Almost 1.8 million farmed and captive birds have been culled in the past three months because of the spread of avian flu across the UK, the BBC has learned. There have been 33 outbreaks of the virus on farms and, on Wednesday, the government said bird-keepers in five more English counties would have to house flocks indoors from Sunday.

It also could mean there are more “virus–infected dairy cattle in states where infection in dairy cattle has not yet been identified,” the CDC says. Human cases of bird flu still remain rare, with only 68 cases confirmed in the United States by the CDC.

The new study is more evidence that the official U.S. tally of confirmed human bird flu infections—68 in the past year—is likely a significant undercount.