The H5N1 strain of avian flu isn’t new. It was discovered in China in 1996. It is now rife on cattle farms in the United States.

Dr Caitlin Rivers, epidemiologist at the Johns Hopkins School of Public Health, recognized the problem in March this year,  the record showed 46 affected herds in 9 states. In January cows giving less milk, caused concern for farmers. In April a farm worker in Texas tested positive for bird flu, the virus does not typically jump from cows to humans. Herds in Texas, Michigan, New Mexico, Kansas, Idaho, Colorado, and one each in Ohio, North Carolina, and South Dakota.
The virus reached North America in  2021 via wild birds, and then passed to cattle, this is not normally something you see, most likely by natural contact with a bird carcass in a water supply. Possibly spread from cow to cow via milking equipment. Also, the movement of cows across state lines, sent south in the winter months and north in the summer. Very little reporting or tracking of cows in US in contrast to tag systems used in Europe.
The virus was first recorded in China 1996 with reports of humans infected in wet market. Since then it has continued to mutate.  Deadly for chickens, but in mammals the virus goes back into the brain causing mad cow disease 

Mad cow disease could be dormant in people
June 22, 2006,
https://www.nbcnews.com/id/wbna13483989
People could be infected with the human form of mad cow disease for more than 50 years without developing the illness, which means the size of a potential epidemic may be underestimated, state UK scientists.

"...There were still people silently infected.
July 11  2019
https://www.bbc.com/news/uk-scotland-48947232

Introducing The Cows Are Mad
Mon 23 Oct 2023
https://www.bbc.co.uk/sounds/play/p0glvxc3
Written, presented and produced by Lucy Proctor.