Search results for: cat-to-cat

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It’s nice to finally be able to talk about H5N1 influenza without discussing ostriches, but unfortunately, we still have things to talk about.

1) Peacocks. Specifically peacocks that don’t have influenza and aren’t going to be culled.

Last month I posted about an animal sanctuary in BC (Critteraid) that was dealing with an outbreak of H5N1 influenza in poultry on the property. The outbreak lead to investigation of the risk to other animals on the same premises, including their peacocks. My understanding is the peacocks were physically well removed from the infected birds, so there was clear rationale to try to avoid culling them. It was a great example of how to respond to H5N1. Critteraid was transparent, worked with CFIA and followed their guidance, and took appropriate measures to contain the infection and decontaminate the affected areas. As a result, CFIA was able to assess the situation through their established processes, the peacocks were tested and were negative for the virus, and they were therefore able to avoid culling them.

2) More sick cats with the flu. This story doesn’t have such a good ending. We know cats are susceptible to H5N1 flu and often develop fatal neurological disease. Recently a lot of attention has been paid to exposure of pet cats to the virus via raw diets and raw milk, but cats can also get infected through contact with infected birds. A recent cluster of fatal H5N1 infections in a group of kittens in the Netherlands reminds us of that risk:

  • On November 19, a kitten from a dairy goat farm was found dead and tested positive for H5N1 flu. Samples were collected from other animals on the property the next day as part of the investigation. Three adult cats (including the kitten’s mother) were tested, as were the goats (the goats probably being the main concern, given the issue with H5N1 in dairy cattle in the US). All of the animals were healthy at the time, and all tested negative.
  • However, the other 7 kittens from the litter weren’t on the farm anymore as they had all been adopted. Further investigation revealed that they had all died; none of them were tested, but it’s quite likely they all could have had H5N1 flu as well.
  • The source of the virus in this case couldn’t be identified, but the mother cat was seen with a dead bird October 27, and it would be logical to assume that the kittens may have eaten part of the bird or been exposed to it it some way. The timing doesn’t quite fit though, as that was 23 days prior to the one kitten being found dead on the farm, which is a very long timeline for flu. I’d have to wonder if there might have been exposure from another dead bird, since the mother cat likely caught birds on more than one day, or she may have been infected by the bird caught on October 27, and then the kittens were subsequently infected by her.

This case is a good reminder of the potentially devastating impact of this virus on cats and one of the reasons why we’d prefer to keep cats away from wild birds. If cats have outdoor access, flu must be a consideration in any cat that develops acute neurological disease or dies suddenly.

We don’t know what’s the risk is to people from infected cats. I think we have to assume there’s some degree of risk since there is evidence to support that cat-to-cat transmission can happen. Fortunately the risk to people seems to be low, given the lack of identified cat-to-human transmission. However, we can’t rely on that to say there’s no risk, both in the present and in particular in the future, as this virus continues to evolve.

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The H5N1 influenza situation in North America has been quieter lately (apart from saga with the BC ostriches, but that’s a different story). There’s been less spillover into domestic animals recently and less attention in the news, but the virus is still out there and causing problems. (H5N1 flu activity has already started to increase in some areas as the fall wild bird migration gets underway.)

I’ve written about the risks of H5N1 flu (and other pathogens) from raw pet food many times, so the latest report about 2 cats from a household in California that got sick and died after eating commercial raw pet food isn’t a surprise. It doesn’t change anything, but it’s yet another reminder that this virus is still a risk, and there are completely avoidable death occurring in pets (primarily cats).

Both of the cats from the report were nominally indoor only. That narrows down the potential sources nicely to start. Both cats were fed a commercially available raw pet food. The first cat got sick and rapidly deteriorated and developed severe neurological disease. That’s quite common in cases of H5N1 influenza in cats that have been reported to date, but the first cat wasn’t tested. Five days after the first cat got sick, the second cat became ill with similar signs, starting with fever and loss of appetite and ending with severe neurological disease. It tested positive by PCR for H5N1 influenza after it died.

  • With that time frame, we have to consider whether the second cat could have been infected from the diet or from the first cat. The latter would be the worst scenario, as cat-to-cat transmission would be a significant concern, and also indicate potential risk of cat-to-human transmission. Unfortunately there isn’t any way to determine the transmission route in this type of situation.

