USDA orders testing of milk for bird flu

The national milk supply must be tested for bird flu under a federal order announced Friday by the U.S. Department of Agriculture.

Entities handling raw milk, such as bulk milk transporters or dairy processors, must collect and share samples with the USDA upon request.

The goal is to quickly identify which dairy herds are affected by H5N1 — the strain of bird flu that's causing outbreaks in poultry and dairy cows — and prevent transmission among livestock. As of Thursday, 718 dairy herds have been affected in 15 states.

“Among many outcomes, this will give farmers and farmworkers better confidence in the safety of their animals and ability to protect themselves, and it will put us on a path to quickly controlling and stopping the virus’ spread nationwide,” Agriculture Secretary Tom Vilsack said Friday in a press release.

The initial round of testing under the order, which was first reported by Reuters, is set to begin on Dec. 16. The rollout will start with six states: California, Colorado, Michigan, Mississippi, Oregon and Pennsylvania.

In addition to the testing mandate, the order requires dairy herd owners with cattle that test positive for bird flu to provide information that can help with surveillance. 

The USDA previously issued a federal order in April that required lactating dairy cows to be tested for bird flu before being shipped across state lines, and required private labs and state veterinarians to report positive cases based on those tests. Those requirements remain in place under the new order as well.

The order responds to renewed urgency to address the rapid spread of bird flu among livestock in recent months, which scientists worry could eventually pose a more widespread risk to human health.

Drinking raw, or untreated, milk has risen in popularity, despite warnings from the Centers for Disease Control and Prevention that it might be possible to contract bird flu that way. Several studies have shown that commercial pasteurization — using heat to treat milk — inactivates the virus, making milk that’s safe for consumption.

The Food and Drug Administration regulates the sale of raw milk across state lines, but as many as 30 states allow it to be sold locally.

In November, California health officials detected bird flu in multiple batches of raw milk from a local dairy farm. On Tuesday, the California Department of Public Health announced a broad recall of the farm’s raw milk and cream on retail shelves — though no human bird flu cases have been linked to the products.

To date, the CDC has confirmed 58 human cases of bird flu in the U.S. The vast majority were exposed to infected cattle or poultry. Arizona reported an additional two cases in poultry workers on Friday — the first in the state.

Symptoms so far have been mild, and often include pinkeye, coughs or sneezes. However, the CDC reported last month that some cases may be asymptomatic, so the agency recommends that anyone who has been exposed to bird flu get tested.

Research on dairy farms has shown the virus spreads efficiently between mammals, including from cows to other species like raccoons and cats. Scientists think the virus likely spreads between farm animals through raw milk, because infected cows shed large amounts of the virus through their mammary glands. 

There is no evidence yet that people can pass bird flu to each other, but scientists are concerned that H5N1 might evolve in that direction, which could lead to the next human pandemic.

Research published in the journal Science on Thursday bolstered those concerns. The study found that a single mutation to the bird flu strain that's circulating in dairy cows would allow it to spread more easily between people.

This story first appeared on NBCNews.com. More from NBC News:

• Bird flu detected in raw milk sold in California, health officials say
• Mexican authorities investigate deaths of 13 children possibly linked to contaminated IV bags
• Anthem Blue Cross Blue Shield halts anesthesia payment policy after backlash