I was at the airport the other day and, as per usual, there were lots of dogs in transit 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 people looked (a key reason 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 shit show for contact tracing” trickled in.

The vast majority of random dog-human encounters are benign. A miniscule fraction aren’t, and those can be a real pain. Tracking down human contacts in a specific area is tough, but we can flood local social and conventional media with “if you might have touched this dog, give us a call” notices. If we’re just targeting an exposure in a town where most people are still around, that’s one thing. When we’re dealing with exposure of people that hopped on planes to various cities and countries….well….that’s not great.

A recent paper in Zoonoses and Public Health (Williams et al 2024) describes an easier to contain situation, exposure on an airplane. The bug? An uncommonly discussed but concerning bacterium, Brucella canis.

Brucella canis is a bacterium that kind of flies under the radar. It’s more common that most people recognize, as we found out a few years ago. In dogs, it can cause a range of diseases, with reproductive disease being a big 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’s the rundown from the paper…

A 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 2nd 12 month old dog (Dog B) from the same facility was on the same plane, flying in cargo.

  • It’s stated that they were to be imported by a breeder but importing a pregnant young dog is sometimes a way to bypass the ban on importing lucrative puppies, so I have to wonder if this is really a breeder purchasing new, high quality breeding stock or a puppy mill situation. The fact that they were French bulldogs, a common and lucrative puppy mill breed, heightens my concerns]

Regardless, the dog’s distress was reported by the pilot 1 hr before the plane landed (I’m impressed with that) and the dog was evaluated at 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 were pregnant or children.

Potentially contaminated areas of the plane were disinfected (but that’s tough to do effectively with the types of surfaces on a plane) and presumably the plane took off with a few hundred more people not long after.

Both dogs were sent to a vet clinic, where Dog A aborted a 4th fetus. Both dogs were otherwise stable (yet underweight). Dog B was also pregnant (more “puppy mill” alarm bells going off).

Samples were collected from both dogs. 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.

Dog B was negative but if exposure was recent, we have to wait at least a month and retest to have more confidence. So, rather than isolate the dog and retest it, the importer decided to ship the dog back to Poland (great…two long trips for a young pregnant dog), and it presumably was lost to follow-up. It quite plausibly 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 the aborted pup). Staff members at the vet clinic were also evaluated, including two that 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 cost of all this?

CDC estimated their costs at >$22,800 USD. The importer was on the hook for $16,5000 in vet expenses for both dogs. They also estimated costs to the clinic (where the dogs took up space while being isolated that could have been used to house clinical patients (at $10,000). Overall, they estimated the cost of this 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.

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

This scenario raises another question….what if the dog hadn’t aborted on the plane? If it had happened the day after, it’s quite possible that no one would have known, no one would have investigated and we’d have more ongoing exposure risk. So, it was a bad luck situation for the people on the plane but maybe a good luck situation more broadly.

Regardless, this type of incident won’t be prevented by the new CDC importation rules (which mainly target rabies) but increasing the number of hoops and health measures will probably have some impact as it’s a disincentive to ship dogs to the US. Unfortunately for us north of the border, shipping them to Canada is easier and, as we saw when the US upped their rules a few years ago, dogs of questionable health status came to Canada more often because we were more welcoming.

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.

Dairy cows produce a lot of milk. That’s great when you’re producing it for sale. It’s not great when you have to get rid of it.

It’s clear that this H5N1 flu virus has an affinity for the udder, and a lot of virus is shed in the milk of infected dairy cattle. It appears that it’s often obvious when a cow’s udder is affected: milk production drops and the milk looks abnormal. Abnormal-looking milk is disposed, so it doesn’t go into the human food chain. With the odd sick cow on a farm, diverting this amount of milk isn’t hard. With a lot of sick cows on a farm, it becomes more problematic – it can be a lot of milk.

There’s also the issue of the clinically healthy cattle on farms with H5N1 infected cows. At this point, we don’t understand enough about the virus in cattle to know if exposed cows could be shedding the virus in their milk before they look sick. With influenza infections in other species, we know that virus shedding in respiratory secretions is common prior to the onset of illness. This “pre-clinical” period is a big problem when it comes to infection control, because individuals can be infectious before anyone has any clue they’re infected.

