My post earlier this week about culling considerations for control of disease outbreaks in animals generated some good discussion and ideas (and a surprisingly small number of “you’re an idiot” emails). Although that post was focused primarily on culling in response to influenza outbreaks, we can also think about it little more broadly and how it would apply to other disease outbreaks.  Australia has a really nice related guidance document that I hadn’t seen before, and it was great to receive it from a couple of people: Risk based assessment of disease control options for rare and valuable animals. It’s a good read for anyone interested in the topic. 

So, I’ve expanded the assessment table I created with a few more rows, with some more explanation below:

Likelihood animals/group have been exposed

If most of the group has been exposed, that’s bad. If it’s possible or likely that there’s only been limited exposure, containment is more feasible. 

For example, if a farm brought in a new animal and kept it isolated after arrival (a good but underused routine infection control measure), or it was kept with a small, separate group, then the rest of the farm/group might not have any realistic risk of exposure if that new animal turns out to be a disease carrier. It takes work to sort out what the risk is depending on how long the new animal has been there, the quality of the infection control practices, management issues and physical proximity to the other animals, but it can be done (and may well be worth the work in some cases).

Rapid and effective vaccination

Vaccination is often “too little, too late” for a facility-level outbreak. However, some vaccines (usually modified live vaccines) can produce very rapid and effective protection. Vaccination of this kind could also be considered for adjacent facilities to help avoid the need for culling (in some situations culling may be applied to all susceptible animals within a certain radius of an affected property, not just on the one property). If layered onto a situation where there hasn’t likely been widespread exposure of other animals on the farm or adjacent farms, rapid and effective vaccination of these other animals makes managing the outbreak without a call a more reasonable and feasible option.

Effective post-exposure prophylaxis

Although not very common, there are some situations where we might have an effective post-exposure treatment that can prevent infection in exposed animals. If we can do that (without causing more risk or damage), then it decreases the likelihood that culling is necessary.

International trade and regulatory restrictions

As I said before, I hate to see us acting based on regulations that might not be up-to-date, logical or fit the situation. However, I’m practical enough to know that doesn’t always matter. While we want to make sure regulations are practical and reasonable (and updating regulations is typically a very slow process), it doesn’t change the fact that they can be important. Trade issues can be even more important if there’s a potential loss of a multi-million or multi-billion dollar export market because of the presence of a disease.

Immunity after infection

This factor is perhaps a bit more obscure, but it’s worth considering. It comes down to how likely it is that the pathogen can be effectively, rapidly and safely contained and eliminated. If infected animals have solid post-infection immunity, the disease will typically burn through a group and die out as the number of susceptible animals decreases. If animals don’t have good post-infection immunity, they can get infected again (and again, and again), which can lead to ongoing cycling of the infection in a group. That makes it much harder to contain. 

Testing options

Accurate, rapid and accessible testing for the pathogen in question is important. Without it, we’re flying blind: we have less confidence in our understanding of the disease status of individuals, which makes it harder to implement control measures and determine when an animal or facility is no longer a risk. 

Is the species known or reasonable suspected to be able to transmit the pathogen?

This overlaps with some of the rows in the first version of the culling assessment table (i.e. transmission risk to humans/domestic animals/wildlife). However, it probably deserves its own row since since it’s a key factor and “transmission risk” covers a broader range of considerations. Being infected is not always the same as being infectious. It’s possible for animals to be infected but to pose little or no risk of transmitting the pathogen. If they are infected but are unlikely to be shedding the pathogen, the risk they pose to others is very low, as is the value of culling such animals.

Risk to people involved in culling

Culling may mean a lot of people having a lot of contact with infected animals, which is a significant risk, especially if it’s a zoonotic disease. Culling can be done with lots of infection control measures to mitigate the risk, but there’s always still some degree of risk to consider. With some animals there may also be physical risks/dangers to people, depending on the mechanism used for culling, which also ties into the last factor on this list.

Humane aspects of culling

Culling needs to be done effectively and humanely. That’s always a controversial topic, but it needs to be part of the equation. Human health risks will always win out over animal health (and usually welfare) concerns, but we need to make sure both sides are fairly assessed. The flip side of this is also humane aspects of not culling, i.e. what is the likelihood of animal suffering with highly infectious and very severe / fatal diseases (like highly pathogenic avian influenza in poultry) if the animals are not culled.

There are probably other things we could add to the list so the table might get even bigger. That makes it messier, but it’s important to consider all the different (and sometimes competing) factors to make sure we make the best decisions. 

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The recent situation has raised more discussion about the role of mass culling in response to H5N1 influenza infection in domestic / captive birds. After H5N1 flu was identified on an ostrich farm in British Columbia, the CFIA ordered all 400 birds to be culled, but the owner is trying to find options to preserve her herd. 

It’s a tough decision, and views on the matter vary drastically depending on perspective and mandate (e.g. animal owner versus public health versus protecting trade and treaty obligations and and broader animal health across the country). So it gets messy.

I have not been involved in any discussions about this particular situation, so this post is more of a generic discussion of when, where, how and why culling can be used for disease control.

  • On one hand, culling can be necessary to eliminate a pressing and poorly controllable threat in order to protect other people and animals.
  • On the other hand, culling can (at times) be an overly heavy hammer that’s used to simply remove concerns altogether. Killing affected animals is a very definitive (and irreversible) response to a disease concern, but question is whether it actually achieves much in terms of reducing risk, depending on the specific situation.

