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It’s nice to finally be able to talk about H5N1 influenza without discussing ostriches, but unfortunately, we still have things to talk about.

1) Peacocks. Specifically peacocks that don’t have influenza and aren’t going to be culled.

Last month I posted about an animal sanctuary in BC (Critteraid) that was dealing with an outbreak of H5N1 influenza in poultry on the property. The outbreak lead to investigation of the risk to other animals on the same premises, including their peacocks. My understanding is the peacocks were physically well removed from the infected birds, so there was clear rationale to try to avoid culling them. It was a great example of how to respond to H5N1. Critteraid was transparent, worked with CFIA and followed their guidance, and took appropriate measures to contain the infection and decontaminate the affected areas. As a result, CFIA was able to assess the situation through their established processes, the peacocks were tested and were negative for the virus, and they were therefore able to avoid culling them.

2) More sick cats with the flu. This story doesn’t have such a good ending. We know cats are susceptible to H5N1 flu and often develop fatal neurological disease. Recently a lot of attention has been paid to exposure of pet cats to the virus via raw diets and raw milk, but cats can also get infected through contact with infected birds. A recent cluster of fatal H5N1 infections in a group of kittens in the Netherlands reminds us of that risk:

  • On November 19, a kitten from a dairy goat farm was found dead and tested positive for H5N1 flu. Samples were collected from other animals on the property the next day as part of the investigation. Three adult cats (including the kitten’s mother) were tested, as were the goats (the goats probably being the main concern, given the issue with H5N1 in dairy cattle in the US). All of the animals were healthy at the time, and all tested negative.
  • However, the other 7 kittens from the litter weren’t on the farm anymore as they had all been adopted. Further investigation revealed that they had all died; none of them were tested, but it’s quite likely they all could have had H5N1 flu as well.
  • The source of the virus in this case couldn’t be identified, but the mother cat was seen with a dead bird October 27, and it would be logical to assume that the kittens may have eaten part of the bird or been exposed to it it some way. The timing doesn’t quite fit though, as that was 23 days prior to the one kitten being found dead on the farm, which is a very long timeline for flu. I’d have to wonder if there might have been exposure from another dead bird, since the mother cat likely caught birds on more than one day, or she may have been infected by the bird caught on October 27, and then the kittens were subsequently infected by her.

This case is a good reminder of the potentially devastating impact of this virus on cats and one of the reasons why we’d prefer to keep cats away from wild birds. If cats have outdoor access, flu must be a consideration in any cat that develops acute neurological disease or dies suddenly.

We don’t know what’s the risk is to people from infected cats. I think we have to assume there’s some degree of risk since there is evidence to support that cat-to-cat transmission can happen. Fortunately the risk to people seems to be low, given the lack of identified cat-to-human transmission. However, we can’t rely on that to say there’s no risk, both in the present and in particular in the future, as this virus continues to evolve.

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Hot on the heels of the H5N1 influenza in ostriches debacle in BC comes a reported outbreak of H5N1 influenza at an animal sanctuary in the same province. The sanctuary (Critteraid Animal Sanctuary) is not one I’ve ever heard of before, but based on their website, it looks like a good operation, and their response to this situation elevates them further in my mind. Unfortunately, even the best run facilities can be at risk from H5N1 influenza.

The sanctuary said on Facebook that one of their roosters got sick on Saturday October 25 and died; they sent the body for diagnostic testing (which is great). By Monday October 27, three more chickens had died; they were sent for testing too.  Ultimately testing identified H5N1 influenza as the cause. Overall, 8 of their 10 chickens died, and the other 2 were sick and had to be euthanized (probably both because of the severity of disease and because of the H5N1 flu test result on the other chickens, which would trigger a cull order).

The CFIA confirmed the test result and the farm closed. Five ducks had to be euthanized too. It’s not clear if they were sick (it doesn’t sound like they were sick, at least not yet) but because they’re poultry and there was potential exposure to the virus, they would be included in the cull order. It’s heartbreaking, since these were truly pet ducks that had likely been at the sanctuary for a long time. However, the sanctuary didn’t try to hide the birds or argue with the authorities – they accepted the unfortunate but necessary response to control this high consequence disease. They worked with CFIA and euthanized the birds themselves (versus having CFIA do it), saying “we made the choice that to ensure our policies of compassion were upheld and they deserve that dignity, that we did it ourselves.”

