Finding H5N1 avian influenza in mice in the US has caused a lot of angst amongst some – some angst is warranted, but some of it is overblown. That’s not because H5N1 isn’t an issue, or that more species being involved isn’t relevant, but because there are bigger issues to address. Adding yet another species to the susceptible list isn’t a doomsday scenario, even though we’d rather that list didn’t get any longer.

The latest APHIS report involved detection of H5N1 in an additional 36 house mice in Roosevelt County, New Mexico. They’d already found 11 infected mice there earlier in May. Typical of this ongoing outbreak in the US, available details are sparse. I haven’t seen a clear statement about where these additional mice were collected. I assume they were from infected dairy farms, and that’s a pretty basic but critical piece of info. (The first 11 infected mice were reportedly from an infected poultry premise). If the new mice were from farms with infected cattle, it’s not surprising to find the virus in mice at the same location. If they were from other areas, that would be more confusing and more concerning.

How do mice get infected?

I haven’t yet seen any genomic information on the virus found in the mice; it will be helpful to know if they were infected with the dairy cow-associated H5N1 strain, or whether some of the infections might be linked to exposure to wild birds. If we go on the assumption that these mice were from dairy farms, cattle are the most likely source, because we know infected cattle shed lots of virus in their milk, which would make it easy for mice on the farm to be exposed to the virus in the environment. Even though flu virus doesn’t survive long in the environment, if there’s lots of milk loaded with lots of virus (especially in areas where mice are looking for food), mice are likely to encounter some active virus. Fecal shedding of H5N1 in cattle seems to be low, but data are pretty sparse; we need to clarify that risk more since that would be another possible means of exposure for mice (and other animals, and people). It’s obviously highly relevant since cattle produce a lot of feces, and that manure needs to be stored and/or spread somewhere.

If we think about the risks from finding H5N1 in mice, I’d consider four main areas:

1) Risk to people from H5N1 in mice

Yes, there is a risk to humans, to some degree – but we need more information, like the amount of virus the mice were shedding, and how (e.g. fecal shedding vs respiratory shedding). Some infected mammals shed a lot of virus, but others are likely dead end hosts that don’t shed enough virus to spread the infection any further.

Even if mice shed appreciable amounts of H5N1, we don’t tend to have close contact with (wild) mice, so the risk from direct exposure is presumably really low. If someone’s on a dairy farm with infected cattle, mice are very low on the risk scale. Cattle are the biggest risk. Cats are probably #2 on the list.

However, mice do get into peoples’ homes as well, and the risks from that are completely unclear. Flu virus doesn’t survive long outside the host, so it’s not like a virus like hantavirus, where mouse poop in the environment is a significant concern, but we need to know more about virus shedding.

At this point, I suspect the direct risks to people are very low, but not zero.

2) Risk to other species from H5N1 in mice

The biggest risk from this new finding might be mice acting as a bridge from wildlife / livestock to humans, through their potential to infect cats. Cats catch mice, and eating an infected mouse is presumably high risk for H5N1 transmission (just like eating an infected bird). Cats are susceptible to infection, and have close contact with both mice and people (and other domestic species), so anything that increases the risk of cats being infected is a concern.

3) Risk of mice spreading H5N1 farm-to-farm

As we start to (slowly) get more information about H5N1 on dairy farms, we’re seeing more reports of infected farms that did not bring in cattle from other infected farms. That makes us wonder about other sources of introduction, like humans tracking the virus around or spread via wild birds. Fortunately mice, like coconuts, do not migrate (yes, that’s a niche reference – see link below), so they probably pose limited risk for broad geographic spread of the virus because they’re not that mobile (unless they hitch a ride on a human conveyance of some kind…). Mice tend to have very small ranges, so it would probably be tough for them to spread H5N1 even between farms, unless the farms were very close to each other. The bigger risk would be bringing the virus from the barn into the farm house.

4) Risk of virus mutations

The more avian influenza spreads to and within mammals, the more opportunities it has to adapt to mammals. It would take a number of specific genetic steps for an avian flu virus to evolve to effectively infect and spread between mammals (including people), but the more it’s transmitted, the greater the risk that could happen. This is why we don’t want to see avian flu spreading in any mammalian species.

So, while I don’t like seeing more H5N1 infections in more mammalian species, and even though if H5N1 became endemic in mice they could be a long term reservoir, I’m still more worried about birds, cattle and cats at this point.

Back to H5N1 flu in cats

The good news is that infections in cats are still rare. The bad news is that most reported infections in cats have been very severe or fatal. Whether severe disease is the norm in cats, or whether we’ve mainly just tested really sick cats is hard to say. There have been approximately 21 cases of H5N1 influenza identified in domestic cats in the US since the outbreak in cattle was first detected. Other cases have been reported in cats in various parts of the world over the last 2+ years, including in Canada. However, those cases are probably just the tip of the iceberg. We need more surveillance, including testing of healthy and sick cats from locations where H5N1 is present. Cats on dairy farms with infected cattle are at the highest risk, but any cat with outdoor access that might encounter an infected wild bird is at some risk.

What do we need?

More surveillance and more communication. We need broader testing on affected farms, thorough epidemiological investigation of the spread on and between farms and clear (and timely) communications.

