Labrador retriever in a field

I have Labrador Retrievers, so I’m well versed in dealing with the implications of what my dogs have been eating (Labs being prone to considering just about anything fair game for ingestion). Owning such a pair of environmental vacuums, the question of whether dogs might be at risk of H5N1 avian flu exposure from bird feces or other things in the great outdoors has a personal spin.

It’s a really common question these days, and we don’t have a really solid answer, and it’s a very hard thing to study. In natural settings, teasing out environmental exposure from direct bird-to-bird or bird-to-other-animal exposure is tough. We rarely have situations where we can clearly say “this individual was exposed to this and only this type of environment, and then did (or did not) get infected.” There’s also a species susceptibility component. Environmental transmission of H5N1 influenza to birds is likely very different from environmental transmission to mammals because of the lower susceptibility of mammals. Even within mammals, there’s likely a lot of variation in susceptibility of different species, and types / level of exposures they’re likely to have.

So is there cause for concern regarding exposure of dogs (and other mammals) to H5N1 flu from bird feces in the environment?

Yes, to some degree. Environmental exposure has been identified as a risk factor for human H5N1 infection in the past, but that’s usually from contact with very high risk environments like infected farms or markets, or where people live with poultry (e.g. in the household). More recently, environmental exposure was deemed to be the most plausible source of infection with H5N1 influenza in a person in Chile.

So, there’s a plausible risk of environmental exposure to this virus, but it’s hard to extrapolate those reports to common casual outdoor exposure to bird feces or other sources. We also have to remember that these are rare incidents during an outbreak where millions of birds have been infected, and have been shedding a vast amount of flu virus into the environment.

What types of environments are of greatest concern?

The more infected birds in a particular area, the greater the risk the virus will be present in the environment. That’s pretty obvious. One infected bird can shed a little bit of virus, ten thousand infected birds can shed a whole lot more. But the risk doesn’t depend solely on the amount of virus expelled by birds; it also depends on things like how well the virus survives in the environment, and how people or other animals are exposed.

Does the H5N1 influenza virus survive long in the environment?

Yes and no. Fortunately, flu viruses don’t usually survive well outside the body. If it’s dry and sunny, the virus will die pretty quickly (typically within hours). But, if the virus is in a nice protected environment, it can survive for longer periods of time (potentially days or weeks).

Can a dog get H5N1 influenza from eating bird poop?

Potentially. It would have to be fairly fresh poop from an infected bird, with enough of a viral load to cause infection (but we don’t know how much of a load that is for a dog). I’d say the risk is pretty limited in most situations. However, if a dog is “grazing” in an area with a lot of infected birds and a lot of bird poop, there’s going to be some risk.

Can a person or dog get infected from virus on someone’s shoes if they walk where infected birds have been?

This is getting into the “theoretically possible but extremely unlikely” area. It’s not impossible but there are bigger things to worry about. That said, if there are lots of birds in an area and flu is active, it’s wise to stay away (and not just to keep your shoes clean).

Overall, I’d say the risk of H5N1 influenza infection in dogs from normal activities like walking in a park is low. It’s not zero, but it’s low enough that people shouldn’t panic. Like most things, it’s a matter of balancing costs and benefits of preventive measures. A bit of common sense goes a long way:

  • Stay away from areas with lots of birds.
  • Be more restrictive about outdoor activities when flu is active in birds in the area.
  • Avoid direct dog-to-bird contact (including both live and dead birds).
  • Avoid situations where direct dog-to-bird contact is more likely to happen (e.g. lots of territorial or habituated geese, like in many public parks).
  • Avoid areas with obvious accumulations of bird poop.

And the most important measure:

  • Keep dogs away from sick or dead birds.

This can be filed under “concerning but not surprising,” but H5N1 avian influenza has been identified in a dog in Ontario

It’s concerning because any spillover into mammals raises concerns about continued adaptation of this virus to spread outside birds, and because spillover infections in mammals can be severe. 

It’s not surprising because when you have millions of infected birds internationally, it’s inevitable that domestic and wild mammals will be exposed. Even if transmission is rare, when there’s a lot of exposure, transmission becomes more likely to occur and to be detected.

