#18,994
Despite the recent lull in new infected dairy herds reported to the USDA, 5 days ago that agency announced that Wisconsin had Become the 18th State to Detect HPAI H5N1 in Dairy Cattle, after a positive bulk milk sample was detected under the National Milk Testing Strategy testing program.
Throughout 2024, only genotype B3.13 was detected in cattle, and the USDA worked under the assumption that this was likely the result of a single spillover (in Texas in late 2023) and that the virus was subsequently transported by cattle shipped to other states.At the time, only the clade (2.3.4.4b) and subtype (H5N1) were available, with results pending on the genotype.
They believed this to be a `rare', one-off event, sparked by a single rogue genotype, and unlikely to be repeated. A hope that was widely embraced by many other countries, comforted by the lack of spread of B3.13 outside of the United States.But in July 2024, in Germany: FLI Statement On Experimental Infection Of Dairy Cows With European H5N1 Virus, researchers reported `. . . not only the US isolate but also a recent H5N1 virus from a wild bird in Germany was able to multiply very well in the udder.'
This time, it was genotype D1.1, which has emerged the previous fall in wild and migratory birds, and had sparked numerous poultry outbreaks and roughly 2 dozen human infections, some serious (1 fatal).
Several weeks later a 3rd spillover into cattle was announced by the USDA (see APHIS Statement On HPAI Genotype D.1 In Arizona Dairy Cattle), where they reported:
Whole genome sequencing indicates that this detection is a separate wild-bird introduction of HPAI to dairy cattle, now the third identified spillover event into dairy cattle.Since the first cattle outbreak in Texas we've also seen - in addition to (1) HPAI H5 in goats and (2) alpacas in the United States - (3) serological evidence of HPAI exposure in goats and sheep in Pakistan, (4) the UK's Defra reported H5N1 Detected In Domestic Sheep with Mastitis last spring, and again in May; (5) we saw serological evidence of H5N1 in sheep in Norway.
This finding may indicate an increased risk of HPAI introduction into dairies through wild bird exposure.
This growing evidence that it wasn't just the North American genotype B3.13 that posed a threat to (dairy) cattle finally led to the following WOAH Statement last October on High Pathogenicity Avian Influenza (HPAI) in Cattle, calling for increased surveillance and testing.
Early yesterday evening (Friday) the USDA announced that the aforementioned Wisconsin herd has also tested positive for the D1.1 genotype, calling it yet another spillover event.
Update: Genetic Sequencing Results for Wisconsin Dairy Herd Detection of Highly Pathogenic Avian Influenza
Contact: aphispress@usda.gov
WASHINGTON, D.C., December 19, 2025—On December 14, 2025, USDA’s Animal and Plant Health Inspection Service (APHIS) announced the first detection of highly pathogenic avian influenza (HPAI) in a dairy herd in Wisconsin. On December 17, the National Veterinary Services Laboratories (NVSL) completed whole genome sequencing and confirmed that the virus is H5N1 clade 2.3.4.4b genotype D1.1. Analysis indicates that this detection is a new spillover event from wildlife into dairy cattle, separate from previous events.
Key Points
Public Health and Food Safety
- Most detections in U.S. dairy herds have resulted from movements linked to the original spillover event that occurred in the Texas Panhandle in late 2023, involving the B3.13 strain.
- In early 2025, through the National Milk Testing Strategy, USDA detected two spillover events in Nevada and Arizona dairy herds. Both were identified early, and no further herd infections occurred through animal movements. These events involved the D1.1 strain.
- The Wisconsin herd, also detected through the National Milk Testing Strategy, represents a new, separate spillover event and involves the D1.1 strain. At this time, no additional dairy herds have been identified as infected in association with this event.
This detection does not pose a risk to consumer health or affect the safety of the commercial milk supply. Pasteurization effectively inactivates HPAI virus, and milk from affected animals is diverted or destroyed to prevent entry into the food supply. The Centers for Disease Control and Prevention (CDC) continues to consider the risk to the public to be low.
The Importance of Biosecurity
USDA remains committed to working with state partners to monitor, investigate, and mitigate the spread of HPAI in livestock. The detection does not change USDA’s HPAI eradication strategy. Biosecurity is still key to mitigating the risk of disease introduction or spread between premises.
APHIS recommends enhanced biosecurity measures for all dairy farms. Producers should immediately report any livestock with clinical signs, or any unusual sick or dead wildlife, to their state veterinarian.
Phenotypes of the Reassortant viruses observed in this study
Currently the threat of the `bovine' B3.13 genotype to humans is perceived as being less than from other, more virulent, strains. But this expansion of genotypes capable of spilling over to bovines and other mammals could increase that threat.
This is, as far as I can tell, the first large-scale international acknowledgement that a mammalian livestock driven novel flu epidemic was plausible.So much so, that two weeks ago we looked at WPRO Table-Top Exercise Crystal: A `Bovine' Novel Flu Outbreak Scenario, which envisioned `. . . a novel influenza A infection related to dairy farms, with sustained human-to-human transmissions and further spread to many countries.'
As to how likely it might be, only time will tell.
