The U.S. will ramp up bird flu testing in dairy cows amid growing concern about spread of the virus among livestock.

Why it matters: While U.S. health officials say the risk to humans remains low, there are indications the outbreak may be more widespread among cows than known.

Driving the news: The Biden administration on Wednesday said it will require that all dairy cows receive negative flu tests before they can be transported over state lines.

• Positive tests would spark further investigation and reporting to a national network of labs thats track animal disease.

Spread of the virus has been detected between cows of the same herd, from one dairy operation to another when cows travel between them, and from cows to poultry, said Mike Watson, administrator for the Animal and Plant Health Inspection Service. There have also been positive tests among cows with no symptoms, he said.

• "This federal order is critical to increase information available for [the Department of Agriculture] because we've mentioned this as an evolving situation, and we're treating it seriously," he said.
• Bird flu has been detected in dairy herds in eight states: Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, Texas and South Dakota.

Between the lines: The order also comes one day after the Food and Drug Administration reported trace amounts of the bird flu had been detected in pasteurized milk samples taken from grocery store shelves.

• "Based on information currently available, our commercial milk supply is safe," Don Prater, acting director of the FDA's Center for Food Safety and Applied Nutrition, told reporters Wednesday.
• Existing data indicates the pasteurization — in which milk is heated to cut down pathogens to a level that will not pose a risk to consumer health — is effective against the bird flu virus known as H5N1.
• The diversion and destruction of milk from sick cows should also help protect the U.S. milk supply, Prater said.

Yes, but: Health officials on a Wednesday call did not offer more details about where it had detected virus fragments in milk or where the products originated from.

• Public health experts have criticized the federal government for not providing more information about the outbreak.
• New genetic data suggests bird flu began spreading in dairy cattle several months before it was first reported in late March, per the New York Times.