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).

There’s not a lot new to report since last week about the finding of H5N1 influenza in dairy herds in multiple US states (see map below). That’s often a good sign.

  • I haven’t heard of any new affected farms.
  • There have been a few more reports that the virus is not being found (or is present at only low levels) in respiratory secretions from infected cattle.
  • No additional human cases of H5N1 flu have been reported.

The investigation is focusing more and more on milk as the key means of virus transmission on affected dairy farms. Viral loads seem to be quite high in milk, which is really surprising, as is the limited to no apparent shedding by the typical respiratory route. These two factors support the hypothesis that transmission has been driven by milk, or more specifically by milking practices that result in cow-to-cow exposure via virus in milk, such as milking equipment that is used on multiple cows. If that is indeed the case, I’d expect the cases to be limited to milking cows, and not younger cows (heifers) or cows not currently lactating (dry cows). I haven’t seen any data on that yet, but it will be informative to learn more about case distribution on affected farms.

With the same strain of H5N1 flu being reported on multiple farms across multiple states, contact tracing will be critical. If there’s a solid link between these farms, then cow-to-cow transmission (probably via milk) would explain things. If there’s no known movement of infected cows between all the affected farms, that would suggest there’s a variant circulating in birds that may be more amenable to infecting cattle, and that wild birds on each of the farms are the most likely (but possibly not the only) route of entry. I can argue for either situation being both good and bad:

Scenario 1: The virus is moving with the cows

If this was a single introduction of a cattle-friendly strain with multistate spread, then it might be containable through good testing and management of exposed and infected cattle. That would be good. If the outbreak is being driven by transmission on farms, and not repeated reintroduction from birds, then containment in cattle might work. Flu virus isn’t usually shed for long in most animals, so I assume that will be true in cattle as well, which makes containment more feasible than for some other viruses that can be shed for a long time. The bad part of this scenario is the unexpected transmissibility of this strain of the virus in a manner and species that really wasn’t on the radar, and the potential that this has spread more widely between dairy farms than we know yet.

Scenario 2: The virus is moving with wild birds

If there’s actually only been limited cow-to-cow spread and the virus wasn’t tracked between farms on cattle, that’s good from the standpoint that it would mean the virus actually does not spread well between cattle. However, it would also mean that there’s a broader reservoir of infected birds across the US (and presumably beyond) with a strain of H5N1 that’s amenable to infecting cattle, such that multiple farms were infected by birds. So, we couldn’t just use a really aggressive containment strategy in cattle and expect that we could eradicate the strain. If we eliminated it from cattle, there’d still be a risk of exposure from bird reservoir.

Both scenarios need good on-farm infection control to limit cow-cow spread via milking and perhaps other contacts. Scenario 1 focuses more on the need for strict measures to prevent farm-to-farm spread. Scenario 2 means the long-term focus needs to be on preventing re-introduction from birds into more cattle herds. That’s really tough since it’s hard to strictly isolate cattle from birds, especially cattle on pasture. So, scenario 1, while it has some concerning aspects, is probably what we’d prefer at this point. Given the bits of epidemiology that I’ve seen and the strain similarity across farms, it’s probably the leading candidate too, which I guess is good.

Regardless of which scenario turns out to be true, what’s needed right now is:

  • Continue and increase H5N1 surveillance in cattle (and other animals).
  • Be on the lookout for new infections in cattle, where disease is likely going to be subtle.
  • Work with farms to increase routine practices that help prevent cow-to-cow spread (e.g. routine hygiene and infection control, review of milking practices and equipment management).
  • Work with farms to reduce the risk of inter-farm spread through movement of cattle (e.g. short term quarantine after arrival, health checks before introducing new animal to the main herd). That includes precautions when bringing animals onto dairy farms, as well as when sending animals off dairy farms (such as when excess calves are sold).
  • Ensure farmers and veterinarians are using routine, and when necessary enhance, infection control practices to reduce the risk of occupational infection.

As with any emerging disease, we have more questions than answers, but we can make some interim assessments based on general principles of infection control, and what we’ve learned over the past couple years about this particular H5N1 influenza A virus that’s been circulating in wild birds all over the world, and spilling over into many different species of mammals (including cats), along the way. The details are likely to change over time as we learn more, but this is where I see things at the moment.

