In some ways, the approach to rabies vaccine is easy. In other ways, it’s complicated. To some degree, the medicine is easy, but other considerations (like regulatory requirements) cloud the picture.

The medicine:

Rabies vaccines are highly effective. A single initial dose provides at least 1 year of protection. The first dose is supposed to be given at 12 or 16 weeks (depending on the jurisdiction – rabies vaccines in Canada are labelled for use in dogs and cats 12 weeks of age and older), with a booster one year later. Getting that first shot is critical, so the animal is protected as soon as possible against this deadly – and zoonotic – disease.

After that, we can use 1 yr or 3 yr vaccines.

  • For a 1-year vaccine product, if the animal is overdue, we’d just give another dose as soon as possible, but the yearly cycle does not change.
  • For a 3-year vaccine product, after the initial dose and the first 1-year booster, the animal can safely go 3 years between boosters.  However, if the 1-year booster is missed, then we’d have to restart that primary series to stick to the manufacturer’s instructions, meaning the late booster would only be considered good for a year. Then we’d go to every 3 years after the next dose a year later. If the animal misses any of its 3-year boosters, the same would apply, i.e it would need a booster a year later to get back on the 3-year schedule.
  • Note: As of March 2019, there is only one 1-year rabies vaccine product for cats still available in Canada, and none for dogs; all other rabies vaccines for dogs and cats in Canada are 3-year products.

That’s all pretty straightforward and by-the-book (or label, in this case).

The challenge is what constitutes “late.” Strict interpretation of the label would be that even a single day overdue would require the primary series to be repeated. Many would consider that overkill since rabies vaccines are so highly effective. However, there’s not much appetite for guessing when it comes to rabies. Once an animal is more than a couple of months overdue, it’s harder to say that the booster should count as a 3-year dose, since we’re deviating ever more from the label recommendations. So, prudence would dictate we go back to the start. If there’s concern that the provincial/state/regional authorities (especially border authorities) would use a strict interpretation and consider any lapse an indication that the vaccination series had to be restarted, then it’s safer to be more conservative and do that in your practice as well. More on that below).

From a rabies protection standpoint, I don’t worry about late vaccines (within reason) since rabies vaccines are so good, especially in an animal that has received multiple doses over the course of its life.

The “other considerations”:

What makes rabies vaccination delays complicated is most often regulatory/public health rules and interpretations thereof. In many areas, rabies vaccination is required by law; for example, in Ontario, all dogs, cats and ferrets over 12 weeks of age are legally required to be kept up-to-date on rabies vaccination, regardless of whether they have indoor or outdoor lifestyle.  I doubt there would be a fine applied in most cases for reasonable delays in vaccinating a pet due to COVID-19 restrictions over the last year (but each jurisdiction is its own…). However, the bigger issue is the response to a potential rabies exposure in such an overdue animal. When an dog or cat is exposed to a rabid (tested) or potentially rabid (untested suspicious) animal, the response depends on the pet’s rabies vaccination status. That response varies a bit by region; some default directly to the guidelines in the NASPHV Rabies Compendium, some adapt those guidelines, and others are… well, a bit unpredictable.

In Ontario, the response could range from simple observation with no strict confinement (e.g. a fully vaccinated pet that gets a booster within 7 days of the encounter) to a strict 6 month confinement (e.g. an unvaccinated pet that does not get a rabies vaccine within 7 days of the encounter). Animals with a history of lapsed vaccination are dealt with on a case-by-case basis, with things such as the time since the last dose and the number of lifetime doses being considered (among other factors). The pet could ultimately be treated as fully vaccinated or unvaccinated, depending on the details, and that can be the difference between life-and-death, as euthanasia is sometimes elected by owners in lieu of having to strictly quarantine the pet.

Understanding how your region addresses overdue vaccination is useful, to determine how strict you need to be about getting animals in for their boosters. If they’re not flexible and consider an animal’s vaccination status to be lapsed the minute they pass the 1- or 3-year mark, then we need to make sure we’re prioritizing rabies vaccinations so there are no gaps.