I know seals are mammals.

I know rabies virus can infect all mammals.

I’m pretty tuned into rabies and rabies prevention.

Yet, I’m not sure how quickly I’d clue in to any rabies risk from a seal bite. (I’d hopefully get there eventually, but I doubt it would jump to mind like it would with a bite from a dog or raccoon.)

Rabies in marine mammals is rare, but it happens. There are no marine mammal rabies reservoirs, so a rabid seal would have to have been bitten by a terrestrial mammal, survive the encounter, develop rabies then be noticed and tested. That’s a lot of steps to detect a case. But, as we know with infectious diseases, rare doesn’t mean it won’t happen. It means that it will happen… eventually.

A Cape Fur Seal in South Africa was recently found to be rabid. There’s very little detail in the media article, so we don’t have a lot of context. It’s not clear why the seal was tested, or if it was seen to have had neurological disease. I assume they sequence the virus to see what strain it is, which can help infer the source. The two main rabies strains circulating in South Africa are dog variant and mongoose variant. Presumably this seal tangled with another infected wild mammal or an infected dog, initially survived but got infected with this fatal virus, and for some reason was tested after it died or was killed. While this is a really rare situation, it’s probably fair to assume it happens a more than we realize because of the low likelihood that we’d test a marine mammal for it.

Regardless, post-exposure prophylaxis is being recommended “for anyone who has unwanted contact with the creature.” I guess any bite would classify as “unwanted contact.” They also advise the public to stay as far away from seals as possible. That’s never bad advice.

This is yet another reminder that wildlife should be left alone.

  • Watch them? Great.
  • Touch them? No.
  • Any bite from a mammal? Ask about the need for rabies post-exposure prophylaxis.

Image from:


There’s a great new early-release paper in Emerging Infectious Diseases (Oguzie et al. 2024) about the emerging situation with H5N1 influenza in dairy cattle. It provides some important new information, and more details that somewhat challenge the early narrative.

The authors explain some of the limitations of the study to keep in mind…

  • The study was only done on one farm
  • They didn’t direct the response (samples were sent to them)
  • They got involved a bit late in the outbreak

…yet they still have some really interesting and useful findings.

They studied one of the earlier affected dairy farms in Texas. Interestingly, the research team was contacted because of “rumours among cattle veterinarians of possible influenza A virus detection among cattle and conjunctivitis among dairy farm workers.” They also say that cattle had transient respiratory and gastrointestinal signs. That’s different from earlier information that initial concerns were focused on drops in milk production and unusual cat deaths. In particular, the mention of an early concern about human illness is worrying, from the standpoint that we might still be underestimating human infections. No cat deaths were reported on this particular farm.

There’s not much clinical information in the paper, since it’s based on test results, but they report cattle had “decreased appetite, lethargy, increased respiratory secretions, high temperatures (up to 105°F or 40.56°C), abnormal bowel movements, and decreased milk production.” Most of that is unsurprising, but I’m not sure I’ve seen much about respiratory signs before. An increase in respiratory secretions (a runny nose) doesn’t tell us much about the degree of respiratory involvement but suggests there was some. That’s important because it brings in questions about respiratory transmission.

They only got nasal and rectal swabs from cattle for this study, even though milk seems to be the prime source based on what we’ve learned since then. Timing of sampling wasn’t ideal, as is often the case early in outbreaks: illnesses was observed in cattle starting March 6, and several people were sick March 4-6, but samples weren’t collected until March 21, March 28 and April 1. That’s really late to be testing for a virus like influenza that tends to have a fairly short shedding period. They indicated that the outbreak was already waning at the time of sample collection, yet there were 7 PCR positive nasal swabs from 29 sick cattle. No healthy cattle were positive.

It’s been stated from the start that there’s limited respiratory shedding in cattle, and this study doesn’t necessarily contradict that since the PCR Ct values suggested the viral load was quite low in those nasal samples. However, even low shedding in 7/24 (29%) cows is still relevant, especially if samples were collected quite late in the disease process. Peak respiratory virus shedding with flu tends to be very earlier in disease. After a few days, I’d expect shedding to drop, both in terms of the percentage of positive animals and the amount of virus being shed by each one. So, we have to be careful not to take <50% PCR positivity with a high Ct value (i.e. low viral load) as an indication of limited risk of viral shedding.

Rectal samples were all negative for flu. That’s good. The same sample timing disclaimers as above still apply, but with no positives, it suggests that we don’t have much (or as much) to worry about in terms of exposure to feces and manure handling. That simplifies matters.

The human component raises a lot of questions. Details are sparse, but the authors say “Several workers experienced influenza-like symptoms and missed work during March 4–6. A maternity worker visited a local clinic and received treatment for influenza-like symptoms; 2 milkers also experienced influenza-like symptoms and stayed home.”

There’s no mention of anyone being testing and I’d guess that none were. Routine flu testing would be uncommon in mildly ill people at that time of the year if there wasn’t yet any communication about potential influenza exposure from dairy cattle. This furthers the ongoing concern that we are underestimating the amount of cattle-to-human transmission because of limited testing. It’s critical to know how often cattle-to-human transmission occurs and to continue to look at the genome of H5N1 when it infects people (to look for more human/mammalian adaptations).

