Details have now been released about the information that was briefly posted then removed from the US CDC website a couple weeks ago regarding suspected household transmission of H5N1 flu to and from cats. It’s great to have more details, but (as is common with emerging diseases) the full story probably raises more questions than answers. The gist of the story is similar to what we inferred from the leaked information, but the full report is now available online (Naraharisetti et al. MMWR 2025), and includes some useful details. Here’s the more complete rundown:

Household 1

  • This household contained 2 adults, 2 kids and 3 indoor cats. One of the owners worked on a dairy farm that was not known to have infected cattle, but there were infected farms in the area.
  • Cat 1: One of the adult indoor cats developed loss of appetite, lethargy and disorientation, then worsening neurological signs.  After a few days, she was euthanized because of rapid and severe disease progression. H5N1 influenza virus (genotype B3.13) was detected in the cat’s nasal passages and brain tissue; this is the predominant strain currently circulating in US dairy cattle. 
  • Cat 2: One of the other cats then developed signs of respiratory disease four days after the first cat got sick – that timing is generally consistent with what we’d expect for transmission of a flu virus. The owner was asked to collect a nasal swab from the cat because it was too sick to be taken to a veterinary clinic, but this wasn’t done. Fortunately, the cat recovered after 11 days.
  • Cat 3: The last cat just watched everything unfold and appeared healthy the whole time.

The household investigation:

The owner who worked on the dairy farm didn’t have direct contact with cattle. He reported that he took off his farm clothes and boots when he returned home and that they were stored in an area that the cats could not access. The cats were not fed a raw diet or raw milk.

One of the kids had mild upper respiratory tract disease (cough, sore throat, headache and muscle pain) six days after the cat got sick. The other kid had a dry cough around the same time that was attributed to allergies. 

Eleven days after Cat 1 got sick, samples were collected from three of the people in the household. That’s a bit later than ideal, but still reasonable (logistics often dictate when samples can be collected). All the people tested were negative for flu. The sicker kid had a positive test result for rhinovirus/enterovirus, which could explain the illness, but it doesn’t rule out flu completely. 

Unfortunately, the person who worked on the dairy farm declined to submit a sample. He reported a day of vomiting and diarrhea before Cat 1 got sick… Was that potentially related to flu infection? Maybe, since flu can cause gastrointestinal disease, but it’s far from classic, and most that episode was unrelated. The unanswered question is whether the person had a mild or asymptomatic H5N1 flu infection and was responsible for infecting the cat(s).

The people who were tested for flu also got oseltamivir prophylaxis (antiviral medication) because of their close contact with the infected cat (and potentially the initial source of the virus in the household…); the farm worker declined prophylaxis.

Putting Household 1’s story together:

  • The most likely source of H5N1 flu exposure of Cat 1 was the farm worker. Whether he unknowingly tracked the virus home from the farm or whether he had a mild infection that he transmitted to the cat can’t be discerned, but that’s a really important thing to figure out in future cases.
  • It would also be really useful to know if the farm where the person worked had confirmed H5N1 flu in the cattle. Hopefully follow up testing of the cattle was done. If flu wasn’t present, that raises more concerns about how the farm worker might have gotten infected, or if the farm worker was not the initial source in the household, then how did an indoor cat with no other apparent risk factors get infected? 
  • It’s unclear whether there was cat-to-cat transmission of flu. Spontaneous, transient upper respiratory tract disease is uncommon in adult indoor cats. Maybe stress caused a recrudescence of feline herpesvirus infection in Cat 2, but it sounds like the cat was a lot sicker than we’d expect for that. We therefore have to consider Cat 2 to be a strong flu suspect; based on the timing of illness relative to Cat 1’s illness, if it had flu, it presumably got it from Cat 1.

Ideally, serological testing would be performed to detect anti-H5N1 antibodies in blood from individuals (cats and people) in the household, to determine which individuals ultimately were infected, even though serology can’t tell us the when or how.

  • If the farm worker was seropositive, it still wouldn’t tell us if they infected the cat directly or if the cat was exposed to virus on clothing or other fomites (though the former seems more plausible, and would be concerning). If the farm worker was seronegative, that suggests that clothing or fomites was the source (which is a good reminder of why infection control measures like wearing and changing personal protective equipment are so important).
  • If Cat 2 was seropositive, that doesn’t confirm cat-to-cat transmission, though that would still be most likely.
  • If the kids or other adult were seropositive, then there was almost certainly either cat-to-human or human-to-human transmission, neither of which would be good news.

Household 2

This household consisted of a dairy worker and two indoor cats. It’s not clear whether the person had contact with cattle, but he collected raw milk from farms and reported being frequently splashed with milk. He didn’t remove his work clothing before returning home, and one of his cats liked to roll on his work clothes (that were presumably contaminated with milk).

  • Cat 1: The worker’s six-month-old indoor cat was taken to a referral veterinary hospital with a one-day history of progressive neurological disease. It was seriously affected and died in hospital within 24h. H5N1 influenza virus (genotype B3.13, the dairy-associated type) was confirmed in this cat. Whether it was a coincidence or not, this was the cat that liked to roll in the work clothes.
  • Cat 2: The other cat remained healthy, and swabs collected from this cat the day after Cat 1 got sick were negative for flu. 
  • The worker reported signs of eye irritation that began 2 days before Cat 1 got sick. The person was not tested and declined antiviral treatment. So we don’t know if the person had H5N1 flu, but eye inflammation is consistent with the conjunctivitis seen in other human cases of H5N1 infection in the US in the last year, and would be a likely presentation in someone getting splashed in the eyes with influenza-contaminated milk.

There was little further follow up because the owner shut down contact with public health officials, stating a fear of losing his job. 

As with Household 1, timing of sample collection is always a challenge. While we’d ideally get serial samples over time, it’s simply not possible in many cases. Testing Cat 2 at the start was logical and useful, but if there was cat-to-cat transmission, that was probably too early to test. We’d need at least one more sample a few days later to account for the incubation period if Cat 1 infected Cat 2. The initial sample only tells us that Cat 2 likely wasn’t exposed at the same time as Cat 1.

It’s also hard to say if the owner had H5N1 flu or if his clothing was the source of virus for Cat 1. Ideally they could have done serological testing after the fact to confirm whether or not the person was infected, but it doesn’t sound like the person would have been willing to come back to provide a blood sample.

What about the veterinary staff involved in these cases?

Twenty-four (24) veterinary clinic personnel were potentially exposed to the two confirmed H5N1 flu-infected cats from the two households, 18 of which were contacted and monitored for signs of flu. They were all deemed to have had limited exposure. Hopefully that’s a win for routine infection control practices and quick identification of higher-risk situations warranting use of enhanced personal protective equipment (PPE). Because they were all deemed low risk, they were not offered antiviral prophylaxis. Seven individuals reported signs or symptoms of illness after exposure, of which five were tested for flu, and all were negative.  

To sum up:

  • Was there direct human-to-cat transmission of H5N1 influenza? Maybe.
  • Was there direct cat-to-cat transmission of H5N1 infleunza? Maybe.
  • Was there transmission of H5N1 influenza via contaminated clothing? Maybe.

Unfortunately, we need to learn more about transmission by investigating disease events. It’s almost certain there are undiagnosed infections, and maximizing our recognition of H5N1 flu spillovers between species (including humans) is critical, as it helps inform proper care and management of people and animals (including antivirals, when indicated) and helps us figure out transmission patterns and risk factors to prevent more infections. The more we understand this virus, the better we can control it, all the while trying to walk that ever-changing fine line between protection and practicality.

If I asked 100 random people on the street “should we be giving antibiotics to healthy animals?” I’m pretty sure most or all of them would say “no.“

That makes sense in a lot of ways. We should save antimicrobials to treat sick individuals (especially people), and we shouldn’t use antimicrobials in healthy individuals… except when we should.

If I asked to the same 100 people “Should your dog get an antibiotic if it eats something it shouldn’t have, and then needs to have surgery to open up the intestines to remove the object?” I’m pretty sure most would say “yes!” – even though that dog doesn’t have an infection. That’s prophylactic use of an antimicrobial.

Like most things in the antimicrobial resistance (AMR) space, this issue of prophylaxis is complicated and messy. Too often, the sound bites we hear about approaches to antimicrobial use (AMU) in animals miss the nuances and complexities that play a huge roll in the necessary discussions. 

  • We don’t want to overuse antimicrobials.
  • We don’t want to use antimicrobials unnecessarily.
  • We don’t want to use antimicrobials to compensate for poor animal management or lack of veterinary care.
  • But, there are situations where antimicrobial prophylaxis makes sense and logical prophylaxis can improve animal health and animal welfare, and reduce the need for therapeutic use of antimicrobials (that might be higher tier drugs) with limited risk.

There’s always some risk with the use of antimicrobials in any animal or person, but part of optimizing antimicrobial stewardship (AMS) is using them when we should, and avoiding them when we shouldn’t. Use antimicrobials as little as possible, but use enough.

Because we keep running into the same misconceptions and misunderstandings about AMR and AMU at various levels (including high level international discussions) we teamed up with the World Organization for Animal Health (WOAH) and the quadripartite‘s AMR Multi-stakeholder Partnership Platform to write a primer on antimicrobial prophylaxis in animals. The document discusses when antimicrobial use is bad, when it’s clearly indicated, and a few levels in between.

Prophylaxis can include scenarios such as:

  • Routine administration of antimicrobials to groups of animals in the absence of evidence of need, largely because of historical practices. 
  • Routine administration of antimicrobials for a prolonged period of time to a group of animals because of a high endemic rate of a specific disease in the group.
  • Routine administration of antimicrobials to most or all animals at a specific stage in life or production to reduce a specific disease or syndrome (e.g. tetracycline treatment of pigs at the time of weaning to prevent post-weaning diarrhea, administration of intramammary antimicrobials to dairy cattle at the end of lactation to prevent mastitis, tetracycline with ITM vaccination for prevention of East Coast fever).
  • Targeted administration of antimicrobials to a group of animals in response to a specific, defined disease threat that is known to be mitigated by antimicrobial prophylaxis.
  • Administration of antimicrobials to a specific animal at a specific and well-defined high-risk time (e.g. peri-operative antimicrobial prophylaxis for prevention of surgical site infection).

Some of these are good uses of prphylaxis, some are bad, and some are situational. But it shows the wide range of possible prophylaxis scenarios. We need to make sure we use antimicrobials when they are needed, but not prophylaxis as an excuse to overuse them or try to avoid scrutiny. It can be challenging to have discussions about why we need prophylaxis when some people think we’re just being apologists for the agriculture industry.

Do we need to improve antimicrobial prophylaxis?  Yes, especially in agriculture.

Can we massively reduce antimicrobial prophylaxis without negative impacts on animals? Yes, if we do it right, and if we do the other things we should be doing to prevent infections and optimize animal health.

Do we need to use some antimicrobial prophylaxis in animals? Yes, but we need to find that “use as little as possible but use enough” sweet spot. That’s not easy, but it’s clear we can reduce what we currently use a lot.

We need to have informed discussions about prophylaxis, not dogmatic debates with “all prophylaxis is bad” as an entrenched starting point. 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Back in March 2024, I wrote a post about a systematic review of the efficacy of antibiotics and probiotics and the associated treatment guidelines for acute diarrhea in dogs from the European Network for Optimization of Veterinary Antimicrobial Treatment (ENOVAT). Now, we have an ENOVAT/WSAVA infographic to put all that into a (hopefully) quick reference for veterinarians.