Cats can get infected with this H5N1 influenza strain

We’re known this for a while. There have been infected cats (big and small) in various countries over the past couple of years. It’s a bit cumbersome, but there’s good tracking of avian influenza cases in mammals worldwide on the WOAH (World Organization for Animal Health) website.  As with most infections, we no doubt only diagnose a small subset of infections that actually occur (just the tip of the iceberg).

Infections with H5N1 influenza in cats are rare

Notwithstanding my comment above about underdiagnosis of cases, we need to keep this in context. There have been millions upon millions of infected birds all over the world in the last few years. There are lots of cats in areas where infected birds have been present, and cat-bird contact is far from rare. So, there’s likely been lots of exposure, but disease (at least serious disease) in cats has been rare; that’s a bit of good news.

Although H5N1 infections in cats can be severe, social media claims of “100% mortality” are overblown

As we’ve seen in many other mammals with spillover infections (but fortunately not humans so far), infected cats can have very severe disease, including fatal infections, typically with severe neurological signs. It can be very bad, but it’s presumably nowhere near 100% fatal.

  • We’re missing a lot of context because of testing bias, because we’re mostly only testing cats with severe signs of illness, or cats that are found dead in areas where birds have avian flu.
  • We don’t do enough testing of other cats that have been exposed but are still healthy, or only have very mild disease. We just don’t know enough yet to say what the true morbidity or mortality rates are in cats.
  • For all the hype, getting samples from exposed cats to test is a challenge. I’ve been set up for a while to get samples from cats with wild bird contact, but despite there being lots of outdoor cats and lots of infected birds, I haven’t been able to get any samples. (But when the dairy cow news broke this week, I made sure my PAPR was charged and my sampling kit was ready, in case there’s now more motivation to test.)

We don’t know whether H5N1 infected cats can be infectious to others

With only a small number cats tested, it’s hard to gauge the risk of transmission from cats to other animals (or people). Hopefully we’ll get more information about the cats on infected dairy farms. A challenge with multiple cats being infected in a situation like this is sorting out if they were all exposed to infected birds, all exposed to infected cattle (especially contaminated milk) or whether there may have been some cat-to-cat transmission of the flu virus. There’s very little we can do to sort that out when investigating a single farm at a single point in time. We can infer some things from testing results (particularly from quantitative viral loads in respiratory and fecal samples), but it’s still a bit of a guess without more testing and epidemiological investigation.

What can the average cat owner do?

If possible, keep your cat inside, as it minimizes any risk of exposure to infected wild birds (which are still the main source of H5N1 influenza). That’s not always possible though, since some indoor-outdoor cats simply won’t tolerate being inside 24/7, and some outdoor cats can’t be moved indoors.

Take our three cats as examples:

  • Milo is an indoor only cat. He’s low risk.
  • Rumple was adopted through the Guelph Humane Society’s working cat program as a barn cat since he was deemed unsuitable for indoor living. He’s actually a huge suck and now spends a lot of time inside, but he wouldn’t tolerate it full time (I’m not sure he’s ever used a litter box).
  • Alice is an outdoor cat that Rumple adopted. She was a scrawny, completely feral cat who started living with Rumple in the garage, and on our deck. She’s a sweetie around us now, but only on her terms. She will take a few steps in the house (very warily) and then dart back outside, but when outside she’ll roll around on us and purr her head off. She cannot be moved inside. We can make sure she and Rumple are well fed (they’re both on the chunky side), but can’t guarantee they won’t hunt anyway. So they (especially Rumple) are a risk as a bridge from outside to inside. We know that and accept the risk.

People who have indoor/outdoor cats should assess the risk, the ability to change their cat’s living arrangements, and their risk tolerance.

We can also try to discourage mixing of cats and birds. Removing bird feeders from yards is a simple step that I’d recommend at this point.

What if an indoor/outdoor cat gets sick?