What this report tells me overall:

  1. We still have a poor understanding of what’s happening with H5N1 flu in dairy cattle.
  2. There is a lack of transparency; we are still not getting all the available information.
  3. Consistent with some other recent reports, there may be more of a cattle health component beyond the drop in milk production than was initially flagged.
  4. Human disease from cattle-to-human transmission may be underestimated.

There’s nothing to panic about here, but it shows that much more work and communication is still needed.


Adding to the “that was not on my bingo card” theme of the 2020s, there was a recent fatal H5N2 avian influenza infection in a person in Mexico.

First important point: this was H5N2, not the H5N1 strain we’ve been focused on for the past few years in bird (and spilling over into mammals, including most recently dairy cows). This H5N2 strain is very different, and this is the first reported case of H5N2 infection in a person. H5N2 has caused a few outbreaks in poultry in Mexico so far this year, so there’s likely a wild bird reservoir, and potential for more spillover into domestic poultry, but this strain hasn’t caused human disease before. When we see rare spillovers of avian flu into people, they’re usually in individuals with close contact with infected poultry, but that wasn’t the case here.

The infected person was a 59-year-old who was hospitalized in Mexico City after a short illness with signs including shortness of breath, fever, diarrhea and malaise. Surprisingly, the person has no known exposure to domestic poultry, wild birds or other animals. The person had actually been bedridden for 3 weeks, so their recent exposures could be fairly well characterized. It’s a bit unnerving not to have no idea from where an infection like this came. It could be a one-off that we never figure out and never see again, but we have to try to investigate as much as we can in case it’s an early harbinger of yet another new flu problem.

What could have been the source of the H5N2 virus in this patient?

1) Another person: This patient could have been an infected by another person (healthy or sick) who got the virus from a bird. That would indicate bird-to-human spillover (which we know happens occasionally) followed by human-to-human transmission (a bit of a concerning scenario, especially if it wasn’t a very close household contact). Seventeen (17) human contacts at the hospital were identified and tested and were all negative, including one that reported having a runny nose 4-5 days after the patient got sick. Twelve other human contacts near the person’s home were also tested and were all negative. Even so, it’s possible one of those people was the original human source but had eliminated the infection by the time they were tested. It’s good to have the testing information (and good that they looked), but it does not rule out a human source.

2) Birds: Despite the patient history, I’m not sure we can completely rule out bird contact. I’d want to know more about where the person lived and how people and animals moved around on the property. For example, was there any chance a bird got into the home at some point? If there were open windows, there’s still a chance of direct or indirect bird exposure.

3) Other animals: As for birds, I’d want to know how solid the “no animal contact” determination was, and if it covered more than just birds. Cats are great potential bridging hosts, since they can be infected when hunting wild birds and then have close contact with people. I’d want to make sure there was clearly no dog/cat contact, and if there had been any sick or dead cats seen in the area.

4) Food: This is lower on the list, but contamination of food would have to be considered. I don’t mean properly cooked meat (even if it came from an infected bird), but more likely contamination of prepared food with bird poop (possibly from someone’s hands) or improperly cook meat from an infected bird. I’d investigate where the patient’s food was prepared; if birds could have gotten into the area, contamination of food is possible.

That’s all speculation, and I hate to do too much of that, but it’s important to brainstorm possible sources to come up with surveillance plans.

Hopefully this was just a one-off infection. However, it shows us the ongoing risks from endemic avian flu viruses (not just H5N1) and the need to support good surveillance systems in people, domestic animals and wild animals. Despite continued emergence of infectious diseases, we’re seeing underfunding and active dismantling of our public and animal health surveillance and response systems, and inadequate willingness to address these big issues.


When an unvaccinated person is exposed to rabies, they typically receive post-exposure prophylaxis (PEP) consisting of a dose of anti-rabies antibodies and four (4) rabies vaccines over the course of two weeks. In dogs and cats, it’s a different story. We don’t use formal PEP protocols in pets in most regions.

  • Why not? I’m not sure. Probably because nobody has put that much effort into figuring out whether it would be effective (and it’s not an easy – or cheap – thing to study).

Texas has approached rabies exposure of domestic animals somewhat differently than the rest of the US, particularly when it comes to unvaccinated animals. If an unvaccinated domestic animal is exposed to a rabid animal, they can undergo a PEP regimen consisting of immediate vaccination, followed by a 90 day confinement period with additional rabies vaccine boosters in the 3rd and 8th week of confinement. This is in contrast to most other jurisdictions, where the two options are most commonly euthanasia or a single vaccine dose followed by a 3-6 month quarantine.

Texas also does a really good job of publishing data on their rabies response and control programs in domestic animals, so others can learn from the information. Their most recent publication (Wilson et al, J Am Vet Med Assoc 2024) reports on 1218 unvaccinated animals that underwent the Texas PEP protocol after possible exposure to a confirmed rabid animal between 2010-2019. The animals included 570 dogs, 138 cats, 347 cattle, 93 horses, and a smattering of other species. One cat and two previously unvaccinated dogs (that received PEP) went on to develop rabies.