There are a lot of issues to consider when determining the best response to an infectious disease in animals. There’s no magic rule that says “if X happens, then the best thing to do is always Y.” There are lots of grey areas, nuances and case-by-case variations, which makes it a big challenge from a regulatory standpoint, because regulations and policies are usually designed to be as black and white as possible (“do this, or do that” full stop). That’s good for clarity, but it runs the risk of inappropriate or unnecessary responses in cases that fall in the grey.

While lots of factors come into play, in general, culling makes more sense when there’s an imminent and substantial risk to human or animal populations from the affected animals that’s too hard to contain by any other means. In the case of H5N1 influenza, culling is easier to justify in poultry where those infected birds would not likely ever end up going into the food chain anyway (due to illness and mortality from the disease), so keeping them alive would just put the people handling them at risk (as well as potentially birds on nearby or linked properties) for little net benefit to the birds.

As H5N1 influenza is now well-established in wild birds in Canada and internationally, there’s less argument for culling from the standpoint of controlling disease. Previously, if we had rare incursions of highly pathogenic flu, culling would make more sense as it would quickly eliminate a large infected group of birds when there’s limited or no known infection in birds outside of that group. Here, we can’t say that. Now if we have H5N1 in a group of domestic birds, we still likely have lots of H5N1 circulating in wild birds in the area. If an individual group of birds is a drop in the bucket now that H5N1 is widely distributed, culling probably has little benefit to the larger population. There’s still some impact, but it’s probably low, and if it’s a small group, transmission isn’t sustained and there are some basic infection control measures in place, the risk that they will infect other birds or mammals is low.

Human health risk is a major consideration, but the risk is only for a short period of time with a group of domestic birds, because H5N1 influenza would be expected to burn through the population quickly and then be gone. If they can limit contact with the birds and use good infection control practices for necessary contacts, the risk is lowered further. However, it’s “lowered” not “eliminated.”

The value (economic, conservation, emotional) of the birds or animals is also part an important factor. The conservation aspect is particular important when it comes to rehabilitation facilities or zoos have infected animals, where culling could have important impacts on some uncommon species. The same doesn’t apply to ostriches or many other poultry and non-poultry species, but economic and emotional values might.

Ultimately, whether or not to cull birds on an infected premises is a cost-benefit decision where we can’t really quantify the costs or benefits well. The easiest thing from a risk aversion standpoint is to cull, but that’s not necessarily always the best thing from a broader standpoint.

I sketched this out for a synopsis of considerations for culling. The more answers end up in the red, the greater the value of culling:

Here are my answers for commercial chickens:

Here are my answers for ostriches:

How about for dairy cattle?

Cattle infected with H5N1 influenza probably pose at least as much (or likely more) risk to domestic animals and people as poultry. I’d score them similarly or sometimes a bit more to the right compared to chickens and ostriches, apart from the value row. The economic value of a dairy cow is much higher than a chicken, and I doubt culling would be on the table just because of that. Economics are likely going to be a major driver of any decision of this kind.

Does that mean we should or shouldn’t cull the ostriches? That’s beyond my pay grade, but I think these are some of the things we need to seriously consider when making those decisions.

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Blue Ridge Beef has (again) recalled a diet because of Salmonella contamination – this time a kitten diet. As is often the case, the issue was identified after a pet that was fed the diet got sick. This follows a recall of a puppy diet from the same company in December for Salmonella contamination was identified after a litter of puppies got sick that was identified after a litter of puppies got sick. (It seems like a bit of a “stealthy” recall too, as I see no information on the company’s website about it.) Of note, the company states on its website that its diets are high pressure-pasteurized.

A recall of raw pet food for Salmonella isn’t surprising. Bacterial contamination rates in these diets are high, and recalls are only the tip of that iceberg. Recalls of high pressure-pasteurized diets are more noteworthy but unfortunately are also far from rare.

High pressure pasteurization (HPP) is a process used to reduce pathogen burdens in raw food. It should markedly reduce (or perhaps sometimes even eliminate) bacteria like Salmonella and viruses like influenza. However, as we repeatedly see in situations like this, contamination can still occur. Whether that’s because of an inadequate method for HPP or post-processing contamination is unclear, but it probably relates to using methods that are not adequately tested and validated for the specific food matrix.

These repeated recalls raise a number of issues. One is the standard concern about contamination of these diets with bacteria like Salmonella, that cause disease in dogs and cats, and sometimes owners who have contact with the diets or animals. More recently, we now also have important concerns about H5N1 avian influenza virus contamination of raw meat diets for pets, as multiple cat deaths have been linked to consumption of such raw diets (including diets that were high pressure-pasteurized).

I recommend not feeding raw diets to pets. We cook food for people to kill things that can hurt us, and the same concept applies to pet food. If someone insists on feeding a raw diet, I recommend a high pressure-pasteurized diet to help reduce the risks, but this certainly does not eliminate the risks. Pet owners need to be aware that (despite all the claims on various companies’ websites) there are still infectious disease risks to pets and people with these diets, and safe food handling practices in the home are critical to reduce the risks to people in particular.

Since there’s no health benefit from raw diets beyond equivalent cooked diets, it’s best to go with a cooked diet. Post-cooking cross-contamination of food can also still occur, so while they still aren’t zero risk, they are much lower risk than raw diets.

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Issues with raw diets and H5N1 influenza risk in cats (and dogs) have gotten a lot of attention in the last week or two. Like most emerging disease situations, it’s still fluid, and we’re learning more as time goes on, but we know enough at this point to at least make some basic assessments and recommendations. Here’s my current take on it.

