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:


 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.