and more research has just solidified that. More on that below.

How often do cats get infected?

There are a lot of papers now about SARS-CoV-2 in cats. Some are very good. Some are interesting but low-impact single case reports, and some are rushed studies (“I want to be first, not the best”) that use small sample sizes, indistinct populations or cherry-pick interesting results from what should have been more comprehensive, bigger studies.

Overall, it’s apparent that human-to-cat transmission is common in households where people have COVID-19. A small number of studies have looked at active infection using PCR testing +/- virus isolation, which is tough to do logistically. It’s a lot of work to identify infected people, arrange to sample their pets and (typically) go to the household to do that. A study from Texas (Hamers et al. 2021) identified the virus in 3/17 (18%) of cats in infected households. The results of our Canadian study (which have been presented but not yet published) were fairly similar.

Testing for virus only tells us part of the story, because of difficulty with sampling infected cats soon enough to catch them during their short-term active shedding period. We assume that we often miss infections because we get into the household to sample too late. That’s why more studies are based on looking for antibodies in the blood of house cats as an indicator of previous infection. It’s less definitive than detecting the virus, since the performance of antibody tests can be variable, but with good tests it can really help our understanding of the situation.

When we look at antibodies to SARS-CoV-2 in cats in infected households, the apparent infection rates go up. The Texas study reported a seroprevalence (the percentage of cats with antibodies) of 44% (7/16), and the seroprevalence in our preliminary data from Canada was even higher at 67%.  Other studies have had variable results (for example,  a study from Peru found a seroprevalence of 17-30% among cats from infected households, depending on how the testing was interpreted), but the take-home message is that human-to-cat transmission of SARS-CoV-2 is pretty common.

There are also many studies that have looked at antibodies against SARS-CoV-2 in the general cat population, usually without any information about whether the cats were exposed to an infected person. These studies are fairly easy to do (for example, by testing leftover blood from samples collected for other purposes, or collecting convenience samples from cats presented to veterinary clinics or shelters), but their value is variable. Typically, these studies report low seroprevalence among cats. One study reported close to 10% prevalence, but <2% is more common (e.g. Dileepan 2021, Klaus 2021, Smith 2021, Stranieri 2021, Udom 2021, van der Leij 2021). Positives could be cats that actually had infected owners, but the information wasn’t known or collected, or false positives, due to an imperfect test. In our surveillance study of cats from shelters or spay/neuter clinics, we found there was often very limited history about the cats (e.g. cats recently acquired off Kijiji), so we can’t use the history to put the results into context.

There’s always a lag between disease occurrence and publication of reports, so it would be expected that rates of infection in cats would increase over time as the human pandemic continues and more cats become exposed.

Risk factors for infection in cats haven’t been carefully investigated yet, often because of fairly small study sizes. A Brazilian study reported that cats that slept in the bed were at higher risk of being seropositive (Calvet 2021), something we also found in our Canadian study. That’s not too surprising as things that increase close contact (direct or shared airspace) presumably increase the risk of human-to-cat transmission.

Do cats get sick from SARS-CoV-2?

They can, but most often if appears they don’t. Experimentally, clinical signs in cats have been pretty unremarkable. Most infected cats have been reported to be healthy, but it’s not always the case. There are reports of sick cats, including a paper describing a fatal infection in a cat in the UK.

In our surveillance, cats that had antibodies against the virus were more likely to have been reported as being sick at the same time as the COVID-19-infected owner, but most of the time any illness in the cats was mild (e.g. coughing, sneezing, quieter than normal). I get lots of anecdotal reports about sick cats that have been exposed to the virus, and I suspect many of them really are due to to SARS-CoV-2. When an otherwise healthy adult indoor cat with no contact with other cats develops signs of upper respiratory tract infection around the time its owner had COVID-19, there aren’t many other probable causes for the cat’s illness. However, at the same time, since infection of cats seems to be quite common, we’d expect to find incidental infection of cats that get sick or die from various other unrelated things. A small study by the US CDC (yet to be published) explored this, and the take-home message was that some cases of severe disease seemed to occur but much of the time, cats that died while infected didn’t die from the effects of SARS-CoV-2.