The case at hand is a dog from Oshawa, Ontario that died several days after being found scavenging a dead goose. Both the dog and goose were tested for H5N1 highly pathogenic avian influenza (HPAI) virus, and both were positive. Sequencing of the virus at the National Centre for Foreign Animal Diseases was performed and the virus from both the dog and goose were the same, and were consistent with the H5N1 strain that’s circulating in wild birds and domestic poultry. Further testing is being performed to confirm the cause of death in the dog. Given what we know about spillover infections into related species like foxes, it’s certainly possible that avian flu could have contributed to the dog’s death.

What should people in Ontario (or anywhere else avian flu is circulating) do?

  • Relax. That’s the first thing. This is concerning but not a doomsday scenario. We know that spillover into various mammals is happening and it will continue to happen. Also, this was a pretty high-exposure scenario where a dog had a lot of direct contact with a bird that had died of avian flu. It’s a reminder of why we’ve been emphasizing the need to try to better understand this virus since the outbreak was first identified, and to try to prevent more spillover infections from wild birds.

The next step is to just take (or continue to use) some basic common sense measures to reduce the risk of exposure.

What can be done?

  • The big thing is keeping dogs (and other domestic animals) away from wild birds.  It’s a good general rule to keep dogs away from wildlife anyway (alive or dead). That’s particularly true when there’s avian flu activity in an area.

Can dogs be vaccinated against this flu virus?

  • No, at least not at this point. Canine flu vaccines target different flu strains (canine H3N2 and H3N8) and there’s not likely any relevant cross protection. 

What’s the risk to people from infected dogs?

  • It’s probably very low but this is an unknown. Spillover infections into other species are often “dead end,” where the infected individual can’t/doesn’t infect anyone else. However, there have been some wild mammal outbreaks where limited mammal-to-mammal transmission has been a concern. When litters of wild canids have been infected, it’s hard to say if they were all exposed to the same infected birds or whether there was mammal-to-mammal spread.
  • So, it’s a big unknown. With that, it’s reasonable to take precautions to reduce contact with potentially infected mammals. However, the risk is probably quite low.

Should sick dogs be tested?

  • Testing would be considered in situations where there’s a plausible concern about H5N1 flu, based on likely exposure and the signs of illness in the dog. Lots of dogs have respiratory disease from various viral and bacterial causes and there’s no use testing every coughing dog (especially since a mildly ill coughing dog isn’t going to be a classical presentation for this viral infection). Testing for H5N1 influenza can be done through veterinarians, typically by PCR testing of oro-pharyngeal (throat), nasal and/or rectal swabs. 

What about cats?

  • Basically, replace everything above with “cat” instead of “dog.” The risks and preventive measures are pretty similar. Keeping cats indoors (when possible) to reduce their exposure to wild birds, is the main measure. That will help protect both them and their human contacts.

We’ve been dealing with a major shortage of canine influenza vaccine for a while. That’s caused a few different hassles, some related to disease and others related to vaccination requirements.

A quick recap: There are a couple of different strains of dog-adpated flu internationally, but currently we’re really just worried about H3N2. This strain is present in Asia and the US (to which it was imported from Asia). It was introduced to Canada (specifically Ontario) a couple of times in 2018, but we were able to contain it and I’m not aware of any spread of it up here since then, but we remain on the lookout.

From an epidemiological standpoint, canine flu is different from human flu in that it’s not particularly seasonal, and it’s not as widely distributed. In the US, we see sproradic cases and outbreaks that seem to jump around the country (likely related to dog movement), often causing local outbreaks that tend to burn out. In contrast, human (seasonal) flu is more widely disseminated and most people are at some risk of exposure. Currently most dogs probably have little chance of encountering canine flu, but we have a hard time predicting which dogs will get exposed, since the virus jumps around geographically.

Canine flu vaccines are, well, just like other flu vaccines, in that they’re reasonably good at what they’re designed to do: reduce the likelihood and severity of disease. They’re not meant to stop the spread of the virus altogether, or contain an outbreak. They’re used to reduce severe impacts on dogs that get infected.

