The avian flu in an elephant seal from Antarctica signals further expansion of the virus in the region's vulnerable wildlife.
In other developments, a challenge study in dairy cows finds that the virus can spread by both mammary and respiratory routes.
Colorado health officials are closely monitoring and testing workers, and the CDC has a field team on the ground to assist with the response.
Alongside more dairy herd confirmations, the USDA noted three more H5N1 outbreaks in poultry, all on Minnesota turkey farms.
The additional cat detections are from 2 states battling the virus in cows: Minnesota and Michigan.
The rise in invasive H influenzae disease came amid eased COVID measures, with babies and seniors the most affected groups.
The outbreak at the poultry farm, with more than 1.7 million birds, prompted an emergency declaration from Colorado's governor.
The authors say the virus can bind to both avian and human-type cell receptors but doesn't easily spread through respiratory routes.
The patients are young cousins from the same household who had contact with dead poultry before they became ill.
The infected farm worker is from Colorado, which has had the most affected dairy herds in the past month.