Similar to people, most exposed cats probably don’t get sick or get mild disease. A subset get more serious disease, and a smaller subset may even die from the infection. The relative size of those different groups is completely unknown.

Can cats infect other animals with SARS-CoV-2?

Yes. Experimentally, cats have been shown to infect other cats. That’s also been seen outside the lab, such as the high-profile outbreak in lions and tigers in the Bronx Zoo, where cat-to-cat transmission was more likely than all the big cats being individually infected by people. We also investigated one large group of infected cats, and it’s most likely there was cat-to-cat spread there too, rather than all human-to-cat infections.

Can cats infect people with SARS-CoV-2?

We still don’t know for sure if cat-to-human infection occurs. Since cats can infect other cats, we have to assume there’s some risk of them infecting people, but sorting out how much of a risk there is is a challenge. If someone got infected by a cat, it would be very difficult to determine that they got it from a cat vs a human contact, because the virus is still circulating widely in people, and contact with the infected cat would probably coincide with contact with that infected cat’s (probably infected) owner.

I think we have to assume that cat-to-human transmission is biologically possible and has probably happened. However, in the real world, it’s probably very rare given the dynamics of cat-to-human contact. If my cat gets infected, he got the virus from me, my wife or my kids. In that event, transmission from the cat to other people in the household is possible, but transmission between people is far more likely. Most cats don’t encounter a lot of different people, especially when their owners are sick. The biggest risk is likely when a cat leaves the house, such as to go to a veterinary clinic, or is surrendered to a shelter. We’ve detected infected cats in shelters, so it’s a plausible scenario, and it’s why we recommend asking about owner infection status prior to bringing animals into clinics, shelters or other places outside the home.

Do we have a SARS-CoV-2 vaccine for cats, and should we consider vaccinating cats?

My current answers are “kind of” and “no.”  There’s a SARS-CoV-2 vaccine (of unknown safety and effectiveness) licensed for use in cats in Russia.  In North America, there’s an experimental vaccine that has been used in mink and some zoo animals, and it would be the best option if we needed a vaccine. However, I don’t see a need at this point given the apparent rarity of severe disease. There’s more information on the possible utility (or not) of SARS-CoV-2 vaccines in pets in an earlier post. 

Could cats be an important reservoir of SARS-CoV-2 once it’s controlled in people?

Probably not. Cats are pretty susceptible to the virus, but they don’t shed it for long. To maintain the virus in circulation in the cat population, an infected cat would have to interact with another susceptible cat within a few days (and on and on…). Most cats don’t do that. In community cat colonies, I could see it spreading through the group, but it would likely burn out quickly as most of the cats became infected and recovered, assuming there’s some degree of immunity to re-infection (which seems to be the case) . In order to maintain a virus in a population when it’s only carried for a short period of time, you need a lot of animals and a lot of animal-to-animal contact. That’s more of a concern with some wildlife species (but that’s a story for another day).

Could cats be a source of new SARS-CoV-2 variants?

Probably not. Variants occur because of random mutations. These occur when the virus replicates. So, the risk of variant emergence is directly proportional to how much transmission (and therefore virus replication) is going on. Since we don’t expect sustained transmission in the cat population, there’s limited risk of variants emerging in there.

So, should we worry about SARS-CoV-2 in cats?

Worry, no, but we should pay attention to it.  There’s a cat health risk, and we want to avoid that by reducing contact of infected people with cats. It’s probably most important with older cats and cats with underlying diseases that may make them more susceptible to severe disease.

The risk of cats spreading the virus in a household is limited, but can’t be ignored. When you have someone isolating from the rest of the household (e.g. living in the basement), we want to make sure pets like cats are considered, so they’re not tracking the virus from the infected person to the rest of the family. It’s easy to see how someone might do a great job staying away from other people, but not think about the cat that runs back and forth between them and the rest of the family.