Further testing showed that it the genotype of the virus from the second cat was the B3.13. That’s the main strain found in dairy cattle in the US, though it is also still possible to find it in wild birds and poultry. Presumably a poultry-based raw diet was the source, but since it was B3.13, I assume they investigated raw milk exposure too, as well as direct or indirect exposure to dairy farms.

The implicated raw diet is currently being investigated. It’s the logical source of infection in these cats, but confirmation would require detection of the virus in the food. Unfortunately, the time lag from ingestion to illness to diagnosis to investigation in cases like this often means the food that was the potential source is long gone before testing can be done, especially with raw diets that are often made in small batches (vs kibble diets). If the lot of the diet is known, then there’s the potential to find other samples from the same lot, but we also don’t know how evenly distributed the virus (or any other contaminant) would be within a batch of food. It can therefore be hard to show a definitive link, but it’s been done before so we clearly know raw food-associated H5N1 flu is an issue.

The report does not name the commercially available food that’s being investigated.

  • On one hand, you can understand why: the link is not confirmed at this point.
  • On the other hand, if there is contaminated food out there that’s yet to be eaten, or if there are sick cats that haven’t been diagnosed because people don’t know about the risk, holding back the information can be a problem.

I understand why this particular information is being withheld at this point, but hopefully more information will be released as the investigation continues.

What should cat owners do?

  • Avoid feeding raw poultry-based diets to cats (and other pets that may be susceptible).
  • If a raw poultry-based pet food is to be fed, use a high pressure pasteurized diet; this will be lower (but not zero) risk for pathogens like H5N1 flu.
  • If signs of illness occur in a cat that’s eaten a raw poultry-based diet, a veterinarian should be contacted ASAP and the raw diet exposure history should be explained. Fever, lethargy and decreased appetite are probably the most common initial signs. Respiratory disease may not be apparent. Neurological disease is a big concern but typically occurs later. If these signs occur in a cat that has been fed a raw diet, H5N1 should be considered, testing is warranted, and there might be value in providing antiviral therapy (if it can be given early enough).
  • We should assume that cats that might have H5N1 are infectious to other pets and possibly people. The risk is probably highest to other cats (since cats are quite susceptible to the virus and cat-to-cat transmission has been shown experimentally) but we can’t rule out the potential for cat-to-human transmission. Keeping the sick cat isolated and using good infection control practices are critical.

The LA County Deptartment of Public Health is monitoring household members and veterinary personnel who were in contact with the cats from this report, and are offering flu testing to anyone who gets sick, but there is no evidence of human disease at this point. That’s great. The risk is probably low, but it’s not zero. When we see a low risk situation over and over again, it eventually ceases being low risk, and then it becomes a bigger problem.

One last thought: Why are we seeing another report of H5N1 in cats from LA County?

  • Do they have lots of people? Yes, it’s a huge county.
  • Do they have lots of cats? Yes, more people generally means more cats.
  • Do they feed raw diets to pets more often? Maybe, but probably not that disproportionately.

The main reason is likely the fact that LA County has an exceptional public health system that does at great job integrating animal health. That raises the concern that more situations like this are likely occurring elsewhere but are going unrecognized. The less robust the local surveillance and communications, the fewer cases are found. I assume there are many more cats that have died from H5N1 flu after eating contaminated food than we know about.

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Last week, I wrote about the use of oseltamivir in dogs and cats infected with H5N1 influenza. We have to be aware of the risk of drug resistance risks whenever we use anti-infectives, especially when the same drugs (like osteltamivir) are used in people, and assess the risks and benefits in order to “use as little as possible, but use enough.”

That’s lead to some questions about use of the antiviral GS-441524 in cats. This drug has been a game changer in the management of a previously almost invariably fatal disease in cats called feline infectious peritonitis (FIP). 

When considering use of oseltamivir for treatment of H5N1 flu, there are a few things to our advantage:

  • It’s a narrow spectrum antiviral 
  • H5N1 infection and virus shedding are short term 
  • There’s no endemic H5N1 flu virus circulating in the cat and dog population
  • Cats and dogs are not common (if ever) sources of H5N1 transmission to people
  • We can implement infection control measures during the short treatment course required and estimated virus shedding period to contain the risk of spread of any resistant virus

The risk of a resistant H5N1 virus emerging during use of osteltamivir in a pet is therefore low, and the risk of any such virus spreading is even lower. While there’s some risk, with basic precautions, I think we can justify its use in infected pets under the right circumstances.

