We know a lot more about the situation with H5N1 influenza in dairy cattle than we did a couple of weeks ago, thanks to ongoing research and (more importantly) better disclosure of information that has been held pretty tight up until now.

Current situation with infected dairy herds in the US

Rather than focus on numbers, I’d rather just say it’s on lots of dairy farms in lots of US states. As discussed below, this is likely much more widespread than the official numbers suggest, so those numbers aren’t very useful.

When/where the outbreak of H5N1 flu in dairy cattle likely started

Genomic data suggest that the current H5N1 spillover into cattle likely occurred much earlier than was known. It was unlikely that we’d caught the first (or at least one of the first) affected dairy herds in March. Rather, it looks like the spillover into cattle in the US likely occurred in December 2023 (but maybe as early as October 2023). That was presumably from a single bird-to-cow spillover, with an associated mutation of the virus that made it better able to infect cattle. Where did this happen? Somewhere in the US is all I’d say at this point.

After that first bird-to-cow transmission, subsequent transmission is thought to have been from cow-to-cow, with spread on farms through contaminated milk, and spread between farms and states through movement of cattle. Given the limited evidence of virus in respiratory samples from cattle and the large viral load in milk, spread on farm is probably through human-associated milking practices, based on the high likelihood of tracking milk between cows during milking.

There has been subsequent spillover of this H5N1 strain from cattle into cats and poultry flocks. Some farm cats have had severe disease (and even died) from the virus; farm cats could possibly be good sentinels in this situation (i.e. if you see dead cats (more than usual) on the farm, consider looking for influenza in the cattle). Spillback into wild birds is also a concern, since if this strain goes back into wild birds, the situation becomes even harder to control: We can much more effectively monitor and control cow-to-cow transmission than an ongoing risk of exposure from wild birds (that also don’t respect political borders).

What do we know about potential for human infections with H5N1 flu from cattle?

The human case of H5N1 flu associated with contact with dairy cattle in Texas raises concern. Although it is the only one identified so far, testing of exposed people (e.g. farm workers) has been limited. There are lots of anecdotal reports of farm personnel in the US avoiding testing and not telling anyone when they are sick. That’s in part because a lot of dairy farm workers in the US may be undocumented and therefore have concerns about getting on the radar of anything related to government.

There are some interesting aspects of that one human case, too. The person was a dairy farm worker so there’s obviously a link to cattle, but surprisingly there was no testing of cattle on that particular farm. The strain that caused the human infection is genomically a bit different from the main strain circulating in dairy cattle, which supports concerns that there might be more widespread and ongoing transmission than we’ve realized. With more time and transmission, the the virus gradually accumulates genomic changes. The strain from the person had one of the more common mutations (PB2:E627K) that helps the virus adapt to humans; that raises a bit more concern, but it’s still a long way from being a “human-adapted” virus. Odds are that strain died out in that person, but it shows how there could be cow-to-human transmission and that there can be continued concerning mutation of the virus when this happens.

The risk of H5N1 flu (or lack thereof) in pasteurized milk

An earlier report that described finding H5N1 virus material in commercial milk samples caused a stir, since about 20% of samples were positive. Most of us were pretty unconcerned since the test used (PCR) also detects dead virus, and we’ve been confident that pasteurization will kill flu. Subsequent testing confirmed that live H5N1 virus was not present in those commercial milk samples, which is good news. The most important aspect of the report was that it supported the thought that this virus must be MUCH more widely established in US dairy herds than current testing suggests.

Concerns about raw milk remain, but there are lots of other infectious diseases risks from raw milk regardless (so just don’t drink raw milk).

Is there a risk of H5N1 flu in beef?

There’s not much reason to think that contamination of commercial beef with H5N1 flu would be common or high level, but a small initial study didn’t find evidence of the virus in beef samples.

What is the risk of H5N1 flu in cattle in Canada?

We don’t know. Since the virus was probably flying under the radar in US dairy cattle for months, we have to be careful thinking “we haven’t found it in Canada, so we’re good”. It might not be here. It might be here and we don’t know. That’s why we have to look.

If H5N1 flu has not ye gotten into the Canadian dairy herd, can we prevent this from happening? Maybe.

• If the virus is only moving via cow-to-cow spread in lactating cattle, we can contain that.
• If spread is via cow-to-cow transmission beyond lactating cattle, that makes control harder, because that’s more cattle and recent restriction on importation of cattle into Canada are focused on lactating cattle…
• If this strain of the virus spills back into wild birds, then we’re in trouble. Wild birds don’t respect borders so it would probably just be a matter of time before it found its way onto a Canadian dairy farm and into a Canadian cow.