There are two main groups I think about in terms of dogs that benefit from canine flu vaccines:

  1. Dogs that are at increased risk of exposure to flu. Those include dogs that have contact with lots of other dogs, dogs that travel to areas where flu is spreading and dogs that have contact with dogs imported from high risk areas (e.g. Asia).
  1. Dogs that are at greater risk of severe disease or death if they get infected. Unfortunately this group is often overlooked. Older dogs are the main concern, since flu deaths tend to occur mostly in senior dogs. However, it also includes the very young, pregnant, immunocompromised, brachycephalic breeds (smushy faced breeds like bulldogs) and dogs with underlying severe heart or lung disease.

The first group is more likely to be exposed but the second group is more likely to be seriously impacted. So, if push comes to shove, I want to focus on vaccinating that second group. That protects more dogs from serious disease and doesn’t negatively impact control much (if at all), since flu vaccines are not a good tool to prevent transmission in the first place.

However, if we look at what dogs actually get vaccinated, it’s often dogs in low risk areas that go to kennels or group events. It’s not necessarily because of a high risk of exposure to flu or a high risk of complications, it’s because someone has made it part of admission requirements.

What’s the benefit of requiring flu vaccination for boarding or day care?

  • The benefits are pretty limited, from a facility standpoint. It might help reduce transmission if an infected dog gets in, but that’s probably marginal. Flu is really transmissible, so I doubt even having good flu vaccine coverage in a group of dogs will really prevent an outbreak. It will reduce the degree of illness in individual dogs, but not do a lot to protect the facility (and that’s the premise for requiring vaccination).

That means by vaccinating all these dogs, we’re burning through our limited vaccine supply for very limited gain. In a time of shortage, I’d rather see vaccine diverted to situations where it is more likely to have an impact:

  • Dogs at higher risk of serious disease
  • Situations where vaccination is needed for dog movement (e.g. someone moving to a country that requires flu vaccination of dogs prior to importation)

Would I like all dogs vaccinated against the flu? Sure. It’s not going to happen, though. So, while we’re in a period of vaccine shortage, here’s how I’d prioritize our limited supply:


An abstract in the upcoming European Conference on Clinical Microbiology and Infectious Diseases (ECCMID) has gotten a fair bit of press and led to a lot of questions because of some sensational headlines. I’ll hopefully be able to check out all the details at ECCMID (I’ll be there for the Global Leaders Group on AMR meeting), but let’s take a quick look at what we know so far…

First, we know there’s regular exchanging of bacteria between people and pets. That’s no surprise. We have close and prolonged contact with our pets, and many bacterial are adept at surviving in a variety of animal species in addition to humans. I’ve done various studies over the years looking at this with specific bugs like methicillin-resistant Staphylococcus aureus (MRSA) and Clostridium difficile.

There are some specific issues with certain bacteria, but there are also some common themes about how many of these bacteria behave:

  • Bacteria are periodically passed between people and pets, in both directions.
  • Bacteria may be more human-associated (e.g. Staphylococcus aureus) or dog-associated (e.g. Staphylococcus pseudintermedius), so when they move to another species, it’s often just a transient visit, such that they soon disappear again on their own.
  • If we look at multiple people and pets in the households, it’s typically more common to find human-associated bugs (like MRSA or C. difficile) in other humans, not pets.
  • When we drill down to see if the same strains of a bacterial species are present in both people and pets in a household, we often don’t find too much overlap.
  • Transmission of many concerning bacteria, particularly multidrug-resistant (MDR) bacteria that are most commonly associated with people, is probably mainly human-to-pet.

None of that is meant to dismiss the issue of transmission of MDR bacteria between people and pets. These strains that emerge and spread in people can spread to pets, and it’s possible for pets to spread them back under the right conditions. However, just finding the same bacterium in people and pets doesn’t mean the pet is an imminent risk. Much of the time, pets are innocent bystanders or victims, with humans being the source and posing a greater risk to other people that the pets. .

The ECCMID study, coordinated by Dr. Carolin Hackmann from Charite University Hospital Berlin, is a nice study of human-animal sharing of MDR bacteria. Here’s a summary of what we know so far about their work:

  • Researchers collected swabs from 2891 human hospital patients, 30% of whom harboured a resistant bacterium of some sort (no surprise).
  • Patients were then asked to submit fecal samples from their pets. Over 300 samples were collected from dogs and cats, of which 15% of dogs and 5% of cats harboured at least one resistant bacterium (no surprise there either, especially if they used a broad definition of a resistant bacterium).
  • In only four instances was the same bacterial species found in the human patient and their pet(s).
  • In only ONE instance (out of 2891 humans and over 300 pets) did a pet and person actually harbour the same bacterial strain, based on genomic testing. This was a MDR E. coli in a person and a dog.
  • In this one instance, nothing is reported in the press reports about directionality of spread. Presumably, there was no way to figure out who infected whom (or whether both person and dog were infected by someone/something else).

