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:

As I mentioned recently, we’re tracking anecdotal reports of increased respiratory disease activity in dogs in Ontario.  If you have observations or cases to contribute/report, please take a few minutes to fill out our short survey here:

Infectious respiratory disease is endemic in dogs everywhere, so there’s always some disease activity going on. Sometimes we see what are likely true outbreaks, caused by either by our usual suspects or by something we haven’t yet identified. Sometimes the “outbreaks” are actually just more attention being paid to the normal level of disease.

Usually infectious respiratory disease is self-limiting in dogs – they cough, have a runny nose and eyes and feel run down for a few days, and then they get over it on their own. It’s really just like people with the common cold. However, sometimes more severe disease can occur, and I’ve had what I’d consider a lot more reports of serious disease lately, so we’re trying to sort out what’s happening.

So far, there’s not much to report. Some of our preliminary case maps are below; the size of the dots corresponds to the number of cases reported from that area. At this point, most reported cases are in Toronto, but there are also more dogs and more people there, so we have to be cautious not to over-interpret that.

Most reported cases have also been in dogs not vaccinated against “kennel cough.”  Those vaccines only cover some of the potential agents of canine infectious respiratory disease complex (CIRDC), namely Bordetella bronchiseptica, +/- canine parainfluenza virus, so it’s hard to interpret that result with the limited information we have so far too.

Limited diagnostic testing has been performed, which isn’t surprising. Testing doesn’t usually influence care of the individual dog so most owners don’t want to pay for it, and we don’t actually recommend it as a standard tool (although it would potentially help us in situations like this).

More updates will follow if I get more reports.  Please fill out the survey if you have additional cases to contribute.

Key points for any concerned pet owners:

  • If your dog is sick, keep it at home.
  • Don’t let your dog interact with sick dogs. In particular, try to stay away from dogs that are coughing, or have runny eyes and a runny nose. (I know, that’s not easy in some situations.)
  • Consider “kennel cough” vaccination if your dog regularly encounters other dogs (talk to your veterinarian).
  • Preventing exposure at places like parks where your dog may encounter many other dogs of unknown health status is tough. If you’re concerned or your dog is at higher risk for complications (e.g. old, pre-existing respiratory or heart disease, brachycephalic breed like a bulldog), be more restrictive about your dog’s contact with other dogs, and talk to your veterinarian about the value of a “kennel cough” vaccine.

I get calls about concerns regarding increased respiratory disease in dogs all the time.

  • Sometimes, they’re the result of local disease outbreaks.
  • Sometimes, they’re just a result of increased awareness of the normal “baseline” disease rate, since “kennel cough” is always occurring at some level.
  • Sometimes, the circumstances just seem different, and we need to get more information.

The last of these is where I stand at the moment. I’ve been getting more reports of canine infectious respiratory disease, including a concerning number of reports of severely affected dogs (with some fatalities). My impression is that something unusual is happening. Most often, the cause is actually one of our “usual suspects,” – the bacteria and viruses that normally cause canine infectious respiratory disease complex (CIRDC). It’s often very difficult to sort these things out because limited testing is done, there are limitations on the testing that is done, and there’s no formal tracking system for these infections.

However, we want to figure out what’s going on if we can, because sometimes we do see new (or new-to-us) pathogens (like when canine influenza hit Ontario in 2018), or we can identify hot spots for disease transmission (we’ve implicated specific dog parks in the past).

So, once again, we’re trying to track respiratory disease cases in dogs in Ontario (and beyond Ontario, if people want to report them). We have a quick online survey to collect more information and hopefully figure out if something unusual is happening and what it might be.  The survey can be accessed here:

I’ve recently received a few reports of serious (including fatal) respiratory disease in dogs in regions east of Toronto, Ontario. We often see localized variations in the incidence of “kennel cough” (aka canine infectious respiratory disease complex (CIRDC)), including sporadic outbreaks, but in this case there is particular concern about the number of severe infections and deaths. No cause has been identified yet, so I’m trying to collect some more information to facilitate the investigation and get a better idea of what’s happening.

If you are a veterinarian or or owner of a dog with recent respiratory disease in Ontario, you can help by filling out this quick survey:

I’ll post more information about these reports soon.

We continue to track cases of canine infectious respiratory disease in various parts of Canada, for what it’s worth. The data are obviously a bit dodgy because it’s primarily from self-reporting, but I think we’re getting some interesting information. Cases seem to be slowing down, but we continue to get reports from the two main areas in Canada (and a trickle from the  US). Part of the clustering we’re seeing is probably due to local increased awareness and reporting, but I don’t doubt that a couple of reasonable-sized outbreaks have been ongoing.