  • We don’t know yet if this pre-clinical / sub-clinical virus shedding happens in cattle.
  • We might be lucky if when virus is shed in milk, it’s always identifiable by visible changes in the milk.
  • We might not be lucky if virus can also be shed in milk that looks normal (and our luck when it comes to infectious diseases hasn’t really been great in the 2020s).

If cattle have a period where they are shedding virus in milk without any outward signs of illness, we have to consider what that means. We’re pretty confident that pasteurization is highly effective against this virus (the other day @SafeFoodCanuck and I wrote a commentary on why the risk of H5N1 flu from pasteurized milk is likely still low in The Conversation). However, raw milk still poses a risk, and we also need to think about exposure of people who handle milk before it’s pasteurized. That raises the question about whether all milk from infected herds should simply be dumped. That’s a lot more milk. I can argue both ways at this point.

Regardless, with influenza circulating in dairy cattle, we’re going to have to dump milk. Maybe a lot of it. An unfortunate waste to be sure, but dumping that much milk is also not as simple as it sounds.

Dairy farms aren’t plumbed into municipal wastewater systems, and don’t have their own septic systems to handle waste. You can’t just flush hundreds of litres (or more) of milk down a drain. It usually goes into manure pits or lagoons, then is eventually spread on fields. That’s fine for the relatively small volumes of milk that typically are dumped from sick cows, but when we have large amounts of milk potentially contaminated with a concerning virus like H5N1 flu, what do we do with it all? There are a few options, but none are great:

  1. Dump it in the manure pit / manure lagoon as usual.
    • That’s the easiest and most practical means of disposal. However, at this point we don’t know how long the virus would survive in a manure pit / lagoon, or on a field after the manure is spread. So this could result in exposure of lots of wildlife, including more mammals (bad for continued mammalian transmission and adaptation) and wild birds (bad for spillback into birds and subsequent transmission over wide areas).
  2. Pasteurize the waste milk before it’s dumped into the pit / lagoon.
    • Some farms have small pasteurizers on site for milk that’s used to feed to calves. Probably no farms have pasteurizers that could handle their full production capacity, so this isn’t a realistic option if all the milk has to be dumped.
  3. Send the waste milk away for disposal.
    • Sure, farms could conceivably contract someone to come pick up the waste milk and dispose of it another way (perhaps into a wastewater treatment plant?). But, that’s not cheap or easy, and might open up a whole new can of worms.
  4. Cull the affected cows (so they’re not producing milk that needs to be dumped).
    • Not a viable option for many reasons.
    • Animal welfare is one reason. Killing an animal that has a short-term, usually mild, infection is extremely hard to justify.
    • Economics is another reason. Individual dairy cows are valuable animals; dairy cows don’t start milking until they’ve had their first calf, which is usually around 2 years of age, so each one represents a significant investment of time and resources. Some can also have very high genetic value. You can’t just clear out a herd of dairy cattle and repopulate the farm next week and be up and running, like you can with poultry.
    • In addition to the animal welfare issues and economic costs, if the cows were culled then farmers would also need to figure out what to do with hundreds or thousands of dead animals.
    • Last but not least, if a dairy farm was depopulated but the virus is still circulating nearby or present in the environment, any new cattle brought to the farm could be re-infected at any time, and it would all be for nothing.

There might be other options, but none jump to mind as practical to me. For example, there might be some other potential on farm virus inactivation approaches, but the cost, logistics and timeframe would likely not make sense in this scenario.

So, we’re most likely left with the option of dumping the contaminated milk into manure pits, going on the assumption (hope) that the virus will die quickly (since it’s not very tough) and it won’t be a source of further spread. It’s not an unreasonable approach, and is probably the least-bad way, but isn’t ideal.

When we talk about “worms” in dogs or cats, we’re usually talking about parasites that can infect pets or (less commonly) that harbour other pathogens. However, there are also certain worms that can cause other problems for our furry friends. For example, the hammerhead flatworm (Bipalium adventitium) produces a very potent paralytic neurotoxin, tetrodotoxin, which is the toxin famously associated with human deaths from improperly prepared pufferfish.

(Disclaimer: I’m neither an entomologist nor a toxicologist, so I’m drifting out of my lane here.)

Headlines about this worm can be pretty sensational…

…but the buzz may be greater than the actual risk.

