Zoonotic diseases from pet fish are very rare, but rare doesn’t mean never.  That can make messaging around the disease risks from pet fish challenging, because there’s a need to balance costs vs benefits, and being proactive vs paranoid.

I have a saltwater fish aquarium, and I don’t stay up at night worrying that it’s an impending source of doom. For managing the aquarium, I use some really basic and non-disruptive common sense, though. I did some work on it a couple days ago and paid attention to two things: I kept my hand with a few cuts on it out of the tank, and after I was done I washed my hands.  Common sense and hygiene 101.

The most commonly reported zoonotic disease issue from aquariums is probably Mycobacterium marinum infection, also known as “fish tank granuloma.” A recent paper in Emerging Infectious Diseases (Dawson et al. 2021) describes a case of another type of infection acquired from a household aquarium, caused by a bug that’s gotten some press recently for other reasons: Burkholderia pseudomallei, the cause of melioidosis.

Melioidosis is a nasty and potentially life-threatening disease. In North America, melioidosis is typically associated with travel to tropical or subtropical areas where the bacterium can be found in water and soil.  People are exposed through ingestion or inhalation of the bacterium, or contamination of broken skin. However, local sources in North America have been identified, such as the recent outbreak of human melioidosis cases in the US that was linked to contaminated aromatherapy products.

The case in the recent EID paper was a 56-year-old woman in the US that was hospitalized because of fever, cough and chest pain. She had a somewhat complex medical history, and had been taking immunosuppressive drugs up until approximately 1 month before she got sick. Culture of her blood identified B. pseudomallei.  Fortunately, she responded well to a few months of antibiotic treatment.

The first question asked after the bacterium was isolated was likely “where have you traveled recently and when?” However, she had no history of travel.

As part of the next stage of the investigation, they found out the patient had two freshwater aquaria, from which they collected samples for testing. (It’s not clear how quickly they moved from “no travel history” to “maybe the fish tanks are the source” and how many steps were in between.)

There’s nice description of events as they relate to the aquaria. The patient had purchased two small aquaria about 3 months before she got sick. In one (tank A) she kept cherry barbs and in the other (tank B) she kept guppies. She also purchased some tiger barbs for tank B, but that was after she got sick.

Interestingly, she reported that the water in tank B had been persistently cloudier than the other tank, and it required more cleaning, which she did with bare hands.  That’s the aquarium from which B. pseudomallei was isolated.

Presumably, the woman was exposed while cleaning the aquarium, either via a break in her skin, contact of contaminated water with her nose, eyes or mouth (e.g. splashes, contact with wet hands) or maybe contact with areas that got contaminated by water from the aquarium (e.g. areas around a sink down which water was poured).

There were also dead fish in tank B when public health investigators came to visit. That raises the question of whether the fish might also have been sick. We’re not going to recommend testing all dead pet fish for melioidosis as a surveillance tool, but it once again illustrates the potential inter-relatedness of human and animal health.

The article concludes “To prevent or reduce risk of exposure, particularly among persons who have major risk factors, simple precautions can be taken when handling freshwater fish, snails, aquatic plants, aquariums, or other materials in contact with aquarium water, such as gravel, substrate, decorations, filters, and other equipment. CDC recommends thorough handwashing with soap and water before and after handling or cleaning aquariums and feeding fish, wearing gloves to cover any cuts or wounds in the hand while handling fish or aquariums or allowing wounds to fully heal first, avoiding cleaning fish aquariums if immunocompromised or in areas where immunocompromised persons might be present, and not allowing children <5 years of age to clean fish aquariums.”

  • All pretty standard statements, although glove use is a challenge if you’re putting your hand into the water past the wrist, unless you use full arm gloves.  You have need to be aware that there will often still be some exposure of the hands through leaks in gloves.

So, don’t freak out about your fish, but use some common sense, and remember that if you’re sick, it’s always good to make sure your healthcare providers know about any animal contacts you’ve had recently (in your household or elsewhere).

I’ve been a laggard with blog posts lately.  It’s a bit crazy and it’s been easier to get things out quickly via Twitter (@weese_scott). However, one interesting topic I wanted to get back to is SARS-CoV-2 in deer. It’s caused a big stink in some areas and, like a lot of things involving this virus and animals, we don’t really know the big picture implications yet.  I wrote about SARS-CoV-2 in deer a few months ago, but some new data have come out recently that raise even more issues.

