(Updates story first published on 3 April)

By Julie Steenhuysen

CHICAGO, April 25 (Reuters) - U.S. officials have

strengthened measures to contain the further spread of the first

known outbreak of H5N1 or bird flu in dairy cows, which has now

spread to 33 herds in eight states and bled over into the

country's milk supply.

So far, only one person - a Texas farm worker - has tested

positive for H5N1, and while health officials and scientists say

the risk to people remains low, many questions remain.

WHY IS H5N1 OR BIRD FLU A CONCERN?

The spread of bird flu among dairy cattle reflects an

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

virus that typically infects birds.

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. It has long been on

the list of viruses with pandemic potential, and 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 several recent

developments suggest it may be in more herds than documented.

On April 23, the U.S. Food and Drug Administration said it

has detected particles of H5N1 in the milk supply. Tests suggest

they are remnants of virus killed during the pasteurization

process, and that the milk is safe. Early testing has not

detected any live virus in milk samples, and U.S. health

officials believe people cannot get sick from drinking

pasteurized milk and warn not to drink unpasteurized milk.

Officials have learned the virus can be present in cows that

show no signs of infection, and milk from those cows does not

show signs of infection such as being thicker and yellow.

To contain the outbreak, the U.S. government is requiring

dairy cattle moving between states to be tested for bird flu.

WHEN DID THIS BIRD FLU OUTBREAK START?

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

information suggests it may have started late last year.

After the U.S. Department of Agriculture released some raw

genetic data onto a public database on April 21, a team of

evolutionary biologists led by Dr. Michael Worobey of the

University of Arizona pieced together the viral genetic codes.

The team mapped the outbreak by tracking changes in the

virus as it spread among herds. Based on their analysis, Worobey

said in an April 24 interview that the outbreak in dairy cattle

occurred through a single transmission event from a bird to a

cow in late 2023, likely in December.

HOW IS H5N1 SPREADING?

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

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

one case of cow-to-human transmission. There is no evidence of

human-to-human transmission.

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.

Whether the virus can spread through respiratory droplets

that infect the airway, as flu viruses typically spread in

humans, remains unclear.

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 antivirals that

could be used to treat human infections. For a major epidemic or

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

Flu vaccine suppliers Sanofi, GlaxoSmithKline

and CSL Seqirus have said they are monitoring

avian flu and stand ready.