The farm has been working with CFIA and BC Interior Health, and they provided information about human contacts for contact tracing. Twelve people were potentially exposed to the infected birds.  They are all healthy and presumably are being monitored by public health for a couple of weeks to make they don’t develop any signs of influenza.

As per standard policy, the facility is quarantined and is undergoing decontamination to further contain the virus. The other animals on the farm consist of pigs, goats, cows, dogs and cats, which are not subject to culling, as they are not poultry. They will presumably be closely monitored. Ideally we’d test any that had very close contact with the infected birds or their environment, but that would be primarily to gain more knowledge about the inter-species transmission risks, rather than as a control measure on the farm. Testing the other animals is not required, but it would be an opportunity to learn more.

Even though the sanctuary is in the middle of this highly stressful and draining situation, they’re thinking ahead, as they indicate “now more than ever we feel the need to educate ourselves further to ensure we have a safe environment for birds in the future.”

This is the antithesis of the Universal Ostrich Farm.

Sadly, but not surprisingly, what should have been a relatively small local story has blown up, with anti-vax / anti-CFIA / anti-public health activists and influencers making up stories about the CFIA “coming for all the animals” (and more broadly, anyone’s pets). I assume they were hoping to move the convoy to another location to continue to party, protest and ask for money.

The situation also seems to have (again, not surprisingly) led to threats against the facility. On Facebook, they outlined the time frame and their response (see below). As part of that, they address a lot of things that they shouldn’t have to, including that they have nothing to do with a couple of petitions that are just spreading fear, misinformation and mistrust. That’s the last thing with which they should have to deal right now. Clearly the conspiracy fringe is in full motion:

I don’t know why the sanctuary is being threatened. It could be because they’re not being vociferous against CFIA, and thereby making it hard for that crowd to leverage the incident for attention and fundraising.

Regardless, the situation and the facility’s response are worth noting. From all I’ve seen, this is a great example of what should be done. The farm has been transparent, fair and taken responsibility for what needs to be done. It’s not their fault they got H5N1 flu in their birds. Particularly with birds that live outside, a big component of it is just bad luck. How they responded is key, and that seems to be going well.

Rather than sending more donations to grifters, dodgy “reporters” and everything else we saw with the ostrich debacle, donating to this sanctuary that is navigating a very tough situation would be great. I just did.

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As the H5N1 influenza debacle at the Universal Ostrich Farm in British Columbia finally comes to a close, part of me wants to ignore all the crap that’s coming in, but there’s value in recapping this incident in an effort to help move us forward.

Recap: A large flock of captive ostriches was found to be infected with H5N1 influenza in December 2024. Contrary to federal legal requirements to report this disease immediately, the farm hid the issue and didn’t do any of the things typically required to respond to a detection of this high-consequence virus. It’s been reported that 69 birds died and 300-400 birds survived (they never could come up with an exact number). When the outbreak was discovered, the Canadian Food Inspection Agency (CFIA) got involved, as is their mandate. As per standard protocols for detection of H5N1 flu in poultry (captive ostriches are considered poultry by international designation), a cull order was issued immediately. However, the farm fought the order by trying to appeal and sue the CFIA, while taking no measures to try to control the spread of the virus – if fact they did the opposite, showing pictures of people posing with the birds without personal protective equipment (PPE), and inviting a convoy of farm supporters onto the grounds. Ultimately, the case made it to the Supreme Court of Canada, which issued one last temporary stay order on the cull in late October, but ultimately declined to hear the last appeal. As a result, CFIA completed the cull of the remaining birds on November 7.

Make no mistake, the ostriches are the victims here. They had bad luck in being exposed to H5N1 flu in the first place, and they had little chance of avoiding a cull because of the mess of a farm on which they lived. But this case moved quickly from being about disease control to being about politics and fundraising, and the birds got caught in the middle.

Why was a cull done?

A cull is a standard approach to controlling H5N1 influenza in poultry because of the risk to animals and people. It’s done to contain the virus and reduce the risk of further transmission to other birds, spillover into mammals (which increases the risk of the virus adapting further to transmission between mammals), infection of people (given this virus has a historically high mortality rate) and, of most concern, recombination with other animal and human flu viruses (which is how all pandemic flu viruses originate).