Most of the time, the cat won’t be sick from flu, but it’s a possibility, and the risk would be higher if the cat is a known hunter and if there’s recent flu activity in local wild birds. In that event, I think it’s reasonable for owners to limit close contact with the cat, consider wearing a mask if close contact is required, and talk to their veterinarian about testing (for flu and/or other causes). We can’t freak out every time a cat gets sick, but acute onset of severe respiratory and/or neurological disease in an adult indoor/outdoor cat would raise a lot of concern, since that’s uncommon in otherwise healthy mature cats. (Young kittens are a completely different story – they’re upper respiratory snot factories at the best of times.)

What should veterinarians when presented with a sick cat?

I don’t think we’re at the point of saying respiratory PPE should be worn for handling every sick cat. A risk assessment is always appropriate, and ideally there’s a triage process over the phone prior to any sick cat entering the clinic. If the cat has outdoor exposure, especially known exposure to wild birds, and the cat has an acute onset of respiratory or neurological disease, it makes sense to start off with enhanced PPE (e.g. mask, eye protection, gown, gloves) until the situation is sorted out

Are there any concerns about H5N1 influenza and raw meat diets?

Maybe. There are a variety of reasons why raw diets create disease risks (Salmonella being a big one), but there are some specific concerns about the H5N1 virus in these diets too. Last year, there were reports of outbreaks of H5N1 infection in cats linked to raw diets in Poland and South Korea. However, confirmatory data has been lacking, so it’s unclear how strong the link is. It’s probably also mainly or solely a risk from very fresh diets.

I’d use this as yet another reason to avoid raw diets. If someone wants to feed a raw diet, high pressure pasteurized diets should be considered as that likely eliminates influenza virus.

What am I doing about H5N1 influenza in cats right now?

I’ll keep an eye on Rumple and Alice for any signs of illness. If they get sick, I’ll keep them away from everyone, sample them, and go from there. If they have severe illness, I have a plan to manage that, but that’s more medical than I want to get into here. Beyond that, I’m staying aware of the situation and will act (and adjust) as necessary.

What about the risk of H5N1 influenza in dogs?

Separate species. Separate story. But, we know dogs can be susceptible to H5N1 influenza too (but likely even less commonly than cats). Keeping dogs away from potentially infected birds is important at this point, especially dead birds that are higher risk for having died from infection. (That’s something that’s important for me since we live in the country and own Labradors that consider basically anything (live or dead, organic or not) to be a potential snack.)


There’s been a lot of concern about the recent identification of H5N1 avian flu in cattle in the US, along with a single human infection in a person working with infected cattle. Some of the concern is warranted, but some is overblown.  We need to balance awareness versus paranoia, and try to better understand the problem and reduce the risk, while avoiding excessive, ineffective or harmful responses. That sweet spot is hard to define early on with emerging diseases. We should liberally apply the precautionary principle, but also ensure we keep re-assessing the risks based on emerging evidence.

Here are some initial thoughts on what we can all do regarding the current situation at this stage:

Cattle farmers

  • Be on the lookout for sick animals. Based on what we’ve seen do far, flu will likely cause mild signs in cattle such as decreased appetite and decreased milk production.
  • Call your veterinarian as soon as possible if you suspect a problem in your herd (good advice at any time!), and allow testing of suspect cases (good to know what’s going on, whether it’s influenza or not).
  • Although it’s tough, try as much as possible to keep wild birds out of barns. Also try to discourage migrating waterfowl in particular from entering cattle housing areas or taking up residence in fields.
  • If you’re sick, stay away from cattle. We don’t want human and animal influenza viruses mixing, and putting an infected person in a barn with potentially infected cattle increases this risk. Farmers are notorious for “sucking it up” and working as long as they’re not fully dead, but we need to encourage a culture shift that limits contact of sick people with animals. If that’s not possible (which is often the case), farm personnel that might have the flu should minimize contact with animals as much as possible, and wear a mask to reduce the risk of spread.
  • If there’s flu activity in wild birds in the area, it’s not unreasonable to wear a mask around cattle, but I realize that’s unlikely to happen.
  • Consider adjusting grazing practices to reduce exposure of cattle to infected birds or their feces, especially if there is flu activity in wild birds in the area, and particularly if dead birds are found in the pasture.
  • If your cattle might have flu, definitely wear a mask and eye protection around and sick animals, and limit close contact. Also, think about high risk items and surfaces that may get contaminated by sick cattle, and take additional measures as appropriate (e.g. increasing routine disinfection, reducing direct contact, wearing appropriate PPE when contact is required). Contamination with respiratory secretions from sick animals were the main initial concern, but recent information suggests that milk could be more important for potential cow-to-cow transmission during milking (e.g. shared milking equipment). Contact with milk and milking equipment might pose a risk for people too, meaning we should take extra precautions with contact with the udder, milk and any in-contact surfaces. I’d also take care around cattle feces until we better understand if cattle can also shed the virus that way (but there are lots of other reasons to avoid contact with cattle feces besides flu too).
  • Keep cattle away from any other species that might carry or acquire an influenza virus, which basically means keep cattle away from all other mammals and birds, but with a particular focus on avoiding higher-risk flu species (i.e. domestic poultry and pigs).
  • If you develop flu-like illness, make sure your physician knows you have contact with cattle. If your cattle are sick at the same time, absolutely make sure your veterinarian, physician and public health know so the situation can be properly investigated.