When considering use of GS-441524 (GS) for treatment of FIP, there are some very important differences:

  • GS is a broader spectrum antiviral
  • Enteric feline coronavirus (the virus that mutates to ultimately cause FIP in cats) can be shed by infected cats for months
  • Enteric feline coronavirus is a cat-adapted virus that can spreads very efficiently from cat-to-cat via fecal-oraltransmission

The risks of resistance when using GS in cats therefore differ according to the scenario.

1. Using GS to treat a cat with FIP

    • There’s a risk of emergence of resistant FIP virus within a treated cat. This would be bad news for the cat, but probably of limited broader risk since once enteric feline coronavirus becomes a cause of FIP, it’s not readily transmitted anymore. Odds are that the cat would not transmit the resistant virus further. We can’t say there’s no risk, but it’s low risk.
    • If the cat had concurrent intestinal infection with feline coronavirus, then there would be a risk of that virus becoming resistant and then spreading. One study reported fecal shedding of feline coronavirus in 61% of cats with FIP that were being treated with GS . Shedding dropped fairly quickly in most cats, which shows some likely impact of the drug, but it also shows that there’s some plausible risk of resistance emergence and transmission.
    • Since FIP is devastating, GS is highly effective, and the risk of resistance spreading is low, this is clearly a high-benefit / low-risk use situation. However, it’s not no risk so we need to study it more and optimize our treatment approaches.

    2. Using GS to treat cat with enteric feline coronavirus infection

    • Feline coronavirus is widespread and continually circulating in cats. There’s been some discussion of use of GS to knock that back, and to try to eliminate it from groups of cats (e.g. catteries). Treatment will reduce fecal shedding of the virus, and less shedding would likely have some downstream reduction of FIP, but I have my doubts that we can do much to control spread in the grand scheme with an antiviral. Reducing and eliminating a virus are different, and reducing while creating a substantial risk of resistance isn’t usually a good combination. In general, we are rarely able to use anti-infective drugs for effective infection control approaches in a population, especially for a virus that’s host adapted and endemic.
    • If we are treating cats with enteric infection, there’s a lot of virus, a lot of cats and a lot of chance for resistance emergence. If resistance emerged in a cat, it could shed large amounts of virus for long periods of time, releasing GS-resistant virus into the cat population and hampering our ability to treat FIP when it occurs. That’s a big concern for me.
    • Since enteric feline coronavirus infection isn’t a major health issue, treatment is not likely to have a major impact on enteric virus circulation, GS is so important for cats with FIP, and resistance would result in cat deaths, I have a hard time finding an indication for use of GS for enteric feline coronavirus.

    Dr. Niels Pedersen, a (or The) leader in development of antiviral approaches for FIP has a nice commentary entitled “Inappropriate use of GS-441524 in an attempt to eliminate Feline Enteric Coronavirus (FECV) from healthy cats.” The title gives away his thoughts on the matter. It’s a good, impassioned summary of why we need to be good stewards of FIP antivirals and why targeting feline enteric coronavirus is likely a bad idea.

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    We don’t talk a lot about antiviral resistance in animals, particularly compared to antibacterial resistance, primarily because we don’t use antivirals a lot in animals – but we do use some.

    When we use any anti-infective medication, we have to think about the risks of resistance developing to that medication, and how we can try to optimize use in order to maximize clinical benefits while minimizing resistance risks. I say “minimizing,” not “eliminating,” on purpose, because we can rarely guarantee that there will be no risk. Usually we’re aiming for a risk that is low enough to justify use of the drug, and ideally we can lower that risk even further through some basic infection control practices.

    I’ve written a lot over the last year about antivirals for the treatment of feline infectious peritonitis (FIP) in cats, since we now have incredibly effective options (now in Canada) for what was previously an almost invariably fatal disease. The antiviral drug GS-441524 (a relative of the COVID-19 antiviral drug remdesivir) is what we can now use in most regions to treat this disease. It’s a complete game changer.

    However, anytime we have a revolutionary new anti-infective (be it for bacteria, viruses, parasites, fungi or other nasties), we need to take steps to try to maintain its effectiveness long term – primarily through good antimicrobial stewardship.  

    Are there scenarios where GS-441524 resistance could emerge in cats and be a problem? Yes.

    Does our routine treatment of FIP pose much of a risk for emergence of antiviral resistance? No.

