Health officials have confirmed the first known severe illness in the US caused by bird flu, and California's governor declared a state of emergency as the virus rampages through dairy cattle in that state.

A person in Louisiana is hospitalised in critical condition with severe respiratory symptoms caused by a bird flu infection, state health officials said. The person, who is over 65 and has underlying medical problems, had been in contact with sick and dead birds in a backyard flock.

It's the first human case in the US linked to exposure to backyard birds, according to the Centers for Disease Control and Prevention.

Since March, more than 60 bird flu infections in people have been reported in the US, but previous illnesses have been mild and most have been detected among farmworkers exposed to sick poultry or dairy cows. In two cases — and adult in Missouri and a child in California — health officials have not determined how they caught it.

CDC officials said that bird flu is still mainly an animal health issue and that the risk to the general public remains low. There has been no documented spread of the virus from person to person, said the CDC's Dr Demetre Daskalakis.

The Louisiana illness comes weeks after Canadian officials reported that a teen in British Columbia was hospitalised with a severe case of bird flu. In both instances, the illnesses were caused by a strain of the virus found in wild birds, not in cattle, officials said.

On a call with reporters, CDC officials did not answer a question about whether the new US case and the Canada case had any similarities or differences.

In California, Governor Gavin Newsom declared the state of emergency as cases of the virus were detected in dairy cows on Southern California farms. Previously, the virus had been detected in state's Central Valley.

"This proclamation is a targeted action to ensure government agencies have the resources and flexibility they need to respond quickly to this outbreak," Newsom said in a statement.

The virus has been detected in 865 dairy herds in 16 states, with nearly 650 of them in California. Raw milk from infected cattle was recalled late last month after the virus was found in samples sold in California stores.

Today, Los Angeles County health officials confirmed the deaths of two cats from bird flu infections after drinking the recalled raw milk.

Flu experts said the trajectory of the virus in people remains unclear, but they urged people who have contact with sick or dead birds to take precautions, including wearing respiratory and eye protection and gloves when handling poultry.

"We may see more cases of severe disease in the US since cases seem to be growing more frequent," said Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada.