Highly pathogenic avian influenza A H5N1 has spread globally in wild birds and led
to outbreaks in increasingly diverse mammalian species (1). Since March 2024, an outbreak of H5N1 clade 2.3.4.4b has spread among dairy cows
across the United States (1). Given that there is little to no pre-existing immunity in the human population,
acquisition of efficient human-to-human transmissibility by H5N1 could lead to rapid
global dissemination, resulting in substantial morbidity and mortality (2, 3). As of August 2025, 70 confirmed human cases have been detected in the United States,
65 of which have reported high-risk exposures to infected dairy cows or poultry and
two of which reported exposure to backyard flocks, wild birds, or other mammals (4). There have been three reported infections in individuals without known exposures:
one in Missouri, one in San Francisco, California, and another in British Columbia,
Canada (1, 5, 6). There has been one fatal case in Louisiana (1).