There’s been another round of new reports recently following identification of the hammerhead worm in southwestern Ontario this spring, but it’s not actually a new problem. The first report of these worms in Canada actually dates back to 2018 when they were found in Montreal. Some interesting crowd-sourced tracking of hammerhead worms shows that they’ve likely been present in Ontario for at least a couple of years (including just down the road from me).

What’s the actual risk from hammerhead worms in the environment?

It’s hard to say. These worms are small and the amount of tetrodotoxin toxin in them is limited. I haven’t found good data on how much worm exposure would actually pose a health risk to an animal. Hammerhead worms are present in other parts of the world, yet I can’t find any reports of disease in humans or animals linked to them, despite lots of media reports saying “they’re toxic to kids and pets.

As they say, “absence of evidence” isn’t “evidence of absence” but a lack of reports of something as dramatic as acute paralysis suggests that the risk from exposure to these worms is limited. I’d still avoid eating hammerhead worms of course, and I wouldn’t dismiss the potential that ingestion of one or more worms by a small animal (and we’ll include kids in the small animal definition here) could cause a problem.

What should pet owners do?

  • Relax (as is often the first step with topics like this).
  • Ultimately there’s not a lot that can be done specific to these worms. The main prevention measures are awareness and avoidance. We’re concerned about the potential impact of ingesting or touching hammerhead worms, so try to avoid any direct contact with them; unfortunately that may be easier said than done in some cases, especially with dogs like mine (Labradors) that consider anything (organic or non-organic) to be a viable food source.
  • The good news is that these worms are pretty obvious if you find one (since they have a very unique head). The bad news is, as with most wildlife, if you see one, you can be pretty sure there are lots more in the area that you don’t see. If you come across a single hammerhead worm, they are probably already well established in your area.
  • If you know that this worm is in a particular area, avoid the area if you can, or at least prevent uncontrolled (e.g. off-least) access to it by your animals. Walking a dog through an area where hammerhead worms are present is low risk. Letting a dog root around in areas like that increases the risk of worm contact. Knowing the dog’s behaviour (and any tendencies to eat random things on the ground) also helps with the risk assessment and determining how strict to be about controlling animal access.

An advantage we have in Canada is our (historically) tough winters (yes, there is a bright side to really cold weather), because cold weather kills a lot of parasites. However, we’re losing some of the protective effects of winter with climate change. We’re seeing the potential for expanding ranges of various critters (large and small) and a greater ability of those critters to survive Canadian winters. Given the number of reports of hammerhead worms over the past few years, and the massive underestimation of how common any particular worm is based on the number of reports, we have to assume that hammerhead worms are well established in various parts of Ontario and Quebec (and maybe beyond), and that they’re probably here to stay, at least in some areas.  Common sense would dictate that we should raise awareness and take some basic measures to avoid contact with these worms. We should probably also add Bipalium-associated tetrodotoxin exposure to the differential diagnosis list in the very rare situation when we see unexplained acute paralysis (or weakness) in an animal (or child) with potential exposure to worms. My guess is that this is a minor- or non-issue around here, but more information would be nice.

Image from: https://en.wikipedia.org/wiki/Bipalium_adventitium – by Sanjay Acharya – Own work, CC BY-SA 4.0


As H5N1 avian flu continues to infect dairy cattle in the US, and because we still don’t really understand some important aspects of how it’s being spread on and between dairy farms, questions keep coming up about the risk of transmission of this strain of the virus from milk.


  • Relax. There’s really nothing to be concerned about.

For a more detailed answer, let’s start with a quick refresher on pasteurization:

Pasteurization was a revolutionary public health intervention that had huge impacts on human health. It was first developed in the 1800s, and ultimately became used as a way to practically and effectively reduce the risks of infectious diseases transmitted through milk, and extend the shelf life of milk (and other food products). Typically pasteurization of milk involves heating it to 72 Celsius (162F) for at least 16 seconds before cooling, or heating it to 63C (145F) for at least 30 minutes before cooling.

Does pasteurization kill H5N1 influenza virus?

We don’t have specific data on the effectiveness of pasteurization to inactivate the current circulating strain of H5N1 in milk. The US Centers for Disease Control has stated (repeatedly) that “… pasteurization has continually proven to inactivate bacteria and viruses, like influenza, in milk – which is an accurate statement. It doesn’t specifically say we know it kills H5N1 influenza, but we are confident in it because we know what pasteurization does to other organisms, including those that should be harder to kill than a flu virus

Why can’t we get a defiinitive answer to this question?