  • As for farmers, consider flu in cattle and on beef and dairy farms, and be on the lookout for it.
  • Communicate with farmers, infectious disease specialists, labs and government agencies if you have concerns about flu in a particular herd.
  • Use standard infection control practices to minimize the risk of farm-to-farm spread of flu (and other pathogens). That route of transmission is pretty unlikely for flu, but there is potential for veterinarians to track flu between farms, which can’t be ignored.
  • As for farmers, if you might have the flu, stay away from animals (at least birds and mammals) as much as possible. Yes, that’s tough since that means not working, but we don’t want co-infections of people or animals with different flu viruses.
  • Consider wearing a mask around cattle. I realize that’s not likely to happen routinely, but definitely wear a mask and eye protection if you suspect flu might be present in cattle you’re seeing. Take particular care around handling the udder and with milk sampling.
  • If you develop flu-like illness, do the same as mentioned above for farmers.


  • Support testing for influenza A in cattle. We don’t know what we don’t know. We need a fairly wide net of testing to understand this issue. That costs money, and it’s hard to expect farmers to cover all the costs of this kind of testing, especially when there’s limited direct benefit to them (because so far flu only causes mild disease in cattle, and there’s no specific treatment).
  • Don’t make it hard to get testing done. Sometimes, we run into barriers when we want to test for emerging diseases. Don’t make veterinarians jump through permission processes to limit testing. We need more information, not push back against getting more information.
  • Support farmers who have suspected or confirmed cases of influenza in cattle. We need to see a balance of measures that are adequately restrictive to contain flu, but not so extreme that they drive the situation underground. If the response is over-the-top, there will be a strong disincentive for anyone to test for flu, and that just makes the situation worse.


  • Relax.
  • Don’t drink raw milk (for lots of reasons).
  • Avoid fearmongering Twitter threads.
  • Relax.

Public farm events/petting zoos

  • I really have no idea what to say here. Petting zoos or other similar animal exhibit events can result in large numbers of random people have contact with cattle, with limited hygiene, no health screening  and no contact tracing. There’s a risk, but it’s too early to say how much of a risk. We need to see how the situation evolves.
  • Fortunately, these kinds of events are most common in the summer and early fall, so we have a bit of time. At the moment, if there’s flu activity in wild birds in the area, it’s reasonable to say that we shouldn’t allow random access to cattle, pigs and other flu-susceptible species.
  • If visitor contact is allowed, make sure cattle are assessed daily to confirm none look sick, try to keep sick people away, try to maximize ventilation in animal areas, and facilitate (and encourage) hand hygiene.

As with any emerging disease, guidance will change. People don’t like that, but it’s actually a good sign – it means we’re learning and responding. We shouldn’t be so arrogant to think that what we recommend now will ultimately be the optimal approach. Hopefully we’re pretty close, and we can make good recommendations based on what we know now, but I’d be disappointed if we don’t change them at least a bit (because that would more likely mean we didn’t adapt than we knew it all from the start).


H5N1 avian influenza has been found in cats on a dairy farm in New Mexico where cattle were infected with the same virus. As part of the farm investigation, some (number not reported) sick (type of illness not reported) cats were tested, and three were positive. There’s no way to know for sure, but I’d guess that the cats were infected by the same source as the cows (i.e. from infected wild birds), not from the sick cows themselves.