While we’ve historically used antimicrobials like metronidazole by rote to “treat” acute diarrhea in dogs, a lot of dogs were probably getting better despite what we were doing rather than because of it. Antimicrobials are needed in dogs with severe disease, but that’s to target systemic infection in those cases, not necessarily what’s going on in the intestine.

It’s pretty straightforward, as the infographic shows:

  • Dogs with mild diarrhea: no antimicrobials needed, just basic supportive care like a GI diet.
  • Dogs with moderate diarrhea: fluid therapy first, and if that resolves the systemic signs then no antimicrobials required. If signs persist that might be attributable to sepsis, then it’s considered severe and systemic antimicrobials are warranted.
  • Dogs with severe diarrhea and systemic illness: systemic antimicrobials are warranted.

The standard disclaimer is that guidelines are meant to cover most cases, but there can be nuances to individual cases that indicate the need for a different approach. That’s fine, but we still want to try to use an evidence-based approach as much as we can to determine the default treatment.

Of all the guidelines with which I’ve been involved, this one is by far the hardest to get people (including veterinarians and pet owners) to accept, since we are so conditioned to treating diarrhea with antibiotics. We do it because we’re risk averse, because it’s habit, and because we are conditioned to want to do something – even when there’s no evidence that the something is useful. A large percentage of the metronidazole that is used in dogs is psychotherapeutic… for the pet owners and veterinarians, because it makes the people feel better, but not the dog. In fact, those antimicrobials might actually make the dog feel worse.

We’ve made big strides in veterinary antimicrobial treatment guidelines in recent years, and this is one more step in the right direction, but we still have a long way to go.

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H5N1 influenza has been widespread is US dairy cattle for close to a year, so you might wonder why a recent report of H5N1 in dairy cattle in Nevada is garnering extra attention. Well, it all comes down to the strain. TLDR:

H5N1 influenza isn’t one specific virus. There are already numerous known subtypes and new ones continue to emerge. The “dairy strain” that’s spread among US dairy cattle is called clade 2.3.4.4b, genotype B3.13. 

Finding the B3.13 strain in cattle in Nevada wouldn’t have been surprising, but multiple herds in Nevada tested positive for a stain of H5N1, subtype D1.1. This “new kid on the block” is a recombination of one of the H5N1 strains that moved from Asia to North America in late 2021 and early 2022 and a low pathogenicity avian flu strain that was already present in birds in North America. It was first identified in September 2024 and has now emerged as a dominant strain in wild birds (and spillovers into poultry). 

The first detection of the D1.1 strain in Nevada cattle was from samples collected January 6 and 7, 2025, so (as is typical) we’re playing catch-up. It will be important to see if these farms are linked, and whether there are other affected farms. It will also be important to see if shedding patterns (lots of virus in milk, little in respiratory secretions) and virulence in cats on these farms (lots of dead cats) are similar to B3.13. Obviously the risk to humans must also be tracked(more on that below). 

This is a noteworthy event because it represents a new spillover into cattle from wild birds. Infection of a single cow wouldn’t be too remarkable, since rare spillovers to mammals are expected given how widespread the virus is in birds. However, infection in multiple cattle on multiple farms suggests either effective bird-to-cow transmission on multiple different premises, or another single bird-to-cow spillover that has subsequently been spread cow-to-cow and farm-to-farm like B3.13.

Another concern is the potential for more severe disease in people. Human infections with D1.1 have been previously identified in people who were depopulating infected poultry farms. They had mild disease, like most of the human H5N1 flu spillover infections in the North America to date. But D1.1 is the strain that was involved in a fatal H5N1 flu infection in a person in Louisiana, and that caused very severe disease in a teen from BC. On one hand, we can say any H5N1 strain can cause severe disease under the right circumstances and maybe those were just rare events. On the other hand, those two severe infections show that we can’t sit back and just say but human infections are always mild. When something spreads widely, rare events become more common, so we can’t ignore them. It also raises concerns about what could happen if this strain becomes better able to spread human-to-human. A more transmissible strain that can cause severe disease in people is the big concern.

Highlighting these concerns, a human infection has already been linked to these Nevada dairy cattle. Fortunately, the person had mild disease (conjunctivitis), akin to what’s been seen in dairy workers with B3.13 infections. Still, it shows that this strain also poses a risk to people who work with infected animals. 

There’s also a concern about evolution of this virus in cattle, even over this short timeframe. The Nevada cattle D1.1 strain has already acquired one mutation (PB2 D701N) that makes it more able to infect mammalian cells. Lots of things still have to happen to make this a mammal-adapted virus, and even more for it to become an effective human pathogen, but this is a potential step in that process. Spillover into farm workers raises the stakes further, since any new infection of a person increases the likelihood of human adaptation or, worse, recombination with a human flu virus in someone coinfected with two flu strains at the same time (e.g. H5N1 and a human seasonal flu). A recent CNN article has a nice description of some of the issues related to the H and N changes in these H5N1 subtypes.

More information about the effects of this “new” strain on cattle is also needed. It’s reported that the cows were not obviously sick when the positive results were first obtained as part of a state screening program, but that disease subsequently occurred. That might suggest that disease is mild and only found when someone is really looking for it, or that they just picked up these infected farms very early with their surveillance program. The US recently started a national Milk Testing Strategy surveillance program, which should help detect problems earlier. We’ll also need more information about the spread of this strain within farms and whether it’s spreading between farms (and if so, how). 

The sky is not falling, despite some social media reports, but it’s another concerning development. The more this virus infects mammals and spreads between mammals, the more risk to humans and domestic animals. Surveillance is a key part of our response but there also needs to be a concerted effort to limit mammal-to-mammal transmission and exposure of people to infected animals. 

From a Canadian context, this highlights a few things too. We can’t just focus on preventing H5N1 flu in dairy cattle through movement of cattle from the US. If D1.1 can move from birds to cattle in Nevada, it can do it anywhere.

We have to maintain a robust milk surveillance program to identify early incursions of this virus. We also have to be ready, able and willing to act decisively if/when H5N1 flu is identified in dairy cattle in Canada. That doesn’t mean culling cattle, but it does mean using strict controls to prevent farm-to-farm spread, and reduce the risks to humans on farms.

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Earlier today I wrote about a New York Times story about some interesting H5N1 flu transmission data from the US CDC that was posted briefly on Wednesday and was then quickly removed from the CDC website. The information suggested they’d identified cat-to-human and human-to-cat transmission of H5N1 flu in two separate households, both of which would be noteworthy events. A bit more information about the cases has since come to light, from outside official channels (the lack of transparency regarding these findings is a separate story).

Household 1

  • Cat 1 got sick, died, and tested positive for H5N1 influenza.
  • Cat 2 got sick 4 days after Cat 1 got sick. That’s pretty consistent timing for cat-to-cat transmission, but it appears Cat 2 was not tested for flu.
  • Cat 3 was tested for flu and was negative. Sorting out the testing for this cat is important, including when the cat was tested and what samples were collected, as this can impact how likely a the result is to be a true negative or a false negative.
  • An adolescent got sick 6 days after Cat 1 got sick / 2 days after Cat 2 got sick. The adolescent tested negative for H5N1 flu 6 days after the onset of illness. It’s not clear if there was testing for other diseases (I’d guess either that was not done or results were negative).
  • An adult dairy farm worker was in the household. That person was healthy and was not tested. There’s no mention of whether the cattle on the farm where the person worked were known to be infected with H5N1 flu (but I wonder if a farm exposure history is why Cat 1 was tested initially).