  • That’s a pretty impressive 99.8% success rate, but we have to temper that a bit. We can’t say the Texas protocol is 99.8% effective overall, since it’s pretty certain that not all potentially exposed animals were actually exposed to the rabies virus. Only 22% of animals that underwent PEP had “direct exposure” to the rabid animal. Another 30% had “probable exposure” and 48% had “low probability” exposures (see table below). Almost half of the low probability exposures were cattle, since sometimes whole herds were managed as “exposed” when a rabid animal was found on the property.

We always err on the side of caution with rabies, and lots of low risk or unknown circumstances get called potential exposure to make sure we don’t miss any relevant exposures.

  • If we just include high risk exposures, the success rate drops to 91% for cats, but stays high at 99.7% for dogs, but it’s possible that even some of those were not true exposures.
  • So, it’s hard to put any type of accurate number on the success rate, but we can reasonably assume that the risk of developing rabies when this protocol is applied is low.

Looking at the PEP “failures” is also important, and in this case it’s a actually a bit of a good news scenario:

  • All 3 PEP failures were in animals less than 12 weeks of age. We don’t normally start rabies vaccination until 12 weeks of age because that’s the age that we know that it will work. In younger animals, there’s the potential that lingering antibodies inherited from the dam can interfere with response to vaccination.
  • Two of the three PEP failures only got their initial vaccine dose (see table below).
    • One dog died before the time of the dose in the 3rd week.
    • One dog just didn’t get its second dose for some reason, and died 31 days after exposure.
    • The cat got a 2nd dose, but signs of rabies developed the next day, so that dose was obviously too late.
  • It’s quite possible that these 3 PEP failures were animals that had no ability to respond effectively to a vaccine based on their age, and the onset of disease was quick enough that they didn’t get a chance to have multiple doses of vaccine during their confinement period.
  • No dogs or cats that were over 12 weeks of age and no dogs or cats that underwent the full PEP regimen got rabies.

Take home messages

Rabies PEP should be considered in domestic animals. There’s no guarantee that it can prevent rabies in an exposed dog or cat, but odds of rabies developing while being managed using this approach appears to be really low.

We can also flag the cases that are of greater risk of PEP “failure” (in this case, young animals that started PEP before 12 weeks of age).

I wonder whether the 3 and 8 week booster timing is too late. If we’re going to give multiple doses and we don’t have the option for anti-rabies antibody, why not approach it more like they do in people: give the vaccine ASAP (which is already part of the protocol), and then on days 3, 7 and 14? I can’t see any reason not to use that type of approach if it could possibly help avoid PEP failures.

Vaccine failures

One “true pre-exposure vaccination failure” was reported, which is an important case to note.

  • It was in a 3-year-old dog that was initially vaccinated at 16 months of age and got a booster 1.4 years later, meaning it was properly vaccinated and not due for a booster at the time it was exposed to rabies. Nonetheless the dog developed rabies and died.
  • On one hand, it’s just one case. That’s great. Rabies vaccine is one of the best vaccines we have. But no vaccine is ever going to be 100% effective, since we can’t guarantee that every animal will respond to a vaccine the way we want them to.
  • This is a reminder that while rabies vaccination is really useful and really important, we can’t dismiss the potential for rabies in a vaccinated dog or cat. It’s just really unlikely.

Two other previously vaccinated dogs also developed rabies. Both dogs were overdue for vaccination, so they are not considered true vaccine failures, but they’re also important to note.

  • One was a 7-year-old dog that was overdue following a 3-year vaccine (last dose given 5 years earlier). I’d really like to know if this dog had multiple vaccines before the age of 2, or just one dose, since I expect pretty solid, long-lasting immunity with a couple doses – but this also illustrates why you can’t rely on that in all dogs, and regular revaccination remains important.
  • The other case was a 2-year-old dog that had received a rabies vaccine as a puppy but did not get the 1-year booster.

The conclusion of the report was “Results indicated that this protocol is a viable option for unvaccinated domestic animals exposed to rabies. Alternative protocols warrant additional consideration.”

I think that’s a fair evaluation. We don’t really know how well PEP works in domestic animals, but it’s likely quite effective. It is certainly much better than doing nothing, and should be an option for exposed animals. However, I’d like to see a more aggressive PEP regimen than those currently used.

On a closing note, we don’t need to worry about PEP regimens for unvaccinated animals if we don’t have unvaccinated animals. All dogs and cats in rabies endemic areas should be vaccinated from 12 weeks of age and up. We’ll still see exposures in animals younger than that, but maximizing vaccination in animals that can be vaccinated is critical.


For such a potentially big problem, we’re received disappointingly little official information about H5N1 influenza on dairy farms in the US. The more we know, the better we can plan for containment and control, wherever it pops up next. If information isn’t being gathered or isn’t being shared, our ability to address this problem is significantly compromised. It was therefore really nice to see some details from Michigan State University about the H5N1 outbreak on one Michigan dairy farm, but also concerning to see that the situation may not be as straightforward as we were hoping based on the information available to date.