It’s totally unsurprising, since there are various reports of cats from several countries around the world being infected with this virus, which continues to circulate widely in the wild bird population. Cats hunt birds, and birds that are sick with avian flu are a lot easier to catch. Eating a sick bird is a clear way to get infected, so it’s not surprising at all to find infected cats in areas where there’s lots of flu activity. I’d guess there have been hundreds of infections of cats worldwide, but most of them have gone unrecognized.

This isn’t a game changer, but it’s yet another reminder to pay attention. Fortunately, cats (like cows) don’t have their own influenza A virus, and they tend not to be very susceptible to human flu strains. So, there’s less risk of them being infected with the H5N1 virus and another flu virus at the same time, and acting as a mixing vessel for creation of a new, more problematic recombinant flu strain. However, every spillover into any mammal poses some extra risk, and cats are potential bridges between wildlife and people, since people often have close contact with cats. That’s why we’re on the lookout for these spillover infections, and ideally want to limit exposure of cats to infected wild birds.

If you have cats that go outside and you can’t prevent that (not all cats can be indoor only), be on the lookout for flu. If an outdoor cat gets sick, especially with respiratory or neurological disease, and it’s had potential contact with flu-infected birds, it’s important to talk to your veterinarian (and ideally get the cat tested so we can gather more information on the virus’ behaviour, and also try to contain the infection).


Following the recent report of H5N1 avian influenza in cattle in the US, the CDC has now reported H5N1 infection in a person from Texas who was exposed to infected dairy cattle. This is bound to get a lot of attention. To a large degree, it’s warranted, but it’s probably going to get overblown.

The infected individual developed conjunctivitis (i.e. redness and swelling around the eyes) – that was the only clinical sign of illness, so clearly the disease has been mild in this case. The person was told to isolate and is being treated with an antiviral drug. This is the second known infection of a person with this strain of H5N1 influenza in the US (the other case was a person in Colorado who got infected with H5N1 influenza while culling infected poultry in 2022; that person also only had mild disease – just a couple days of fatigue).

I’d sum up my initial thoughts as such:

  • Not good, but
  • Not unexpected, and
  • Not something to panic about, but
  • Not something to ignore.

Not good because we don’t want people to get sick from this virus. It’s also not good because the more time this virus spends in people, the more chance it has to adapt to be better able to infect people. It also increases the risk of recombination with a human flu virus anyone gets infected with multiple flu viruses at the same time (fortunately, we’re not in peak human flu season now). Mixing viruses like that creates the potential for emergence of a new virus with a human flu strain’s ability to spread widely in people, but one against which we don’t have any population immunity to help protect us, assuming it’s different enough from currently circulating strains with which a lot of people have been previously infected or vaccinated against.

Not unexpected since we know that people can get infected with this H5N1 strain. Human infections have been surprisingly rare given the scope of infections in myriad other species around the world, but we know they can occur. More contact with infected animals means more exposure risk. It’s another reminder of the value of routine infection control practices and the need to ramp those up when there’s exposure to potentially infected animals.

Not something to panic about because panic rarely helps. More than that, it’s good that disease has been mild in this person and at least some of the other human cases. The virus is not, at the moment, able to readily infect and be transmitted between people. We have a very small number of known infections in a global ecosystem with millions of infected animals. That’s not meant to be dismissive, since this is a still a very concerning event, but it’s meant to provide balance.

Not something to ignore because of what could happen. A rare infection from direct contact with an infected animal isn’t a big deal. Adaptation of this virus to more readily infect people or recombination with a human flu virus would be a very big deal. That might not happen, or it might have occurred already and it’s just not been identified yet. We don’t know. That’s why we need to be proactive and try to control the virus in domestic animals as much as we can (we’re kind of screwed when it comes to trying to control it in wildlife), and do all we can to make sure infected people and animals do not get exposed to other flu viruses at the same time.

Time will tell. Hopefully we keep taking appropriate measures (and up our game) to better contain this virus, and keep preparing for the possibility that it becomes a new human flu virus. We should have learned these lessons from the COVID-19 pandemic, but I fear we’ve forgotten many of them already. Things like public health measures and vaccination have become so politicized that we could be starting even farther behind than we would have a few years ago if something major develops.