Were the ostriches infectious in December 2024?

Yes they certainly were, and likely at great risk of infecting wild birds, wild mammals and any people or other domestic animals on the farm. It was subsequently determined that the strain found in the ostriches was a variant that has caused human infection and had some potentially concerning mutations.

Were the ostriches infectious when they were culled in Novemeber 2025?

Probably not. In a group like this, the virus probably burned through the entire population by January or February. Long term shedding of flu virus in infected birds is unlikely, so with a group this size over 10 months, realistically the flock would have had to be reinfected to sustain transmission. Could that have happened? Maybe, but probably not. Unfortunately ” but “probably” isn’t an acceptable threshold for control of this disease. If we don’t know the virus is gone, prudence dictates that we assume it’s still there. That’s the approach that CFIA (understandably) takes with highly pathogenic avian influenza like this.

Was there an alternative to culling the birds?

That’s a big “maybe” with lots of caveats. While culling is the international expectation, countries can take alternative approaches if they can show they’re taking other measures to adequately contain spread of the virus. That’s not expressly defined though, so it’s a grey area for a regulatory agency that doesn’t like to work in the grey zone for a lot of good reasons. It would require reliable isolation of the birds, strict infection control practices, testing, monitoring and documentation. That’s tough to do in general, but t’s really tough with outdoor birds and uncooperative owners.

At the time the outbreak was happening, it was fair to say trying to contain the virus by any other means was not reasonable. The risks were too high with the setup they had. But the ball got dropped, and so there we were ten months after the fact… at that point, testing could have potentially made more sense. That’s no easy feat either, though. Ostriches are big birds that can cause significant injury to people handling them, especially where there are poor (to non-existent) handling facilities. Trying to stick a swab down the throat of a large bird that can split you open with a kick is hard to sell.

In order to make testing work, each bird would have needed to be caught, identified, and tested. If there were zero positives in the group, I’d be pretty confident the group was negative. That’s theoretically possible. However, there would likely have been a need for ongoing monitoring to have confidence in the negative group status. Since the farm hid the disease initially and showed no interest in implementing disease control measures, it would be hard to convince anyone such an approach could have been effective.

Furthermore, even if the World Organization for Animal Health (WOAH) thought such an approach for controlling the virus was adequate in this flock, that doesn’t mean all countries would agree. Any country could then restrict importation of poultry (or other food products) from BC or even all of Canada on the premise that Canada was not protecting its food supply. Even if it’s a dodgy claim, a country with an agenda or that wants leverage for negotiations could play that card, and be successful.

When there’s potential risk to animals, humans and multi-billion dollar industries, the bar for disease control is obviously going to be very high. So was an alternative to culling the birds possible? Yes. Would it have been easy? Certainly not. While easy isn’t a requirement, in this case it wasn’t that it was just a hassle, it was that there was a good chance it might not ultimately be possible.

Did the ostriches have herd immunity to H5N1 flu?

For a group of supporters who were comprised of a large proportion of individuals who were anti-vax (and anti-PCR, and anti-public health and some people who don’t believe viruses exist at all), they put a lot of attention on herd immunity (which we typically try to achieve in a given population of people or animals through vaccination). It’s very unlikely this group was immune to H5N1 flu. the birds that were exposed and fought off the infection would have some immunity, but we know that immunity to influenza after natural infection isn’t great or long-lasting – that’s why people and animals can be repeatedly infected with flu. Some of the birds might have been immune, or at reduced risk of severe disease. Some were probably previously infected but are not immune. Some might not have been exposed and were not immune. As a group, it’s very likely that there would have still been susceptibility to re-infection if there’d been another exposure to the virus, so there was still risk.

What about the “research value” of the ostriches?

The farm talked a lot about how these are no longer meat birds, but rather a “research flock.” However, they were never able to present any information about valid research for which the birds were being used. They didn’t indicate that there was ethics approval for research on the farm (a basic requirement for research involving animals). They didn’t even know exactly how many birds they had, and the birds had no form of identification (tags, microchips); without basic information like that, the farm can’t be a legitimate research operation.