What are the concerns with H5N1 influenza and raw diets for pets?

The unprecedented pandemic of H5N1 highly pathogenic avian influenza (HPAI) has ongoing for more than 2 years. Massive numbers of wild birds have been affected around the world. Spillover into domestic animals is an ever-present risk where this virus is circulating in wild birds, with domestic poultry being the most severely affected. Millions of domestic poultry have died or been depopulated due to infection, and spillover infections have also occurred in mammals, including many cats and at least one dog.

Recently, H5N1 influenza infection has been linked to consumption of contaminated raw meat diets in at least two cats. It’s been well established for some time that ingestion of infected birds can lead to H5N1 influenza infection in cats, based on earlier studies and field observations. It is therefore unsurprising that infections could occur if infected poultry makes its way into raw diets fed to cats, and unfortunately the infection in cats is often fatal.  The risk to dogs is less clear, but infection was reported in one dog that died shortly after close contact (chewing on) an infected bird. Dogs may be less susceptible to H5N1 influenza than cats, but they are still susceptible and infection can have dire consequences, so the same concepts apply to both species.

What types of raw diets for pets pose a risk of H5N1 influenza infection?

Poultry-based diets are the main concern, including chicken, turkey and duck, as all poultry are highly susceptible to H5N1 influenza. However, the virus is now also widespread in dairy cattle in the US, particularly in certain states (like California at the moment). Work done by the Food Safety and Inspection Service (FSIS) in the US has found no evidence of virus in retail beef samples and only very low levels of virus in samples from 1 of 185 cull dairy cows tested (the positive cow did not enter the food chain). More investigation is needed, but it is likely that viral loads in muscle of infected cattle are far less than those in poultry. Risks from beef are presumably low, but the potential for the virus to be present in meat from infected dairy cattle cannot be dismissed.

Does use of “human grade” meat in raw diets eliminate the risk?

Human grade meat does not mean pathogen-free – it only means that the meat would have been allowed for sale for human consumption based on more rigourous requirements for the animals, facilities, processing and handling. These presumably reduce, but do not eliminate,  the risk of H5N1 infected animals entering the pet food chain.  

Does high-pressure pasteurization of raw diets eliminate H5N1 influenza virus?

High pressure pasteurization (HPP) of food products uses high pressure (rather than high heat, as in cooking) to reduce contamination with bacteria and viruses. Many commercial raw diets for pets are high pressure pasteurized (which is good). The pressures achieved during this process should inactivate influenza virus, but there are no standard methods for HPP. The effectiveness of HPP depends on the pressure, temperature, and composition of the food matrix. It is unclear whether manufacturers have developed and validated the method for their wide range of diets and pathogen risks. Recalls of high-pressure pasteurized raw diets because of Salmonella contamination have been regular occurrences, and a recent infection in a cat was linked to such a diet. High pressure pasteurization should be considered a risk reduction step, not a risk elimination step.

Are frozen or freeze-dried raw diets lower risk for H5N1 influenza?

Freezing and freeze drying are effective preservation methods for viruses, so its unlikely that these methods substantially reduce the risk of viral contamination in raw diets. Ultralow freezing is used for longterm preservation of viruses, but shorter-term survival is also possible at temperatures achieved using normal freezers (-20C). A study of the survivability of H7N9 influenza on raw chicken meat (Dai et al. Lancet 2022) reported that viral infectivity was maintained for 9 days at -20C, 4 days at 4C and 4 days at 25C. This was a rather small study, so it is possible that somewhat longer survival could occur in some situations. In nature, long term (e.g. overwintering) survival of influenza virus in ice has been suspected.

While the survival kinetics of this virus with freezing are not clear, it should be assumed that the virus could survive frozen for at least a week, and possibly much longer. Freezing should not be assumed to be a risk mitigation measure for viral contamination of raw diets.

Freeze drying is a highly effective virus preservation method. Survival of virus in freeze dried food has not been assessed, but in the absence of specific evidence, it is reasonable to assume that influenza virus would survive for long periods of time in such diets.

My cat is doing well on a raw diet and I don’t want to change. What can I do?

Raw diets can be cooked so that the cat receives a diet with the same ingredients but without the risk from H5N1 influenza (and other pathogens). There is no evidence that cooking reduces the health benefits of a diet. Cooking the food to an internal temperature of 165F/74C will inactivate influenza virus and other pathogens such as Salmonella.  

My cat has eaten a raw diet that’s been recalled diet. What should I do?

You should observe your cat closely and contact your veterinarian in you are concerned or your cat develops any signs of illness. Antiviral prophylaxis could be considered in particularly high-risk situations where it is likely that the cat has eaten contaminated food. This may not be the case for all recalled diets and would depend on the nature of the recall. Early signs of H5N1 influenza in cats are not well established, but often seem to be vague signs such as lethargy, malaise and decreased appetite. Rapid progression to severe disease can occur so veterinary care should be sought as soon as possible.

My cat has eaten a raw diet that’s not been recalled. What should I do?

Unless there is evidence that the diet has been implicated in disease, the odds of a problem developing are low. However, it would still be ideal to transition the cat to a cooked diet, whether that’s a commercial wet or dry diet, a home-cooked diet or cooking the cat’s current raw diet. Monitoring of your cat for illness, as above, is still indicated, as always.

What are risks to people from contact with contaminated raw diets?