Click here for the latest version of our canine infectious respiratory disease complex (CIRDC) map (December 31), now with the ability to display cases reported by month.  A snapshot of the map is also shown below.

Here are some additional details from the data we’ve collected via the reporting survey:

65% of affected dogs had been vaccinated against “kennel cough” in the past year.

  • That’s not too surprising. Kennel cough vaccines protect against one, two or three of the many potential causes of CIRDC, but not all of the causes, by any means. Furthermore, no vaccine is 100% effective. These data don’t tell us anything about how well those vaccines work (they actually work quite well).

Of that 65% of affected dogs that were vaccinated in the past year:

  • 40% were vaccinated orally: The oral vaccine only covers Bordetella bronchiseptica, which consistently comes in as the #2 cause of CIRDC in Canada. It’s a good vaccine for that bacterium but has less coverage than intranasal vaccines.
  • 29% got an intranasal vaccine: Intranasal vaccines in Canada cover Bordetella bronchiseptica and canine parainfluenza virus, giving protection against the top 2 causes or CIRDC. Some also include protection against canine adenovirus type 2.
  • 35% received an injectable vaccine: Injectable vaccines are less protective when it comes to CIRDC. Oral and intranasal vaccines provide better protection where the infection occurs – in the upper respiratory tract.
  • 26% were unsure of the vaccine type: So whether these dogs were truly vaccinated against kennel cough is unclear.

Over half of affected dogs had visited a dog park shortly before they got sick.

  • That’s not surprising at all, since CIRDC is spread dog-to-dog, and parks are a place where dogs congregate.  Groomers came in as the #2 most common previous contact, followed by doggie day care.
  • Since we just looked at sick dogs, we can’t say anything about risk factors (e.g. we don’t know if visiting a dog park was more common among sick dogs since we couldn’t compare them to healthy dogs).
  • There were a couple specific parks that were frequently named, so it’s likely there were some true hot spots of transmission at those parks.

Diagnostic testing was performed on 17% of sick dogs, but nothing remarkable was apparent in terms of diagnosis.

  • That’s actually a pretty high percentage for testing in cases like this. Testing isn’t commonly recommended for routine cases of CIRDC since the cost is hard to justify where there’s little impact of test results on individual patient care.
  • Testing is more useful when there’s an outbreak (to figure out what the culprit is and see if there are any control measures that might be applied), with imported dogs (worried about bringing in influenza strains), kennels (outbreak potential) and breeders (outbreak potential, risk of more severe disease in young and pregnant dogs).
  • Limited test results were provided on the survey but nothing remarkable was present.

Most of these outbreaks of CIRDC die out over time and we never find the cause.

  • Canine parainfluenza is always high on my list since it’s common (common things occur commonly) and can be missed with routine testing because the virus isn’t shed for long. By the time the dog is taken to a veterinarian and sampled, PCR tests looking for the virus may be negative (and other approaches like antibody-based testing aren’t usually done).
  • A “new” or (more likely) established but unknown cause of illness is certainly possible. There are undoubtedly many canine respiratory viruses out there that we don’t know about.
  • Introduction of canine influenza from imported dogs is always a concern. It’s a “foreign” disease, but canine influenza was introduced to Ontario a few years ago, and was ultimately eradicated (as far as we can tell).  Here, since there haven’t been any positive test results, it’s unlikely to be the cause. That virus is shed for a while in infected dogs, and I’d expect to see a positive result with a reasonable number of tests. Introduction of influenza into areas where few to no dogs have immunity to the virus would almost certainly result in more widespread disease. So, I think flu is pretty unlikely here, but the potential for flu is a reason to test. We’ve shown it can be controlled when it’s caught early, but if it’s not, it can cause a lot of damage.

Disease tracking like this won’t provide clear answers, but helps identify and refine things we need to look at, so I think there’s a role for it  as an easy, low-cost surveillance tool.

Not uncommonly, I get questions about outbreaks of “kennel cough” (more formally known as canine infectious respiratory disease complex (CIRDC)). It’s not uncommon for us to see little outbreaks of this kind of respiratory illness in dogs, and we don’t have a good handle on the amount of “background” disease that’s always present in the community at large. So, it’s hard to interpret whether an “outbreak” is really an outbreak of disease versus an outbreak of awareness of disease, and to figure out much about true outbreaks.

In the past 24 hours, I’ve had a few calls about what seem to be fairly large outbreaks of respiratory disease in dogs in two different parts of Canada: southern Ontario and Calgary, Alberta.