    Those statements seem to conflict, but let me explain:

    Feline infectious peritonitis is caused by feline coronavirus (FCoV), whichbis very common in cats, especially especially cats living in large groups. It infects the intestinal tract and then cats shed it in their feces, usually with no signs of illness The virus is spread cat-to-cat through fecal-oral exposure. The clinical disease (FIP) develops when an intestinal-dwelling FCoV mutates into a tissue-infecting FIP virus (FIPV) within that cat.

    While FCoV is highly transmissible, FIPV is not traditionally considered to be transmissible between cats (though I’m not sure we can actually say there’s no risk of transmission; I think there is some cat-to-cat transmission risk, but it’s probably very low).

    Antiviral resistance can emerge when a virus is exposed to an antiviral. Random mutations occur that allow viruses to resistant the action of the drug, and then the mutated virus multiplies while the susceptible version may die out. The key is what happens next, and that varies a lot depending on the disease.

    A critical component of the risk assessment for GS resistance is that is that we don’t consider FIPV to be transmissible (or at least we consider it really rarely transmissible). So if GS resistance develops in a cat with FIP, while it could be very bad for that cat (because the infection won’t respond to the drug), the odds of that cat transmitting the resistant virus to another cat are very low, so the resistant virus would likely die out with the first host.

    The main concern is if cats with intestinal FCoV infections are treated with GS, then resistance could develop in FCoV in the gut of a cat, and that resistant virus would be shed in feces and be transmitted cat-to-cat (like FCoV normally is) through the cat population. Then, if any of those cats develops FIP, the virus would already have GS resistance and wouldn’t respond to treatment.

    That means that appropriate treatment of cats with FIP poses an exceptionally low risk of causing resistance issues. For there to be a significant problem, it would require rare cat-to-cat transmission of FIPV (or emergence of resistance of FCoV from a concurrent, unrelated FCoV infection in the intestine). The benefits of treatment massively outweigh this small theoretical risk.

    Empirical treatment of cats for FIP (without a solid diagnosis) is also generally low risk of resistance. If the cat doesn’t have FIPV or FCoV, there’s no risk. It’s not like the situation with bacteria where there’s always a massive and diverse pool of bacteria in the body that could develop resistance through exposure as “bystanders” and where resistance genes can then be transmitted between different bacteria. If the cat doesn’t have FCoV or FIPV to start, it can’t develop resistant FCoV or FIPV. The only risk would be (as above) if there was a concurrent incidental intestinal FCoV infection.

    Does use of GS in cats with enteric FCoV pose a risk of resistance? That’s a big “hell yeah!” from me. We don’t have data to prove it, but the last thing I want is to be treating lots of cats with highly transmissible enteric FCoV infections, because spreading resistant FCoV through the cat population is how we end up with a lot of dead cats from resistant FIP.  The cost-benefit just doesn’t add up in this scenario. Enteric infections are mild or cause no disease at all, and we are not going to eradicate FCoV from the overall cat population, or likely even from a local cat population (e.g. cattery), using antivirals (the virus is just too common and transmissible to be thinking about eradication). So we’re talking about treatment with little to no clinical benefit and no eradication benefit, while posing significant risk of selecting for resistance to a game-changing drug that is currently saving countless feline lives. That makes no sense to me.

    Are there any human health issues related to use of GS in cats?

    GS is related to remdesivir, a drug used to treat COVID-19 in people, but transmission of antiviral resistance from FCoV to SARS-CoV-2 isn’t a risk. The only theoretical risk would be if a cat had FIP AND a SARS-CoV-2 infection at the same time, AND resistant SARS-CoV-2 emerges in the cat during treatment AND the cat then infects a person with it. That’s a really unlikely scenario, so the risk is negligible compared to risks from direct use of remdesivir in humans.

    Use as little as possible but use enough. That’s my line for antibiotics, and it applies equally well to antivirals. We don’t want to miss treatment opportunities, but can’t be reckless either and jeopardize this incredible new opportunity to be able to treat this terrible disease.

    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.

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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.

    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.)

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    Back in August, I wrote about a high-profile but poorly described “outbreak” of feline infectious peritonitis (FIP) in cats in Cyprus. Now, a preprint publication (Attipa et al. 2023) has come out with some interesting information, but lots of questions still remain. That’s the nature of emerging diseases – a single report gets us farther down the path but rarely answers all of the important questions.