The risk to people from H5N1 influenza from handling contaminated raw diets is likely quite low but not zero. Exposure could happen from inadvertent ingestion of the virus from contaminated surfaces (e.g. food prep surfaces, refrigerator, food bowls). That would likely be low level exposure, but the infectious dose of the virus is not known so it could still be relevant. There are similar risks with these diets from pathogens such as Salmonella, so good food handling practices are always warranted. These include avoiding cross-contamination with human foods and kitchen surfaces, careful cleaning and disinfection of in contact surfaces, dishes and utensils, and thorough handwashing (or use of an alcohol-based hand sanitizer) after contact with the raw diet or food bowls.

The greatest risk to people would be from exposure to a cat (or less likely a dog) infected with H5N1 influenza. An infected animal might be shedding enough virus to infect people, although the risk of this is still unclear. Any cat or dog with suspected influenza should be handled sparingly, and with use of good infection control measures, including a mask (ideally an N95 respirator) and eye protection.  

Closing thoughts

My opinions on raw diets have been pretty clear all along. I don’t think there’s any evidence that raw diets have any health benefits over an appropriate cooked diet, but raw diets come with numerous infectious disease risks. I’ve seen enough dogs and cats (and sometimes owners) sickened from raw diets to be confident in that. At the same time, I’m a realist and know that some people will continue to feed pets raw diets regardless. That’s why we have a infosheet about raw diets that includes ways to reduce the infectious disease risks for those who choose to feed them to their pets.

The current situation doesn’t really change anything for me. I’m still opposed to raw diets for pets, and this simply adds yet another layer of risk to pets and people that consume/feed them.

If someone is adamant that they are going to feed their pet a raw diet, here’s what I currently recommend:

  • Avoid diets that contain poultry (including duck)
  • Use a diet treated with high pressure pasteurization (realizing it reduces risk, but does not eliminate it)
  • Take care to avoid cross contamination of human food, and use good food handling and hygiene practices (always)
  • If your pet has been fed raw meat and gets sick, make sure your veterinarian knows about the diet so they can consider whether that might be relevant
  • Pay attention to the news and recalls so that you can stop feeding a diet if there are any known issues (but realize that we don’t typically know about any issues until one or more animals gets really sick or dies from the diet)
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Hot on the heels of the recent death of a cat in Oregon due to H5N1 influenza infection linked to a raw diet (that some raw diet proponents are trying to deflect or downplay), we have confirmation of a very similar case in California. Los Angeles County Public Health has issued a notice to avoid feeding Monarch Raw Pet Food “due to detection of H5 bird flu in product samples.” (note the plural). H5N1 was confirmed in one cat that was fed this diet and is suspected in four other cats from the same household.

The diet type was not reported, but the company’s website indicates “Our pet food is made of human grade USDA free-range poultry that is raised in the San Joaquin Valley.” Free range poultry are at increased risk of H5N1 infection from wild birds. Nonetheless, it would be interesting to know how infected poultry made it into the food chain (even the pet food chain), since H5N1 influenza usually causes widespread illness and death in domestic poultry, so it’s pretty obvious when it hits a poultry farm (and sick birds cannot be sent to slaughter).

The status of the infected cat wasn’t reported, but H5N1 infection in cats is often fatal based on what we know to date. Hopefully the cat had a milder infection, but I assume it was at least worse than a routine upper respiratory tract infection, since testing is usually limited to pets that are pretty sick.

The good news is that food-associated H5N1 influenza risks are totally avoidable… just don’t feed pets raw diets. The cost:benefit calculus is pretty easy since this virus kills cats and there are no health benefits of raw diets. More information about raw meat-based diets for pets (beyond the risk of H5N1 flu) can be found in the Worms & Germs / OAHN Raw Meat Diets infosheet, available on the Worms & Germs Resources – Pets page.

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The Oregon Department of Health has issued a warning to pet owners about contaminated raw pet food after H5N1 avian influenza virus was detected in turkey-based raw (and frozen raw) pet food from Morasch Meats. Testing was performed after the death of an indoor cat from H5N1 flu in Washington County, Oregon. The virus from the cat and from the pet food were a genetic match, making a solid link between the two. The pet food company is voluntarily recalling some of their Northwest Naturals diets.

This highlights (yet another) risk posed by poultry-based raw pet food at the moment in areas like the US where H5N1 flu is actively circulating. There could also potentially be risk of H5N1 contamination of raw pet food made with beef, if the meat came from infected US dairy cattle. Risks could also extend to other raw pet food formulations if there is cross-contamination from these higher-risk ingredients.

There are no health benefits to raw diets beyond a properly formulated cooked (be it commerical or homemade) diet. H5N1 flu is now yet another potential threat to animals fed raw diets. The risk may extend to owners as well, through exposure to virus from handling food, and potentially from infected pets (especially cats, but we still don’t know the level of cat-to-human transmission risk).

We have seen foodborne H5N1 influenza infection in cats in multiple situations, often resulting in fatal disease. Foodborne disease probably helps explain some of the earlier reports of H5N1 in indoor cats, and in that respect it’s good to have an idea of how all these infections are occurring. The more infections we can’t explain, the more we worry about serious issues like unknown human (owner) infections with subsequent human-to-cat transmission. I’d much rather be able to attribute cases to straightforward food-associated transmission than have to explore other transmission pathways, or think that there may be silent human-associated spread.

The mitigation approach here is simple: just avoid raw pet food diets (especially poultry-based diets).