Story #1

Earlier this year, there was an experimental study that showed white tailed deer are susceptible to SARS-CoV-2 and, importantly, are able to spread it deer-to-deer.  At the time that put deer onto a fairly large and growing list of species that are susceptible this virus, but there are some specific concerns with a wildlife species like deer.  We really don’t want SARS-CoV-2 to become established in wildlife, as that creates the potential for a reservoir of infection for people and other species, and the potential for emergence of new mutants (variants of concern) that could cause issues if they spread back into people (spillback). One key thing that’s needed for an animal reservoir is a large enough group of susceptible individuals to allow continued circulation of the virus, and there are in fact large relatively mobile populations of deer across wide ranges in North America.

However, “susceptible” and “relevant” aren’t the same, and we didn’t know whether there was much chance deer were actually going to be exposed to SARS-CoV-2, and how likely infection in deer would be in the real world.  The answer surprised us (or at least me).

Story #2

SARS-CoV-2 in deer went from a theoretical risk to a real world issue when the USDA released a study reporting high rates of seropositivity (antibodies against the virus in blood) in deer in various parts of the US. Overall, 40% of tested deer from 4 US states had evidence of previous infection. A subset of samples was also tested with the standard virus neutralization test, with good agreement. That, and the lack of positive results in samples collected pre-pandemic (see figure to right) suggested the results were reliable.

Story #3

Some recent pre-print studies have taken this issue a step further.

Hale et al. (2021) reported detection of SARS-CoV-2 by PCR in 36% (129/360) of deer sampled in northeast Ohio between January and March 2021. Interestingly, at least three different lineages (strains) of virus were identified, meaning at least three different introductions into the deer population. As expected, they were lineages common in people in the same areas, and people were the most likely source of the introductions.

Kuchipudi et al. (2021) performed PCR testing on retropharyngeal lymph nodes (i.e. the lymph nodes at the back of the throat) from wild and captive deer in Iowa. SARS-CoV-2 RNA was identified in 33% of samples from April to December 2020, and a whopping 83% of samples from November 23, 2020 to January 10, 2021. Twelve different lineages were identified. None of 17 samples collected earlier (April-August 2020) were positive.

Does this mean people can get infected with SARS-CoV-2 from deer?

  • We don’t know.

Does this mean the SARS-CoV-2 virus is actively circulating in deer?

  • We don’t know.
  • High rates of seropositivity could mean that the virus gets into a deer population, spreads quickly, infects a lot of deer but then burns out and disappears. That would be good. However, it could also mean that the virus circulates through the population as it continues to find new susceptible deer at a slower pace. This is a big question that needs to be answered.

Does this mean the SARS-CoV-2 virus is likely in deer populations elsewhere?

  • We don’t know. (Sensing a pattern here?)
  • It seems likely and it’s being actively investigated.  So far it’s only been found in deer in the US.

Are “deer variants” of SARS-CoV-2 being created?

  • We don’t know. None have been found to date.
  • The likelihood of “deer variants” emerging really depends on the amount of virus circulation in the deer population. Mutations occur during virus replication. The more transmission, the more replication and the greater the risk of variants emerging.

What’s the risk to the general public from SARS-CoV-2 in deer?

  • Pretty low. (I could say “we don’t know” but I’ll mix it up.)
  • Few people have close contact with deer (although every time I say or write that, I get flooded with emailed pictures showing deer hanging out on front lawns, porches, lounging in the yard with dogs, etc. so it may be more common for some than most).
  • It’s direct contact or close range aerosol transmission that I’m worried about, and since deer are outdoors in well ventilated spaces, simply being in the same area as deer shouldn’t pose any risk.

What’s the risk to hunters from SARS-CoV-2 in deer?

  • That’s a big question. Hunters will have the closest direct contact with deer and have the potential for exposure to respiratory aerosols in wounded animals, as well as close contact with carcasses. We have no idea what the risks might be.
  • To be prudent, we should assume there is some risk from this kind of close contact associated with hunting in areas where this virus might be circulating in deer.

Should hunters do anything different?

  • It’s hard to say, but a little bit of practical prevention makes sense. It has been recommended that hunters wear a mask when handling deer carcasses . That’s reasonable.
  • However, since dead deer don’t breathe, we’re not worried about exhaled respiratory aerosols when people handle carcasses. We’re mostly concerned about splashes or direct contact with respiratory tissues or secretions (+/- feces). So, wearing a mask when dressing a carcass, for example, still makes sense, to protect from splashes and prevent hand-to-mouth contact (which occurs more often than most people realize) when a person’s hands may be contaminated.
  • Keep in mind that if we’re worried about splashes, eye protection also makes sense.
  • Being particularly careful around wounded deer, avoiding close contact and using a mask and eye protection if such an animal must be approached would also be reasonable.