We still need more details to assess this further. The timing of disease in the cats and the adolescent certainly fit with H5N1 flu transmission within the household. The negative tests decrease the concern a bit, but sample quality and especially timing of testing can make a significant difference. False negative tests can occur when we sample too late in disease if the virus has been eliminated by the time we test (even if the individual has not fully recovered yet). That’s a particular concern for a virus like H5N1 flu (in its current form) that’s not well adapted to humans and other mammals (hopefully it stays that way); I’d expect longer viral shedding with a well-adapted virus. So in this case sampling later in the disease course doesn’t rule out H5N1 flu entirely. If the adolescent had flu-like disease and tested negative for other potential causes (e.g. SARS-CoV-2, human seasonal flu), then I’d still be concerned about H5N1 flu.

Based on the limited information available so far, I’d consider this to be potential but as yet unconfirmed H5N1 flu transmission from cat-to-cat and cat-to-human.

Knowing the strain of H5N1 flu in the cat will help too. If it’s the dairy strain but the cat had no direct contact with an affected farm (e.g. indoor only cat on a farm, or an off-farm cat owned by a farm worker), that would suggest that the cat got infected from its owner, either because the person had and asymptomatic / unrecognized infection or they tracked the virus home on their clothes. Hopefully they’ll do some sequencing, and serological testing too. Antibodies against H5N1 in either the cats or the people could tell us if infection occurred in the past, even if they weren’t sick. Positive serology in the remaining cats or the kids would be strongly supportive of household transmission.

Household 2

  • Cat 1 got sick, died a day later and tested positive for H5N1 influenza.
  • Cat 2 was healthy and tested negative for H5N1 influenza on the same day as Cat 1.
  • The owner was a dairy farm worker who got sick 2 days before Cat 1, but it appears the person was not tested (at least not for H5N1 influenza).
  • The household was lost to follow up 3 days later.

We can’t rule out transmission from Cat 1 to Cat 2 in this case because the time frame for both the initial test and the monitoring period were too short to say for sure that Cat 2 did not get infected, but there’s nothing to indicate cat-to-cat transmission here either.

The timing of illness in Cat 1 could fit with human-to-cat transmission, because people can be infectious before they start to feel sick, and/or have only mild signs of illness at the start that they don’t recognize as being flu-related. Only 2 days between the owner and the cat getting sick is on the short side, but still within the range we might anticipate for flu transmission with very close contact in a household.

Knowing the cats’ lifestyles and other risk factors is important too. If Cat 1 was indoor only and not fed raw meat or milk, the owner is the logical source of the virus. If sequencing shows the cat was infected with the dairy strain of H5N1 and the cat didn’t have access to dairy cattle or their environment, that would be a very solid link too. With the person being sick first, it’s strongly suggestive of human-to cat transmission, but the smoking gun (testing the person before the cat and finding the same strain in the cat and person) is missing.

What does this all tell us?

We still need more details. Presumably most or all of this information is known to someone involved in the investigation. Getting the information out is important to help others identify risks and keep messaging balanced, in order to avoid panic but also help prevent additional infections. Cat-to-human transmission concerns can raise a lot of fear (and potentially bad consequences for cats). Human-to-anything transmission would be really noteworthy, so we need to know if it was almost certainly occurred, maybe occurred or almost certainly didn’t occur.

I’ve already been working under the assumption that cats with H5N1 influenza can be infectious to others, and this doesn’t change that. It gives a little more support to the concern (and weight to the recommendations to help avoid it) but doesn’t answer the question completely. It definitely does not make me less concerned.

As usual with emerging diseases like this, we need more surveillance, including the details.

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Cat-to-human or human-to-cat transmission of H5N1 influenza would be concerning but not too surprising.

Suppression of information by the US government would be concerning but not surprising given what’s going on at the moment.

But, did those occur?

The sharp eyes at the New York Times have raised concerns, because they spotted a CDC report that mentioned potential cat-to-human and human-to-cat transmission of H5N1 flu that was available online briefly on February 5, but then removed. 