There wasn’t a lot of interest in testing the efficacy of pasteurization to inactivate flu in milk until now. We haven’t recognized cattle as a relevant source of flu virus in the past, so it’s not something anyone would study (or, probably more importantly, get funding to study).

Is testing now being done to confirm the efficacy of pasteurization against H5N1 flu?

Hopefully. We’ve had some discussions about it here, but it’s complicated because of the need to handle this virus using containment level/biosafety level 3 conditions. Most laboratories that work with milk pathogens (or most infectious diseases in general) use a lower level containment (Level 2); level 3 labs are much less common (because they’re also much more expensive to run). We’ve been arguing for years to get one at the university here… despite a series of new threats over the years when we’ve said “this is why we needed Level 3 lab!” no one’s ponied up the money; so we still can’t do this kind of testing here.

There are Level 3 labs elsewhere, and while they tend to be heavily used due to their limited number, presumably (hopefully) someone is working on this now. With the right level of containment, it’s not too complicated to do.

Does pasteurization work against other viruses?

That’s not been studied very intensively, since the main public health risks from milk are bacterial. Antibacterial effects of pasteurization are well described, and pasteurization methods are designed to kill those important bacteria. The good news is that influenza viruses tend to be much less stable than many of the bacteria that pasteurization is designed to kill.

What do we know about pasteurization and flu at this point?

We know that flu viruses aren’t very heat tolerant. That’s a good start. Based on that, and knowing how pasteurization is performed, we can come up with some reasonable interpretations of the available information.

There is a study that examined thermal tolerance of an older H5N1 strain in chicken meat (Thomas & Swayne, 2007) – thanks to @SafeFoodCanuck for sending this. It’s not the same as milk or pasteurization, but still gives us useful heat tolerance information. It showed that at 61C (lower than milk pasteurization temperature), the D value (the time required to eliminate 90% of viable virus) was 33 seconds. At 70C, the D value was 0.28 seconds. When we consider that milk pasteurized at 72C is held for at least 16 seconds and milk pasteurized at 63C is held for at least 30 minutes, that provides pretty good confidence that the process will inactivate flu virus.

What about that study that showed pasteurization wasn’t good for killing viruses?

There’s been a lot of social media buzz about a study that showed incomplete inactivation of foot and mouth disease virus (FMDV) in milk by pasteurization (Tomasula et al. 2007). However, FMDV is a very different and much tougher virus: it’s a non-enveloped virus that’s tolerant to lots of stressors, including heat.

Even so, there was still pretty good reduction of the viral load in milk; it’s not like it didn’t do anything. They found “residual infectivity was still detectable for selected pasteurized milk samples,” so pateurization didn’t consistently and completely eliminate this really tough virus, but still had a pretty big effect. They concluded “Although HTST pasteurization did not completely inactivate viral infectivity in whole and 2% milk, possibly because a fraction of the virus was protected by the milk fat and the casein proteins, it greatly reduced the risk of natural transmission of FMDV by milk.

As a result, we can’t use that study as an indication that flu virus won’t be inactivated by pateurization. Actually, I find it a bit re-assuring that there was such an impact on a much tougher virus like FMDV.

Beyond pasteurization, there are other layers of protection when it comes to the risk of transmission of H5N1 through milk:

1) Diversion of milk from sick cows

Dairy cattle and their milk are very closely monitored. Milk from sick cattle should not make it into the food chain. It’s usually discarded. That’s not a complete guarantee that all potentially contaminated milk will be discarded, since we don’t know whether cattle infected with H5N1 can shed the virus before they show signs of disease or if there can have subclinical infections (infections that don’t result in apparent disease). However, such cases would presumably be uncommon and contribute a proportionately very small amount of milk on any farm.

2) Dilution

A typical glass of milk doesn’t come from a single cow. All the milk from dairy cows on a farm that’s acceptable for human consumption first goes into a big bulk tank on the farm. That milk is then picked up and loaded into an even bigger tank on a truck. Then, it gets mixed with more milk from other farms, so there’s a massive dilutional effect. If a small amount of flu virus got into the bulk tank on farm, it would end up at a really, really low (and probably irrelevant) concentration in milk, even before it got to the pasteurization step.

Can we guarantee that there’s no ability of H5N1 to evade pasteurization?