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The Los Angeles County Department of Public Health has released more information about recent cases of H5 influenza in cats, including links to raw milk exposure and potentially to raw meat diets. This expands on the information I wrote about yesterday regarding H5 influenza cases in California cats linked to recalled raw milk, and provides some important new information about cases in two separate households.

Household 1:

  • Eight cats were fed raw milk that was subsequently recalled because of H5N1 flu contamination (because the milk came from an infected dairy and was not pasteurized). Seven of the cats got sick and five of them died. The other two are being isolated while they recover.
  • Four cats were confirmed to have H5 influenza via PCR testing of samples from multiple body sites.
  • The virus from the cats was sequenced and was the same as the virus from the recalled milk (and dairy cattle).
  • Clinical signs in the cats were consistent with what we’ve seen so far in other infected cats (e.g. fever, lethargy, anorexia, respiratory distress ad neurological disease (mainly seizures)), but with more specific mention of a respiratory component in these cases.
  • Viral shedding, both in terms of amount and time period, is hopefully being assessed in the surviving cats. That will be really valuable information to help with management of future cases.

Household 2:

  • Five cats got sick after eating two different types of raw pet food, which contained poultry and beef. Both of these types of meat are potential flu sources if fed raw (because both chickens and cattle can be infected). Raw diet-associated fatal influenza has been previously reported in cats.
  • Two of those cats were presented to a veterinary clinic in respiratory distress and were euthanized.
  • Two other cats showed signs of lethargy, anorexia, neurological, respiratory and ocular disease. One of these cats was tested and was positive for H5 influenza. Further testing (and sequencing) of the virus will help infer the source.

Ideally, more testing of the surviving cats (from both households) will be done. That’s not a given since owners have to consent to further testing, and people have to be motivated enough to collect samples over time (which takes money, time and creates at least some degree of risk for people doing the sampling).

I’m spending a lot of time talking to veterinarians about how to flag potential H5N1 flu cases in pets coming into the clinic, both to help make a quick diagnosis and to help ensure any risk of further transmission from infected pets is contained. Our lack of understanding of the disease in cats is a big challenge, including the potential scope of illness, how common it is, and what the main risk factors are. This report from California actually helps a lot, as it provides more information about a broader range of clinical signs in cats, and once again raises the concern about the potential for exposure via raw pet diets.

The LA County Department of Public Health is recommending that veterinarians ask owners of sick pets about raw milk and raw diet exposure.

  • That’s good advice all the time, and is particularly important now.
  • I’d add a need to query outdoor access as a potential risk factor for exposure as well (both intended outdoor access (i.e. indoor-outdoor cats) or unintended access (i.e. indoor cat sneaking out)).

They are also recommending the use of appropriate PPE “when in contact with animals suspected or confirmed of being infected with H5 bird flu; acutely sick animals that have history of exposure to raw milk or raw milk products, wild birds or raw or undercooked animal products.”

  • That’s good advice too. “Appropriate PPE” isn’t defined but I consider it to be a mask (ideally an N95 respirator or equivalent), gown and gloves, and eye / face protection (which often gets overlooked).

They don’t give a specific disease type to focus on in terms of screening cats for flu infection. That’s fair, because we just don’t know what to expect in all cases at this point. We know that flu sometimes kills cats showing signs of severe neurological and/or respiratory disease. We don’t know if some infected cats may have less severe signs, but we should approach it as if they can. So, cats with risk factors and non-specific disease should also be handled with care, at least for now.

They are also recommending flu vaccination of veterinary personnel. “Animal health staff are strongly recommended to receive their seasonal flu vaccine. Although this vaccine is not proven to prevent infection with H5 Bird Flu, it can help protect staff from seasonal human flu which is circulating right now.”

  • That’s a good idea too. Seasonal flu vaccination may not do anything to protect against H5N1 infection, but may help to to avoid people getting infected with H5N1 and human seasonal flu at the same time. That helps to decrease the risk of H5N1 flu mixing with a human flu strain to make a new (and potentially nastier) strain that is more easily transmitted to and between people.

I suspect we’re going to learn a lot more about H5N1 in cats in the next few months, especially from California, where a state of emergency has been declared because of their poorly contained dairy cattle H5N1 flu situation.

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There’s a reason we pasteurize milk: there’s always a risk of contamination of raw milk with infectious agents, and pasteurization is a cheap, easy and effective way to mitigate disease risks from drinking milk and other dairy products. There are no clear benefits to raw milk (despite what some may say) and the risks from drinking raw milk are clear (and have been known for decades).

However, raw (unpasteurized) milk can still be legally sold in some states in the US (it is not allowed in Ontario). It’s getting more attention as a certain high-profile US politician talks about raw milk alongside his anti-[lots of things] agenda. That’s a huge concern for human health. There are some animal health issues too, and since that’s my turf, I’ll talk about those.

A recent report from LA County highlights the risk of H5N1 influenza in raw milk from infected cows in California (where hundreds of herd have been affected since the summer). So far, this seems to just be a problem in the US, as they are the only country where H5N1 flu has spread to (and between) dairy cattle. Lack of an aggressive initial response, in combination with how dairy cattle are managed and moved across the US, led to widespread infections on dairy farms in certain states. The strain of H5N1 flu affected the cattle has a unique predilection for the bovine udder. Infected cattle shed huge amounts of virus in their milk, and milk (and milking equipment) therefore seems to be the main source of transmission from and between cattle.

Milk from infected cattle shouldn’t enter the food chain, but not all infected cattle are identified right away. Pasteurization will effectively inactive influenza virus, so pasteurized milk is safe regardless. The concern is with consumption raw milk (whether it is sold legally or illegally, depending on the jurisdiction).