As an example, here’s the situation in my household:

  • My dog, Merlin, has had multiple doses of rabies vaccine and is probably effectively protected for life. Him being overdue is probably low risk, but I wouldn’t want to let it go very far. I’d booster him when feasible but not stress about it, and would keep him on the 3-year cycle. If he’s not too far overdue, I’d have a strong case to consider him completely protected if he tangles with a rabies suspect.
  • My cat, Milo, is less than a year old and has had a single dose or rabies vaccine. He’s higher priority to get boostered. If I miss the 1-year booster by much, I’d have to consider re-starting the primary series again, which means I’d give him a dose, and then another in 1 year, before switching to every three years. (That’s a picture of him… he doesn’t look very stressed about it).
  • The outdoor cats (Rumple and Alice)  have received multiple doses or rabies vaccine over their lifetimes. They’re like Merlin, but higher priority for boosters since they likely have a greater risk of exposure to rabid animals.

The take home messages on rabies vaccination delays:

  • Don’t let rabies vaccination lapse, if possible.
  • I’d prioritize pets for rabies vaccination in this order
    • Animals that may have been exposed to rabies in the last 7 days because of an encounter with a suspicious animal that couldn’t be tested for rabies (or that was tested and was confirmed to be rabid.  This is a medical urgency.
    • Animals that have never been vaccinated for rabies (or unknown if they’ve ever been vaccinated). They need a dose ASAP to make sure they’re protected and won’t have to risk a long confinement period if they’re exposed to rabies.
    • Animals that have only had one dose of rabies vaccine in their lifetime.
    • Animals that are significantly overdue for their rabies booster (more than a couple of months).
    • Other pets that are due for a rabies booster (if there’s really a need to prioritize further, focus on dogs/cats that go outside unsupervised).

I’ve been dealing with questions (and some threats) about this issue for many months:

How to handle overdue vaccines because of COVID-19 delays

It’s a complex issue that’s hard to navigate because of limited data. We know how most vaccines work when used according to the label instructions. We don’t know much about what happens when use of those vaccines differs from the label instructions.

Dr. Michelle Evason and I did a webinar for CommuniVET on this topic the other day, and it made me think a lot more about it.

The main questions are:

  • How long do vaccines actually protect an individual animal?
  • Does being overdue for a vaccine mean you have restart the primary vaccination series, or can you just give a single booster later than normal and continue with the regular vaccination schedule?

Manufacturers can’t give too much guidance because they are legally bound to the licensing (label) claims. They also don’t have much data about uses that differ from the instructions on the label, because no company is going to pay for expensive vaccine trials to test various combinations of delays and boosters once the product is licensed.

Owners want their animal protected, but they also don’t really want to come back in / pay for another vaccine, and many want to avoid giving any more vaccines than are absolutely necessary.

Veterinarians want to make sure that their patients are protected, that their owners are happy (or at least not overly unhappy), and that they can provide veterinary care during a time when that might be complicated, and when most clinics (around here at least) are completely swamped.

Our two main vaccination guidance providers (the American Animal Hospital Association and World Small Animal Veterinary Association) want to help, but are restricted by the limitations above, i.e. there’s not much hard evidence on which to base off-label recommendations.

This could become a monstrously long post, so I’ll break it down. Today’s post will cover scenarios for “core” vaccines (other than rabies). These are vaccines for diseases against which we vaccinate pretty much all dogs and cats, often using combination vaccines. Core vaccines for cats are for panleukopenia,  feline viral rhinotracheitis  (aka feline herpesvirus type 1) and calicivirus. Core vaccines for dogs are for distemper, parvovirus and canine adenovirus-2

Comments below refer to vaccines and vaccination strategies used in Canada, which are similar to many other countries, but there can be some variation in vaccines and label instructions in other jurisdictionsThe discussion will also refer primarily to modified live virus (MLV) vaccines, which are the most common type of vaccine used in dogs and cats, as they are highly effective and generally result in an excellent immune response.

1. Puppy or kitten with a delayed initial series

With MLV vaccines, a delay in the initial series is not a big deal. Core MLV vaccines are highly effective and should provide long-lasting immunity even with a single dose, but only IF the puppy/kitten didn’t have a lot of maternal (mother-derived) antibodies at the time of vaccination.