    Here’s a quick refresher about FIP: Feline infectious peritonitis is a nasty disease caused by feline coronavirus (FCoV), a very common virus in cats that circulates widely in the cat population. Whenever you have a large group of cats, you can be pretty sure there’s FCoV circulating somewhere in the group. It’s usually not a problem because the virus doesn’t typically make most cats sick. However, FIP develops in a small percentage of infected cats when there’s a random mutation in the spike protein of the virus that occurs after the cat is infected. So FIP is not spread cat-to-cat, it’s FCoV that spreads between cats, with an unpredictable change to the virulent version of the virus causing FIP in some cats.

    In the past, FIP has been considered an almost invariably fatal disease. There are some new treatment options, but those are mainly black market-sourced in North America. I’ll get back to that topic later as there are some new developments.

    Back to the cat deaths in Cyprus…

    Reports came out of Cyprus earlier this year of thousands of cats dying from what they suspected was FIP. The numbers varied a lot and tended to decrease when people started asking questions, but still, there were lots of cats dying. It’s tough to assess the significance of reports like that when we have little data about what’s going on now and little historical data for comparison. There are lots of cats on Cyprus and most are feral. If you have a million feral cats, I’d expect a hundred thousand (or more) deaths per year in that population under normal circumstances, many due to FIP among other things, but we almost never hear about those deaths in the population because people don’t look, count or test. However, I also don’t dismiss boots-on-the-ground observations, so when people start talking about remarkable increases in deaths, I pay attention.

    The preprint (Attipa et al. 2023) provides some more epidemiological and clinical information:

    • They only had 165 confirmed FIP cases, which is a very small number for a country with so many feral cats, but since diagnosis requires lab testing which costs money, only a subset of cats ever get tested.
    • This rate is considerably higher than the 3-4 cats per year in Cyprus that are normally diagnosed with FIP, but we don’t know if that’s because of a marked increase in disease, a marked increase in testing, or a combination of both.
    • Disease in the confirmed cases was pretty typical for FIP (i.e. no unusual clinical findings).

    So was this a huge outbreak or not?

    When there’s more discussion and awareness of a disease, you get more diagnoses of endemic disease that may have been there all along. Often these “outbreaks” are combination of a small overall increase or a small local cluster of disease and increased testing and reporting, leading to over-estimation of the true increase in the disease rate. Only a few cats a year were diagnosed with FIP in Cyprus before this, but how many did they test? If you start to look, you find things that have always been there.

    However, the genomics data included in the paper support concerns that this could be a true, new, large FIP outbreak. The virus they found appears to be a recombination of a feline coronavirus with a pantropic canine coronavirus strain (see diagram below). Those are both alphacoronaviruses, which are different from SARS-CoV-2, which is a betacoronavirus. This kind of recombination event can happen, so it’s not a shocker. What it means and whether it will play a role in changes in disease or disease patterns is unclear, but it will need to be studied.

    The big question is whether this new strain might be transmissible between animals as an FIP virus, versus the typical situation where a random mutation needs to occur in the FeCoV in a previously infected cat in order to cause FIP.

    • Based on some genomic characteristic and the very close similarity of different virus isolates, the authors suggest that this could be a virus that is transmissible in a FIP-causing form. If so, that would be a big concern, and could plausibly cause a true, large outbreak of FIP.

    It’s also unclear if this is a new issue or whether it’s a newly discovered issue that’s already been there for some time. If it’s driving an outbreak, we’d expect that it’s a new emergence. However, due to lack of surveillance before the outbreak, we can’t be sure it wasn’t there before. I’d guess it’s fairly new based on the similarity between the strains they tested, as viruses tend to develop more population diversity over time (e.g. you find a wider variety of strains the longer the virus has been circulating, but all the strains they found were very similar).

    This virus strain was also found in a couple of cats imported from Cyprus to the UK. That’s not surprising since lots of cats get moved around and they bring pathogens like this with them. It raises concern about whether this could result in seeding of a new, more virulent, strain in other countries around the world.

    It’s still too early to make a definitive call on whether this is a transmissible FIP virus, whether it’s likely to cause other outbreaks or whether it’s spread beyond Cyprus. Unfortunately, as is often the case with infectious diseases, it’s a “time will tell” scenario, but we need to keep investigating to figure out what’s going on, what the risks are and, if this is an issue, what we can do to reduce there risk and impacts.