This is how the story went for this Michigan dairy farm:

Widespread fever in the cows was the initial finding. It was detected quickly since most cattle on the farm have a monitoring bolus (in their rumen) that tracks things like body temperature, among other things. Temperature increases of 4-5oF were typical. Shortly after the temperature spike, rumen activity decreased. (The rumen is the largest of the cow’s four “stomachs;” as the cow goes, so does the rumen, so a decrease in rumen activity indicates that something is off with the cow). Fever typically persisted for 2 days. The cows were mostly treated with aspirin and IV fluids.

After the problem was identified, the farm staff tried to contain it. Milk has been implicated as the main route of spread, so the farm started washing all the milking equipment after the affected cows were milked. Unfortunately that didn’t help, and it eventually spread to all the groups of milking cattle on the farm.

  • The subsequent spread to other groups on the farm could indicate that the routine equipment washing procedure was not adequate to stop the spread, or that the virus had already spread before they changed their procedures, or that there are other routes of spread. Some people have said that this indicates milk is not the main route of spread, but I think that’s a bit premature. We need to keep investigating. Some people online are using this as a “gotcha” moment to say that USDA is hiding respiratory spread, but I don’t think there’s enough detail here to make any conclusions like that either.

Over the first 9 days of illness, milk production decreases were fairly mild. Daily milk yield per cow dropped about 5 pounds (with the average cow on the farm normally producing 95-100 pounds of milk). However, by day 12, milk production had dropped by 21 pounds per cow, the somatic cell count (an indicator of inflammation in the udder) increased, and many cows became severely dehydrated. This is more severe than the general descriptions provided to date.

Overall, they suspect that 40% of cattle on the farm were infected.

  • 40% affected is surprisingly low for flu. With all groups having one or more infected cow, I’d expect pretty much all the cows to get infected. I wonder how many of the “unaffected” cows actually had mild disease that simply wasn’t detected.

Unsurprisingly, the impact on farm management was severe. A lot more work was required to manage so many sick cows.

  • That also increases the risk to farm workers, since it means more cow-to-human contact, and that contact is mainly with sick cows. They had to stop breeding cows because they were too busy managing the sick milking cows, which will presumably have at least some long term impact on farm operations.

Some abortions occurred in pregnant cows; it’s suspected that this was because of the high fevers.

  • Flu isn’t something we typically link to spontaneous abortion, but anything causing severe disease can do it. That’s another aspect of disease severity that hasn’t been previously reported in this outbreak.

The good news is all the farm workers remained healthy.

  • It’s not clear whether there was any surveillance for mild or subclinical infections, but no illnesses were reported. Since the farm is providing detailed information, I assume that their assurance of no human illness (so far) is pretty solid.
  • The US CDC recently issued guidelines for working with infected animals, including cattle. They’re pretty standard from a control standpoint, but I also look at them and say “not gonna happen” on a standard, busy and potentially hot farm. In this case, “the farmer encouraged them to wash their hands frequently and avoid touching their face and eyes. All employees were offered safety eye wear or face shields.” That’s probably as good as we’re going to get.

Cost to this 500-cow dairy herd was estimated as $30,000-40,000 USD.

  • That may not seem like a massive number, but if it’s your family farm, that’s a big chunk of income, and you also lost money while having to work a lot harder and in a higher risk situation. It also doesn’t include long term costs from any ongoing loss in milk production, and other impacts on farm operations, such as missed breedings. Some cattle will be culled because of sustained impact on milk production despite apparent recovery from infection. That’s not been reported before either.

We need to remember that people on farms are people. They have to work and live through this. If it’s a family farm where there’s an even closer association with the animals and more direct dependence on income, it’s even harder. The farmer in this case was quoted as saying “It has been a lot of work, stressful on the cows and frankly overwhelming.” I suspect that’s an understatement of the potential impact on the mental health of farm personnel.

The article states “[The farmer] believes it is important for the industry to understand the disease. He knows that his is not the only farm to get HPAI and hopes that the more we can learn from his experience, the better we can prevent more herd infections, reduce the impact and potentially be better prepared against other diseases.

  • We need more people like this. They managed a new and potentially scary problem to the best of their ability and were willing to tell their story. That’s too rare. We also need more groups like the MSU Extension Service who will write about events like this, so we understand more about this outbreak. A lot of questions remain and a few new questions and uncertainties have come up based on this information, but information is power. We need good information to plan and to contain. There’s been too much hiding of information, unwillingness to share, and unwillingness to cooperate (at many levels), and that’s compromised our ability to respond.

We need to thank people like this and support people/farms/veterinarians/government officials who are willing to stick their necks out to provide critical information, despite the risks (social, political, economic or other).


I was at the airport the other day and, as per usual, there were lots of traveling dogs there too. As I was watching one dog getting lots of random attention while in line to board, I could only smile given how happy the dog and the people looked – a key reason why we have pets and work hard to keep them healthy!

However, the infectious disease side of my brain never completely shuts off, so “this would be a nightmare for contact tracing” also trickled into my thoughts.

The vast majority of random dog-human encounters are totally benign, but a miniscule fraction are not, and those can be a real pain. Tracing human contacts even within a specific neighbourhood is tough. We can flood local social and conventional media with “if you might have touched this dog, give us a call” notices, and if most people are still around town that can be reasonably effective. When we’re dealing with exposure of people who hopped on planes to various cities and countries around the world… that’s a whole other ball game.

