On March 25, 2024, the USDA announced the first confirmed instance of highly pathogenic avian influenza (HPAI) spreading between dairy cattle herds in the United States. This followed reports from dairy producers of an unusual illness including low appetite and decreased lactation in their lactating cows over the preceding 2-3 months.
Virus whole genome sequencing and modeling performed by the USDA suggested a single spillover of HPAI H5N1, clade 2.3.4.4b, genotype B3.13 from wild birds into dairy cattle likely occurred between October 2023 and January 2024.
Since that time, federal, state, and industry partners have collaborated to address the HPAI threat in dairy cattle, resulting in two federal orders and the implementation of the National Milk Testing Strategy (NMTS), in which states began enrolling in December 2024. As of March 2025, 45 states are enrolled and performing active surveillance.
The NMTS facilitates comprehensive H5N1 surveillance of the U.S. milk supply and dairy herds, though a combination of dairy processing plant silo monitoring and State-specific surveillance, ongoing testing for case investigation and response, and the mandatory testing for interstate movement of lactating dairy cattle.
It also provides a strategy to identify which States and herds within them are affected with H5N1 through a five-stage roadmap to demonstrate the elimination of this virus from their dairy herds.
On January 31, 2025, the USDA Animal and Plant Health Inspection Service (APHIS) National Veterinary Services Laboratories (NVSL) confirmed by whole genome sequence the first detection of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b, genotype D1.1 in dairy cattle.
This is the first detection of this virus genotype in dairy cattle (all previous detections in dairy cattle have been HPAI H5N1 clade 2.3.4.4b, genotype B3.13). Genotype D1.1 represents the predominant genotype in the North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry.
Genotype D1.1 is a reassortment of the A3 genotype, one of the six separate introductions of Eurasian HPAI H5N1 clade 2.3.4.4b genotypes in migratory birds. This genotype was first detected in September 2024 and has quickly expanded to all North American flyways. D1.1 is the current predominant genotype in migratory wild birds, making up 6.07% of the total detections since 2022 despite first occurring late 2024.
The D1.1 genotype found in the Nevada dairy cattle is closely related to the genotypes in recently detected migratory birds in the region, however a change in PB2 D701N, commonly associated with mammalian adaptation of HPAI virus, was identified in viruses sequenced from four separate dairy cattle. This change has previously been associated with mammalian adaptation because it improves RNA polymerase activity and replication in mammalian cells and has the potential to impact pathogenesis in infected mammals.
On February 13, 2025, the USDA Animal and Plant Health Inspection Service (APHIS) National Veterinary Services Laboratories (NVSL) confirmed by whole genome sequence a detection of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b, genotype D1.1 in dairy cattle in Arizona.
The detection of this virus genotype in dairy cattle is not unexpected, because genotype D1.1 represents the predominant genotype in the North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry. 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.
This finding may indicate an increased risk of HPAI introduction into dairies through wild bird exposure. Biosecurity is still key to mitigate 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.
Data Source: USDA-APHIS HPAI Detections in Livestock
The National Milk Testing Strategy (NMTS) facilitates comprehensive H5N1 surveillance of the Nation’s milk supply and dairy herds. It provides a strategy to identify which States and herds within them are affected with H5N1.
Specifically, the NMTS gives States a five-stage roadmap to demonstrate the elimination of this virus from their dairy herds. It is designed to:
The map provides State status and testing and surveillance activities for each State in the contiguous United States for the NMTS.