Was that dog a risk to other dogs or people? Potentially, just like many other dogs. The owner was probably a greater risk to others, but we can’t dismiss the potential for the dog to be a source of re-infection of the owner either (i.e. were the owner to get rid of the bacterium but then get re-exposed from contact with the dog).

It’s an interesting study and we need more work like this.

But, is this concerning?

Not really. It’s another piece of evidence that we live in a polymicrobial world as part of a vast ecosystem. Sometimes we share bacteria (and other microbes) with other members of that shared ecosystem. Usually, it’s benign; sometimes, it’s disastrous (i.e. SARS-CoV-2); and there are lots of gradations in between.

This doesn’t change my thinking or my behaviours, but adds one more piece of evidence to the notion that we have to look beyond people to control certain human diseases – something that is overlooked surprisingly often. At the same time, we need to maintain perspective and not over-react when we find certain microbes in pets that have close contact with people. There’s always going to be some infectious disease risk with any animal contact, but we can mitigate that risk significantly. When it comes to sharing MDR bacteria in households, the best things we can do are optimize health (both human and animal), improve antibiotic stewardship (to reduce the likelihood one of these bacteria will be present in any individual, human or animal), and use common sense basic infection control practices, like hand hygiene.

So, pay attention but don’t panic (and take headlines with a grain of salt).

Being asked to coordinate a paper for the Journal of Law, Medicine & Ethics wasn’t on my academic bingo card for the year, but like a lot of things in recent years, I’ve come to expect the unexpected. The paper in question is part of a special edition entitled Addressing Antimicrobial Resistance through the Proposed Pandemic Instrument. It has a great series of papers addressing a range of pandemic issues, the threat of antimicrobial resistance (AMR), how a pandemic instrument (meaning an international instrument like a treaty) can address pandemic prevention, and why AMR needs to be part of that.

Why are we talking about AMR and pandemics?

Antimicrobial resistance has been called the “silent pandemic,” since it currently causes substantial but often largely unrecognized harm around the world, and the scope of the problem is only getting worse. While peoples’ first thoughts about pandemic risks almost invariably jump to new viruses (which are absolutely important too), the ongoing pandemic of AMR is killing huge numbers of people, affecting countless lives and livelihoods, compromising advances in healthcare, impacting animal health and welfare, and having a tremendous economic impact. It doesn’t get much press, but it’s one of the biggest global health threats we are currently facing. If we don’t act aggressively, it’s going to get much worse.

The special edition of the Journal of Law, Medicine & Ethics has lots of great information about why AMR needs to be in the pandemic instrument, governance issues related to that, what a pandemic instrument might achieve, and what barriers might be present. If you’re interested in this area, I’d recommend checking out those articles.

Here I’ll comment on the article that we wrote for the series, entitled “Governance Processes and Challenges for Reservation of Antimicrobials Exclusively for Human Use and Restriction of Antimicrobial Use in Animals“. We were asked to write about how restriction of antimicrobial use (AMU) in animals and designation of certain antimicrobials exclusively for human use could be built into an instrument. However, we took a bit of a different tack, partly because governance isn’t my area, but more because there are a lot of nuances to the issue that need to be considered by people with expertise in governance.

Ultimately it sounds simple: just use fewer antimicrobials.

But that’s just part of the story, and there are many related issues. If we don’t dive into those, we can end up with ineffective or impractical efforts that may not be optimal, or worse may not help at all, or even worse yet may cause unintended negative consequences. We’ve seen all those outcomes in the past.

Why? The short answer is, it’s complicated.


Too often, the approach to AMU in animals is focused more on sound bites than science and practical measures that will actually achieve anything positive. That’s partly because of the complexity of the larger problem, partly because of big data gaps, and partly because of a lack of understanding of key issues within the problem.