The LA County Department of Public Health has reported the deaths of two cats from H5 influenza (that will almost certainly be confirmed as H5N1 when all the testing is completed). Both cats had consumed recalled (contaminated) raw milk, and had what seems to be a typical disease course for cats with this virus. They had some initial non-specific signs (e.g. loss of appetite, fever) then developed severe, progressive and ultimately fatal neurological disease. Presumably public health is closely monitoring the people and other animals in the household.

They are also investigating three other feline cases from a different household. One initially tested positive for influenza already, so it’s pretty safe to assume this is another H5N1 cluster. Two cats in that household died “after worsening respiratory illness.” More details about that would be nice since we still don’t have a good handle on the scope of disease that occurs in cats and dogs, but it would help us to better flag potential flu cases. There’s no known link to raw milk for these three cats yet, but they are investigating potential sources of exposure, including raw milk.

Almost certainly they are trying to sequence the virus from these cases.

  • They’ll presumably find the dairy-associated strain of H5N1 in the first household; that would make a solid link to the raw milk as the source.
  • Sequencing from the other household will be important to help sort out potential sources (e.g. dairy strain vs strains in wild birds in the area).

While avoiding raw milk doesn’t eliminate all H5N1 flu risks (since infected birds are still a source), it’s a completely avoidable risk factor. There are no health benefits from raw milk, but there are very clear risks (which now include influenza as well). The cost:benefit calculus is pretty straightforward here.

What about risks from raw milk in areas outside the US where there’s no known H5N1 flu in dairy cattle?

There are still other infectious disease risks from raw milk. Also, just because we don’t know that there are cases now doesn’t mean that there are actually none or that there will be none forever.

It’s pretty simple to me: just avoid drinking raw milk (and other unpasteurized dairy products), but remember that pasteurized milk and dairy products are safe!

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Warning: Long post. Lots to unpack here.

As H5N1 avian influenza continues to spillover into domestic animals and people, the question of when to test animals (especially cats) keeps coming up. Testing isn’t just a matter of swabbing a bunch of animals and sending the samples to the lab (which unfortunately is not uncommonly done, and it often leads to problems). There are lots of things to consider before we start testing, which I’ll bin into 4 major questions: what is the goal, who will be tested, what test will be used, and what will be done with the results?

What is the goal of testing?

This should always be question #1. We need to know why we’re testing to figure out how to do it and what to do with the results. Specifically are we testing:

  • to understand more about how common the disease is, what it looks like clinically, risk factors, virus variants and transmission risks? Those are surveillance goals.
  • OR to inform how we’ll care for and manage individual animals? That’s clinical testing.

These goals can overlap, but they are not the same. If we think first about what we want to achieve, we run the risk of wasting time and money, and not answering the right questions.

Who will be tested?

In the case of H5N1 testing in companion animals, do we test all cats and dogs? All the cats? All cats and dogs with certain signs of illness? All cats and dogs with certain risk factors? Any pet that looks abnormal?

What we learn will vary a lot based on who we test. It will also impact efficiency and costs. More testing can provide more information but also costs more (time and money). For rare diseases (and I still consider spillovers of H5N1 influenza into pets to be rare overall at this point), you can do a lot of testing and get no positive results. That’s still informative in some ways, but broad testing can be pretty low yield in this type of scenario.

What test will be used?

For H5N1 influenza, PCR is our standard go-to test for a number of reasons, but there are also some “rapid” point-of-care tests that get talked up periodically (more on that below).  Different tests perform differently, so we need to pick the right test for the right population based on the goal of the testing.

What will be done with the test results?

This is the biggest question.

  • If we’re doing surveillance testing, is the design of the testing scheme robust enough to actually allow us to infer anything relevant beyond the animals sampled? Poor study design leads to poor (or no) conclusions.
  • If we’re doing clinical testing, what will be the response be, and will it differ if the test is positive or negative?

Too often, people do a test, and only think about what to do after they get the result. That can be problematic. What will we do with pets that test positive for H5N1 influenza?

Will we euthanize positive pets?

I hope not, but it’s a knee jerk response we handle sometimes. I can’t see any value in euthanizing infected pets. We can typically isolate them effectively, and then they’ll either get better and eliminate the virus, or die. Either way, the infection is contained, and there’s no concern about long term shedding of the virus. Yes, there’s some short-term risk from the pet while it’s infected, so we need to do it right, but there’s risk in most of what we do, and it can be managed.

Will we treat positive pets?

If we have an effective treatment for a given disease, early clinical testing can be really useful. For H5N1 influenza, it’s less useful since we don’t have a treatment we’d typically use in a pet, and by the time we get test results back, it might be too late for it to have any effect anyway. We can potentially use antivirals (e.g. oseltamivir) in dogs and cats, but that would be reserved for rare situations where the animal has early and severely progressive disease, or where it’s at high risk for severe disease (e.g. an immunocompromised animal on chemotherapy). Most often, we’re just going to give them supportive care, whether they have influenza or not.

Will we isolate positive pets?

This would be useful to help prevent spread of the virus. However, if we think there’s a reasonable chance that a pet has influenza (of any kind), we should isolate them anyway. Since tests are not 100% sensitive and we don’t get results back right away, we have to err on the side of isolating sick animals from the start. If the case is high risk for flu (i.e. compatible clinical signs and realistic risk of exposure), I would want to test it, but even if it tests negative I would still consider it a flu suspect and manage it accordingly to be on the safe side.