A recent paper in Zoonoses and Public Health (Williams et al. 2024) describes a slightly easier-to-contain situation involving international travel: exposure of passengers on an airplane to the uncommonly discussed but concerning bacterium, Brucella canis.

Brucella canis is a bacterium that flies under the radar a bit. It’s more common that most people recognize, as we found out a few years ago when we did a surveillance study of B. canis in dogs in breeding kennels in Ontario, and we found quite a bit of it. In dogs, it can cause a range of disease, with reproductive problems being the biggest issue. Human infections are uncommon, but can be serious and hard to treat. The greatest risk of infection is when people are in contact with infected dogs that are giving birth or aborting stillborn puppies.

Here are the highlights from the paper (Williams et al. 2024):

  • A pregnant 10-month-old French bulldog (Dog A) flying in the cabin of a commercial airliner started to abort three fetuses during a flight from Poland to Chicago.
  • A second 12-month-old dog (Dog B) from the same facility was on the same plane, flying in cargo.

It’s stated that the two dogs were to be imported by a breeder, but importing a pregnant dog is sometimes a way to bypass rules against importing puppies (which are very valuable in terms of sales), so I have to wonder if this was really a breeder purchasing new, high quality breeding stock versus a puppy mill situation. The fact that they were French bulldogs, a common and lucrative puppy mill breed, heightens my concerns.

  • Dog A’s distress was reported by the pilot 1 hour before the plane landed (I’m impressed with that) and on arrival the dog was evaluated at US CDC’s Chicago Port Health Station.
  • An infectious cause of abortion was on the list of possible causes, so testing was performed and Port Health Officers interviewed the crew and passengers that sat by the dog, paying particular attention to whether there were any who were pregnant, or children.
  • Potentially contaminated areas of the plane were disinfected (but that’s tough to do effectively with the types of surfaces found on a plane…) and presumably the plane took off with a few hundred more passengers not long after.
  • Both dogs were sent to a veterinary clinic, where Dog A aborted a fourth fetus. Both dogs were otherwise stable, though underweight. Dog B was also pregnant (more “puppy mill” alarm bells going off).
  • Samples were collected from both dogs for testing. Dog A was positive for B. canis and was euthanized at the request of the importer. (I suspect that’s because one of the key components of treating this infection is spaying the dog, and spayed dogs don’t generate profitable puppies.) Testing for B. canis can unfortunately be complicated by a few factors (veterinarians can access fact sheet on B. canis from the Ontario Animal Health Network).
  • Dog B was negative for B. canis, but if she was only recently exposed (e.g. from the other dog), we’d have to wait at least a month and retest to have confidence that she wasn’t infected too. So, rather than isolate the dog and retest it, the importer decided to ship the dog back to Poland (making it two long trips for a young pregnant dog… not great from the dog’s standpoint). The dog was then presumably lost to follow-up. (Unfortunately it’s quite plausible the dog ended up back on another plane to the US, Canada or elsewhere the next day. Who knows.)
  • Five people (3 crew and 2 passengers) were determined to have had high risk exposures because of direct contact with Dog A (including one crew member who apparently tried mouth-to-mouth resuscitation on one of the aborted puppies). Staff members at the veterinary clinic were also evaluated, including two who were pregnant. Exposed people were provided with information about the risks, told to monitor for symptoms of disease and to discuss post-exposure prophylaxis with their healthcare providers. It’s not clear if any got post-exposure treatment.
  • The CDC estimated the cost of their investigation at over $22,800 USD. The importer was on the hook for $16,500 in veterinary expenses for both dogs. They also estimated costs to the veterinary clinic (where the dogs took up space while being isolated that could have been used to house clinical patients) at $10,000. Overall, the authors estimated the total cost of the entire incident to be close to $50,000. It would have been higher if the importer hadn’t shipped Dog B back.

Fortunately, it doesn’t sound like anyone got sick as a result of exposure to these dogs.

There’s an inherent risk associated with dog (or any animal) movement, but there are also things that increase the risk. Here, there were several red flags that this was a higher risk situation. The combination of a high risk dog in a densely packed airplane is a recipe for problems.

This scenario raises another question too: what if the dog hadn’t aborted on the plane? If it had happened the day after, it’s quite possible that no one associated with the flight would have known, no one would have investigated, and there’d be more ongoing exposure risk to these dogs in the community. While it was a bad luck situation for the people on the plane, perhaps it was a good luck situation more broadly.

Regardless, this type of incident won’t be entirely prevented by the new US CDC canine importation rules (which mainly target rabies risk), but increasing the number of hoops and health measures for imported dogs will probably have some impact, as it’s a disincentive to ship dogs to the US. Unfortunately for us north of the border, shipping those same dogs to Canada is much easier and, as we saw when the US increased dog import restrictions a few years ago, that meant more dogs of questionable health status coming to Canada instead.