Even though the remaining birds survived H5N1 influenza, that still doesn’t make them inherently useful research animals. We don’t know how many fought off the infection (something we know many birds of some species can do) or weren’t actually exposed. The birds that survived an infection may have antibodies against H5N1, but so would any other animal that survived natural or experimental infection, so there’s no unique value there.

Where do we go from here?

Hopefully we can look back on this case and salvage something positive from this mess and the unfortunate ending for the birds. There are a few key components:

The Supreme Court has re-inforced the appropriateness of CFIA’s disease response. This is important since it might reduce the chance of future lower court successes with appeals and stays. It’s fair to criticize CFIA and demand that they have oversight. It’s also fair for CFIA to say “we have to contain this disease for the protection of animals and people, and we need to do it quickly.” No one at CFIA celebrates when they have to cull animals. They do it as an unfortunate but necessary part of their mandate to protect animal health, human health, agriculture and the food supply.  Allowing people with agendas to weaponize the court system creates risk.

There may be valid arguments for approaches other than a cull, in certain situations, as H5N1 flu is now an endemic disease in wild birds. However, this needs to be done carefully, with a lot of transparency, effort, structure and excellent care. In other words, exactly the opposite of what happened here. A mechanism to promptly evaluate a potential alternative, with a clear approach, set criteria to review (what, who, etc.), a very rapid response and no appeal process would be interesting to investigate.

As part of any alternative approach, there would need to be excellent interim containment. If a farm wants to convince CFIA to take a different approach, they need to do absolutely everything possible to contain the disease when an outbreak occurs. If it can be done with minimal risk, it’s potentially viable. If it creates risk in the interim, it’s not. Even if this farm had done the exact opposite of what they did, and diagnosed the issue quickly, reported it, properly disposed of dead birds and enacted strict infection control practices, it’s debatable whether an exemption during the acute disease period would have been viable because of their set up. You can’t contain a few hundred large birds (producing lots of virus-laden snot and feces) outside in an area with abundant wild birds, with no indoor isolation options or handling facilities. Without appropriate facilities and management, it’s not an option. That’s not CFIA’s fault. That’s a choice by the farm about how they operate. Raising birds outdoors isn’t bad, it just create risk that any farm needs to accept. And when it results in disease, there are consequences.

We had a high consequence virus on this farm, they handled it horribly, the virus in question had some mutations that created even more risk, and nothing bad seems to have come of it (that we know of, at least) in terms of disease spread. Does that mean none of this made sense?

  • No – we just got lucky, and you only get lucky so many times with a high consequence infectious disease that has pandemic potential.

It’s really sad how infectious disease control (and beyond) has gotten so politicized. This went from an issue with one group of infected birds to having a convoy on the (quarantined) farm, massive misinformation, dodgy “media” stirring up hate, threats to experts and people doing their jobs, massive fundraising and grift, largely by people who likely don’t care whatsoever about those ostriches. It also cost taxpayers a huge amount of money, and took up time a resources that could have been much more effectively spent elsewhere. The ostriches were political pawns and an excuse to complain, raise money and have a party at the farm. We need to move beyond that component of this issue and come back to the valid disease control questions that can and should be discussed going forward.

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When it comes to food safety risks with H5N1 influenza, we know it’s a problem with raw (unpasteurized) milk from cattle, so I often get asked if it’s a concern with milk from other animals, such as goat’s milk. It’s a good question. My typical response has been that we really don’t know. There haven’t been any reports of H5N1 flu in other dairy species (except for one report at this time last year of infection in neonatal goats on a farm in Minnesota that had infected poultry), but we have a lot more dairy cattle living on much bigger farms in North America. So it’s hard to say if cattle are actually at greater risk, or if we’re just more likely to recognize problems on a big dairy where the animals are monitored closely.

There is now a report from the UK about H5N1 infection in a single ewe (a female sheep) in Yorkshire, which was on a farm that also had infected captive birds. While this doesn’t tell us about the risk in goats or the overall difference in risk between dairy cattle and other dairy animals, it shows that cattle are not the only dairy species of concern. 

The ewe tested positive on a milk sample, which was collected as part of the response to the infected birds on the farm (a smart surveillance approach that helps with early detection of spillovers and helps us understand transmission risks and patterns). Only one sheep from the flock was positive, but we have to assume the risk is likely broader – if one sheep can be infected, then other sheep could be too, if exposed to enough of the right virus.

