Roughly 70 Americans in Colorado are being monitored for bird flu due to potential exposure.

The dairy farm workers will be tested for the virus if they show any symptoms, though this has not been the case so far, local health officials said.

Details about their ages, gender and conditions have not been revealed, but they all worked on a farm in the northeastern part of the state.

Only one person so far - a farmer in Texas - has tested positive for H5N1 virus this outbreak, but the CDC fears many more could be infected and not coming forward.

It comes as the FDA's top official revealed the agency is gearing up for a bird flu pandemic in people that could kill one in four of those it infects.

Dr Robert Califf, the FDA's chief commissioner, told a Senate Committee that officials were drawing up plans to roll out tests, antiviral drugs and vaccines in the event the virus jumps to humans and causes a large-scale outbreak.

But he emphasized the risk of it spreading to people was still low.

'This virus, like all viruses, is mutating,' he told the policymakers. 'We need to continue to prepare for the possibility that it might jump to humans.'

'[The] real worry is that it will jump to the human lungs where, when that has happened in other parts of the world... the mortality rate has been 25 percent.'

Officials in Colorado said it still was not clear exactly how or when the workers were potentially exposed, but bird flu was detected in the herd of cows in late April.

'We're still putting the puzzle pieces together,' said Colorado's state veterinarian Maggie Baldwin in a public town hall on Zoom on May 8.

Texas has tested around 20 dairy workers with flu symptoms, with one positive test, while Idaho tested one person and New Mexico said it had tested three people.

But the CDC says there has been a reluctance by states and farms to cooperate with the agency, because farmers are worried about financial losses if their cattle are found to be infected.

After causing a record outbreak in birds over the past few years, H5N1 appears to be spreading in cattle for the first time.

A total of 36 farms across nine states have so far detected the virus among their dairy herds. Only one of the 36 diary herds is in Colorado. 

As the virus spreads, particularly between two different species, it acquires more mutations which may allow it to infect people more easily.

Fragments of the virus have already been detected in products including one in five grocery store milks — as well as cottage cheese and sour cream.

But officials say these are still safe to consume because the virus inside them is inactivated during the pasteurization process.

Dairy farmers work closely with raw milk, which contains a high concentration of bird flu when it is produced by infected cattle.

Only two humans in the US have tested positive for bird flu ever, with both being in close contact with animals infected with the virus.

There have been no signs in either case of the individuals spreading the disease to others.

Some scientists suspect that bird flu can spread when cows are milked, as equipment may become infected or the virus may enter the air during cleaning.

People who have close and lengthy contact with infected animals may become infected when the virus enters through their eyes or mouth, or when droplets are breathed in through the nose.

It comes after CDC officials restated their warning that bird flu had 'pandemic potential' in a new report.

Writing in the New England Journal of Medicine, they made the warning in a section about rolling out vaccines if the virus spills over to humans.

Also last week, a separate study by the US Department of Agriculture released genetic data showing the H5N1 strain tearing through dairy farms had acquired dozens of new mutations.

These changes could make the strain more likely to spread from cows to other animals, including people, and make the virus resistant to antivirals.