I don’t want to take the lazy way out and just say “read the paper if you want more details” but I recommend you do if you want to understand the area more. It’s far too complex to summarize neatly in a blog post, but here are a few key points to consider as a start:

  • “Antimicrobial use” encompasses a wide range of uses, including treatment, metaphylaxis, prevention and, in some countries still, growth promotion. The issues differ between those, but even within each category, there can be lots of variation leading to different issues. For example, it’s common to hear “Antimicrobials shouldn’t be used for prophylaxis in animals.” However, prophylaxis is very broad, including things such as medication of thousands of healthy animals at once based on historical use more than data, or a single dose of antibiotics in an animal undergoing a high-risk surgical procedure (where infection is a reasonable concern and would result in a longer therapeutic course). We want to reduce prophylaxis as much as possible but there are some situations where it’s needed for animal health and welfare reasons. There are also situations where use is unnecessary, done based on fear or habit versus evidence, or used in lieu of good management practices. We definitely want to curtail use in those cases.
  • Ideally, we’d have all antimicrobial use in animals driven directly by a veterinarian. That works in areas where there is good access to veterinary care, but it doesn’t work everywhere, at least not at this point. We need to improve veterinary access and animal health systems in parallel with efforts to reduce AMU and bring all AMU under the auspices of a trained veterinarian (or allied health professional). “Stop over the counter access to antimicrobials” makes sense in many places, but in some, it would significantly compromise animal health and welfare. It’s even a challenge in some parts of Canada that are under serviced in terms of access to veterinarians.

There are many similar components that sound simple at first, but get much more complex when you delve into them. Failure to understand that complexity dooms approaches to address the issue of AMR.

At the same time, there are lots of potential interventions, some of which are amenable to an international instrument like a treaty. We can’t solve this problem with a treaty, since there’s no single thing that can eliminate the scourge of AMR. However, the way we will solve the problem is through myriad interventions by myriad groups, each providing a small piece of the greater prevention puzzle. An international instrument can be a key part of that by driving some changes and by emphasizing that this is an issue that needs to be addressed across the globe.

If you’ve managed to make it this far through this post, maybe that’s a good indicator you’d be interested in reading the article or the full special edition. Enjoy.

I doubt you’ll be shocked to hear that the normal host of Staphylococcus felis is cats. It’s a bacterium that can often be found in healthy cats and periodically causes disease in cats (e.g. urinary tract infections). Overall, though, it’s a pretty innocuous bug. Human health risks related to S. felis haven’t been well investigated, but it’s been reasonable to assume there’s a potential (but low) risk of infection in people.

Some supportive evidence of this is provided in a recent case report about suspected transmission of S. felis from a cat to its owner in the form of a wound infection (Sips et al, J Med Microbiol 2023).

The case involves a 58-year-old woman who had back surgery. Two weeks after surgery, she delevloped a mild surgical site infection, with some drainage from the incision and mild redness. She was otherwise fine. They did a routine culture of the wound that identified Staphylococcus felis that was susceptible to all of the antibiotics tested. She received some wound care and was sent home with antibiotics.

While Staphylococcus spp. are leading causes of surgical site and wound infections, S. felis isn’t one we’d expect to find here. It’s a coagulase-negative species, part of a large group of staph that typically only cause infections in higher-risk situations. Coagulase-negative staph can also be found as contaminants in cultures when samples pick up bacteria that just happen to be present on the skin, but aren’t actually part of the problem. So, it’s not definitive that S. felis was the cause of the wound infection in this case, but it’s pretty likely.

When they got the culture results, medical staff queried animal exposure and it turned out that the patient had three cats.

Too often, case reports in medical journals stop here. They say “the infection happened in a person… this bacterium is usually found in animals… the person lives with an animal… they must have gotten it from the animal. Eureka, my job is done, let’s write a paper.” In many cases the link is probably real, but no always, so we need to investigate such things properly.

Fortunately, this case report highlights a great investigation of the situation.

After the owner was diagnosed, they tested her three (healthy) cats. Not surprisingly, all three cats had positive oral swabs for S. felis, and there were mixed results from axillary (armpit) and perineum (bum region) samples as well.

But they didn’t stop there. They looked at the genetic sequences of the bacteria and found that the three cats harboured different strains of S. felis. One cat had the same strain as the owner, so that animal was presumably the source of the wound infection. The others were innocent bystanders, even though they harboured the same bacterial species.