Puppies and kittens get these antibodies from their dam, which provide protection while the animal is very young and its own immune system gears up to produce its own antibodies. Unfortunately, the maternal antibodies also decrease the ability of the animal to respond to a vaccine.  We usually give a series of 3-4 doses of MLV vaccine to puppies and kittens; this is not because they need an initial dose and a series of boosters, but rather, it’s because we want to make sure they’re protected as soon as those maternal antibodies run out, and that they get long-lasting immunity. So, we start the vaccination series early, and if their immune system responds that’s great, they’re protected early. If not, they get another dose in a few weeks, and then another dose a few weeks later… By 16 weeks of age, we assume they are able to fully respond to vaccination because their maternal antibodies have disappeared (please note that puppies and kittens can be vaccinated for rabies at 12 weeks – you do not (and ideally should not) wait until 16 weeks for rabies vaccination, in order to get them protected as soon as possible – more on that in Part 2 tmrw).

So, the key is we want to get a dose of MLV vaccine into a kitten or puppy at 16 weeks of age or older. If they missed one or more shots in the series, as long as they get a dose at 16+ weeks of age (even if their very first one), they should be good to go because that’s the one that gives them lasting immunity. In some high-risk situations, such as a dog that’s going to encounter a lot of dogs or be in a high risk environment like a shelter, we’d still consider giving another dose if they got their only shot at 16-20 weeks, but that’s case-by-case.

The take home for this group: Get them vaccinated when they’re young if at all possible, but once they’re 16 weeks of age or older, they may just need a single dose.   If there are delays getting a young animal vaccinated, care must be taken to reduce the risk of exposure as maternal antibodies drop, as there is a risk the animal will be unprotected for some time. That means limiting contact with other dogs/cats and high risk environments (e.g. parks, puppy classes, kennels) until vaccinated.

2. Puppy or kitten with a delayed 6 month/one year booster

Unlike killed vaccines for which the timing of the booster is important to get the full vaccine response, boosters of MLV vaccines can be given almost any time and probably results in a similar response. After the initial puppy/kitten vaccination series with a killed vaccine, these animals are usually vaccinated again 6 months or 1 year later, only because we are less confident about long-term  (multi-year) protection from the single initial vaccine.

The risk of being a bit overdue is probably low. The MLV vaccines we use are very good and immunity is probably fairly long-lasting.  We just don’t have as much confidence in it (because we don’t have the data to back it up). I’d prioritize these boosters below a younger animal needing its 16-week vaccination, but above an older dog/cat waiting for its 3 year booster.

The take home for this group: Get them boostered when you can, but don’t worry about a delay. If there is a delay, they still likely just need the single booster they were going to get.

3. Adult dog or cat with a delayed 3 year booster

After the initial vaccination series (or single dose), and the subsequent dose 6-12 months later, we now typically give dogs and cats core booster vaccinations approximately every 3 years. Being late for the 3 year booster isn’t a big deal. These vaccines are highly effective and produce an excellent response with a single dose, regardless of vaccination history. We rarely see the core vaccine diseases in vaccinated adult dogs and cats.

If there’s a need to triage who gets in for vaccines, these animals are lowest on my priority list (but remember we’re not talking about rabies vaccination – see Part 2). They’re unlikely to get these diseases as adults, and a delay will not impact how well they respond to the next vaccine. I want to get them done, though, and the need for a rabies booster may bump them up the priority list.

The take home for this group: When it comes to just the non-rabies core vaccines, get them boostered when when you can, but don’t worry about a delay. They just need the single dose they were going to get, whenever they can get it.

MLV core vaccinations are the easiest part of the decision process. It’s basically just a matter of getting them done when possible, but not changing anything else (beyond the potential for needing fewer puppy/kitten doses).

I didn’t talk about killed vaccines (yet) but I’ll mention them quickly because the story is completely different for them. With killed vaccines, we’re much more dependent on properly timed boosters. If those aren’t given, the default is to re-start the initial series to ensure there’s a robust immune response. So, if a vaccination series is started and then delayed, the whole series may need to be re-started.

More on other vaccines soon.