We also don’t want cats tracking the virus out of the household and exposing other cats or wildlife. The odds of this causing a big problem or creating a wildlife reservoir are very low, but not zero. A little prudence makes sense.  Keep cats indoors if they’re in contact with any infected people.

What should be done with cats?

This hasn’t changed from the first post….

  • Cats are people too, when it comes to SARS-CoV-2.
  • If you are infected, try to stay away from animals – all animals, human and otherwise.
  • If your cat has been exposed to SARS-CoV-2, keep it inside and away from others.

For the past year or more, we’ve been trying to track infectious upper respiratory tract disease (officially known as “canine infectious respiratory disease complex (CIRDC)” but more commonly called “kennel cough”). It’s a tough thing to do since testing is limited, the disease is always present to some degree in the dog population, and there’s no formal reporting system. Enquiries about CIRDC in different areas seem to fill my inbox in waves, but that’s probably more related to reporting (especially social media rumours) vs actual frequency of illness. This week’s been busy so far  with a dozen or so emails asking about things like “new” respiratory diseases, or specific things like canine influenza (and it’s only Monday…).

We’re still not sure what’s going on. It does seem like there’s increased CIRDC activity over a lot of North America right now, and it’s been going on to some degree for quite a while. When we think about increases in respiratory disease reports, there are a few  potential causes (as I have mentioned many times before):

Increased disease caused by the usual suspects

  • This is my main guess at this point for what’s currently going on. Common things occur commonly, and that’s particularly true for the variety of bacteria and viruses that cause CIRDC in dogs.
  • A few potential reasons for the increased disease from these pathogens can be postulated. One is there’s more dogs mixing with each other now as people start to increase activity and get together post-lockdowns, and as people prioritize safer outdoor activities (often with their dogs). Combine that with a surge in new dogs and potentially decreased vaccination (due in part to overloaded veterinary clinics and access difficulties from earlier restrictions), and it’s easy to see how we might have more disease.
  • Another potential dynamic is increased use of oral “kennel cough” vaccines, as they are easier to administer to some dogs compared to intranasal vaccines. The problem is oral vaccines only protect against one cause of CIRDC (Bordetella bronchiseptica) while intranasal vaccines protect against Bordetella and canine parainfluenza virus (CPIV). That’s important because CPIV is the most commonly diagnosed cause of CIRDC in many areas.

Increased disease caused by a new pathogen

  • We’re always on the lookout for something new, but nothing is apparent yet. With a new virus, we’d be more likely to see widespread transmission in exposed groups, since no dogs would have any immunity. We’re not really seeing that. The cases being reported are more sporadic, as we’d expect with our typical causes of CIRDC. However, we can’t rule out a new pathogen completely, and there are undoubtedly various causes of CIRDC (mainly viral) that we simply haven’t identified yet.  I don’t think it’s the explanation for the current situation, though.

Increased reporting of disease

  • This is probably part of what we’re seeing. There’s more social media use these days so word spreads quickly. One voice can be amplified disproportionately and unsubstantiated claims can be disseminated easily. Further, it feeds on itself. When there’s more buzz about sick dogs, more people that otherwise wouldn’t have said anything chime in. So, we probably hear about a greater percentage of sick dogs simply because people are talking about them when they otherwise wouldn’t have.
  • Also, as more people are at home with their dogs, we probably hear more about the typical mild cases of CIRDC, because owners pay more attention when the dog is coughing beside them all day.

What about SARS-CoV-2?

  • SARS-CoV-2 is very unlikely to be playing a role. We can never say never, since the COVID-19 pandemic is a dynamic situation and we don’t know much about recent variants in animals. However, what we know so far is that infection of dogs and cats with SARS-CoV-2 is quite common, but disease is uncommon in cats and rare in dogs.

What about canine influenza?