What does this change?

Not much, but it’s a good reminder that infections from various bacteria that are present on/in healthy animals can happen, and that proper investigation is required to truly understand any transmission links.

Staphylococcus felis infections haven’t been reported before in people, but I’d guess they have occurred. Until recently, most identification of bacteria in diagnostic labs was done with biochemical tests. Often, testing would stop at “coagulase-negative Staphylococcus spp” on the assumption that the bug was probably not overly relevant and/or it didn’t really matter which one of those different coag-negative staph it was for treatment purposes. In recent years, more advanced testing has become standard, so we now get more specific identification of bacteria that otherwise might have been missed or generically reported. As a result, my guess is that previous incidents of such infections were just not identified. Regardless, it’s still fair to assume this is a rare event.

There’s no mention of what happened to the cats. Hopefully, nothing. I wouldn’t treat the cats in a case like this because we have no evidence it helps. It’s hard to eliminate a bacterium from its normal ecological niche, and treatment might cause more problems, such as selecting for resistant bacteria that could pose a bigger risk to the owner. Staphylococcus felis is just one of many bacteria present in these cats that can potentially cause disease, and even if was eliminated, the risk posed by the cats wouldn’t drop by much. In these types of situations, I focus on basic hygiene and infection control, and owner awareness about risks.

The antibiotic development pipeline is drying up. More companies are abandoning the area because antibiotics are expensive to develop and license, but they are low profit drugs that we try to use as little as possible.


This is a big issue for human medicine, and was the focus of a lot of discussion at this week’s Global Leaders Group on Antimicrobial Resistance meeting.

It’s a complex issue. I won’t get into the details here, but there are initiatives to support getting X number of new human antimicrobial products to market by year Y. That leads to the question of how many antimicrobial products we should be aiming to develop for animals, and whether we should aim for completely new “animal only” drugs.

At first glance, having antimicrobials that are only used in animals would be ideal, so that they are distinct from those used in humans, and therefore any resistance that developed in bacteria from use in animals would not impact resistance to the unrelated antimicrobials used in people.

Unfortunately, as with most things about antimicrobial resistance (AMR), it’s not that straightforward. It’s not “drugs” we need to think about, it’s “drug classes.” A new drug that’s just a slightly different version of (and works the same way as) drugs we currently use in people isn’t what we want. We need new drug classes that are different in how they work and how bacteria become resistant to them. That’s a much tougher order.

If I had to characterize a “great animal-only antimicrobial class,” I’d say it should be:

  • Effective against a range of bacteria that cause disease (in animals)
  • Able to be given orally (vs injectable)
  • Low cost
  • Safe (in the target species, or ideally in multiple species)
  • Not persist in the body for long after treatment (to shorten withdrawal times for meat and milk)
  • Not excreted into the environment in urine or feces

And the big one: if resistance develops to our animal-only drug, it doesn’t also confer resistance to human drugs.

That would be great.

However, what else did I just describe? A perfect ‘human-only’ drug.

I’d rather use that drug in people than launch it for use in animals.

So, unless we have a new drug class that fulfills those properties and is toxic in people but not animals OR only works on a pathogen that’s of relevance in animals, we’re not likely to get a completely new useful animal-only drug class that would not impact resistance in people.

That doesn’t mean we shouldn’t keep it in mind and explore new drug classes that have been rejected for use in humans (which is already a prime source of many animal drugs). However, it means our main focus should be saving the drugs we have so we don’t need to worry about finding new drug classes. That’s why we need to focus on better animal health systems to reduce the need for antimicrobials, better education and support systems to optimize antimicrobial use, and more study about what situations contribute to more or less resistance risk. That’s antibiotic stewardship.


As the current (and unprecedented) H5N1 avian flu outbreak continues, there’s the ongoing threat of transmission to other species. The extent of spread to mammals is hard to say since it’s hard to know how many wild mammals have been infected. However, we know that an impressive range of species has been infected. Spread to mammals is a concern because the more widely this virus spreads, the greater the chance for recombination with other flu viruses to create a “new” strain that could cause serious problems in humans or other species.

A recent news report and the corresponding WOAH report are light on details but describe H5N1 infection in a domestic cat in France from late 2022.

