Bird flu: US detects new type of H5N1 virus in dairy cows

Dairy cattle in the US state of Nevada have been infected with a new type of bird flu that is different from the version that has been spreading in the country since last year, according to US officials.

The detection indicates that distinct forms of the virus, known as Type A H5N1, have spilt over from wild birds into cattle at least twice in the US.

Experts said it raises new questions about wider spread – and the difficulty of controlling infections in animals and the people who work closely with them.

“I always thought one bird-to-cow transmission was a very rare event. Seems that may not be the case,” said Richard Webby, an influenza expert at St Jude Children’s Research Hospital in the US.

A version of the H5N1 bird flu virus known as B3.13 was confirmed in March 2024 after being introduced to cattle in late 2023, scientists said. It has infected more than 950 herds in 16 states.

The new version, known as D1.1, was confirmed in Nevada cattle last week, according to the US agriculture department, after it was detected in milk collected as part of a surveillance programme launched in December.

“Now we know why it's really important to test and continue testing,” said Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada who helped identify the first spillover.

The D1.1 version of the virus was linked to the first US death tied to bird flu. A person in Louisiana died in January after developing severe respiratory symptoms following contact with wild and backyard birds.

It was also tied to a severe illness in Canada, where a teen girl was hospitalised for months with a virus traced to poultry.

At least 67 people in the US have been infected with bird flu, mostly those who work closely with dairy or