While the COVID-19 pandemic and its associated controversies continue to dominate our collective attention, much older, more familiar diseases are still out there too, causing their old familiar – and deadly – problems.  Rabies has once again reared its snarling head, this time in the small Arctic coastal community of Tuktoyaktuk, NWT. Rabies is considered endemic in arctic foxes in northern Canada and historic outbreaks have occurred cyclically. The current outbreak is shaping up to be higher in terms of the peak seen in past cycles, so the  surrounding communities in the Beaufort Delta region are on high alert as well. Since November 2020, there have been 8 or 9 reports of foxes attacking domestic dogs in the area. Foxes involved in biting incidents and those exhibiting concerning behaviours within and around the local community have been caught and submitted for rabies testing (which is no mean task when you need to get the samples out of a remote northern community in the dead of winter).  So far, no less than 5 foxes have tested positive for rabies, and more results are pending.  At least 8 dogs have been euthanized due to confirmed or suspected rabies exposure through fox bites, or in some cases, due to signs of rabies in the dogs themselves following a known or suspected fox interaction. Unfortunately, coordinating mandatory quarantine for dogs in these situations is often not feasible. Access to veterinary care is limited to absent in high Arctic communities, so most dogs are unvaccinated, despite living in a rabies endemic area. Strategies that are being implemented for the protection of human and domestic animal health, in cooperation with the territorial Departments of Health and Social Services and Environment and Natural Resources, include distribution of dog vaccines, post-exposure prophylaxis (PEP) for human exposure cases, and human rabies vaccines for at-risk individuals; increased education/awareness in communities regarding bite prevention, rabies awareness, and dog husbandry considerations; and enhanced monitoring for wildlife rabies.  A local community member and a few officers have also been trained as lay community vaccinators to facilitate ongoing preventative vaccination of dogs (due to the remoteness and inaccessibility of communities like Tuktoyaktuk).

For those of us living a little (or a lot) further south, this situation is an important reminder of what can happen when endemic wildlife rabies meets a large population of unvaccinated dogs, and the risk this centuries-old virus still poses to people and animals alike.  In Ontario, by law, all dogs and cats over 3 months of age must be kept up-to-date on rabies vaccination.  Even animals that live almost exclusively indoors can be exposed to rabies through contact with bats, so it’s still important to vaccinate them in order to protect everyone – people and pets – in the household.  It’s also critical to be aware of the risk of rabies in dogs imported from endemic areas (including northern Canada), especially when they could have been exposed to the virus before they were vaccinated.

That may seem like a strange question, but bear with me and read on.

Mink are back in the news, mainly with respect to vaccination against SARS-CoV-2. Mink are very susceptible to this virus, and it’s been shown that they can transmit it back to people. Perhaps more of a concern is that several mutant strains of the virus have also emerged in mink, though it’s not really surprising, since more transmission (especially with a species jump) means more risk of mutation.

The question of vaccinating mink against SARS-CoV-2 keeps coming up, and efforts to develop vaccines are underway. I’m no vaccinologist, so keep that in mind as you read this.

Vaccines can be used for a number of reasons, but I’ll focus on two broad categories:

  • Reduce/eliminate disease
  • Reduce/eliminate infection

There are important differences between these purposes.  Ideally we want a “sterilizing” vaccine, which prevents infection altogether. That means when the virus encounters a vaccinated individual, it doesn’t do anything – it can’t replicate enough to establish an infection.

Unfortunately, most vaccines aren’t that good. Most vaccines can reduce the likelihood of disease or severity of disease, but the virus can still commonly infect vaccinated individuals to some degree.  That can still be useful, as the primary goal is usually to reduce illness.  However, if those infected but somewhat protected individuals can still transmit the virus to others, it’s not as good for disease control overall.

But even if a vaccine isn’t great, it can’t hurt, right?

  • That’s not clear. There are a few potential concerns with vaccinating mink with a non-sterilizing vaccine.

Maintaining a reservoir

If a vaccine just reduces disease but not infection, that’s good for the individual mink, but could be bad for people. It would mean the mink are still susceptible to infection (which is typically introduced to the herd by infected people).  If the virus then spreads widely and silently on the farm because the mink aren’t getting sick, it’s harder to detect and control. That means farms may be more likely to become silent reservoirs of the virus.

More risk of mutations?

Virus mutations are random events, but the more a virus spreads and replicates, the greater the risk that these random events can occur. If a mutation results in increased transmissibility, increased virulence or poorer vaccine effectiveness, and that strain spread back into people, that’s obviously bad.  If the virus is circulating silently on a farm, it is likely to do so for longer before it’s detected and brought under control, providing more opportunity for mutant strains to emerg.