I’ve tried different ways to investigate reports of CIRDC outbreaks in the past.  Getting reliable information is always a challenge, but we’re going to try again. This time, we’ve launched a very short survey that owners/veterinarians with dogs showing signs of infectious respiratory disease can complete. It asks a few basic questions, like location, date of onset of signs, whether the dog has been vaccinated for “kennel cough” and if a potential source of exposure to other sick dogs is known. I know the data will be really biased, so it’s not intended for for research use. It’s merely meant to identify changes in disease patterns, such as disease rates or potential point sources, that we can investigate further.

So, if anyone has (or is treating) a dog with suspected infectious respiratory disease, completing this short, anonymous survey may help us figure out what’s going on. While the recent reports are from Calgary and the Guelph area, anyone can participate.

The survey can be accessed using the link or the QR code below:

This one’s easy. Birds are not susceptible to SARS-CoV-2. Stop reading here if that’s all you want to know.  If you’d like a little more detail read on…

The SARS-CoV-2 virus originated in mammals (most likely in bats, which will be the topic of the next review) and has spread to other mammals (especially people, of course). Birds are, well, birds, so they’re not mammals. Some viruses like both birds and mammals, but most don’t.

Researches have looked at this, and experimental infection studies have not resulted in evidence that birds are to any degree susceptible to SARS-CoV-2. This includes a study that looked at chickens, one that investigated chickens and ducks, and one that looked at chickens, turkeys, ducks, quail and geese.  Those fit with the predicted poor susceptibility of chickens to SARS-CoV-2 based on their ACE2 receptor (the cellular structure the virus uses to attach to and invade cells – no attachment, no infection).

So, why bother investigating birds, since it wasn’t likely that they’d be susceptible to a mammalian coronavirus in the first place?

It was really important to check and verify that birds are low risk for a few reasons:

  • There are massive numbers of domestic birds all over the world. That means lots of potential for exposure to infected people. If this virus got into a large group of birds (like on a poultry farm) and they were susceptible, there’d likely be big risks for the birds, as well as for transmission back to farm workers and issues with contaminated manure (just like we’re seeing in mink).
  • Jumps to new species in large groups is a perfect recipe for unpredictable mutations (which has also been a concern in mink).
  • Perhaps the biggest reason for wanting to know is simply that birds live everywhere we do, and beyond. There’s lots of potential for direct and indirect exposure of domestic (and wild) birds to this virus from human sources, and additional potential for contact between domestic birds and wild birds, which can then rapidly spread pathogens over long distances (as with avian influenza viruses).  So, we needed to be confident that this virus couldn’t establish itself in any bird populations.

That’s why the work was done quickly at the start, and thankfully birds seem to be completely resistant to SARS-CoV-2.

(All that said, if you have COVID-19, don’t cough in your bird’s face. The non-scientific part of my brain still never wants to tempt fate.)

Moving on from cats and dogs, let’s talk about one of our major livestock species, pigs.

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

  • Kind of, but not really. There are conflicting experimental data that show no or very little susceptibility to the virus.

Why did we talk a lot about pigs and SARS-CoV-2 initially?

At the start of the pandemic, we were worried about the potential for this virus to infect pigs because of their ace2 receptor, which is used by SARS-CoV-2 to invade pigs’ cells. If the virus can’t enter an animal’s cells, it can’t infect them. Different animals have slightly different ace2 receptors on their cells. The pig ace2 receptor is quite similar to the one people have,  suggesting there could be similar susceptibility to SARS-CoV-2. Looking at ace2 receptors has been interesting, but we’ve also seen the limitations of this method, with some purportedly low-risk species being susceptible and some purportedly high-risk species being resistant. Ace2 is only a part of the picture, so while it’s worth considering, it really doesn’t answer the question of whether there’s a concern with pigs.

Trying to grow the virus in a laboratory in cell lines from a particular animal species can provide some additional information on potential susceptibility. In one study, SARS-CoV-2 was grown in 2 of 3 pig cell types, but did not damage the cells. In another study, the virus was grown in pig cell lines and caused some cell damage.  These all raised concerns about the virus’ ability to infect pigs, but there are limitations to what in vitro studies can tell us. To get the real story, we need to look at real pigs.

So, forget about pig cells – are actual pigs susceptible to SARS-CoV-2?