Another thing to ponder: Is this the first instance or first diagnosis of H5N1 in a person associated with cattle contact? That’s always an interesting question. It could be that this is the first person that’s (likely) gotten H5N1 from a cow. It’s also possible that this has happened many times and we didn’t know because the cattle weren’t known to be infected, the person’s disease was mild and, because of that, there was no testing. That kind of context is important, but is often lacking with emerging issues.


Recent reports from federal and state agencies in the US of H5N1 avian flu in cattle in Texas, Kansas and now other states have gotten a lot of attention, and for good reason; but, as is typical, some of the media coverage has gone a bit over the top. This is certainly a noteworthy event, and some aspects of this are concerning, while many aspects remain unclear. At this point, it’s not a game changer in terms of what this H5N1 avian flu virus is doing, but it’s yet another example of why we need to pay attention to animals and influenza viruses.

Remember: For the last 3 years or so, we’ve been seeing an unprecedented, sustained global transmission of highly pathogenic H5N1 avian influenza virus (HPAI) in myriad bird populations internationally. The “highly pathogenic” classification is based on how the virus affects domestic poultry specifically; highly pathogenic strains (as the name suggests) can cause devastating mortality in poultry flocks. That doesn’t necessarily mean it’s highly pathogenic (or transmissible) in people or other mammals, but that depends on the specific virus.

While over 99.99% of infections with this pandemic H5N1 avian flu strain have been in birds, there’s been spillover into a really wide range of mammals, including a lot of wildlife but also some domestic species such as dogs, cats, goats, and now cattle. Often these spillover infections have been fatal to the affected animals (cattle being a notable exception), but whether that’s because infections are always severe when they occur, or we just find the most severe infections more easily isn’t clear.

Spillover into mammals raise a few concerns:

  1. Animal health: animal illness and death are obviously concerning.
  2. Adaptation of the virus to new mammalian species, creating a new flu virus that is transmissible within the species: spillover infections into atypical host species are usually dead end infections, because the virus can infect the host but doesn’t replicate well enough to be able to spread to other individuals. However, flu viruses can change and adapt to other species, with the potential to become an endemic flu virus for that species. The H3N2 canine flu strain that’s endemic in parts of Asia and the US started off as an avian flu strain; then it became host adapted to dogs, and was able to spread widely in the dog population.
  3. Animals acting as a mixing vessel to create a new and even more problematic flu virus: this is the biggest concern. In species that can be infected with multiple flu strains (including human flu strains, there’s the potential for the viruses to recombine with each other and make a new strain. What we don’t want is co-infection with human flu and this H5N1 avian flu (and/or other flu viruses) that could result in creation of a new flu virus that has all the human flu virus bits it needs to readily infect people, but is different enough from other human flu strains that our population won’t have any existing immunity to it. That’s how we get a new human pandemic flu virus. If a new strain like that happens to also cause severe disease in people, that’s even worse.

Back to the US cattle with H5N1 avian flu:

On March 25, the USDA reported H5N1 avian flu infections in cattle from two dairy herds in Texas and two in Kansas. Disease was most often in older cows but it was described only as “decreased lactation, low appetite, and other symptoms.” The virus was identified in unpasteurized milk and oropharyngeal samples from sick cows. Dead wild birds, which had presumably died from the virus (as has been happening all over the world), had been noted on the properties, and wild birds were therefore considered the likely source of exposure for the cattle. Importantly, genomic evaluation of the virus from cattle did not show any apparent changes that would make it more transmissible to/between mammals.

In the latest update from March 29, additional presumptive positive tests for H5N1 had been found in more cattle in New Mexico, Idaho and Texas, and the virus was also found in a herd in Michigan that had recently obtained cattle from Texas. It’s not stated anywhere I’ve seen whether that means an infected herd in Texas or just Texas. That’s a key factor – did the virus get shipped in with the cattle, or is it just a coincidence that the farm had obtained cattle from the same (very large) state that has some affected farms. The Michigan virus strain is similar to the one found in the Texas and Kansas herds, but that means it’s similar to the strain in wild birds all over North America, so we have to be careful not to over-interpret that.