    They also used molnupiravir in lots of cats during this outbreak, as they were using up stockpiled and expiring COVID-19 drugs for people. That scares the hell out of me given that drug’s crap-tastic performance as a COVID-19 drug and its tendency to cause more viral mutations. But, that’s a separate story…

    Information continues to trickle in about the situation with H5N1 avian influenza in cats in Poland. It’s mainly been updating the number of confirmed infected cats, with 16 cats in 3 cities being the latest number I’ve seen. It’s also now reported that some of the cats are primarily indoor (house) cats.

    Numbers are part of the story, but the bigger question is how the virus was acquired by the cats, and if it’s spreading between cats. I still haven’t seen a good description of the epidemiology, investigation of infected households and clear information about pathways. Presumably (hopefully) that part of the investigation is well underway.

    An OFFLU report about the situation with the infected cats in Poland reads “Early reports indicated that not all suspect cases had outdoor access suggesting that a direct role from infected wild birds is unlikely as a common source. The wide geographical distribution of suspected cases suggests that the primary mode of spread in these cases is not cat-to-cat transmission.”

    That raises the question, “If the virus isn’t coming from birds and it isn’t coming from cats, where is it coming from?”

    • I haven’t seen lab leak, 5G, aliens or Tony Fauci implicated yet on social media, but that may come.

    Unlike most spillover events, the situation here is still pretty cloudy at the moment. I don’t have any inside knowledge, so it’s easy to arm-chair quarterback and be completely wrong, but I’d wonder whether there’s enough data to actually rule out those two sources.

    At this point, there are three main possible sources for discussion:

    Food

     I’ve seen suggestions that food (that is, what the cats are eating) could be the source of the virus. I wouldn’t rule it out, but it’s a stretch. It would have to be from raw diets made from infected birds, obviously. I assume it’s unlikely that infected commercial poultry would be put into any food chain, even for pet food. When poultry are infected, it’s pretty obvious since there’s widespread illness, so it would almost certainly have to be someone making a diet from birds that were known to be sick. People do dodgy things, but that’s unlikely.

    H5N1 virus making it into a cat’s diet is one thing. Actually causing an infection is another. Flu viruses don’t persist well outside the host. I haven’t seen any H5N1 data, but a study of H7N9 showed survival of the virus for up to a week at refrigeration temperatures. So, while it’s possible, “up to a week” means that’s the upper limit that was found, and I’d assume that there was a pretty steady decrease in virus viability (and therefore risk) over that time. We should assume there’s some potential IF a diet is made from infected birds AND it’s not cooked AND it gets to the cat within a few days (and even then, there has to be exposure to a minimum infectious dose).

    Overall, I wouldn’t completely discount food as a source, but I think it’s unlikely. To start, I’d want to know whether these cats were all fed the same diet (and same lot of that diet). A diet history is a pretty easy way to see whether this even deserves investigation.

    Cat-to-cat transmission

    This is pretty easy to rule out since cats were from multiple cities with disease onset around the same time. Given the way cats do (and don’t) move, a common contact pathway is exceedingly unlikely, so let’s toss this one out for now.

    Repeated wild bird-to-cat transmission

    This is still my #1 guess. The kicker is that some cats were indoor cats. A question about this is whether the cats were “indoor”, or actually indoor. Not uncommonly, people say their cats are indoor, but they get outside (e.g. sneak out, are allowed outside on a deck, taken out on a leash). It’s far from rare vet clinics to see an “indoor” cat that’s been hit by a car or tangled with a wild animal.

    If the cats were truly only indoor, that limits the transmission pathways a lot and largely (but not completely) rules out direct transmission from a bird.

    A mix of these

    This might be an explanation for some of the “indoor” cat infections. For example, if there’s a cluster of infections in outdoor cats that’s predominantly or solely bird-to-cat transmission, with maybe some cat-to-cat transmission. But how, if a cat is truly indoor only? Well, it’s maybe a stretch but the picture below of cats facing off through a window is one I put on Twitter the other day, as a reminder that indoor cats need to be vaccinated since they can still have close encounters with outdoor cats. For affected indoor cats, if food is largely ruled out, I’d query the household situation, such as are there screened windows or some other way to have close contact with any outdoor cats (or other cats in the household that do go outside).

    Hopefully the investigation will generate some more information. It’s important for us to know more about transmission pathways and risks, both for cat health and to assess risks to other species (including humans).

    Milo, the indoor cat and Rumple, the outdoor cat, have an encounter through a window screen.