A few months ago, there was a cluster of kittens from a dairy goat farm in the Netherlands that likely all died from H5N1 influenza (only one kitten was tested). That wasn’t too surprising since the H5N1 virus is circulating in wild birds internationally, and we know that cats are highly susceptible to the virus, and can develop very severe (and not uncommonly fatal) disease. Being from a dairy goat farm, there was concern the kittens could have been infected via milk from the goats, because that’s what’s been seen / suspected in cats on dairy farms with infected cows in the US. When the H5N1 virus spilled over into daily cattle in the US in 2024, it spread rapidly and widely amongst dairy farms, often infecting (and killing) the farm cats. Initially it was reported that the goats and several other cats on the Dutch farm were also tested at the time and were negative for H5N1 flu. It was therefore assumed that the kittens had been exposed to the virus via wild birds, possibly indirectly through the mother cat.
More information about this case has recently been released. It turns out the farm was a dairy cattle farm (or at least had cattle as well as goats). When they tested the cattle – which were all clinically healthy – they found one cow that had antibodies against H5N1 influenza. If the test isn’t a false positive (which is always a consideration with rare outcomes and antibody tests), that would mean at least one cow was infected with the virus at some point, but wasn’t sick enough for it to be noticed, and then eliminated the infection without spreading it to the other cows, and leaving antibodies in the bloodstream as evidence.
It’s a good news/bad news situation.
The H5N1 influenza outbreak in dairy cattle in the US has been a mess, both in terms of the impact and the approach to control. We’ve been watching to see if the virus would spread or cross over into cattle again in another country. Sporadic wild bird-to-mammal transmission of the virus in wildlife especially is not uncommon, but the infections don’t lead to further mammal-to-mammal spread. Hopefully that’s what happened on this farm. However, every time H5N1 flu jumps into a mammal, it creates more opportunity for the virus to adapt to spread more easily in mammals, or the strain that jumps could already be adapted to this kind of spread. The more domestic animals get infected, the greater the risk of human exposure, and the more risk of further adaptation or recombination with human flu viruses to make another strain that could be worse for us.
The good news is that this seems to have been a one-off situation on this farm. The virus spread to at least one cow but didn’t cause overt disease, didn’t seem to spread further in cattle, and the situation seems to have resolved itself with no damage beyond the initial cluster of infected kittens.
But this is yet another reminder of why we need to pay attention to H5N1 flu and try to control it as best we can. This situation didn’t cause a problem, but a nationwide outbreak in dairy cattle will likely start with a single spillover on a single farm, so we need to remain vigilant so that we can hopefully stop it before it spreads further from there.