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. 2020Kim 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’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 2021Hamer 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.

The latest version of A Guide to Mitigating the Risk of Infection in Veterinary Practices During the COVID-19 Pandemic (15-Jun-2021) has been released.  It can also be accessed through the Ontario Veterinary Medical Association Coronavirus FAQ webpage (member login required).

The main changes are a new section summarizing various considerations for re-opening of veterinary practices to clients, and minor updates to other areas such as ventilation.

Previous versions of the guidance and other related documents can be found on the Worms & Germs COVID-19 Veterinary Resources page.

I get asked about this topic a lot. Early in the pandemic, I wrote a post about options for caring for pets of people with COVID-19.  Some things have changed a bit now that we know more know about SARS-CoV-2 in animals and the associated zoonotic risks, and we have a vaccine for people.

One example of a commonly encountered scenario is: A dog owner is being hospitalized because of COVID-19 and a friend or family member has been asked to take care of the pet.

What are the risks to the caretaker of the dog?

  • We don’t know. I’d consider it very low but I can’t say it’s zero.
  • Person-to-dog transmission of SARS-CoV-2 is fairly common in households where the owner has COVID-19, based on our and other groups’ surveillance efforts. However, dogs are not really a great host for this virus and infections are probably fairly low grade and transient, and therefore low risk for further transmission.
  • Dog-to-dog transmission of SARS-CoV-2 hasn’t been seen experimentally, but the studies to date have been fairly small so we can’t read too much into that.  Nonetheless, it’s clear that dogs are lower risk than cats for passing the virus on to other individuals. Yet, live virus has been grown from canine respiratory secretions, suggesting there is at least some plausible risk.

What can or should be done in this situation?  There are two key considerations:

1. Should the person in question agree to look after the animal at all?

  • This requires consideration of the risk status of everyone in the household (e.g.  is anyone at high risk of serious disease if they get infected?), vaccination status of everyone in the household, whether other pets are present, how well the dog can be contained in the household, and whether the household members are willing to accept a small degree of risk.
  • The best case scenario is to send the dog to a pet-free household where everyone is vaccinated and where they can keep the dog away from other people or animals (e.g. they have a fenced yard). Asking someone else to look after a pet like this was harder earlier in the pandemic, but now that we have highly effective vaccines, it’s easier.  Asking if someone from a fully vaccinated household will take the job is reasonable.

2. How should the dog be managed?

  • That also depends on the risk status of the household. In a vaccinated household, I’d say it can be “business as usual” in the house, but restricted outside. By that, I mean I’d do nothing different with the human-dog interactions in the household, but when the dog goes outside it should be under control (e.g. on a leash, or in a fenced yard) so the dog doesn’t interact with other people or dogs from outside the household.
  • If there are unvaccinated people in the household, it’s tougher. I still consider the risk of transmission from a dog to be very low, but I can’t say it’s zero. So, it would be prudent to keep the dog away from unvaccinated individuals as much as possible for the first 14 days (7 days is probably reasonable if it’s a major issue, but 14 days is ideal).  That doesn’t mean locking the dog in the bathroom and never going near it, but rather avoiding contact with respiratory secretions, not having the dog in the same small airspace for prolonged periods of time (e.g. not sleeping in the bedroom), and focusing on good hand hygiene.

The main issue is we just don’t know the risk. It’s definitely very low and could be zero, but I don’t think we can say it’s zero at this point. Vaccines are a game changer for these scenarios.

Okay, but what if the pet in question is a cat instead of a dog?

That changes the risk a bit, but not really the overall approach. Cats are more susceptible to SARS-CoV-2 and cat-to-cat transmission can occur, so there’s also more concern about the potential for cat-to-human transmission.  That means the issues above all apply to cats as well, but are probably heightened.