  • Canine flu certainly can cause large outbreaks of respiratory disease in dogs. It spreads quickly because of limited immunity in the dog population. There has been some canine flu activity in a couple places in the US in the past few months, but these seem to have burned out (or at least burned down) relatively quickly.
  • There have been social media reports of canine flu outbreaks in Ontario. As far as I know, that’s false. Canine flu is reportable in Ontario, and no such reports have been received from any lab. We haven’t seen canine flu in Ontario since we eradicated it in 2018. I’m always on the lookout for it, but I’m most concerned about flu when there’s an outbreak that has a very high attack rate, including dogs that have had intranasal kennel cough vaccine. We’re still looking but I doubt canine flu is playing a role currently.

What can people who are worried about their dogs do?

  • Reduce contacts with large numbers of unknown dogs. Just like with other respiratory pathogens, the more contacts, the greater the risk of encountering someone that’s infectious.
  • Reduce contact with sick dogs. This can be harder but it’s common sense: if a dog looks sick (e.g. coughing, runny nose, runny eyes), keep your dog away from it.
  • Keep sick dogs at home. (Duh… but you’d be surprised.)
  • Avoid things like communal water bowls in parks that are shared by multiple dogs.
  • Get your dog vaccinated (ideally intranasally) against kennel cough if it tends to encounter other dogs regularly. My dog doesn’t get this routinely since we live in the country and he has a very limited number of other dogs with which he interacts. If I was in town and/or going to dog parks or other places where he’d mix with lots of dogs of unknown status, I’d vaccinate him (especially as he’s getting older now).
  • Consider testing your dog if your dog gets sick. Testing is useful to help figure out what’s going on and maybe to help control things. However, it rarely tells us something that influences care for the individual dog (since we don’t have specific treatments). So, the cost of testing is (understandably) hard to justify for some.

We’re also still tracking cases so people with sick dogs can provide information by filling out our quick survey here:
https://uoguelph.eu.qualtrics.com/jfe/form/SV_eP6E6AzIiJfnDlY

I’ve meant to write more about SARS-CoV-2 in deer, and USDA’s recent announcement of infected deer made me get my butt in gear (warning: long post approaching).

What do we know about SARS-CoV-2 infection in deer?

We now have three different pieces of evidence:

There are two broader concerns with these findings regarding SARS-CoV-2 in white-tailed deer:

  1. Reservoir for human infection: More infected individuals (human or animal) means more potential exposures for people. If this virus is present in deer (or other wildlife) that creates more opportunities for exposure.
  2. Potential for virus mutation: This is the big concern, but linked to the reservoir concern too. As I’ve said since this pandemic started, we really want to keep this a human virus – we don’t want it to spread to other species. Yes, it’s likely present in the original reservoir host population (presumably bats), but if it gets into other species, especially those that live closer to and interact more with people, and/or  are present in larger numbers, the risk of a significant mutation occurring and spilling back into the human population increases.

Virus mutations are random events that happen all the time, but the more transmission there is, the opportunity the virus has to mutate. If there is sustained transmission in a wildlife population (or any population for that matter), it’s essentially guaranteed that new strains (variants) will develop over time. What that means in the bigger (human) picture could vary. Because they’re random, mutations can be good or bad for the virus, by making it either more or less transmissible, for example. What we’re concerned about is emergence of new variants that are more adept at infecting people and/or harder to prevent or treat, and those variants then finding their way from the animal population (in this case deer) back into people, and then spreading from person-to-person.

In the big picture, is SARS-CoV-2 in white-tailed deer a problem?

It’s hard to say. Currently, human-to-human transmission is still the problem. New mutations are going to develop in people because of widespread transmission internationally, and until we have good vaccine coverage everywhere (not just rich countries) we’ll have persistent and high risk of new variants emerging in people.

Deer probably contribute little to the risk, at least at this point.  For a deer variant to be of concern, it has to find and infect a susceptible person, and human-deer contact is fairly limited in the grand scheme of things. It’s possible, but a susceptible person is still more likely to be infected by another person than by a deer at this point.

While SARS-CoV-2 remains a human pandemic, deer are likely a niche issue. As the virus eventually gets controlled in humans (we hope), then wildlife reservoirs become more important, if they can be a source of new variants.