With multiple dairy herds affected, it raises several key question:

  • Is there more widespread infection of birds resulting in more exposure of cattle?
  • Is there something about this flu strain that makes it more transmissible to cattle?
  • Is it starting to infect more and more cattle? It’s certainly been found more since the first cases were reported because people are now looking for it. Since disease in cattle appears to be fairly mild and non-specific, flu testing would have been rare before now.
  • Has there been spread via cattle between farms?

It’s important to sort out these questions, since the implications and response in each scenario would be vastly different.

At this point, we have a situation where the virus infecting cattle is the flu strain circulating in wild birds, without evidence of genetic markers that it’s better able to infect mammals. That’s good.

However, the more this virus spills over into cattle (and other mammals), the more chances it has to adapt and recombine to form a different strain. Cattle don’t have a common species-specific influenza A strain (they have influenza D, but that’s quite different), and they tend not to be highly susceptible to flu viruses from humans, pigs and other species, so recombination is less of a risk at this point, at least in cattle.

If this strain is actually able to spread between cattle and establishes itself as a bovine influenza A, that would be a concern for cattle health and possibly for people who have contact with infected cattle. But I’d still be more concerned with cattle-to-pig transmission, since pigs are susceptible to lots of flu viruses and are a great mixing vessel for creation of a new flu strains.

And the other obvious question:

  • if cattle are infected with flu, are there any foodborne disease concerns? No, not really (but as usual we can’t say absolutely zero).

Fortunately, flu isn’t a very tough virus. It doesn’t survive well outside the host. “Foodborne” transmission has been shown in some animals, but those are situations like cats or wildlife catching and eating or scavenging infected birds, which is very high level exposure to obviously uncooked tissue. There are pretty strict processes for keeping meat and milk from sick animals out of the human food supply. As long as the virus causes some illness when a cow is infected, that can be flagged. If cattle can be infected but not get sick, it’s more of an issue.

For retail meat, the time lag between when it’s purchased and prepared helps. Proper cooking eliminates any concern. Pasteurization of milk would also eliminates any possible concern.

Raw milk is a wildcard for me, and I think we need to know more but should assume there could be some risk since they were able to find the virus in unpasteurized milk samples (but there are a lot more (and more concerning) infectious disease risks from raw milk… that’s another story).

Raw cheese? I’d guess that flu wouldn’t survive the cheese-making process, but that’s not my area of expertise.

The USDA report indicates “Because of the limited information available about the transmission of HPAI in raw milk, the FDA recommends that industry does not manufacture or sell raw milk or raw/unpasteurized milk cheese products made with milk from cows showing symptoms of illness, including those infected with avian influenza or exposed to those infected with avian influenza.” That’s a pretty reasonable statement.

Time will tell, but efforts to control the spread of H5N1 flu to and between cattle are important – sitting back and just “hoping” that it won’t be a problem is a dangerous proposition. As always with emerging diseases, we need to balance proactivity and practicality, by liberally appling the precautionary principle while not going overboard and panicking.

  • Cattle farmers: Pay attention but don’t panic. Use good biosecurity practices when bringing in cattle from other farms. Report strange disease occurrences to your veterinarian.
  • Veterinarians: Use good routine infection control practices when visiting farms, and keep an eye out for unusual diseases or disease patterns.
  • Consumers: Relax. Handle and cook food properly (for reasons beyond flu).

Infectious diseases of dogs continue to be in the news, and that’s not going to stop. Anytime we have dogs mixing, we have some risk of disease transmission. I wrote about your dog’s social network and why it matters in a previous post.

We are now trying to gather some real data about typical dog social networks. We’re recruiting dog owners to complete a short (3-5 minute) online, anonymous survey that asks basic questions about the number and types of contacts their dog(s) has with other dogs.

This study has been reviewed by the University of Guelph Research Ethics Board for compliance with federal guidelines for research involving human participants (REB #24-01-002). Click here for more information and to participate in the survey.

Reptiles and Salmonella is about as far from a new story as anything on this blog, but talking about it is still worthwhile, because it’s still making people ill. I’ll package two similar stories into one post here.