Encouraging vaccine-resistant mutants

A vaccine that’s only marginally effective might actually help select for vaccine-resistant  mutants of the virus. The big concern with that is if those mink vaccine-resistant mutants are also resistant to human vaccines, and then they spread to people, then that strain could spread even within the vaccinated human population. We don’t know if this is an issue, but it’s been raised in the context of people with suboptimal immunity to SARS-CoV-2 after only receiving a single vaccine dose, while awaiting their second dose.

I’m not saying don’t vaccinate mink.  It might be a useful control tool (for mink and people).

I’m saying:

  • Don’t think about vaccination as the main control measure for SARS-CoV-2 infection in mink. It can’t be done in lieu of other infection control practices.
  • Consider (and investigate) potential unintended consequences of vaccination (and other control measures).
  • Don’t rush to market a crappy vaccine on basis of “it can’t hurt,” because maybe it can.

If it’s going to be done, we need to make sure it’s done right. Cheap, rushed, suboptimal vaccines (in humans or animals) might make things worse. There are enough examples of pretty useless vaccines for animals that are on the market, so it’s a realistic issue. Conditional licencing of animal vaccines usually doesn’t require much data beyond indicating it’s unlikely to be harmful in the individual, but doesn’t account for potential effects at the population level.

The best way to prevent issues in mink is to reduce circulation of the virus in people, so the mink don’t get exposed through infected people in the first place.  Keeping people away from mink and using better infection control practices are also important (or having fewer captive mink).

There’s an interesting report that pets of people with COVID-19 in Seoul, Korea (and soon other areas) will be tested for SARS-CoV-2 for free, if the pets are showing signs of compatible illness. This comes on the heels of the (unsurprising) identification of an infected kitten in the city at the end of January, which was the first (confirmed) infection in an animal in Korea. It’s an interesting response, and it’s nice to see.


  • We get more information about human-to-pet transmission
  • We might get more information about the potential development of mutations of the virus in pets
  • Infected pets can be isolated (whether they pose a significant risk to people or other pets is still unknown at the moment, but erring on the side of caution is prudent)


  • More exposure of of testing staff to infected households in order to collect samples
  • More contact of additional people with potentially infected pets

Overall, I’d say the pros outweigh the cons, as long as there’s a good plan for safe sampling and for handling positive results.

We don’t know if a response like this is necessary, but the fact that we don’t know probably means that it is.  Once again, an Asian country is demonstrating a more proactive response to a disease risk, rather than the “show me there’s a problem, then I’ll think about acting” response we’ve seen elsewhere.

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.

I’ve written about COVID-19 scent-detection dogs before, and I’ve done a variety of interviews on the subject, but it keeps coming up.

Can scent-detection dogs help with COVID-19 control?  The answer is (as with many things about the SARS-CoV-2 virus) less than clear.  My current answer is… maybe… in some situations… potentially.

Let’s break it down.

Can dogs detect SARS-CoV-2 using their sense of smell?  Presumably not. The virus should have no smell.

Can dogs detect SARS-CoV-2 infection using their sense of smell?  It appears that some dogs, in some situations, can actually do this.  There are a few preliminary studies of varying strengths (from laughable to pretty good) indicating that dogs can detect infected people. They aren’t smelling the virus though, they’re smelling something that an infected person’s body produces in response to virus.

What are the the dogs detecting in infected people?  We don’t know exactly what the dogs are smelling, which makes it harder to train and assess them. If we had “eau de COVID-19″ it would make training and assessment much easier.  As of yet, we don’t know the best sample for dog’s to smell (saliva vs nasopharyngeal swabs vs skin wipes vs sweat vs…?) or the best kind of patient (really sick person vs moderately sick person vs asymptomatic person…?) from which to get the samples for training.

Can a dog detect someone with COVID-19 walking by?  We don’t know. The best work has been done with samples taken directly from a person, such as sweat wipes. I haven’t seen any data about dogs detecting infected people from just sniffing around them (despite the Miami Heat trying to use dog walk-bys to screen fans for COVID-19).

Can dogs be used to tell if someone doesn’t have COVID-19, so they can be allowed to do something or go somewhere (e.g. use public transit, or attend an event like a basketball game)?

  • Maybe, but it depends on the scenario.  At best, dogs are going to be a reasonably good screening tool for COVID-19. They won’t be 100% accurate, so we can’t rely on them in critical situations.  But they could be an adjunct screening tool that’s far from perfect but still useful.