What do we need to do?

We need more information, as usual. We need to know lots of things like:

  • How widespread is infection in deer?
  • What’s the likelihood that an infected deer would infect a person through routine contact (e.g. hunting, handling animals or carcasses)?
  • Are variants emerging in deer?
  • Are deer infecting other wildlife?
  • Will transmission in deer be sustained, or (better for us) does this represent repeated short term transmission after introduction from people, or a rapid burn through the population?

This is one of those “let’s pay attention and get more info, but not freak out” situations. Throughout this pandemic, I’ve tried to balance increasing awareness with avoiding excessive concern or paranoia, and that applies here.

What should deer hunters do?

  • Get vaccinated.
  • Hunt with vaccinated people.
  • Use standard COVID-19 precautions around people.

Should hunters do anything specific when handling deer?

I don’t think we have enough evidence at this point to make specific recommendations for hunters handling deer. Good general hygiene is obviously important, but whether hunters should take extra precautions (e.g. mask and eye protection) is completely unclear. As we learn more, it’s possible that guidance will change.

What about venison?

There should be essentially no food safety risk when it comes to consuming venison. This virus does not survive well outside the host, so even if the animal was infected, the risk from handling meat is presumably negligible. Good hygiene practices used when handling raw meat of any kind should cover any theoretical risk.  It’s always important to cook meat properly before consuming it as well, and the virus would not survive that process either.

My usual ending for these posts is a reminder that COVID-19 is ultimately a human issue. Animal infections are a result of human activities and human contacts. The best way to reduce the risk of this virus entering animal populations is to control it in people.

Human vaccination is probably the best protection for deer.

The Canadian Food Inspection Agency (CFIA) has reported the identification of a rabid dog imported into Ontario from Iran. The dog was imported on July 1, 2021 and started showing signs of rabies on July 11. It was euthanized the next day and subsequently diagnosed with rabies caused by a canine rabies virus variant known to circulate in Iran.

  • Canine variants of the rabies virus are not present in Canada. We still have other rabies variants in wildlife (e.g. bat, skunk, raccoon, and Arctic fox variants) but canine rabies variants, the ones that causes most of the human deaths internationally, are only a risk from imported animals.  But make no mistake, all rabies virus variants are deadly and can infect any mammal.

An investigation of the case ensued and identified 24 people who potentially had contact with the rabid dog, of which 14 received rabies post-exposure prophylaxis (PEP) based on the risk of significant exposure to the dog’s saliva. Fortunately, there was no reported contact with other dogs or any other animals.

  • It’s critical that this was diagnosed and investigated expediently. It’s hard to say what the risk was to those 14 people who received PEP, but with an almost invariably fatal disease, we don’t want to leave much margin of error.
  • Overall, this incident would have cost taxpayers thousands of dollars (more likely tens of thousands of dollars). PEP is very expensive, and the personnel time required to diagnose and investigate a case like this are substantial.

This is consistent with our ongoing concerns about poor importation practices.  Our colleagues in the US may be saying “I told you so!”, as Iran is one of many countries from which importation of dogs is no longer allowed because of rabies risk.  (Although prior to the current US ban, they experienced a very similar incident in June 2021 with a rabid dog that was imported into the US from Azerbaijan).

Co-incidentally (or not, I suspect), there was also a recent news report about a group of 5 puppies from Iran being refused entry into Canada in late July. It’s a bit unclear what was wrong, but it seems like there were a few issues with the importation and that it didn’t comply with the recent changes in Canadian importation rules for commercial dogs less than 8 months of age. These puppies arrived in Montreal and were denied entry. After a short layover, they were put on a flight back to their country of origin.

This has always been a concern with increased enforcement of importation rules. Rejecting a non-compliant shipment at the border is a standard approach, but it’s much more complicated when dealing with live animals. However, there’s a reason shipments get rejected and we can’t allow people to get away with ignoring rules. I’ve always said we need a plan to properly assess, quarantine and (when possible) re-home these dogs in cases like this, with substantial fines to the importers both to dissuade them from doing it again, and to cover the costs associated with dealing with the dogs once they’re in Canada. That could help strike a balance between protection from imported diseases and imported dog welfare.

