(Adds details on third human case in paragraph 1, 2; updates

throughout)

By Julie Steenhuysen

CHICAGO, May 30 (Reuters) - A third U.S. dairy worker

tested positive for bird flu after exposure to infected cows,

and was the first to suffer respiratory symptoms, U.S. officials

said on Thursday in the latest expansion of the outbreak.

Bird flu has been found in dairy cattle in nine states. This

second case in a Michigan dairy worker was found through

increased testing of people and cows in the state following the

April 1 identification of a case in a Texas farm worker. Here's

what you need to know about the outbreak.

WHY IS H5N1 OR BIRD FLU A CONCERN?

Health officials say the risk to public health remains low.

But the spread of avian flu among dairy cattle reflects an

expansion of the range of mammals that can be sickened by the

virus that typically infects birds, and influenza experts said

finding bird flu in humans was worrisome.

Scientists are on alert for changes in H5N1 that could

signal the virus is adapting to spread easily among humans. The

virus has caused serious or fatal infections among people in

close contact with wild birds or poultry and has long been on

the list of viruses with pandemic potential. Any expansion to a

new mammal species is concerning.

The infections in cattle are from the same subtype of bird flu

that has been infecting wild birds and poultry flocks globally

for more than two years, also killing several mammal species

that likely contracted the virus from consuming sick or dead

birds.

HOW WIDESPREAD IS THE BIRD FLU OUTBREAK IN CATTLE?

The full extent remains unknown, but it may be in more herds

than documented.

The U.S. Food and Drug Administration has said it detected

fragments of the H5N1 virus in about 1 in 5 samples during a

national survey of retail milk. It said tests of 297 samples

found the remnants of virus were inactivated by the

pasteurization process, and that the milk supply is safe. U.S.

health officials believe people cannot get sick from drinking

pasteurized milk but warn not to drink raw unpasteurized milk.

Officials have learned the virus can be present in cows that

show no signs of infection.

WHEN DID THIS BIRD FLU OUTBREAK START?

U.S. officials had thought the outbreak was recent, but it

may have started late last year.

A study funded by the U.S. Department of Agriculture and the

Centers for Disease Control and Prevention published in May

found bird flu was likely circulating on a limited basis as far

back as late 2023. The study said it believed a wild bird

transmitted the virus to cattle, which then expanded when the

cows were shipped to other states.

HOW IS H5N1 SPREADING?

It remains unclear how the virus is spreading, but there is

evidence of wild bird-to-cow, cow-to-cow, cow-to-poultry, and

three cases of cow-to-human transmission. There is no evidence

of human-to-human transmission, officials reiterated.

Because of the heavy viral load in milk and mammary glands,

scientists suspect the virus is being spread to animals during

the milking process, either through contact with infected

equipment or with virus that becomes aerosolized during cleaning

procedures.

It is not known if the virus can spread through respiratory

droplets that infect the airway, as flu viruses typically spread

in humans. Human symptoms have included conjunctivitis, or pink

eye, and mild respiratory symptoms.

IS THERE A BIRD FLU VACCINE FOR HUMANS?

The U.S. has a stockpile of bird flu vaccines matched with

the strain currently circulating, as well as antiviral drugs

that could be used to treat human infections. For a major

epidemic or a pandemic, the U.S. would have to scale up

production considerably.