A couple of days ago, there was a ProMed request for information following a report of a large number of dead cats in Poland. The cats were reported to have had respiratory and neurological disease, so my thought process went to “that fits with avian flu spillover,” then “but that large of a number would be really surprising.” However, it had to be on the radar given the clinical signs, limited other diseases that would cause that type of clinical presentation in otherwise healthy adult cats, and the dynamic nature of this massive international H5N1 avian flu pandemic.

Details are still pretty sparse, but there are now various reports that “flu” or “avian flu” were identified in at least some of the dozens of dead cats. That obviously raises concern and highlights a need for more detailed information.

It also gets me thinking about a few big questions:

  • Were these actually H5N1 avian flu infections?
  • Was this one transmission event (e.g. a bunch of cats exposed to the same source of virus) or multiple spillover events?
  • Was this likely all bird-to-cat or was cat-to-cat transmission also involved?
  • Have there been less severe infections in cats in the same area?

The last question is a big one. Right now, we know very little about the range of clinical signs in infected cats (and other species). The risk calculus is a lot different if this virus always causes severe/fatal disease versus if it causes a range of disease. We are more likely to find severe disease, especially in wildlife and feral animals, because we don’t notice mild illness as well in those species. Very sick or dead animal are much more likely to be caught and tested.

  • If there was group of 10 dead cats, odds are pretty high that would be recognized and at least some of the cats would be tested.
  • If there was a group of 10 cats with transient fever, cough and lethargy, that’s much less likely to be noticed (and the cats are less likely to be caught and tested).

That second group might be really important though. From a population standpoint, a virus that kills consistently and quickly is generally less likely to spread compared to one that causes more mild, prolonged illness, especially if the animal is still moving around and interacting with other animals.

Said another way, a big question is whether severely ill cats are the norm for H5N1 spillovers or whether they are just the tip of the iceberg. If the latter, then we have a much greater need to figure out what the risk of spread from them is. We haven’t really been able to explore this well yet here in Ontario because of logistical challenges, but the situation in Poland may help shed some light on the matter if more testing of live cats is done as part of the investigation.