No, but there’s not really any basis for concern. Personally, if/when H5N1 hits dairy cattle here in Canada, I’ll still confidently give my kids (pasteurized) milk.

While details about the ongoing H5N1 influenza outbreak in dairy cattle in the US have been really sparse, a new pre-print about the early H5N1 cases in Texas dairy herds posted on bioRxiv earlier this week (Hu et al. 2024) has provided some important information.

  • A pre-print is a non-peer reviewed version of a paper that’s typically posted online on an open access server. This situation highlights the value of being able to release pre-prints: it allows good labs to get good data out faster to let people know, and to get quick feedback before (or while) going through the process to have the study peer reviewed and published in a properly refereed journal. Unfortunately, there’s also a lot of complete crap on pre-print websites, posted by dodgy groups to get publicity or advance their specific cause, when there’s no hope of their material ever being published in a reputable journal.

We have to be careful about relying on information from pre-prints, but with this research group (from Iowa State University) and the quality of the data in this report, I’m not worried about this one. Here are some highlights:

H5N1 disease in dairy cattle

The report describes what has largely been reported to date for clinical signs in cattle: Infected animals had decreased appetite, decreased milk production and abnormal-looking (thickened) milk. Clinical signs peaked about 4-6 days after they started, and cows got better on their own within 10-14 days. That’s pretty “flu-like” when you consider how other strains of flu present in other species (including humans). Affected cows were most often older cows in the middle of their lactation periods. The problems seem to have started in late February 2024.

H5N1 detections in other species on farm

Dead birds and cats were reported on and close to affected farms. Not long after the virus was detected in the cows, it was also isolated from a local skunk, and then from a dairy worker with conjunctivitis. The pre-print mentions virus from “humans in Texas during March 2024”, but to date only one human infection associated with the cattle outbreak has been reported. It’s not clear whether there are more unreported cases, or whether “humanS” was a typo. (Let’s hope it’s a typo and there was just one human infection.)

Identification and characterization of H5N1 flu virus from dairy farms

The virus was identified in milk samples from affected cattle and in lung and brain samples from dead farm cats. The virus was classified as clade, which is the main clade that’s been circulating in wild birds in the area (and across North America). The genomes of the viruses from the cattle, cats, local wild birds and the person with conjunctivitis were nearly 100% identical and shared a common ancestor, confirming they are linked (see figure below and at the bottom of the post).

More detailed study of the virus genome indicated that it was a reassortment of the H5N1 genotype B3.7 and a low pathogenicity avian influenza (LPAI) virus. The B3.7 ancestor emerged in 2023 from an H5N1 virus and a different LPAI strain. This highlights the tendency of influenza viruses to evolve and mix with other flu viruses (reassort), and is a major reason we do surveillance testing of flu viruses. This current strain is a bit of a nothingburger for humans, but we’re worried about what it could become if it keeps spreading, spilling over to other species and reassorting.

Surveillance for important genetic mutations in H5N1 flu from cattle

This is still an avian influenza virus. It can spillover into mammals but isn’t (yet) well adapted for sustained transmission between mammals. There are some genetic markers in flu viruses that indicate a greater ability to infect mammalian cells and therefore spread between mammals, including people. The H5N1 isolates from the cattle and cats on these farms had some changes that might increase their ability to infect people, but none contained the mix of other mutations that are also important for this. So this strain, while potentially able to spread cow-to-cow via milk, doesn’t have the range of genetic signatures (yet) that would suggest it’s going to be a problem for humans.

It’s good to see some more details about what’s been going on, and it’s good to see that the virus hasn’t yet evolved to something that would be expected to cause widespread problems in mammals. The lack of epidemiological data and information about how this virus has spread between dairy farms is still a big issue. It’s been a textbook example of how NOT to do outbreak communications, which is incredibly frustrating, and has hampered risk assessment and contingency planning. We’ll hopefully get more relevant information in the near future, as some scientific reports like this one start to come out.

This is less of a content update and more of a “here’s what I really want to know,” but it’s still an update, I guess. It’s a bit hard to say where we currently stand with this problem, at least in terms of the most important aspects. That’s not uncommon with emerging diseases, but we’re in more of an information vacuum than I’d like.

What do we know about the current outbreak of H5N1 influenza in dairy cattle?