My focus here would be on finding a vaccinated household to look after the cat, if possible.  If not, it’s a matter of restricting contact with anyone who isn’t vaccinated. The good thing about cats is they are (for the most part) easier to contain than dogs.  They can be kept in a large cage or kennel if necessary, or they can be confined more easily to certain areas of the house, and cats don’t need to go outside. So, if someone who’s unvaccinated has to take in a cat, it’s easier to limit contact. The concepts above still apply, with a goal of minimizing the closeness and duration of contact during the first 7-14 days after the cat is removed from the person who was infected with COVID-19.

As we start seeing a light at the end of the (long) tunnel, we have to think about how and when to restart various activities. One particular activity that I get a lot of emails about is pet therapy/visitation programs. These are great programs, where animals (usually, and ideally, dogs) are taken into facilities to interact with residents or patients. They’re not for everyone, but have been shown to have physical and mental health benefits.  Not surprisingly, they were largely stopped during COVID-19 pandemic, but many are now gearing up again.

What’s the risk of animal visitation programs?

People.

The main issue isn’t the animals (at least when it comes to dogs), it’s the human component. Human visitors pose much more risk than most pets, since people are more likely to be infected with the SARS-CoV-2 virus and an infected person poses a greater risk of spreading the virus than an infected pet. That’s particularly true with dogs, who are not uncommonly infected by their owners but who are low (or maybe almost no) risk for transmitting it back to people.  Also, if the animal happens to be infected, it’s quite possible the owner is too.

So, there is some risk from these programs, but the main (if not exclusive) risk is from an infected handler.

When can these programs restart?

The focus should really be on deciding:

  • when COVID-19 is controlled enough in humans the area
  • when resident/patient vaccination rates are high enough
  • when animal handler vaccination rates are high enough
  • what basic practices to identify high risk handlers (e.g. those with respiratory disease) need to be in place.

Should vaccination be mandatory for pet therapy program participants?

(Bracing for the next wave of anti-vaxxer “love” letters as I say this…)

In my opinion, yes, vaccination should be mandatory for the people (not the animals).  A reasonable exemption for people that TRULY CANNOT be vaccinated is reasonable, but the bar has to be high so that “cannot” and “don’t wanna” are differentiated.

Mandating vaccination of handlers would be a logical measure for a voluntary activity that involves close contact with a disproportionate percentage of high-risk participants.  Also, there’s a need to make sure handlers received both doses of vaccine, as concerns about the delta variant increase.

The other approach could be to have a phased re-introduction, e.g. fully (2-dose) vaccinated people can start first, with follow up assessment of when people who are not vaccinated can start as well. That way the policy isn’t saying unvaccinated people can’t participate, it just says they can’t yet. That’s not an unusual approach now as people start to think about re-opening in other areas and how vaccination comes into play for that.

How to we reduce the risk in the event an infected person (or animal) ends up visiting?

Good ol’ infection control and common sense. There are very good guidelines for healthcare visitation programs that apply to most animal visitation settings, and those are the core of risk reduction.  A little basic hygiene and common sense go a long way.

I’ve written before about animal vaccines against SARS-CoV-2 with regard to mink, but with Russia having recently licensed its Carnivac-Cov vaccine for use in several species (namely dogs, cats, foxes and mink) and Zoetis developing a vaccine in the US for mink, there’s continued interest in the subject, so I’ll revisit it.

I’m glad companies are working on SARS-CoV-2 vaccines for animals. It’s good to be prepared and have the work done in case it’s needed in a particular situation. Whether there’s much use for these vaccines in most animals at this time is hard to say.

When I think about SARS-CoV-2 vaccines for animals, there are three main reasons we would use them:

1. Prevention of severe disease

This is the main goal of most vaccines. However, dogs and cats don’t seem to get very sick, very often, from this virus.