The paper’s conclusion raises some good points: “In conclusion, a multifaceted approach is needed to appropriately reduce public health risks posed by B. canis in imported dogs that are sexually intact. Efforts that could reduce the public health risks include: strengthening import surveillance; development of better screening and diagnostic tests for B. canis; increased brucellosis screening and quarantine by importers, breeders and organizations involved in the sale or adoption of dogs; and increased awareness by owners of the importance of procuring dogs from responsible sources. Airlines may also consider adopting policies that promote readiness to respond to ill animals and to prevent the transport of pregnant dogs to reduce the risks posed by B. canis.“


Some dogs cross the border between the US and Canada because the closest veterinary clinic (or closest referral/specialty clinic) is in the other country. In the past, that hasn’t usually been an issue because of the ease of dog movement between Canada and the US. That’s going to change very soon due to new dog importation rules for the US that take effect on August 1.

We’re trying to clarify a few things but here’s the quick run-down:

1. Taking a dog from Canada to the US for veterinary care

Taking a dog to the US for elective veterinary care is still fairly easy, it’s just a matter of treating it like any other Canadian dog going to the US. There will be more paperwork and planning involved, but it’s perfectly feasible, as long as the dog is at least 6 months old.

There’s always some concern about the “must appear healthy” clause in these cases, but dogs going for elective veterinary care usually do appear healthy. That’s also been part of the rules for a while now, and there’s typically been flexibility for dogs that are going to the US for veterinary care.

Taking a dog to the US for emergency care will be an issue. It’s pretty much impossible under the new rules, at least at this point. One barrier is getting all the paperwork done fast enough. The bigger issue is the CDC Import Form that must be done 2-10 days before the dog hits the border. That 2 day minimum is not a timeline that fits with emergencies.

And if the dog is less than 6 months of age… forget about it. They can’t go. (But ideally we’d see an exemption put in place for dogs going directly to and from a veterinary clinic, even if they don’t meet this age requirement.)

2. Taking a dog from the US to Canada for veterinary care

This is something I’ve been working on this morning, since we see US dogs both at our veterinary hospital and at other practices near the US border in Ontario. It’s a work in progress.

As for dogs going to the US, elective cases are easy, as long as they are over 6 months of age. They just need to get the paperwork done.

Emergencies are more challenging. It’s possible under the new rules (unlike Canadian dogs going to the US), because the issues arise when the dog needs to return to the US. A US dog can still get across the border into Canada immediately (with a rabies vaccination certificate). Then there’s time to get the return paperwork together, as long as the dog stays in Canada for at least 2 days (because of the 2-10 day submission period for the CDC Import Form it needs to go back). The bigger issue is the other paperwork. For dogs leaving the US and coming home, the pathway that’s set up is for elective scenarios, where the paperwork is done in the US before the dog leaves. If that’s not done, as would likely be the case in an emergency situation, it looks like when the dog returns, it might be treated as a dog not vaccinated in the US. That means a separate set of paperwork and potentially a need for rabies vaccination in Canada. The rules aren’t really clear for situations like this, so we’ll need some more details to sort that out (or someone with better interpretive powers than me).

Hopefully we’ll get some clarification and maybe some recognition of these issues and ways to address the unintended consequences of the new rules.


The US has been tinkering (for the good) with their canine importation rules for the past few years, primarily in response to concerns about importing rabid dogs. The US had just announces a new set of rules for importing dogs, that come into effect August 1, 2024. The new rules aim to provide more assurance that imported dogs are properly vaccinated against rabies, to reduce the risk of falsified documents (a known problem in many of these cases).

The rules vary depending on the rabies risk in the country of origin, but they’ve increased the requirements for all dogs across the board(er). That’s what’s going to catch some people off guard, because it’s a pretty significant change for some. Specifically, people don’t always think about travelling between Canada and the US with their dog constitutes “importation,” but even if they’re just visiting family for a few days or travelling south for the winter, if they bring their dog, it’s still importation.

As per the new rules:

ALL dogs must:

  • Be at least 6 months of age at the time of entry or return to the US
  • Have a microchip
  • Appear healthy on arrival

A CDC Dog Import Form (including a picture of the dog) must be submitted 2-10 days before arrival.

Additional requirements depend on where the dog has been in the past 6 months. I won’t go over all of the rules, but will highlight two of the most common scenarios involving movement of dogs from Canada to the US. They aren’t too onerous, but will require some planning and effort.

Dogs previously vaccinated against rabies in the US:

In addition to the requirements for all dogs, these dogs must have one of the following:

  • Certification of U.S.-Issued Rabies Vaccine form that was endorsed by the USDA before the dog departed the US (signed by the USDA-accredited vet that gave the rabies vaccine)
  • USDA APHIS-endorsed export health certificate

Note: The dog’s first rabies vaccine must have been given at least 28 days before arrival, so you can’t get away with a last minute rush to the veterinary clinic right before you pack your bags.

Dogs from Canada (that haven’t been to a high-risk country in the past 6 months):

In addition to the requirements for all dogs, these dogs must have one of the following:

  • Certification of Foreign Rabies Vaccination and Microchip form: This requires either a rabies titre (blood test to check the rabies antibody level) or veterinary records from the previous 6 months that include rabies vaccination information. Records must be endorsed by an “official veterinarian.”
  • Certification of US-Issued Rabies vaccination form that was endorsed by the USDA before the dog departed the US
  • Valid USDA export certificate that lists Canada as the destination and the dog is returning directly from here.
  • Certification of Dog Arriving from a DMRVV-free or Low-Risk Country into the United States form, endorsed by an “official veterinarian” from the exporting country and veterinary records  for the dog from Canada for the 6 months before traveling to the U.S.
  • Export certificate that documents the dog is at least 6 months of age, lists the dog’s microchip number, and has been endorsed by an “official veterinarian” of the exporting country, and veterinary records for the dog from the exporting country for the previous 6 months.