The cat lived on a duck farm and was euthanized after developing severe neurological disease. That’s a clinical presentation that’s not been uncommon in mammals that have been infected with H5N1 influenza during this outbreak. That doesn’t mean this virus usually causes neurological disease. It might be a matter of animals with neurological disease simply being more likely to be noticed and/or tested. H5N1 avian influenza infection was confirmed, and the virus recovered from the cat had “genetic characteristics of adaptation to mammals.”

The good news is that cats (as far as we know) don’t have their own flu virus in circulation (unlike dogs, horses, and pigs). That makes it unlikely that a cat would be infected with avian flu and another flu strain simultaneously, which would increase the potential for viral recombination. However, it’s still a concern since cats can (rarely) be infected with flu strains from other species, including human flu viruses.

Overall, the relevance here is mainly to the cat. The odds of this signalling a new problem are low, but it highlights the concerns we have about how far this virus continues to spread. It’s playing with fire.

The other consideration is the potential for cats to act as a bridge from wildlife to humans. Cats that get infected through exposure to wild birds can bring the virus into closer contact with people. It’s another good reason to keep cats indoors whenever possible, particularly if avian flu is circulating.

As a journal associate editor and reviewer, I’ve seen lots of papers about SARS-CoV-2 in animals. Some have been great, ground breaking papers. Many have been small, weak studies rushed out to be “first,” with inadequate depth and lacking critical assessment. Some have been a complete disaster. The latter two groups are a concern beyond just being bad science. Shortcuts can lead to bad conclusions when things aren’t studied properly.

A recent study (Hoppe et al, Infection 2023) highlights the need for proper study to avoid bad conclusions. It describes SARS-CoV-2 infection in a household where both people and a dog were identified as infected.

A human (the father) was the first identified case, and the rest of the family subsequently got COVID-19 as well. The source of the father’s infection was unclear, but he noted that his dog had been sick (i.e. coughing, lethargic) starting 11 days before the onset of his own illness. The dog had been taken to a veterinarian and tested negative for some routine respiratory pathogens, but false negatives are very common with these tests based on timing of sampling and our inability to test for all causes of respiratory disease.

After the owner was diagnosed with COVID-19, the dog was tested for SARS-CoV-2. Samples were taken 18 days after the onset of the dog’s respiratory disease, and all 3 samples from the dog were positive for SARS-CoV-2 at that time.

Timeframe figure of SARS-CoV-2 infection in a dog and 4 family members, with the dog’s infection preceding human infections.

It was suggested that the time frame was consistent with dog-to-human SARS-CoV-2 transmission. I’d say that’s a bit of a stretch based on what we know about infection of dogs, but it’s not impossible. Dogs seem to be commonly infected from their owners, but seem to be pretty resistant to clinical infection and don’t seem to be great hosts for the virus. Virus shedding in dogs tends to be low and of short duration. With a fairly short incubation period in people, the dog would have had to have been infectious for well over a week to be responsible for the father’s infection, and that’s unlikely. The dog also would have had to have been PCR-positive for 18 days after the onset of disease, which I’d consider really unlikely.

However, “unlikely” doesn’t mean “impossible,” so more study was needed. Presumably, at this point, they thought the dog was the source of the family’s infection and were waiting for sequencing results to confirm that.

(Un)surprisingly, the sequencing results didn’t implicate the dog.

There were enough differences in the viral sequences between the dog and owner that the viruses were classified as two different lineages (B.1.1.29 and B.1.1.163, respectively). They concluded the infections were independent, since the series of mutations that would have to happen between the dog and person were “so unlikely, that secondary zoonotic transmission can virtually be excluded.”

It’s pretty clear that the dog was infected. The dog was PCR-positive and antibodies to SARS-CoV-2 were later detected in the dog’s blood.

It’s also pretty clear that the dog wasn’t the source of human infection. The dog’s infection was actually unrelated.

Then how did the dog get infected?

That’s an interesting and unresolved question, since the pet had little outside contact given restrictions that were in place at the time.

  • Maybe he picked it up from unreported contact outside the household.
  • Maybe a person in the household had an undiagnosed (potentially asymptomatic infection) with the strain that infected the dog.
  • Maybe the dog picked up the virus while visiting the veterinary clinic.

