(Adds new details on affected states in paragraphs 1-2, details

on a fatal case in Mexico)

By Julie Steenhuysen

CHICAGO, June 7 (Reuters) - The outbreak of bird flu in

U.S. dairy cows has now been confirmed in Minnesota and Iowa,

bringing to 11 the total number of affected states, which have

been encouraged to ramp up testing efforts at the request of

government officials.

More than 80 dairy herds and three dairy workers have tested

positive for the virus since late March. Other states that have

reported infected herds include Colorado, Idaho, Kansas,

Michigan, New Mexico, North Carolina, Ohio, South Dakota and

Texas. Here's what you need to know.

WHY IS BIRD FLU A CONCERN?

Health officials say the risk to the general public 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 the H5N1 virus strain

that could signal it 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.

IS THE H5N2 BIRD FLU CASE IN MEXICO RELATED?

The World Health Organization this week announced the death of a

person in Mexico with serious health issues who had been

infected with H5N2 avian flu. It marked the first time an H5N2

bird flu virus has infected a human globally and the first H5

case reported in a person in Mexico. It is not clear how the

person was infected. Mexican officials said the man died from

other health conditions, while the WHO said the death was due to

multiple factors.

The H5N1 virus that is currently infecting U.S. dairy cattle

is from a subtype known as clade 2.3.3.4b.

Although bird flu primarily infects and kills birds, human

infections can occur and cause serious or fatal infections.

Recent human infections in other types of bird flu besides the

H5N2 case in Mexico include a case involving a different H5N1

subtype in Australia and an H5N6 infection in May in China.

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. U.S. health officials believe

people cannot get sick from drinking pasteurized milk but warn

not to drink raw unpasteurized milk.

The virus can be present in cows that show no signs of

infection.

HOW IS H5N1 SPREADING IN THE U.S.?

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.

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

scientists suspect the virus is being spread to animals during

the milking process.

It is not yet known if the virus can spread to cows through

respiratory droplets that infect the airway, as flu viruses

typically spread in humans, but studies are underway.

Scientists believe farm workers most likely have been

infected through exposure to infected milk. Human symptoms have

included conjunctivitis, or pink eye, and mild respiratory

symptoms.

IS THERE A BIRD FLU VACCINE FOR HUMANS?

The U.S. maintains a stockpile of prepandemic vaccine candidates

and bulk vaccine against an array of influenza strains. Last

month, U.S. officials said they have decided to manufacture bulk

vaccine closely matched to the H5N1 virus, which will result in

4.8 million doses of vaccine.