Where might scent-detection dogs be used to help with COVID-19 control?

I do NOT see scent detection dogs as a tool to say either:

  • Yes, this person is infected. They need to go home.
  • No, this person is good. Off they go.

If dogs are actually good at detecting people from a distance, then they could be used like rapid antigen tests to say either:

  • Yes, you’re good to go. But still wear your mask and do everything else because I’m not perfect.
  • Hang on a minute… something might be up. You need to go for a PCR test to find out for sure.

Even if dogs aren’t that good for rapid individual screening, they could still be useful. I think the greatest potential is identifying high risk environments. For example, they could be taken into a homeless shelter, workplace, migrant worker housing or other high risk environment, and if they detect a whiff of COVID-19, that would be the trigger to bring in targeted testing (rather than the dog trying to figure out which individual are positive).

Rapid, cheap, person-side antigen testing will probably be more useful than dog screening, but scent-detection dogs could be useful in some situations if rapid testing isn’t universally available.

First ee need to know that COVID-19 scent-detection dogs work, how well they work, and their limitations. As with a lot of things, this is an area where some people have jumped from “idea” to “application,” without all the necessary steps in between.

What we can’t do is try to use these dogs as untested tools that give people false confidence to engage in higher risk behaviours (like having them at the door of a sporting event).  They’re a potentially useful add-on, but not an excuse to do something that wouldn’t be done without them. So, if an event is going on and rules at set, sure, add dogs as an additional measure. If the dogs are being used as the argument to allow people in, allow more people in or allow them to reduce infection control measures, that’s possibly counterproductive.

There are lots of SARS-CoV-2 variants out there. The virus changes a bit all the time.  However, there’s a lot of, well, concern about a few particular “variants of concern” (VOCs).

VOCs are “OC” because they have mutations that could increase their infectivity (i.e. ability to infect people), their virulence (i.e. severity of disease they cause) or decrease the effectiveness of current vaccines or antibody-based treatments. There are a few main VOCs currently circulating, but the B.1.1.7 variant that was first detected in the UK has received the most attention. In people, it seems to be much more transmissible than other “normal” strains. There’s also been some some suggestion in media from the UK that B.1.1.7 might be more virulent as well as more infectious, but that’s still not clear.

So, how infective are these VOCs to animals?

We have no idea. It’s possible that increased infectivity in people could also mean increased infectivity in animals. It’s also possible that increased infectivity in people could mean less infectivity in animals.  We just don’t know at this point.

How will we find out how infective these VOCs are to animals?

Some experimental study is presumably underway, and the WHO has indicated a need to test SARS-CoV-2 VOCs in experimental animal models so we can learn more about them.

We’ll also have to see what results come from field studies, but one of the issues is there are still only relatively small numbers of infected animals (outside of mink farms) in which we’ve been able to find the virus before it disappears.

  • Based on antibody detection, infection with SARS-CoV-2 in pets, at least, seems to be pretty common when their owners have COVID-19.  Yet, in our surveillance work, we’ve only picked up a few animals shedding the virus, in large part because of logistics and timing of sampling.
  • We’re working with our national lab to sequence the few samples in which we’ve found the virus, but the small numbers limit what we can say.  If we find a VOC, we found it.  If we don’t, we can’t say much more, since you need either luck or a high prevalence in the population to detect something with a small number of samples.

Could VOCs (or other variants) infect animal species that are not susceptible to the “normal” strains of SARS-CoV-2?

Probably not, but we can’t rule it out completely.  To start infecting other species altogether would require a much larger change in the virus, which isn’t likely to happen all of a sudden. We still have to pay a bit of attention to species that we think aren’t susceptible, because we can’t really guarantee how fast or how much the virus will change.

What is the best way to limit the impact of VOCs in animals, and the impact of animals spreading VOCs?

  • Control SARS-CoV-2, and all its VOCs, in people.
  • Limit contact of infected people with animals.  (Sound familiar?)

We know cats are susceptible to SARS-CoV-2, and it appears that human-to-cat transmission may be pretty common in households where people have COVID-19. In the big picture, that’s probably not a huge issue, since most cats that get infected show no signs or develop only mild disease, and most infected cats have limited opportunity to spread the virus outside their own household. However, we still need to understand more about infection in cats and other species, to make sure we understand the true risks (if any) and how to mitigate them.