The infected sheep was culled, which is understandable, as it removes the risk from that sheep. Unfortunately but it also removes any ability for us to learn more about what H5N1 flu does in a naturally infected sheep. It would have been ideal to isolate the sheep and monitor it for signs of illness and test it for virus shedding, but that’s not always possible. 

There are a few good take-aways from this report:

  • It shows the value of surveillance. If they hadn’t tested the other animals on the farm, we’d have no idea the sheep was infected, as the sheep did not appear sick.
  • Presumably, this was a direct spillover from the infected birds on the property. Hopefully that means the virus in not established in sheep anywhere else (like it is in dairy cattle in the US), and that this was an interesting one-off infection but not of broader concern.
  • If there really was only one infected sheep, removal of that ewe may have prevented it from infecting other sheep, and the other sheep all tested negative. I hope there will be a bit more testing on this farm to make sure there are no secondary cases.

I guess we can’t rule out the possibility that the sheep infected the birds. However, since they tested all the other livestock and only this one sheep was infected, it’s pretty safe to assume this was a direct bird-to-sheep spillover. Sequencing of the virus will help confirm this. Serological testing of other sheep on the farm would be interesting to see if there’s evidence of earlier infections.

Does finding H5N1 flu in a single sheep change anything?

No, not really. We know that H5N1 has spilled into a wide range of different mammals, and this just expands that list. We’ve assumed there’s some risk from other dairy livestock species, and this shows that’s a reasonable concern. We’ve also talked about risks from raw milk even before H5N1 was concern, there are lots of infectious disease risks with drinking any type of raw milk.

However, this report does raise the stakes a bit. We don’t want continued spillovers into mammals, because that increases the risk of this virus adapting to become better able to infect more mammals (including people). We don’t want endemic transmission in domestic mammals, as that increases human exposure risk. We also don’t want H5N1 flu in the food supply (but remember that pasteurization will kill the virus). 

UK Chief Veterinary Officer Christine Middlemiss’ statement sums things up well:

“We have confirmed the detection of influenza of avian origin (H5N1) in a single sheep on a farm in Yorkshire. Strict biosecurity measures have been implemented to prevent the further spread of disease.  

“While the risk to livestock remains low, I urge all animal owners to ensure scrupulous cleanliness is in place and to report any signs of infection to the Animal Plant Health Agency immediately.” 

The UK Health Security Agency (UKHSA) has said that avian influenza is primarily a disease of birds and the risk to the general public’s health is very low, but people should not touch any dead or sick wild birds they find. 

The Food Standards Agency advises that properly cooked poultry and poultry products, including eggs, remain safe to eat and avian influenza poses a very low food safety risk to UK consumers since the H5N1 virus is not normally transmitted through food.”

Awareness.

Diligence.

Good use of routine infection control and biosecurity practices.

Continued surveillance.

Those are the key factors for limiting the risk from this virus. 

The Washington State Department of Agriculture and the Oregon Department of Agriculture have issued a public health alert because of H5N1 influenza virus contamination of another brand of raw pet food, following the deaths of two more indoor cats from separate households linked to consumption of the pet food.

Details are sparse, but both cats were euthanized due to severe disease from H5N1 influenza. Authorities tested the cats and the food from open containers in the household as well as unopened food samples, and found H5N1 flu virus in all of them. This shows that the food was truly contaminated at the source, and removes the potential that the food got contaminated in the household by the cats or some other source. (In a previous raw food-associated cluster of H5N1 flu infections in cats, the manufacturer of the implicated food tried to suggest the diet was contaminated in the household and was not the source of the virus, which was a very weak argument, and definitely not the case here based on the additional testing).

The recall involves Wild Coast LLC Boneless Free Range Chicken Formula, lots 22660 and 22664, Best Buy day 12/25. However, given that we have multiple instances of fatal raw poultry associated H5N1 infections in cats from multiple companies, the risk probably extends beyond this product.

To avoid the risk of H5N1 influenza from raw pet foods:

  • If raw diets are to be fed, use a non-poultry based diet, and choose one that’s high pressure pasteurized to reduce (though it will not eliminate) the risk.