Does venison pose a risk?

  • Presumably not.
  • SARS-CoV-2 doesn’t survive long outside the body, and high levels of virus wouldn’t be expected in deer meat. Contamination of meat from feces or respiratory secretions is always possible, but good hygiene can reduce that. It’s not really different from how you’d prevent contamination with typical foodborne pathogens.  We should handle meat (from any animal) like is has various infectious agents in/on it, use good hygiene and cook it properly. Any SARS-CoV-2 contamination would be effectively controlled through those same measures.

How do we figure out how SARS-CoV-2 gets from people to deer?

  • That’s a huge challenge. Figuring out how it gets into deer will be very tough and I’m not sure we’ll have a good answer soon.
  • Is it from rare human-deer interactions that involve close contact but leave a live deer? (i.e. not hunting)
  • Is it from deer coming in contact with contaminated items like garbage from infected people?
    • Unlikely, since things like that aren’t thought to be a realistic source of human infection and deer aren’t classical dumpster divers.
  • Is it from deer coming in contact with human wastewater?
    • Probably not. We can use wastewater for surveillance but we typically only detect bits of viral RNA, not viable virus. We don’t have evidence that infectious virus is released in wastewater.
  • Could there be an intermediate animal host that carries the virus from humans to deer?
    • Cats would be a leading candidate, if they have contact with infected people in a household but are allowed outside and then have contact with deer. It seems like a stretch based on cat behaviour, but can’t be ruled out.

Where do we go from here?
We need more surveillance, especially from other regions. It’s critical to determine if the virus is actually undergoing sustained transmission in deer populations, and if so where. If it is circulating on an ongoing basis in deer, we’ll need surveillance to look for emergence of significant variants. Hunting history and deer contact history should be considered when investigating new cases of COVID-19 in people, especially when there’s no other clear source. If the virus is circulating in deer, we’d also want more wildlife surveillance of other potentially susceptible species, to see if it’s spreading into other populations that could also become reservoirs.

Closing notes (the usual)

  • The best way to keep this virus out of deer is to control it in people.
  • The best way for a hunter to reduce their risk of getting COVID-19 is vaccination.

As a journal Associate Editor and reviewer, I see lots of manuscripts about “new” viruses. I tend not to get too excited about most of them, because “new” is usually actually just “new to us” (or newly identified), because as technology improves, we are  able to identify lots of viruses that we’ve been living with for years. Viruses are part of our ecosystem, and most are harmless to us (and animals).

It’s common to find “new” viruses in sick people/animals, but they’re usually just background “noise,” because they can often be found just as frequently in healthy individuals. Differentiating something that’s “newly found but long present and harmless” from something that’s “been around and was an unknown cause of disease” from something that’s “truly a new emerging disease threat” is key (and sometimes easier said than done).

That brings me to a virus I wrote about in May: a “new” coronavirus that was found in 8 people in Malaysia who had pneumonia in 2017-2018. The virus was an alphacoronavirus that most closely resembled a canine coronavirus (notably SARS-CoV-2 and the common canine respiratory coronavirus are betacoronaviruses). The virus was named CCoV-HuPn-2018 (canine coronavirus-human pneumonia-isolated in 2018).  Whether these infections were a rare or one-off event wasn’t clear at the time.

However, a new study has reported finding this virus in a person in Florida who had returned from Haiti in 2017 (Lednicky et al. 2021). The person had pretty mild disease, but it was investigated and this same virus was found.

The fact that the person had only mild disease is important for context – very rarely do people with mild disease get tested, and even more rarely would a hunt for a new virus be performed in people with mild disease that had negative tests for the “usual suspects.” But they did test this person, and they once again found that the virus looks like it came from dogs (at some point), and it was found half a world away the same year.

This second report generates a lot of questions:

  • Was this another rare event, or does it represent a common, mild pathogen that’s circulating internationally?
  • Why were the cases in 2017 for both reports? Is this a virus that spread that year and burned out, or has there been limited study of it since then and it’s still with us (and we’re just not testing for it)?
  • What’s the role of dogs? There’s no information about the epidemiology of disease yet. Is it transmitted dog-to-human or did the dog-origin virus move into people and is now spread human-to-human?
  • Can this specific virus be found in dogs, or is this truly a human variant now?
  • Can the virus be spread back to dogs? If so, can it spread between dogs, can it cause disease in dogs, and can it be spread back to people?

