How worrying is bird flu’s jump to dairy cows?

Inactive viral fragments have been discovered in pasteurized commercial milk samples, but the CDC says risk to the general public remains low for now.

Since 2022, a highly contagious strain of bird flu, has spread across the U.S. at an unprecedented rate, resulting in the deaths of more than 90 million birds in commercial and backyard poultry flocks, plus thousands more in the wild.

Then, in late March, the virus — known as H5N1 — made a jump to a mammal species that surprised many scientists: cows.

Dairy cattle in 33 herds in eight states have since tested positive for the virus, the U.S. Department of Agriculture’s tracker showed on Tuesday. So far, there have been no reported cases in Georgia. But other developments are troubling influenza experts.

Testing throughout the week indicated the cases are likely far more widespread than first thought, according to scientists and federal authorities.

On Thursday, the Food and Drug Administration said about one in five samples of commercially available pasteurized milk it has tested have contained inactive traces of the virus. The agency said it is conducting more analysis to confirm that heat treatment of pasteurization eliminates any possibility of infection, but added that early evidence shows the virus is neutralized in pasteurized, retail milk.

“To date, the retail milk studies have shown no results that would change our assessment that the commercial milk supply is safe,” the FDA said in a statement.

An employee at one of the Texas dairy farms with positive cases in its cows also contracted the virus, the Atlanta-based Centers for Disease Control and Prevention (CDC) confirmed April 1. The worker’s case is only the second-ever human case reported in the U.S. The man, who worked in close contact with cows and likely caught the virus from cattle, developed only a mild eye infection and has recovered.

The CDC says the threat to the general population remains low and the USDA says there are no signs the virus has changed in ways that could allow it to spread more efficiently to and among people. Still, experts say any transmission from mammals to humans is an unwelcome opportunity for the virus to adapt in ways that could heighten the risk to humans.

On Friday, USDA added a ninth state, Colorado, and a 34th herd, to the list of states where the virus has been confirmed in dairy cows. Other states include Texas, New Mexico, Kansas, South Dakota, Idaho, Michigan, Ohio and North Carolina.

The coming weeks could be key to determining whether the virus’ emergence in cows is the start of a growing public health threat or not.

As the situation evolves, here’s what you need to know:

RUTH FREMSON/THE NEW YORK TIMES Since Tuesday, dairy cattle in 33 herds in eight states have tested positive for the bird flu strain known as H5N1. Cases are likely far more widespread.