The case count keeps going up, but there aren’t a lot of details available about the new cases; some media reports about new cases have actually been quicker and more comprehensive than USDA reports. Currently, I think we’re at 21 confirmed infected dairy herds in 7 states. I’m guessing there will be more affected herds, possibly in more states, that we’ll hear about in the near future, as there’s no doubt some reporting lag, there could be continued spread, and in some cases it may be a matter of slower identification of ongoing problems. The latest USDA map is below, but it’s missing the most recent state to identify a dairy herd with H5N1 influenza, North Carolina. The total number of animals affected isn’t reported, but that’s not as important as the farm and state numbers that tell the bigger picture.

What do we not know about the current outbreak of H5N1 influenza in dairy cattle?

Hopefully it’s more of a “what’s not being released” versus “what’s not known” – presumably there’s more information somewhere than what’s been released. That’s not uncommon in a situation like this. Sometimes there are good reasons for certain information to be kept within certain circles, such as information that may be very sensitive or data that are still just preliminary. Sometimes, the information would just be for interest’s sake, but not particularly critical for the average person or stakeholder, so there’s no urgency to release it. But all too often, there’s too much restriction, resulting in information that could inform action being withheld or only slowly released. With emerging diseases, we need balanced and effective action, and if we lack important details, we can’t tailor the appropriate response.

What do we need to know about the current outbreak of H5N1 influenza in dairy cattle?

Inter-farm transmission: We really need to know how this virus is spread.

  • If spread between farms is based on movement of cattle, we can intervene through changes in how cattle are moved and how they are handled after being moved (e.g. isolation requirements at a new farm). We can think about testing associated with cattle movement (e.g. if you’re buying a cow from another farm, test if before it leaves +/- after it arrives) and biosecurity practices to prevent a person from tracking the virus from another farm. This scenario would also suggest that aggressive action now could result in at least temporary eradication of this strain in cattle. Since we don’t have (as far as we know) long term carriers of flu, if we contain the virus on an infected farm, it should die out. If we contain transmission on farms and transmission between farms, we can (theoretically) get this flu strain off farms altogether.
  • If the cases that are occurring on different farms across different states involve repeated wild bird-to-cow transmission, our risk and management approaches are different. It would mean that keeping a closed herd (not allowing new cattle in, and using good biosecurity practices with workers and visitors) would not be enough to prevent exposure. Even if we locked down the affected farms, cattle could still be exposed by contact with wild birds. Preventing exposure to wild birds is really tough on a dairy farm.

From a Canadian standpoint, this is also really important for our risk assessment. If it’s just farm-to-farm spread, we have limited risk of it being tracked across the border if the industry adheres to good basic biosecurity practices. If there’s ongoing spread by birds, we will almost certainly have it up here before long as the spring bird migration continues.

This also ties into the risk to beef cattle. If this strain is being spread by wild birds, beef cattle will be exposed. If it’s being spread via dairy cattle, beef farms will have very limited risk (apart from buying in surplus dairy calves).  

Genomics: Understanding the makeup of the virus is important for a few reasons.

  • Does the strain infecting dairy cattle have more mammalian adaptation markers? There are some genetic changes that make flu more able to infect and transmit between mammals, including people. We want to know if these are cropping up as the virus gets transmitted to more and more cows. The virus is currently a long way from being able to readily infect people, but we need to keep looking to know it’s not continuing to change.
  • Are the strains found in cattle in different states the same? And are they different from birds in some of those areas? This would help sort out the route of transmission. If the virus in cattle from different states is basically identical, and particularly if the strain found in birds from each of those areas is different, that suggests it’s spreading via movement of cattle. If there’s more genetic variation between isolates from cattle in different states, and the strain in one herd is more like those from birds in that state than cattle in another state, it suggests that bird-to-cow transmission is playing a bigger role.

Intra-farm transmission: We could also use more details about how the virus moves around within a farm. It’s been reported that cows seem to mostly shed virus in their milk (with very little in their respiratory secretion), and that transmission between cows on a farm is therefore likely via milking equipment. Having some on-farm epidemiology to support this would be nice. If milk is the main source of transmission, there might be some corresponding disease patterns in the milking cows. I’d also expect to see no (or very few) infections in non-milking cows (i.e. younger animals (heifers) that have not yet calved, older cows that are between lactation cycles). Young calves could also be an interesting part of the story, looking at whether there have been infections in this group, and if so, whether those calves have been fed discarded milk from potentially infected cows (milk from sick cows is not allowed to go into the bulk tank for shipping). If infections are found in non-milking animals, we need to figure out how they might have been infected, because it helps us figure out if the infection control emphasis on farms should be with milking practices or if we have other issues with which to contend.