A recently identified (and likely ongoing) Salmonella outbreak in Canada has been linked to pet snakes and/or feeder rodents. Feeder rodents are rodents (mainly mice and rats) that are purchased to feed pet snakes and other large reptiles. They are usually mass produced and sold frozen. Both feeder rodents and snakes are well known sources of Salmonella.

As of March 19, 70 infections in 8 provinces have been identified involving two different Salmonella types: Salmonella I 4,[5],12:i:- and Salmonella Typhimurium. Diagnosed cases typically represent the minority of actual infections, so I’d guess the real burden of disease is in the hundreds. Infections date back to February 2022, so this has been a long, drawn out outbreak that’s been going on under the radar. The graph below shows the epidemiological curve of the infections:

Ten people have been hospitalized and one has died; 19% of cases have been in kids five years of age or younger. Kids are more commonly affected in these outbreaks because of their increased susceptibility to severe disease, and maybe because of more exposure (e.g. touching the reptiles or their environment and then not washing their hands), and probably also because sick kids are more likely to be taken to a doctor and tested than adults. However, the percentage of young kids affected in this outbreak is actually on the low side compared to many reptile-associated Salmonella outbreaks.

A link to contact with snakes and feeder rodents was identified, but no point source (e.g. a particular vendor for the reptiles or feeder rodents) has been identified.

My guess is that feeder rodents are the problem, since they are mass produced in facilities where widespread Salmonella contamination can develop, and such facilities have been linked to similar outbreaks. The feeder rodents are more numerous than snakes, and are moved across the country more than snakes. It’s harder to see how a common source of snakes would result in an outbreak across so many provinces.

Snake owners that purchase feeder rodents should be aware of the ever-present risk of Salmonella, and Canadian feeder rodent purchasers should pay extra attention right now.

Control measures are pretty basic. Feeder rodents should be considered contaminated items and handled accordingly. Basic infection control measures include:

  • Storing frozen rodents in sealed containers
  • Preventing cross contamination of human food
  • Preventing contamination of general household surfaces, especially those where human food is prepared
  • Paying good attention to hand hygiene after touching feeder rodents (and the reptiles to which they’re fed!)

The other story is one that’s been on my “to-write” list for a while. It’s about a Salmonella Vitkin outbreak in Canada and the US that was linked to bearded dragons (Paphitis et al. 2024). The scale of this outbreak was smaller, but it highlighted some important points and the report describes a nice investigation.

Here are the highlights:

  • Two Salmonella infections were identified in Ontario in June of 2022. Both were in infants and bearded dragons were present in both households. Feeder mice were also present in one household.
  • The strain that was involved, S. Vitkin, hadn’t been seen in Ontario for at least 14 years, so a common source of infection was suspected.
  • This led to a broader investigation of S. Vitkin in Canada and the US. Twelve cases were found in the US between March 2021 and September 2022, across 10 states. Five of those (45%) were hospitalized and 67% were kids less than 1 year of age; 73% reported exposure to bearded dragons.
  • No single breeder of bearded dragons was identified across the US and Canadian cases, but bearded dragons from the two Ontario cases came from the same intermediary supplier.

Bearded dragons have (like most reptiles) been previously linked to human Salmonella infections, so that part is not overly surprising. They’re small, interesting, social reptiles, so they may pose more risk than many other reptile species because they might be handled and socialized more, especially by kids.

The total burden of infection in people from reptiles isn’t well known. Large outbreaks are detected more easily, as are infections with weird strains. Sporadic disease is less likely to be identified and investigated, and infections by more common Salmonella strains would be harder to definitively link to reptiles. So, we probably miss a substantial percentage of reptile-associated infections.

My line about reptiles is the same as it’s always been: They’re interesting critters but are clearly associated with a high risk of salmonellosis. There’s no way to eliminate that risk, so we focus on two things:

  1. Household practices to reduce the risk of exposure, and
  2. Keeping reptiles away from high-risk individuals. That includes kids less than five years of age, people over 65 years of age, pregnant women and immunocompromised individuals.

Saying “the reptile never leaves the cage” is not an adequate control measure when high risk people are in the house. While the reptile may actually never leave the cage, the Salmonella can, e.g. on people’s hands or when contaminated items like food bowls are removed. More information about Salmonella can be found on the Worms & Germs Resources – Pets page.