H5N1 influenza has been widespread is US dairy cattle for close to a year, so you might wonder why a recent report of H5N1 in dairy cattle in Nevada is garnering extra attention. Well, it all comes down to the strain. TLDR:
- The cattle were infected with the D1.1 strain of H5N1, which has been circulating widely in birds and poultry recently, but is different from the B3.13 strain of H5N1 that’s been infected dairy cattle in the US since at least March 2024.
- There’s been at least one associated human infection (mild) in a dairy worker in Nevada.
- This D1.1 strain appears to include a mutation that may help the virus infect mammals.
H5N1 influenza isn’t one specific virus. There are already numerous known subtypes and new ones continue to emerge. The “dairy strain” that’s spread among US dairy cattle is called clade 2.3.4.4b, genotype B3.13.
Finding the B3.13 strain in cattle in Nevada wouldn’t have been surprising, but multiple herds in Nevada tested positive for a stain of H5N1, subtype D1.1. This “new kid on the block” is a recombination of one of the H5N1 strains that moved from Asia to North America in late 2021 and early 2022 and a low pathogenicity avian flu strain that was already present in birds in North America. It was first identified in September 2024 and has now emerged as a dominant strain in wild birds (and spillovers into poultry).
The first detection of the D1.1 strain in Nevada cattle was from samples collected January 6 and 7, 2025, so (as is typical) we’re playing catch-up. It will be important to see if these farms are linked, and whether there are other affected farms. It will also be important to see if shedding patterns (lots of virus in milk, little in respiratory secretions) and virulence in cats on these farms (lots of dead cats) are similar to B3.13. Obviously the risk to humans must also be tracked(more on that below).
This is a noteworthy event because it represents a new spillover into cattle from wild birds. Infection of a single cow wouldn’t be too remarkable, since rare spillovers to mammals are expected given how widespread the virus is in birds. However, infection in multiple cattle on multiple farms suggests either effective bird-to-cow transmission on multiple different premises, or another single bird-to-cow spillover that has subsequently been spread cow-to-cow and farm-to-farm like B3.13.
Another concern is the potential for more severe disease in people. Human infections with D1.1 have been previously identified in people who were depopulating infected poultry farms. They had mild disease, like most of the human H5N1 flu spillover infections in the North America to date. But D1.1 is the strain that was involved in a fatal H5N1 flu infection in a person in Louisiana, and that caused very severe disease in a teen from BC. On one hand, we can say any H5N1 strain can cause severe disease under the right circumstances and maybe those were just rare events. On the other hand, those two severe infections show that we can’t sit back and just say but human infections are always mild. When something spreads widely, rare events become more common, so we can’t ignore them. It also raises concerns about what could happen if this strain becomes better able to spread human-to-human. A more transmissible strain that can cause severe disease in people is the big concern.
Highlighting these concerns, a human infection has already been linked to these Nevada dairy cattle. Fortunately, the person had mild disease (conjunctivitis), akin to what’s been seen in dairy workers with B3.13 infections. Still, it shows that this strain also poses a risk to people who work with infected animals.
There’s also a concern about evolution of this virus in cattle, even over this short timeframe. The Nevada cattle D1.1 strain has already acquired one mutation (PB2 D701N) that makes it more able to infect mammalian cells. Lots of things still have to happen to make this a mammal-adapted virus, and even more for it to become an effective human pathogen, but this is a potential step in that process. Spillover into farm workers raises the stakes further, since any new infection of a person increases the likelihood of human adaptation or, worse, recombination with a human flu virus in someone coinfected with two flu strains at the same time (e.g. H5N1 and a human seasonal flu). A recent CNN article has a nice description of some of the issues related to the H and N changes in these H5N1 subtypes.
More information about the effects of this “new” strain on cattle is also needed. It’s reported that the cows were not obviously sick when the positive results were first obtained as part of a state screening program, but that disease subsequently occurred. That might suggest that disease is mild and only found when someone is really looking for it, or that they just picked up these infected farms very early with their surveillance program. The US recently started a national Milk Testing Strategy surveillance program, which should help detect problems earlier. We’ll also need more information about the spread of this strain within farms and whether it’s spreading between farms (and if so, how).
The sky is not falling, despite some social media reports, but it’s another concerning development. The more this virus infects mammals and spreads between mammals, the more risk to humans and domestic animals. Surveillance is a key part of our response but there also needs to be a concerted effort to limit mammal-to-mammal transmission and exposure of people to infected animals.
From a Canadian context, this highlights a few things too. We can’t just focus on preventing H5N1 flu in dairy cattle through movement of cattle from the US. If D1.1 can move from birds to cattle in Nevada, it can do it anywhere.
We have to maintain a robust milk surveillance program to identify early incursions of this virus. We also have to be ready, able and willing to act decisively if/when H5N1 flu is identified in dairy cattle in Canada. That doesn’t mean culling cattle, but it does mean using strict controls to prevent farm-to-farm spread, and reduce the risks to humans on farms.