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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.