  • I’m still on the fence as to whether this virus really causes significant disease in dogs at all. We have some data that suggests infection is associated with risk of very mild disease but overall, it’s still not entirely clear. However, it is clear that they rarely, if ever, get seriously ill.
  • Cats are commonly infected with SARS-CoV-2, but even cats rarely develop severe illness. There’s evidence that they can, so we can’t dismiss it. However, our research and others suggests that a large percentage of cats from households where people have COVID-19 get infected. So, we have probably had millions of infected cats worldwide since the start of the pandemic. If this was causing severe disease in a reasonable percentage of cats, I think we’d have clear signs of that by now. Since serious illness can occur in a small minority of cases, vaccination could help, but in the grand scheme of things, given the low risk of severe disease, it’s hard to say that the potential benefits justify the cost and potential adverse events.  (We don’t know about any specific adverse effects of vaccination in animals to date. I’m not talking about VITT or the misinformation about mRNA vaccine adverse events in humans. I’m talking about the typical adverse events that we can see in animals with any vaccine.) I’d rather focus on better rabies vaccine coverage and other good preventive medicine things than trying to get cats vaccinated against SARS-CoV-2. I’m not opposed to it. I just think the value is probably limited.
  • Mink are different. They are clearly susceptible to infection and it can cause serious illness and even death. The two points below probably are more important when considering vaccination of mink, but there could be mink health benefits from vaccination.

I’d be most interested in vaccination of endangered species with suspected high susceptibility that have human contact, particularly wild felids (e.g. lions, tigers) and non-human primates (e.g. great apes). Vaccination of captive animals (zoos), animals in rehab facilities and animals in areas where there’s close contact with human populations (mainly applicable to primates) might be worthwhile because of the potential implications of infection in endangered populations.

2. Prevention of transmission from animals to people

Dogs pose little risk to people, if any, in terms of transmission of SARS-CoV-2. Dog-to-dog transmission has not been seen experimentally, and infected dogs seem to have pretty low viral loads, so I doubt there’s much risk. Cats pose more risk to people. Cat-to-cat transmission does happen, so we have to assume cat-to-human transmission can occur too.  But human-to-cat transmission is still far more common. Because most infected cats probably catch the virus from their owners in the first place, they are probably “dead-end” hosts in households and rarely play a role in transmission. Cats get infected from family members, who have also infected each other, and everyone burns off the virus together. I think the greatest risk of cat-to-human transmission is when infected cats leave the house, particularly to go to a veterinary clinic or shelter. However, the risk associated with that can be significantly reduced using basic infection control practices..

The risk of animal-to-human transmission is probably highest with mink, as mink-to-human transmission clearly occurs. Vaccination of mink could be an important way to control mink-to-human transmission (but preventing human-to-mink transmission is most important).

3. Prevention of viral mutation

This is an important aspect for species that might serve as reservoirs and source of new variants. Virus variants emerge because of random mutations in the viral genome, and mutations happen when the virus replicates. So, the more the virus replicates and the more individuals are infected, the greater chance of a “bad” mutation occurring. For dogs and cats, this isn’t really a big concern. We don’t have massive numbers of dogs or cats together where widespread sustained transmission  is of great concern. Yes, a mutation could happen within a single infected dog or cat, but it’s really unlikely (and even then, it’s only relevant if that dog/cat then can pass it on to a person.)

The greatest risk of significant viral mutation in any animal species to date appears to be in mink.  We know that the virus can mutate in mink populations AND spread back to people. When you house thousands of a highly susceptible animals close together and introduce the virus from an infected person, that’s the recipe for widespread transmission and massive viral replication that’s needed for “bad”mutations to result in the emergence of a significant variant. Vaccination of mink farm workers helps reduce the risk of mink getting infected and from passing the virus back to people, but there’s still some degree of risk.

 

Overall, I can’t see a need at this point for vaccination of dogs and cats against SARS-CoV-2. The cost-benefit comparison of vaccination of those species doesn’t seem convincing.  Vaccination of high-risk (e.g. endangered) susceptible animals like large cats and non-human primates might make more sense, such as in zoos, rehab facilities and other places where there’s some degree of human contact with these species.

Mink are a different story. If we’re going to continue to farm mink, vaccination is a reasonable consideration.