These are all reasonable questions that could use more study. While this virus doesn’t seem like a big deal, it’s worth understanding more about the coronaviruses with which we live.  “Relax, but pay attention” is my typical response to new reports like these, and I think that’s fair here. The authors’ conclusion also fits with that:

Our data highlight the potential among coronaviruses for rapid evolution combined with frequent recombination events, leading to periodic emergence of strains capable of crossing species barriers into human populations. In many instances such strains would appear to be of low virulence for humans, as reflected in our work with PDCoV and now CCoV-Haiti; however, the potential for such strains to carry or acquire genes capable of causing severe disease in humans remains of clear concern.

This one’s as easy to write as the first version… we still have no clue.

Overall, the health risk to horses from SARS-CoV-2 is probably very low. If horses were getting sick, or at least very sick, we would have noticed by now. I’ve not had any indication that we’re seeing a disease impact in this species. We still don’t know if horses are getting infected from people or whether, if infected, they can spread it back to people, between horses or to other animals. That’s something  we still need to figure out.

Why don’t we have much information about horses compared to other species?

  • Food animals get studied because of concerns about food safety, food security and economics.
  • Pets get studied because of our close contact with them (e.g. they live in our homes and sometimes even sleep in our beds).
  • Wildlife get studied because we’re worried about this virus getting into wild animal populations that could then act as virus reservoirs and sources of new variants.

Horses are a mix of farm animals, companion animals and competition animals. There’s a relatively small pool of researchers who focus on infectious diseases in horses, and there’s limited research support.  Experimental studies in horses are very expensive, because of their size, cost of housing, value of individual animals, and other factors. Field studies are more practical but require people to do the work (we have that) and interest from horse owners and other relevant parties to participate (that’s often missing).

There’s limited interest in (or downright opposition to) surveillance by some, because identifying potential problems leads to having to deal with those problems. If SARS-CoV-2 was identified as a significant issue in horses, that could mean restricting movements on farms with infected horses (which a lot of people want to avoid), as well as dealing with a lot of additional concerns from the general public (including both horse owners and non-horse owners). As a result, we run into barriers to testing in situations where exposure is plausible. I’ve had multiple situations where it would have been very informative to test horses, but where owners/trainers were wary of what would happen if there was a positive result. Since some people are more wary of the impact of a positive result than the impact of the virus itself, we’ve missed out on opportunities to figure out whether there’s any risk to (or from) horses.

So, what do we actually know about SARS-CoV-2 in horses?

One study from Italy tested 34 horses from 2 farms that were exposed to infected people. None of the horses tested positive. The timing of testing isn’t exactly clear, but the study suggests that horses were sampled fairly late after the onset of disease in people, and it’s quite plausible that short, transient infections could have been missed. In some other animal species, individuals tend to only shed the virus for a short period of time, so if sampling is delayed we may not find the virus (but we can still find antibodies after the fact). This was a good initial study to have in horses, but we need more studies where sampling is done closer to the time of onset of disease in people, and ideally with follow-up antibody testing in case we miss the potentially short window when an animal may be shedding the viral bits that can be detected by PCR.

Beyond that, we have older studies that looked at the composition of the ace2 receptor in different animal species. Ace2 is the structure that SARS-CoV-2 uses to attach to the body’s cells. If the virus can’t attach to cells, it can’t infect them. The structure of this receptor varies between species, and that accounts (in part) for differences in species susceptibility. One study ranked the likely susceptibility of horses to SARS-CoV-2 to be equivalent to cats (specifically domestic cats and lions, both of which we know can be infected) and camels (which we also know nothing about, beyond that they are a host for another zoonotic coronavirus that causes Middle Eastern Respiratory Syndrome (MERS-CoV)). We have to take predictive studies like these with a (big) grain of salt, because the real world situation hasn’t always mirrored what was predicted.  These studies tell me that we should pay more attention to horses and see if there’s a problem, not that a problem is likely.

What should we do?