Will we do nothing different for positive vs negative pets?

That’s probably the case most of the time. So then why would we want to add complexity and cost by testing pets?

  • There are some limited surveillance benefits (discussed below).
  • If the animal is in the clinic, it’s easier to do the testing and the cost/benefit is more reasonable.
  • If the animal is at home, we need to consider whether or not the stress to the animal (i.e. taking it to the clinic, sample collection), the risk to the owner transporting it to the clinic, the risk to clinic personnel handling the pet, and the cost of sampling and testing are worthwhile, if instead the pet could just stay quiet at home (and hopefully recover)?

What I would like to see/do in terms of H5N1 influenza testing in pets

Surveillance testing

  1. Testing of cats/dogs with clinical signs suggestive of H5N1 influenza infection.
    • Right now, the focus is on pets with severe neurological and/or respiratory disease. Testing these animals lets us know how common the infection is in these cases, lets us investigate risk factors and transmission risks, and lets us do viral sequencing.
    • Currently the most widespread testing for H5N1 flu in pets is probably secondary testing of rabies suspects, whereby cats that are being tested for rabies (which often have neurological disease) get tested for flu if the rabies test is negative. This is a convenient and useful surveillance method that can contribute useful infomation, especially if other organized surveillance is lacking, but the sample is limited.
  2. Testing of dogs/cats with more common respiratory or flu-like illness signs.
    • We need this to understand the scope of disease caused by H5N1 influenza in pets. Are severe cases all that occur or are they all that we’re currently diagnosing? We need to know if there’s a fraction of cases that only have milder disease, particularly to help us assess and mitigate transmission risks.
    • We can’t test every sneezing cat, but we can do targeted surveillance by focusing on animals with a realistic risk of exposure to the virus. So, I’d like to test sick cats and dogs in areas where H5N1 flu is active (i.e. in wild birds) and where there’s a reasonable risk of exposure (e.g. outdoor access). This balances throwing a wide net to get a better broad understanding, and being efficient with our time and money.

Clinical testing

This is a tougher call. Most often, H5N1 flu testing would not impact patient care. It could help inform us about risk to the clinic or household, so it’s good information to have, but we can’t dismiss risk based on a negative test, so it doesn’t have a huge impact on what we do regardless.

The less surveillance testing there is, the more valuable opportunistic clinical test is for quasi-surveillance purposes, but that’s usually going to be at the owner’s cost. Spending a couple of hundred dollars to test a severely ill animal with little expectation that the test will change the response or the outcome is hard to justify. If money is tight, I’d rather have owners spend that on treatment.  Some people will pay for the testing because they want to know, or because they are worried about exposure of family members, but relying on owners to cover testing isn’t a great approach. It’s still worth discussing in any case since there can be some limited benefits.

What about rapid in-clinic tests for H5N1 influenza in pets?

It’s been suggested that veterinarians should start stocking rapid in-clinic H5N1 tests and use them liberally when presented with sick dogs and cats. I’m a bit wary of this because we don’t know how good those tests are. If we test a lot of low-risk animals and the test isn’t 100% specific, most of our positives are likely to be false positives. That’s alright if we’re just using it as a screening test and we follow-up with a proper test ASAP, but sometimes people rely too much on the screening test, and that can lead to over-reactions and other problems. On the other hand, if the test isn’t very sensitive, we run a good chance of missing the small number of true positives. If the test is no better than a coin toss, it won’t help.

Collecting good samples can be a challenge (especially trying to swab cats) so that drops the functional sensitivity of any test (i.e. we get more false negatives because of poor samples.

To be useful, we would need:

  •  a rapid test with high sensitivity (reliably detects all the infected animals) AND high specificity (gives us very few false positives)
  • AND good samples
  • AND the capacity to follow up positive tests with PCR testing
  • AND a communication plan for animals that are confirmed positive

But we’re not necessarily there yet. I’d consider rapid testing appealing, but probably not ready for prime time.

What will I do with my own pets?

I have two cats. Milo’s an indoor cat and Alice was a completely feral cat who’s gradually become very affectionate and just recently made the jump to being largely indoor. But we’re still not sure she uses a litterbox, and she’ll be at least partially outdoor for a while, I assume. She’s therefore at greatest risk of exposure to influenza from contact with wildlife. If she had a sudden onset of neurological or other severe disease, I’d test her, primarily because I’d really want to know, given my infectious disease mindset.

  • If she was negative, I’d still assume she might have flu, but that we missed it with the test.
  • If she was positive, then I’d have an early warning of potential exposure in the house. If anyone (human) started to get sick, flu would be on the radar and odds are high that we’d get tested ASAP and would probably be offered antivirals.

Barriers to surveillance

Money $$$

There’s VERY little funding available for companion animal infectious disease research. We try to scrape by with leftovers and token amounts, and we still manage to get good things done, but more support is needed. We can’t test if we don’t have the funding, and we rarely have funding.

Access to animals

You’d think it’d be easy, but field studies are actually very challenging. Veterinary clinics are really busy. When clinics are asked to remember that we have a study, to determine if their patient fits the inclusion criteria, and to contact us about sample collection, that takes time that just might not be available on a busy day. It took us a long time to get a good sample size for our SARS-CoV-2 research project and that was a lot higher profile.

  • Lack of money compounds the issue, since we usually can’t pay clinics for the time required to communicate with us and any extra effort to collect samples (including getting consent forms completed, collecting samples, collecting other data and shipping samples. or helping us arrange to collect samples). We try to do as much as we can to take the burden off the veterinary clinic team. With local cases we can often do almost everything, but the farther away they are, the more we rely on clinics to help out.