Why was the CDC report removed? 

It could be because:

  • they found that the information was inaccurate
  • the story was incomplete and they wanted to get more details
  • the info is coming soon but needed additional sign-off by someone internally or an external collaborator
  • they were told not to release it

Each of those potential reasons causes a different level of concern, but the net result is that there might be some really important information that’s not being made available.

The New York Times article says the CDC report mentioned (likely/potential/confirmed?) transmission of H5N1 flu between cats and people in two households:

  • In one household there was potential transmission from a cat to another cat and a child.
  • In another household, a dairy worker got sick, then their cat got sick a couple of days later and died, raising the question of whether the cat was infected by the person.

I’ve assumed cats would be able to transmit H5N1 flu based on the amount of virus that has been found in respiratory samples from some cats using PCR. Cat-to-cat transmission would be quite likely given how closely some cats interact (e.g. playing / fighting, mutual grooming, sleeping together). Cat-to-human transmission seems possible for similar reasons. If an infected cat is shedding a reasonable amounts of virus in its respiratory secretions, I’d assume there’s some risk to any people who are in close contact, especially those who may have close contact with the cat’s face (e.g. owner of an infected house cat, veterinary personnel). In the case of potential human-to-cat transmission, we don’t really know if cats may be more susceptible to current H5N1 flu strains than people, or whether cats are just more likely to get serious disease when they are infected.

The nature of the evidence and investigation is important to know. 

Sometimes, it’s pretty clear how things were transmitted, based on things like a lack of other potential sources of infection and timing. Other times, it can be very messy. For example, in the second scenario, if the cat was an indoor cat (that didn’t sneak outside), didn’t live on a farm and was not fed raw meat or milk, human-to-cat transmission is by far the most logical source. Transmission by fomites (e.g. clothing worn on the farm) would have to be investigated too.

As I said at the start, cat-to-human and human-to-cat transmission would be concerning but unsurprising. “Concerning” may be a bit of an understatement, as it’s a potentially big deal. If a person can infect a cat, then it’s reasonable to assume that person was shedding a reasonable amount of virus (presumably in respiratory secretions) and therefore could have also infected another person. Human-to-human transmission is a very big concern, because if the H5N1 virus evolves to spread effectively person-to-person, and the general population has no immunity to this virus from previous exposure or vaccination, it could result in rapid widespread transmission (similar to SARS-CoV-2). There are still many gradations in transmission risk, and this one (disappearing) report by no means indicates we’re on the brink of a new flu pandemic, but it would be one more step along the way.

If H5N1 flu ever gets good at human-to-human transmission, cats will ultimately be only a minor risk to people in the grand scheme (even though we know they’re quite susceptible), because we’d be at greater risk of transmission from other people. However, if human-to-human transmission does not occur or remains rare, cats can be a bridge to humans from sources like wildlife, poultry and dairy cows, by bringing the virus into the household. A lot more people have close contact with indoor-outdoor cats than with other major sources of H5N1 flu.

From a personal standpoint, as someone who goes out and samples animals during emerging infectious disease events and who provides guidance to veterinarians, shelters and other groups about management of H5N1 flu suspects, I want more information about this risk ASAP, and it needs to be good, reliable information. I realize that there are inevitably necessary channels and approvals for sensitive information, but these should (hopefully) only cause minor delays in getting the information to those who need it most. The information doesn’t need to be polished, but it should be as close to real time as possible.

Emerging infectious diseases must be managed with active and transparent approaches. Too often, that’s not the case (and the initial US response to H5N1 in dairy cattle showed issues that were present even before the current administration).

Hopefully we’ll get more information about this soon. Hopefully these are rare events that don’t indicate an increased risk of mammal-to-mammal transmission. But, as I’ve said before, hope is not an effective infection control strategy.

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

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

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

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

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

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

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

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

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