It’s impossible to say. That highlights a challenge. Confirming interspecies transmission of a virus that’s widespread in humans is very difficult, and often a really unique set of circumstances need to be present (as was for the cat-human transmission of SARS-CoV-2 that occurred in a Thai veterinarian).

This is a good example of the need to investigate interspecies transmission but the need to do it right. 

Dog image from:

Figure from Hoppe et al, Infection 2023.

The ongoing H5N1 avian influenza outbreak is an unprecedented event in its size, scope and duration (but it’s not getting much press anymore these days). As infections continue to occur is birds in large numbers over a vast geographic range, we worry about spillover events into other species.

There have now been many reports of H5N1 influenza infection in a variety of wild mammals, including foxes, skunks, raccoons and most recently bears. Sporadic transmission into wild mammals that live fairly solitary lives and probably aren’t (currently) great hosts for the virus raises concern, but the broader risks are probably limited because of the low odds that rare infections would result in a relevant change in the virus or recombination with another flu virus. However, more infections create more risk, and infections of species with more animal-to-animal contact, and animal-to-human contact amplify that risk.

That’s why the recent report of H5N1 avian flu on a mink farm in Spain raises some alarm bells. Infection of a farm with tens of thousands of mink is a whole lot different than infection of the odd free-roaming fox or raccoon.

Mink in cages. Source:

The good news is that even though the report just came out, the outbreak occurred in October, and there’s no evidence I’ve seen that this resulted in a broader issue. The bad news is that it shows what can potentially happen (and surveillance is far from good enough to say that this hasn’t caused an issue).

The report is about an outbreak on a farm in Spain that housed over 50 000 mink. Concerns were raised when the mink mortality rate increased in October, suggesting something was going on. It seemed like a pretty classic infectious disease problem, as it started in one barn and was characterized by “multiple ‘hot  spots’ within the affected barns consisting of 2–4 pens where all the animals died within a period of 1–2 days.” Mortality rates then increased in neighbouring barns, then eventually across the whole farm. Infection with SARS-CoV-2 was initially the main concern, as has been seen on mink farms in many countries now, but tests for that were negative. Eventually they confirmed H5N1 flu as the cause of illness in the mink, and sequencing of the virus showed it was the same clade ( that’s been circulating in birds in Europe.

The decision was made to cull the mink, and over the course of about a month, all of the mink were euthanized and their remains disposed.

Farm workers were tested at one point, and all 11 were negative. That was good news, but single point-in-time testing of people exposed to an infected mink farm over the course of weeks doesn’t rule out transmission. (Though it’s worth noting employees were already required to use enhanced precautions, like wearing masks, because of the concerns over SARS-CoV-2 transmission to (and from) the mink, which may have also helped limit transmission of flu.) One of the workers later developed flu-like signs but tested negative for flu virus. Because of the disease transmission concerns, a “semi-quarantine” of the people was performed to limit contact with other people for 10 days after their last contact with the farm.

The source of the outbreak isn’t known. It’s possible that it was introduced by poultry products fed to the mink, but there’s no evidence that any supply farms were infected. It’s much more likely that wild birds were the source, and infected wild birds had already been found in the area at the time of the outbreak. This highlights concerns about mink farms as a wildlife/domestic animal/human interface. It’s hard to keep wildlife away from a mink farm, which creates risk for transmission both from and to wildlife. If wild birds can infect mink, it’s equally likely that wild birds (and other wildlife) could be infected from mink, through contact with the mink farm environment. A virus that spreads to the mink can then spread to human farm workers, domestic animals on the farm, or other “visiting” wildlife. That’s not a comforting scenario.

The report concludes by stating that the mink sector is still important economically and “it is necessary to strengthen the culture of biosafety and biosecurity in this farming system and promote the implementation of ad hoc surveillance programs for influenza A viruses and other zoonotic pathogens at a global level”. I agree with the second part of this statement, but as for the first part, we need to think and risk versus reward. Are the benefits of mink farming worth the risks that come with raising large numbers of animals that are susceptible to various human viruses, in close proximity and with ongoing contact with people and wildlife? The broader societal benefits of mink farming are (to me) negligible, and the risks may be low, but they are not zero and not adequately understood.

Image from