They are not actively monitoring infections in pigs, which are famously effective hosts for evolving flu viruses, and which are often kept in proximity to cattle. And officials have said they have “no concern” about the safety of milk, despite a lack of hard data.

In joint statements in March, the U.S.D.A., the Food and Drug Administration and the Centers for Disease Control and Prevention assured the public that pasteurized milk was safe. But the F.D.A. is still conducting tests to ascertain whether the process eliminates the virus. The agency declined to say when results from those tests would be available.