In both instances, they mainly want solid proof that the dog is vaccinated against rabies and that any records truly belong to the dog.

There are even more requirements if the dog has been to a higher-risk country.


We know a lot more about the situation with H5N1 influenza in dairy cattle than we did a couple of weeks ago, thanks to ongoing research and (more importantly) better disclosure of information that has been held pretty tight up until now.

Current situation with infected dairy herds in the US

Rather than focus on numbers, I’d rather just say it’s on lots of dairy farms in lots of US states. As discussed below, this is likely much more widespread than the official numbers suggest, so those numbers aren’t very useful.

When/where the outbreak of H5N1 flu in dairy cattle likely started

Genomic data suggest that the current H5N1 spillover into cattle likely occurred much earlier than was known. It was unlikely that we’d caught the first (or at least one of the first) affected dairy herds in March. Rather, it looks like the spillover into cattle in the US likely occurred in December 2023 (but maybe as early as October 2023). That was presumably from a single bird-to-cow spillover, with an associated mutation of the virus that made it better able to infect cattle. Where did this happen? Somewhere in the US is all I’d say at this point.

After that first bird-to-cow transmission, subsequent transmission is thought to have been from cow-to-cow, with spread on farms through contaminated milk, and spread between farms and states through movement of cattle. Given the limited evidence of virus in respiratory samples from cattle and the large viral load in milk, spread on farm is probably through human-associated milking practices, based on the high likelihood of tracking milk between cows during milking.

There has been subsequent spillover of this H5N1 strain from cattle into cats and poultry flocks. Some farm cats have had severe disease (and even died) from the virus; farm cats could possibly be good sentinels in this situation (i.e. if you see dead cats (more than usual) on the farm, consider looking for influenza in the cattle). Spillback into wild birds is also a concern, since if this strain goes back into wild birds, the situation becomes even harder to control: We can much more effectively monitor and control cow-to-cow transmission than an ongoing risk of exposure from wild birds (that also don’t respect political borders).

What do we know about potential for human infections with H5N1 flu from cattle?

The human case of H5N1 flu associated with contact with dairy cattle in Texas raises concern. Although it is the only one identified so far, testing of exposed people (e.g. farm workers) has been limited. There are lots of anecdotal reports of farm personnel in the US avoiding testing and not telling anyone when they are sick. That’s in part because a lot of dairy farm workers in the US may be undocumented and therefore have concerns about getting on the radar of anything related to government.

There are some interesting aspects of that one human case, too. The person was a dairy farm worker so there’s obviously a link to cattle, but surprisingly there was no testing of cattle on that particular farm. The strain that caused the human infection is genomically a bit different from the main strain circulating in dairy cattle, which supports concerns that there might be more widespread and ongoing transmission than we’ve realized. With more time and transmission, the the virus gradually accumulates genomic changes. The strain from the person had one of the more common mutations (PB2:E627K) that helps the virus adapt to humans; that raises a bit more concern, but it’s still a long way from being a “human-adapted” virus. Odds are that strain died out in that person, but it shows how there could be cow-to-human transmission and that there can be continued concerning mutation of the virus when this happens.

The risk of H5N1 flu (or lack thereof) in pasteurized milk

An earlier report that described finding H5N1 virus material in commercial milk samples caused a stir, since about 20% of samples were positive. Most of us were pretty unconcerned since the test used (PCR) also detects dead virus, and we’ve been confident that pasteurization will kill flu. Subsequent testing confirmed that live H5N1 virus was not present in those commercial milk samples, which is good news. The most important aspect of the report was that it supported the thought that this virus must be MUCH more widely established in US dairy herds than current testing suggests.

Concerns about raw milk remain, but there are lots of other infectious diseases risks from raw milk regardless (so just don’t drink raw milk).

Is there a risk of H5N1 flu in beef?

There’s not much reason to think that contamination of commercial beef with H5N1 flu would be common or high level, but a small initial study didn’t find evidence of the virus in beef samples.

What is the risk of H5N1 flu in cattle in Canada?

We don’t know. Since the virus was probably flying under the radar in US dairy cattle for months, we have to be careful thinking “we haven’t found it in Canada, so we’re good”. It might not be here. It might be here and we don’t know. That’s why we have to look.

If H5N1 flu has not ye gotten into the Canadian dairy herd, can we prevent this from happening? Maybe.

  • If the virus is only moving via cow-to-cow spread in lactating cattle, we can contain that.
  • If spread is via cow-to-cow transmission beyond lactating cattle, that makes control harder, because that’s more cattle and recent restriction on importation of cattle into Canada are focused on lactating cattle
  • If this strain of the virus spills back into wild birds, then we’re in trouble. Wild birds don’t respect borders so it would probably just be a matter of time before it found its way onto a Canadian dairy farm and into a Canadian cow.