A statement by the importer of the puppies that were turned back in Montreal raises another concern that we’ve had about the business side of “dog rescues.” They said, “So, we can’t ask these people to pay $2,500 to import their puppies from Iran so we’ll have to pay that and that’ll affect our financial situation so badly that we might have to stop rescuing dogs for a while.”

$2500?(!)

If there’s a profit margin built in (which I assume has to be the case), this moves beyond being a “rescue” and clearly into the commercial dog realm, which is certainly not uncommon. Some rescues are shoestring operations that get by on donations and cost-recovery via adoption fees. They do it just to get dogs into homes. Others profit quite nicely from the business of “rescuing.”

A few days ago, I wrote about a potential H1N1 influenza outbreak in dogs in California.  As I mentioned, H1N1 influenza causing an outbreak in dogs would be noteworthy because it’s not a known canine flu strain, and an outbreak would be unexpected given how rare it is to have human-to-dog transmission of human H1N1 influenza.

One of my key questions about the preliminary report was “Is this really on outbreak of H1N1, or is it a different flu strain?”

It turns out it wasn’t H1N1 after all.

Follow-up testing apparently indicated that the outbreak was caused by H3N2 canine influenza virus. That’s still noteworthy, since we haven’t see much (diagnosed) flu activity in dogs for the last year or two in the US. We assumed the virus was still lingering in the dog population and causing smaller, more local issues that weren’t diagnosed or reported.  This outbreak and an outbreak of presumed canine influenza in Florida in June 2021 are consistent with that.

Overall this is good news. I’d rather be dealing with a sporadic, endemic, known entity (for which we have a vaccine) like H3N2 canine influenza, than a potentially new flu virus in dogs.

African Swine Fever (ASF) is a devastating viral disease of swine that isn’t currently present in the US or Canada. Although the ASF virus only infects pigs, it is quite hardy and can be tracked around by humans, other animals and contaminated clothing, equipment, animal feed and uncooked pork products (like sausages).  It can also be spread by certain soft-bodied ticks belonging to the genus Ornithodoros.

Recently, ASF was found in the Dominican Republic, which is a little too close for comfort. If it reaches continental North America, it will cause massive disease and death in pigs, major disruption of the pork supply chain, and have a devastating economic impact on pork producers and result in an immediately loss of export markets for a considerable period of time.

Keeping ASF out of the US and Canada is a huge priority.  There are already a lot of regulations and restrictions on things like pork products and feed from countries that are not known to be free of ASF, as well as rules for travellers  from these countries.  As an additional precaution, the USDA has added more rules for certain dogs being imported from countries where ASF is present.

The new rules don’t apply to dogs being brought to the US as personal pets, but only to those imported for resale / adoption (which includes rescues that don’t game the system by saying all the dogs they import are personal pets, when they’re clearly not), likely because there are already additional rules for this class of dogs, which makes it easier to apply/enforce a few more.  But it would be better if they could have just said “all dogs.”

It’s still a good step.  The new requirements are basic and logical, including:

  • Shipping containers have to be free of dirt, shavings, straw and any other organic bedding materials.
  • Bedding must be disposed of at the entry point “in a way that prevents the introduction or spread of ASF” (e.g. incineration, disinfection).
  • Dogs must have a microchip.
  • Dogs must be bathed at the U.S. post-entry point within 2 calendar days of arrival. (I’m not sure why they’re given 2 days. This could easily be done quicker).

These are straightforward, feasible and minimally disruptive. The risk of a dog being the source of ASF virus entry to the US are very low, but it makes sense to take basic measures like this, since a single case of ASF in the US would have devastating effects on animal health, animal welfare and the economy.

Similar step by Canada would make sense. There’s nothing to lose, and when the outcome could be catastrophic, being proactive is the key, even with low yield activities.