Some surveillance for SARS-CoV-2 in horses would be good. Testing horses that have been exposed to infected people would be interesting and let us know more about the potential interspecies spread of the virus. There have been outbreaks of COVID-19 in grooms in racing stables, a population that’s probably very high risk for infection and for working while sick, and of course they have close contact with their horses, so a situation like that would be a good place to start some equine surveillance.

However, the most important thing we need to do is stay away from any animal, including horses, if we have COVID-19. It’s better to prevent a problem than deal with it, and if we reduce the number of infected people that have contact with horses, we reduce any potential problems.

After a couple of years of very little apparent canine influenza activity in the US, the virus seems to have come back with a vengeance in some parts of California. Canine H3N2 influenza came to  North America from Asia in approximately 2015 and caused outbreaks in many areas, but then seemed to slip into the background. Sporadic outbreaks have been identified in recent years, but were mainly small clusters in individual facilities like kennels.

A recent, prolonged outbreak of H3N2 influenza in dogs in southern California has highlighted the importance of this virus again. The best tracking and reporting of the outbreak has been provided by the County of Los Angeles Public Health:

“Between July and October 2021, approximately 800 confirmed and suspect cases of CIV [canine influenza virus] H3N2 in dogs in LA County have been reported to Veterinary Public Health.  Sadly, 7 deaths in dogs have been associated with this outbreak.  Of the cases reported, most were associated with attending boarding kennels or dog daycare settings.  There are a number of cases that have never visited a boarding or daycare facility, but were exposed while on walks in their neighborhood, at dog parks, groomers, or at veterinary clinics.  This virus has spread rapidly among dogs throughout LA County, affecting many congregate facilities.  Based on interviews with these facilities, many additional cases have not yet been reported to VPH.  We suspect that this outbreak likely involves more than 1000 cases of CIV H3N2 in LA County dogs.  Dogs that appear to have ‘kennel cough’ have a high likelihood of having CIV H3N2.  To date, this is the largest outbreak of CIV H3N2 ever reported in LA County.  To stop the spread of this outbreak, pet owners and veterinarians are strongly encouraged to vaccinate dogs against CIV H3N2 and isolate sick pets at home for 28 days from the first day of illness.  Pets exposed to confirmed or suspected cases should be kept on a home quarantine and observed for clinical signs for 14 days.”

The 7 deaths puts the mortality rate at approximately 1%, which is pretty consistent with what we’ve seen before with this virus in community settings. I’d guess that 20-40% of the affected dogs have received antibiotics and other treatments due to more serious disease (and some overuse of antibiotics, which unfortunately is still common with respiratory disease in dogs, as it is in people).

As is typical with canine influenza (and canine infectious respiratory disease complex (CIRDC), in general) we only have a partial picture of what’s going on. Testing is sporadic and  various other respiratory viruses and bacteria that cause similar disease are always circulating in the dog population.

When canine flu was found in Canada in 2018, we aggressively tested and traced in-contact dogs, and the virus was successfully eradicated in Ontario.  That’s not a realistic goal in this case. LA County has a great veterinary public health team, but you have to be able to intervene early to have any chance of stopping a highly transmissible virus like flu in its tracks.   Once it’s widespread in the general dog population, it becomes impractical to properly trace and test all in-contact dogs for every case (especially when there is rarely much or any financial support to do so). So, containment through education and vaccination is the focus. Presumably, this outbreak will burn out in the area eventually. Whether it will be tracked to other areas outside California is a big question.  We’re still watching for new introductions of canine influenza in Canada. We were able to control the first introductions of H3N2 canine flu in 2018, but I’m not naive enough to think that luck wasn’t a big component of that.

Since respiratory disease is common in dogs, particularly of late, and because we only test a minority of coughing dogs, we need to be aware of higher risk situations. I pay the most attention to:

  • Large clusters of cases, especially in well vaccinated populations
  • Outbreaks that affect virtually all exposed dogs
  • Outbreaks where there’s a known or possible initial exposure to a dog from a high risk region (e.g. Asia, and now parts of the US where the virus is active).

In situations like those, prompt testing, along with good infection control practices, are what we need to identify and contain the problem as much as possible.

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

Makes sense, right?

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

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

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

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

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

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

Where does this leave us?

Antimicrobial stewardship – by everyone – is the key.

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

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

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

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

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

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

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

Nice and clear, eh?

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

How often do dogs get infected?

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

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

Serological studies looking for antibodies against SARS-CoV-2 in dogs have shown that transmission is actually much more common than this, with rates of up to 46% in dogs from infected households (e.g. Stevanovic 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.