Image from https://agr.wa.gov/lookuptypes/recallfile/131

I think it’s fair to say H5N1 becoming seemingly endemic in US dairy cattle in the past year caught us off guard. The virus has spread widely within and between US dairy herds, has caused mild infections in a number of people in close contact with infected cows, has killed a lot of cats on farms (and a few from drinking raw milk from infected cows)… and it isn’t likely to go away any time soon. 

Dairy veterinarians are one of the higher risk groups for exposure from infected cattle because of their close and frequent contact with these animals, particularly when cattle are ill. Surveillance testing of people at high risk for exposure to H5N1 influenza can help us get a handle on how much (if any) under-the-radar cow-to-human transmission may be happening, so it was great to see the release of the results of just such a surveillance study in dairy veterinarians in the US (Leonard et al. MMWR 2025).

In this study, researchers tested blood samples from 150 veterinarians with cattle contact and tested them for antibodies against H5N1 influenza. The presence of antibodies would indicate previous infection, whether or not the person was ever sick from the virus.

Three of 150 (2%) dairy veterinarians were positive for H5N1 flu antibodies, but none of those reported having had signs of illness that could have been attributed to flu, and none reported working with dairy cattle that were known to have been infected with H5N1 flu. If that’s accurate, it could indicate a few things, including possibly:

  • working on farms where there was mild disease in cattle from H5N1 flu that was not recognized
  • working on farms where there was disease in cattle from H5N1 flu but the cattle were not tested for it
  • veterinarians were exposed to H5N1 flu in some other way from animals or the environment, such as through contact with other animals or raw milk. (One of the veterinarians who tested positive also had contact with infected poultry, so that’s another potential source of exposure)
  • veterinarians were exposed through unrecognized human-to-human spread of H5N1 (which would be the most concerning possibility)

One of the seropositive veterinarians worked with dairy cattle in Georgia and other cattle in South Carolina. Neither of those states is known to have H5N1 in dairy cattle (see map above from the report), but the degree of surveillance and (more importantly) reporting is variable across the US. This would suggest that testing of cattle in Georgia needs to be ramped up to see if they have unrecognized infected herds.

All three antibody-positive veterinarians “reported wearing gloves or a clothing cover when providing veterinary care to cattle (including a variety of clinical activities, such as pregnancy checking or surgery)”. That’s strange wording, since those are two distinctly different types of PPE. Virtually every dairy veterinarian is going to wear coveralls (a clothing cover) on farm, so that stat tells us nothing about how many of them wore gloves (nor whether glove use may have been suboptimal, as it often is on farm). There was no use of eye or respiratory protection, which is far from surprising and something the veterinary profession needs to improve, as we do a poor job of using respiratory protection and rarely use eye protection when we’re dealing with animals with respiratory infections, even when they could be zoonotic.

The fact that all three antibody-positive veterinarians reported no obvious consequences of H5N1 flu infection is good news on many levels. However, asymptomatic infections raise some concerns, since if people are asymptomatically infected but still infectious, it may allow the virus to spread silently through the population, at least for a while. We have no idea if infected people shed the virus at levels that can infect others, but it’s something for which we need to be on the look out.

Any H5N1 flu infections in a person is bad, because of the potential for severe disease in the person and, even more importantly, the potential for evolution of the virus to transmit more easily among people. The more H5N1 encounters humans, the more opportunity it has to become adapted to humans. Infection of people concurrently infected with human flu virus strains creates opportunities for recombination of both viruses, which can lead to rapid and significant undesirable changes and emergence of new strains.

This is far from a doomsday report, but it highlights some things that we need to keep watching. It also shows why we need more effort to contain the spread of H5N1 flu in domestic animals. The data here are a bit limited, but they’re an important step in our understanding of this virus. A parallel study of the general population would complement these data, as would more focused study of veterinarians and farmers from affected farms, and veterinarians working with other species.

The more H5N1 influenza continues to circulate in wild and domestic birds around the world, including here in North America, the more we have to be concerned about exposure of pets to H5N1 influenza through raw food diets. Recent documented infections in cats fed raw meat from infected birds have highlighted these concerns. For more information on the risks of H5N1 influenza from raw diets for pets and associated risk reduction measures, check out the latest quick podcast on Worms&GermsPod.