There’s no single approach to surveillance. We need combined approaches both because we have different questions to answers and for practical purposes. We want as much well-designed formal surveillance as possible, but since that’s not always going to be available, we rely on opportunistic information from clinical testing. We’re learning more as more work gets done, but we have a long way to go before we have a reasonable understanding of the dynamics of H5N1 influenza in companion animals and the potential for transmission to and from cats and dogs.

test

As H5N1 avian influenza continues its unprecedented spread internationally among wild birds, domestic bird, different mammals and even people, concerns continue to increase. One of the things that increases risk with influenza viruses is the number of species a strain can infect. The more species the virus can infect, the more risk of exposure to others, and the more risk of the virus changing (for the worse) within a new host. Infection of different mammals gives the virus more opportunity to adapt to and infect other mammals (and humans are included in that group). Some species can also be “mixing vessels” for influenza viruses. If an individual is infected with two different flu viruses at the same time, there’s risk the viruses can get mixed up together and create a new strain.

A recent paper in Emerging Infectious Diseases about the potential for H5N1 flu to infect horses raises some interesting questions, and has resulted in some predictable over-reaction.

The study (Damdinjav et al. 2024) looked at blood samples from horses from two parts of Mongolia: a wetland with a lot of birds, and a dry area with lower bird density. They studied 24 horse herds by testing 10 animals per herd, three times a year between July 2021 and October 2023. There was no evidence that there had been more or any unusual illness in the horses in these areas during the sampling period. They collected a total of 2160 equine blood samples.

  • Nine samples (0.4%) were positive for antibodies against H5, with the assumption that H5 positives meant exposure to H5N1 influenza. That’s a reasonable assumption, but there’s no guarantee as there could be other H5 influenza strains in wild birds in the area as well (however they did rule out cross-reaction with equine flu strains).
  • A further 8 samples (0.4%) were “doubtful.”
  • The rest of the samples were negative for H5 antibodies (indicating no evidence of an immune response to an H5 influenza virus).

They took the 17 positive or doubtful samples and tested them using an H5N1 viral neutralization assay.

  • Only 2 samples were positive using the H5N1-specific test (that’s less than 0.1% overall).

I’m always a bit wary of low-prevalence serological studies. Even if the test was 99% specific (99% of positives are true positives), the low prevalence reported here could be within the error rate. It’s an inherent issue with serological tests where we don’t have (or at least can’t be certain of) 100% specificity. I’m not saying the results are wrong, but we have to consider that some or all of the very small number of positive results could be false positives. More study is needed to know for sure.

If these were true positives, the very low prevalence is important to note. It would strongly suggest that H5N1 is not transmitted between horses.

  • If a new flu strain gets into a population, I’d expect really rapid, widespread transmission. A large percentage of horses would be infected if there was horse-to-horse transmission.
  • The low prevalence suggests sporadic dead-end spillover infections that were likely all bird-to-horse.

What does this tell us?

  • That a small number of horses in an area where H5N1 was circulating were infected with H5N1. Interesting and noteworthy, but that’s it.

What doesn’t this tell us?

  • Whether horses were infected with a currently circulating strain of H5N1 influenza.
  • Whether horses got sick from H5N1 influenza infection.
  • Whether horses can spread H5N1 influenza (to other horses or to other species).

The authors concluded that horses are susceptible to H5N1 infection. That’s probably true, but infection may or may not causes disease. They also suggested that “spillover events are likely frequent,” which seems to be a stretch based on these data.

What’s the risk of spread of H5N1 influenza to people from horses?

Presumably it’s really low. There’s no evidence that horses are a realistic source of H5N1 infection. As discussed above, the low prevalence is strongly suggestive that there was no transmission between horses. We should keep looking, but at this point, nothing suggests to me that horses pose a realistic risk to people or other animals.

What about the risk of recombination of flu viruses in horses?

Horses have their own influenza strains, mainly H3N8 equine influenza. So, concurrent infection with H5N1 could create a recombination risk. But it’s probably much more of a concern for horses than for people. H3N8 equine flu is well disseminated internationally and has been for decades, but horse-to-human transmission of H3N8 is exceptionally rare. So, if H3N8 combines with another virus that has a poor ability to infect people, it’s unlikely that the new virus would be more likely to infect people. We’re more concerned about H5N1 recombining with a human flu virus or animal flu virus that more readily infects people (like some swine H3N2 strains). Recombination of equine H3N8 and avian H5N1 could be a big deal for horses, though, if it becomes a strain that can cause serious disease in horses and for which horses have little immunity (because it’s different enough from the H3N8 their used to).
That said, while the risk may be greater for horses than humans, we don’t want more circulating flu strains; the more infections there are in horses, the greater the risk of that virus changing even more.

Take home message

The risk of H5N1 influenza infection in and from horses is probably very low. Horses rank fairly far down the list of species about which I’m concerned.

However, they’re still on the list. While I don’t have any plans for flu surveillance in horses around here (no available funding being the main barrier), I’m definitely paying attention to them. If we hear about more or atypical disease in horses, especially if it coincides with more flu activity in birds in the area, I’ll want to start testing. The issue with H5N1 in dairy cattle in the US caught us off guard, which shows why we have to remain vigilant and be ready to act quickly and aggressively. (The lack of quick aggressive action in the US with dairy cattle was likely a major reason this virus has spread so widely there.)