Why scientists are concerned about the latest transmission of bird flu to cows

In fact, unlike cows, this bird flu has been deadly to the nearly two dozen other mammal species that have been infected in this U.S., from a polar bear in Alaska, to a mountain lion in Colorado, to raccoons and foxes. Many of those animals were likely infected by eating dead animals that were carrying the virus.

But, by far, the biggest impact here in the U.S. has been on birds. Since this strain of avian influenza first arrived in the U.S. in early 2022, brought here by migratory birds, more than 90 million domestic birds, mostly chickens and turkeys, have died or been intentionally killed across 48 states.

And unlike previous outbreaks, this variant has affected more wild birds and spread across a wider geographic area, crossing down into South America at the end of 2022.

Dr. Ralph Vanstreels, University of California, Davis: I think the alarm really went off when it reached Peru, and that's a massive seabird community, and we saw just unprecedented mortality in the seabirds there.