Find all our podcasts on most major podcast directories, or access them here directly through your web browser.

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Addendum: The Oregon Veterinary Medical Association has indicated in a release that the cat discussed below was euthanized because of the severity of disease. That’s more consistent with the severe disease that’s usually reported but I think the discussion below still applies since this seems to have been an initial primary respiratory disease presentation. It probably still shows that there can be a primary presentation that overlaps with more common presentations so we need to consider H5N1 beyond severe neuro or sudden death cases.

I’m not commenting on every new report of an H5N1 influenza spillover infection in a domestic animal because it’s not really news anymore, but that doesn’t mean they’re not concerning. Spillover infections definitely are a concern with this virus, and we expect these spillovers to continue as long as this virus is circulating in wild birds (or in large numbers of dairy cattle, as it is the the US).

Nonetheless, a recent case of H5N1 influenza in another cat in Oregon highlights something important, because it’s different from previous cases. Most reported cases of H5N1 flu in cats to date have been of severe disease, usually with neurological signs, but it’s been unclear whether this is because infected cats typically get severe neurological disease or whether we’ve only been testing the cats with severe disease. It remains unknown how often infected cats get milder disease, and that’s a really important testing consideration, for both clinical patients and surveillance testing.

Respiratory disease in very common in cats, especially outdoor cats. Knowing whether flu should be a consideration in your average cat with an upper respiratory tract infection is important for determining how they are managed in a clinic (to avoid transmission to staff and other patients) and how they should be managed at home (to avoid transmission to family members and other animals in ad around the home). 

In contrast to previously described severe cases in cats, the recent case of H5N1 in a cat from Oregon was described as having a much more typical respiratory tract infection. “A veterinarian examined the cat after it exhibited symptoms including a fever, runny nose and eyes, lethargy, difficulty breathing and loss of appetite.” Although difficulty breathing isn’t typical for a run-of-the-mill upper respiratory tract infection in a cat, it is consistent with pneumonia, which can occur secondary to any viral infection. The news report is light on clinical details, but if this case was actually was more akin to a typical pneumonia that we might see in cat secondary to other more common bugs, it (long with a few other milder cases where cats have recovered) suggests that we need to vastly expand the cats we should consider potential H5N1 flu suspects. It means we need to focus on more than just the severely ill cats with respiratory and neurological disease. At the same time, it’s tough to say how wide a net we should cast, given the commonness of mild upper respiratory tract disease in cats.

At this point, the key is flagging risk factors for exposure in these animals, including outdoor access, contact with farms and being fed raw poultry diets. In combination with respiratory tract or neurological disease, we should consider the cat an H5N1 flu suspect unless another cause is evident. 

A challenge with this is that cats with outdoor access are also the main risk group for any typical feline upper respiratory tract infection, so including them greatly expands the pool of suspects and can make practical management harder. Nonetheless, at least for now, we should probably still be flagging any outdoor or indoor-outdoor cat presenting with respiratory disease beyond the routine upper respiratory disease complex as a potential flu suspect, with corresponding considerations for testing and infection control.

Should we consider any outdoor/raw fed cat with any signs of respiratory disease a flu suspect? Maybe. It’s certainly possible that H5N1 can also cause typical flu-like disease/upper respiratory infection. I’d recommend not completely discounting it in any case, but paying particular attention the more severe the disease is, and the greater the cat’s risk of exposure.  

As with most emerging diseases, this is a fluid situation and it’s tough to figure out where to draw the line in order to balance protection and practicality. As we learn more, that line will likely move, so we must keep an eye on new developments and take reasonable measures in the interim. Personally, I always prefer to err on the side of testing more and being more aggressive at the start, and then de-escalating when we know more, but there’s also a practical limit to how far we can go with that. 

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As H5N1 avian influenza continues to spread in wild birds and spills over into domestic birds and mammals of many kinds, we’re becoming more aware of the risks to domestic mammals and there are more questions about test selection and interpretation. Fortunately, testing for H5N1 influenza is relatively straightforward.

  • For cats and dogs, the recommended samples for testing are typically oropharyngeal swabs, plus or minus nasal swabs (plus or minus other tissue samples of the animal is deceased. 

What tests are currently being run by veterinary diagnostic labs?