The final disclaimer here is that all this is based on what we know about current virus variants. New variants always have the potential to reset our knowledge to some degree.  Variants that can infect a wider range of animal species, cause more serious disease in animals or be more transmissible from animals to humans might impact the current risks and make vaccination of other species more beneficial.  That’s why I’m glad we have some information about vaccines for animals now. I don’t think we need them currently for most species, but it’s good to be prepared should things change.

Ivermectin is a commonly used anti-parasitic in animals, and it’s also used in people to treat some parasitic diseases. Additionally, it’s still widely discussed in some internet circles for treatment or prevention of COVID-19. That’s based on mainly anecdotes, some in vitro study, and very poor quality “clinical trials”.

It’s led to stories of people using or even hoarding veterinary ivermectin products. I’ve had lots of questions from the general public and veterinarians as a result (including stories of people buying more heartworm meds for their dog  than they’d ever need).

Bad idea. Here are some reasons why:

  1. There no evidence that it works. An in vitro study showed ivermectin inhibited SARS-CoV-2, but only at massive doses, well beyond what would be safe for use in people.
  2. Anecdotes aren’t data.
    • I don’t dismiss clinical observation, as it is the starting point of many important discoveries. However, a lot of things I have thought I observed or initially appeared to “work” didn’t pan out to be true over time.
    • Observations should lead us to take specific steps to figure out if something is real.  They help raise questions, not answer them.
  3. Other factors may be involved that make the drug appear effective in some siutations, but those factors don’t apply broadly.
    • One potential reason ivermectin may seem to work in some areas is because it’s an anti-parasitic. Strongyloides stercoralis is a human respiratory parasite that is common in some countries. People get infected and the parasite can lay dormant in the body, but when they are immunosuppressed, it can be re-activated.
    • Dexamethasone is a common and effective treatment in people with moderate to severe signs of COVID-19.  It reduces inflammation, but also impacts the function of the immune system (since that’s what triggers inflammation).
    • If someone is infected with dormant Strongyloides stercoralis, dexamethasone treatment for COVID-19 might lead to re-activation of the parasite, and that would complicate respiratory disease.
    • I don’t dismiss that potential, but it would mean that ivermectin might be effective in people with dormant Strongyloides stercoralis infection that are also receiving dexamethasone.
    • That’s a lot different than “ivermectin works against SARS-CoV-2 and everyone, everywhere, who is infected should be on it.”  They should not.
  4. Ivermectin isn’t a very commonly used drug in people, but it is important for treatment of certain parasitic infections.
    • Diversion of the relatively small amount of human ivermectin products towards unnecessary use compromises the care of people that really need it. (I realize that isn’t associated with “don’t steal Fido’s stash” but it’s still an important point.)
  5. Your dog’s heartworm preventative is a low dose treatment.
    • To get the levels used in the aforementioned in vitro study, a person would require a dose of  about 3500 ug/kg. Heartworm prevention in dogs is dosed at about 6 ug/kg.
    • So, my dog Merlin gets one 272 ug chewable a month. To get 3500 ug/kg, I’d need 965 tablets per dose. If I had a small dog, I’d need even more of his supply.
    • If I wanted to self-treat for Strongyloides stercoralis (not sure why I would, but let’s pretend), I’d need 52 of my dog’s chewables – per day.
  6. Ivermectin also comes in more concentrated oral, injectable (don’t even think about it) and pour-on (topical) forms. While I have good confidence in mainstream veterinary pharmaceutical companies, I still don’t want people taking a product that is only intended for (studied in and approved for) use in animals.

Ivermectin is one of many so-called “miracle cures” that we’ve seen pushed on the internet. Like most others, there’s limited substance to the stories behind it. There’s no magic bullet for COVID-19. I’d love it if ivermectin was one, as it’s a drug we know how to use and how to produce. Production could be ramped up and it’s relatively cheap. While conspiracy theories abound, no one has come up with any plausible explanation why an effective drug would be suppressed. Dexamethasone is cheap, widely used and widely produced, and it’s standard of care for some patients. That’s because it works. Ivermectin fits all of those except the “works for COVID-19” which is the most important one.