Cattle illness: More information about what clinical signs H5N1 virus causes in cows would be helpful. Some information has been released, but it’s sometimes vague, and I’ve seen conflicting information about illness being very mild versus fairly severe but short-term, and very obvious mastitis. Knowing the typical signs and range of disease is important for farmers and veterinarians on the lookout for disease. More details about test results are also important, particularly whether the early reports of the virus being present at high levels in milk but not in respiratory secretions are holding. That has major implications for transmission, and therefore infection control measures.

Infection in other species: I assume there’s been testing of other animals on these affected farms. It would be useful to know the numbers and results; negative results are as useful as positive results. It was reported that there were affected cats on at least one of the Texas farms. Is that unique or common? Have there been other positive farm cats? Were they sick, and if so what signs of illness did they show? If not, how many cats have been tested and were negative? This has both animal and human health implications. From a veterinary standpoint, I want more information about disease in other species, both to help care better for those animals and to protect people working with them.

Infection in humans: Obviously, we’re worried about human infections. How many people have been tested, and how many have been monitored in terms of their health?

  • If they are monitoring +/- testing hundreds of people with regular contact with cows and their milk, and no one has been sick, that’s great, and it’s important information to gauge risk for others.
  • If there’s limited surveillance and testing in people, we can’t be as confident about the risk.

Concerns have been expressed about the willingness of tenuously employed (and sometimes undocumented) workers reporting signs of illness, so “we haven’t heard of any more sick people” really isn’t a good enough answer in this scenario. Hopefully there’s a robust surveillance program in place, but that’s not something I’ve seen described.

Hopefully a lot of this information is available at least within certain circles, and simply has not yet been released publicly. If that’s the case, hopefully it will be released soon. As I said, there are some reasons to hold some data, but in general, more transparency and more communication makes for a much better response, and also helps to assuage public concerns, fears and mistrust.


In case you need a break from all the discussion about H5N1 influenza and the multitude of species it now seems to be able to infect, there’s nothing like a good zoonotic parasite story to make your skin (or pants in this case) crawl. (I promise that this will be a normal-length blog post… ahem.)

There was a recent research letter in Emerging Infectious Diseases (Hobbs et al. 2024) that caught my eye – particularly because it includes a picture/video which you should definitely check out.  The letter was about the finding of a sizable – and motile – adult roundworm in the diaper of a 2-year-old girl in Mississippi (now that’s got to be uncomfortable…).  The worm was identified from the video taken by the child’s mother (before she disposed of both worm and diaper – can’t say I blame her) as Ascaris lumbricoides.  That may not sound terribly noteworthy, as A. lumbricoides is the primary species involved in human roundworm infections globally, but it is usually spread by ingestion of eggs from human feces in areas where the is very poor sanitary infrastructure (e.g.  where human feces are more likely to be found in the general environment due to “promiscuous defection”) – but not in a region like northern rural Mississippi, on a farm with two flush toilets.  However, pigs have a very closely related roundworm that was previously known as Ascaris suum, that was more recently determined to actually be the same species as the human roundworm, A. lumbricoides.  Ascariasis is not uncommon in some farmed pigs even in the US, particularly those raised outdoors. In this case, no one in the family had traveled outside the US, and there was no reason to suspect there was human fecal contamination in the environment, but there were pigs on the property.  The two young kids were also reported to occasionally eat dirt from house plants, which has to make one wonder if they may have eaten other “dirt” on the farm as well (along with some pig feces and the parasite eggs therein).  Unfortunately by the time the Department of Health conducted their field visit, the family’s pigs had all been sent to slaughter, so it was not possible to confirm that the pigs were carrying the same parasite, but the story all fits together (admittedly it would have been nice to have some more definitive proof).

Fortunately the child was treated promptly by her pediatrician, and did not expel any additional worms, and suffered no further ill effects.  A fecal sample collected within 24 hours was negative for any parasite eggs, so they suspect there was only one adult worm present (I’m sure finding that once in the girl’s diaper was quite enough!). 

One more reason not to eat poop (from any species!) and wash your hands and veggies (or at least do the best you can, if you’re two years old :p).