In three separate studies (Shi et al.,  Schlottau et al., Meekins et al.), pigs were experimentally inoculated with SARS-CoV-2 and mixed with naive pigs. Nothing remarkable happened. None of the pigs got sick and all samples collected were negative for the virus. Antibodies against the virus weren’t found in any animal. All of these results indicated that the pigs were not infected, and there was a collective sigh of relief as it appeared that concerns about pigs were unnecessary.

In another study, pigs were exposed to the virus via the nose, trachea and injection. All the pigs stayed healthy and the virus wasn’t detected in any samples, but antibodies against the virus were found after pigs were injected with the virus. Exposure by injection doesn’t tell us much about natural infection, and the other results are consistent with no natural susceptibility.

However, leave it to Canadians to be disruptive – an experimental study conducted by the CFIA found slightly different results in pigs.  It didn’t raise major concerns, but it suggested things are not not quite as clearcut. In that study, 16 pigs were exposed to a higher dose of the virus than in previous studies.  Once again, 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. However, low levels of virus were detected from respiratory samples by PCR from two of the sixteen pigs, although live virus could not be isolated. The virus was also 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 did not get infected. Overall, 5 of the 16 pigs (~30%) had some evidence of mild infection. So, this study showed some degree of susceptibility, 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 other animals or people.

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

There are no reports of infected pigs to date, but I’m also not aware of any testing of pigs on farms. Field data are always useful because experimental studies don’t tell the full story of what happens in the “real world.” Some data about pigs exposed to infected farmers would be useful to have, to round out the story, but it would probably be low yield research since it’s quite unlikely anything would be found.

What should be done with pigs?

The same general recommendations apply as for other animal species. While the risks are low, we can’t say they are zero. If we keep infected people away from animals, we don’t need to worry about human-to-animal transmission, and 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, the better.  In short, better to be safe than sorry.

Next up for animal reviews: probably mustelids (mink and ferrets).

Image source:

A couple of months BC (before COVID-19), I was planning a live simulation exercise for our hospital. The goal was to see how well we could identify and handle a nasty, reportable zoonotic disease, and to look at our personal protective equipment training and needs (COVID-19 helped with that last one, at least).

Here’s the test scenario we were going to use:  A cat is presented with fever, lethargy and cough, with rapid progression to severe pneumonia. If travel history was queried (another test for the staff, as this step is commonly missed), the “owners” would say they just moved with the cat from Colorado. From there, some good questions would hopefully raise concerns about plague, although it’d be easy to miss initially as plague is a disease we don’t see around here, and most veterinarians have never seen a case. The primary goal of the exercise was to be to look at when and how we identified the concern, and our response to it. Those aspects are themselves educational, but the big-picture goal was to see how we can improve our ability to identify, respond and communicate around such a case.

Once the COVID-19 pandemic hit, it didn’t seem like a good time to do an exercise like this, given the changes and stressors in the clinic.  However, while unlikely, the scenario wasn’t unreasonable. Plague, caused by the bacterium Yersinia pestis, is still present in some regions of North America and other parts of the world.  Pets can be infected and the can occasionally pass the infection on to their human contacts. Veterinarians are at particularly high-risk for this kind of transmission because of our close contact with infected animals during examination and treatment.

A recent case of plague in a cat in Colorado highlights the zoonotic risks. There’s not any detail in the report, beyond saying a person got plague from a cat, but cat-associated plague is well documented.

Plague is now rare (thank goodness), but it can still be found in certain wildlife. People can be exposed from contact with infected wildlife (or their fleas), or via contact with domestic animals that were exposed to infected wildlife (or their fleas). People living in or traveling to areas where plague is present need to be aware of the risk and take some basic precautions:

  • Avoid contact with wildlife (especially species like prairie dogs, squirrels and other rodents) and places where they live.
  • Keep pets away from wildlife and their burrows/nests. In particular, keep pets away from dead wildlife.
  • Keep pets indoors or under control when outdoors.
  • Keep pets on a good flea prevention program.

I frequently harp about the need to query travel history in pets, since a lot of animals move around and knowing an animal came from somewhere else can be a key part of making a diagnosis. Pet owners need to play a role in that too, by making sure they mention places their pet has visited, or if it came from another region. Also (another factor that is often overlooked),  travel doesn’t just mean long distances. From a disease standpoint, it simply means going to a place that is somewhat different microbiologically. The classic example is a dog that lives in the city but visits a cottage that may only be a few hours away by car.  Since we have some regional differences in certain diseases (e.g. blastomycosis) and vectors (e.g. ticks), you don’t always have to go far to be exposed to new risks.

A few basic questions, like “where did you get your pet” and “have you gone anywhere with your pet recently” are easy, free and can make a big difference.