Similar to earlier cases, the virus strain found in Michigan appears to have been introduced by wild birds and transmission between cattle can’t be ruled out, the USDA said. Initial testing hasn’t found changes to the virus that would make it more transmissible to humans, and there continues to be no concern about the safety of the commercial milk supply.

There is concern the flu may have already spread across borders. Florian Krammer, professor at the department of microbiology at the Icahn School of Medicine at Mount Sinai, said on X that with bird flu cases now reported in several US states he “wouldn’t be surprised if there are infections in cows in Europe too.”

On March 27, Mexico’s National Service of Health, Safety and Agricultural Food Quality, known as Senasica, detected highly pathogenic AH5N2 bird flu at a family farm in Michoacan state, according to a statement from the Agriculture Ministry.