Expanding on their earlier work, a research group from Kansas State University looked at susceptibility of experimentally infected cats to re-infection with SARS-CoV-2.  The study has been posted as a pre-print on bioRxiv (Gaudreault et al. 2020).

  • When they re-exposed cats to the virus 21 days after their initial infection, the cats got infected again, but infection was less common and mild.  Also, while infected cats could pass the virus to other cats during their initial infection,  the re-infected cats were unable to infect others.
  • So, while the cats were susceptibile to re-infection, the infection was mild and they didn’t seem to pose a transmission risk on the second go-around.
  • Therefore, natural infection is expected to provide some, but incomplete, immunity in cats (which is not too surprising, given what we know about infection in people).

An important aspect of this study to keep in mind is that the cats were re-infected quite soon after the first infection.  The study showed some protective effect after 21 days, but we don’t know how long this incomplete immunity may persist.  Duration of immunity after infection (and after vaccination) is a big question is humans as well. We’re hoping that natural infection or vaccination provide reasonably long-term immunity, but that may not always be the case. That will have a major impact on how this pandemic progresses, and how we approach vaccination in the long-term.

Overall, this study provides some useful information, but it doesn’t change anything we do at the moment. The key messages remains the same:

  • If you have COVID-19 or have been exposed to the virus, limit contact with animals (just like you would with people).
  • Cats are people too, at least from a disease control standpoint. If your household is isolating, that should include all the non-human critters. It makes no sense for me to lock down all the people in my house but still let my pets have contact with other people and animals.
  • SARS-CoV-2 is (now) a human virus, but it still has an affinity for certain other species. Our goal should be to keep this a human virus and try to prevent it from infecting other animals.
  • If you’ve had COVID-19 once, hopefully you won’t get it again – but you might. Previous infection isn’t an excuse to change your behaviour or stop using basic prevenative measures around other people or animals.

At the start of the COVID-19 pandemic, many major agencies took a head-in-the-sand approach to concerns about the potential for SARS-CoV-2 to infect different animal species. Fortunately,  over the last year a considerable amount of work has been done to help figure out the range of species that are susceptible to this virus, and shed some light on how animal populations might ultimately impact control of the virus, based on the potential for for infecting wildlife in particular (which comes with the risk of creating  wildlife reservoirs, and potential sources of new virus mutants). We now know of a few wildlife species that are susceptible (and can transmit) SARS-CoV-2, but there are so many wildlife species that our knowledge still just scratches the surface.

A recent study in pre-print posted on bioRxiv (Bosco-Lauth et al. 2020) looked at SARS-CoV-2 susceptibility in deer mice, bushy-tailed wood rats, striped skunks, cottontail rabbits, fox squirrels, Wyoming ground squirrels, black-tailed prairie dogs, house mice and raccoons. They did this by catching wild animals and experimentally exposing them to the virus, and then monitoring them in captivity. The study was pretty small (2-9 animals per species) but provides some useful information.

  • Deer mice, bushy-tailed woodrats, and striped skunks were susceptible to infection and shed the virus after infection, but they didn’t get sick (i.e. all infections were subclinical).
  • Cottontail rabbits, fox squirrels, Wyoming ground squirrels, black-tailed prairie dogs, house mice, and raccoons were not susceptible to infection.

That’s a bit of a mixed bag of results. The more wildlife species that are susceptible, the greater the potential problems.  We’re also more concerned about species that may have more contact with people (e.g. urban wildlife), those that live in large groups (where an infected individual can spread the virus to lots of others, potentially leading to sustained transmission in the population and creation of a reservoir), and those that can travel long distances (and could thereby carry the virus to new areas).

One highlight of this study for me is that raccoons were not susceptible, because they’re a very common, social wildlife species that lives in large urban centres where COVID-19 is (at the moment typically) rampant in people. Raccoons are one of the species I’ve been most concerned about in terms of a jump to wildlife.

The authors sum things up nicely “… we will undoubtedly continue to discover more susceptible species as the search for zoonotic reservoirs continues. COVID-19 is just the latest in a series of examples of how the human-wildlife interface continues to drive the emergence of infectious disease.”