Infectious Diseases 2025

Since its introduction into the United States in early 2022, highly pathogenic avian influenza A(H5N1) has undergone a marked ecological and evolutionary expansion. A sequence of unprecedented, first-in-the-world events recorded in the United States illustrates how sustained viral circulation, coupled with insufficient containment, has enabled the virus to cross fundamental host and species barriers at a pace not observed previously.
The most consequential shift emerged in March 2024, when USDA confirmed H5N1 infection in dairy cattle in Texas and Kansas, including direct detection of virus in unpasteurized milk. This was the first global documentation of H5N1 replication in dairy cattle and in raw milk, signaling a substantive change in the virus’s mammalian tropism. One month later, a dairy worker in Texas became the first human case worldwide likely acquired directly from infected cattle, underscoring the immediate occupational and public health implications of this new transmission interface. During the same period, the virus continued to breach additional mammalian barriers. In Minnesota (March 2024), goat kids were confirmed infected with H5N1 - the first recognized involvement of a small ruminant species. In New Mexico, between May and June 2024, investigators documented the first known natural infections of house mice (Mus musculus), demonstrating that the virus had reached a ubiquitous commensal mammal capable of acting as a bridge between agricultural, peri-domestic, and human environments.
The ecological reach extended further into Arctic wildlife. In October 2023, Alaska reported the first confirmed H5N1 infection and death of a polar bear, indicating spillover into apex predators with minimal prior exposure to avian influenza viruses and highlighting the virus’s capacity to infiltrate remote ecosystems.
Most recently, in November 2025, Washington State reported the first documented human infection with H5N5, a distinct avian influenza subtype. The event reflects the growing diversity of avian influenza genotypes now circulating in the United States and their increasing interface with human populations.
Taken together, these events demonstrate a pattern of accelerating viral adaptation and expanding host range. The progression from avian hosts to dairy cattle, small ruminants, commensal rodents, apex wildlife predators, and humans reveals a loss of containment that enabled the virus to explore new evolutionary pathways. The United States has become the location of several global firsts in avian influenza spillover, highlighting the scale of ecological disruption and the heightened risk of establishing novel transmission routes across animal and human populations.