PCR is the most accessible and useful test; diagnostic labs basically offer two types of influenza PCR: matrix / pan-influenza A PCR, and strain-specific PCRs (see below). Different commercial labs offer different tests or combinations of these tests, and the tests offered may also vary by species (i.e. what’s routinely done for dogs can be different from cats). Test offerings may also change over time as labs adapt to the ever-changing situation with flu in different populations. If influenza is a consideration in your patient, check with your lab regarding which tests they will run (particularly if you are submitting samples for a respiratory PCR panel) and how to interpret the results, including whether or not the test will detect H5N1 flu if that’s a concern.

Influenza A matrix PCR (aka pan-influenza A PCR)

This test will detect RNA from any / all influenza A viruses. A positive test confirms that flu virus is present, but not the strain (and not that the virus present is necessarily viable). Knowing the strain is important to understand how the animal might have been exposed and transmission risk. It’s a good first screening step, but if it’s positive we need more testing. If a cat was positive, it could mean it has a human seasonal flu strain (people sometimes infect their cats, and ’tis the season), or it could have H5N1 influenza, or another flu strain (e.g. a low pathogenicity flu strain which can cats sometimes get from wild birds, or potentially a swine flu virus if they have contact with pigs). Really rarely, but importantly, an animal could be infected with a combination of different flu viruses. Our concerns about and responses to these different scenarios are really different, so it’s important information to get. 

Strain-specific influenza PCR

These tests target specific influenza strains in different species, like canine H3N2, canine H3N8, avian H7N2 (found sometimes in cats) and human H1N1 (which can spill over sometimes into dogs and cats). It’s important to be aware that these are strain-specific tests, so an animal that just has H5N1 influenza will test negative on the H3N2 test, for example.

H5N1-specific influenza PCR 

As the name suggests, this test is specifically for H5N1 influenza, and we can be more confident in the result if that’s the strain for which we’re looking. The downside is that it won’t detect co-infections with multiple flu strains. The odds of a dog having H3N2 canine flu or H1N1 human flu and H5N1 avian flu at the same time are REALLY low, but that would be a REALLY concerning situation, so it would be nice to know. It’s more relevant when there’s higher non-H5N1 flu activity in the area as well, since that would mean there’s a greater chance of a hidden co-infection.

In order to provide the best possible information without over-testing every sample, labs will sometimes perform different tests in sequence: 

Run matrix PCR; if positive, then test for H5N1

This adds a step, but it’s usually a quick one, and we find out whether the animal has a flu virus and if so, whether its H5N1. The same issue with not identifying co-infections applies here, because there’s no testing for other flu strains.

Run matrix PCR, then test for non-H5N1 strains; if negative, then test for H5N1 

This works too. It adds a bit of extra time/work, and the more steps that are required, the greater the chance of a test error, but it gets to the same result pretty quickly and gives us a specific H5N1 result. The main theoretical issue is that we could still miss a coinfection (in this case because the H5N1 test isn’t run if any of the other strains are detected). 

Run matrix PCR, then test for non-H5N1 strains; if negative, then refer to another lab for H5N1 testing

This slows things down more and adds in some uncertainty as it requires sending the sample out for follow-up testing, which is another step where human error could affect things. It also misses co-infections.

Run matrix PCR, then test for non-H5N1 strains, then test for H5N1; if negative, stop and call it a generic influenza A positive

This isn’t ideal but still tells us a lot. In most cases nowadays, if the sample is matrix positive and negative for all the other main flu strains, it’s probably H5N1 flu, but pets can get spillover infection of other strains that aren’t included in strain-specific tests. If they stop here, it’s functionally okay but not ideal. I’d want to try to get followup testing of any matrix positive, type-specific negative samples (and would treat them as H5N1 positive until proven otherwise).

Any of these combinations would be okay for testing an animal in which there’s suspicion of H5N1 flu exposure / infection, but I’d rather have a quick H5N1-specific result in these cases, and I’d want to make sure that the lab will forward any positive sample for further genomic testing, so we can better understand the situation with H5N1 flu. 

Take home message

  • Talk to your lab to know what they can (and can’t) do in terms of testing for H5N1 flu, and follow up testing.
  • We can’t just stop at “flu positive.” Any such result needs to be scrutinized to make sure we know the strain (or strains) involved.