Yet no officials have called for quarantines on herds where the infection has been identified in dairy cattle.
It has been identified in eight states and in one person.
Until recently it was believed the only threat to agriculture was the spread of highly-pathogenic avian influenza from migrating birds to domestic chickens and turkeys and in a few notable cases to mink.
The U.S. Department of Agriculture said it still does not know exactly how the virus is being moved around among cattle.
The Canadian Food Inspection Agency has asked veterinarians to be on the lookout for cases in cattle.
“Those of us that have worked with influenza for a long time were fairly quickly saying, ‘Yep it moves cow to cow,'” said Jim Lowe, an associate dean at the University of Illinois College of Veterinary Medicine. “You can’t explain the epidemiology any other way.”
The U.S.D.A . said its investigation into cow infections “includes some cases where the virus spread was associated with cattle movements between herds.” There is also evidence the virus spread from dairy cattle premises “back into nearby poultry premises through an unknown route,” the department said.
The U.S.D.A. said cows shed the virus in milk at high concentrations, so anything that comes in contact with unpasteurized milk may spread the disease. Respiratory transmission is not considered a primary way for the virus to spread in cattle, the department said.
The USDA said it expects that minimizing cattle movement and testing those that must be shipped, along with safety and cleaning practices on farms, should avoid the need for regulatory restrictions.
Officials reported last month that bird flu primarily affected older cows, though additional data now indicates younger cattle have been affected, the U.S.D.A. said.
The World Health Organization has advised consumers that milk should be pasteurized.
