CHICAGO/WASHINGTON, May 22 (Reuters) - A second human

case of bird flu has been confirmed in the United States since

the virus was first detected in dairy cattle in late March, U.S.

officials said on Wednesday.

The infection of a dairy worker in Michigan expands the

outbreak of the virus that has circulated in poultry for years,

though the U.S. Centers for Disease Control and Prevention (CDC)

said the risk to the general public remains low.

The CDC told reporters on a call it has not seen evidence of

human to human transmission of bird flu and that it tested close

to 40 people since March, including the Michigan worker. All the

people who were tested were connected to or had exposures on a

dairy farm, the CDC said.

A Texas dairy worker was confirmed to be infected in April.

Michigan and Texas are among nine states that have reported

bird flu in dairy herds. Scientists have said they believe the

outbreak is more widespread based on U.S. Food and Drug

Administration findings of H5N1 virus particles in about 20% of

retail milk samples.

Similar to the Texas case, the patient in Michigan only

reported eye symptoms, the CDC said. The Michigan worker had

mild symptoms and recovered, according to the state's Department

of Health and Human Services.

The worker had regular exposure to livestock infected with

bird flu, Michigan said. The leading hypothesis is that the case

was a result of cow to human transmission, according to CDC.

The agency said "similar additional human cases could be

identified" given high levels of the virus in raw milk from

infected cows, and the extent of the spread in dairy cows.

Influenza experts said the second case associated with

exposure to dairy cattle was concerning.

"It is worrisome that the virus is spreading widely in cows

because this can lead to changes in the virus that could

potentially increase human susceptibility," said Scott Hensley,

an influenza vaccine expert at the University of Pennsylvania.

Bird flu has caused serious or fatal infections globally

among people in close contact with infected wild birds or

poultry, and scientists have long viewed the virus as being

capable of causing a global health crisis.

"It doesn't appear to be the start of a pandemic but we

urgently need to assess if additional human cases have

occurred," said Dr. Angela Rasmussen, a virologist at the

University of Saskatchewan.

FINANCIAL SUPPORT FOR FARMERS, VACCINE DISCUSSIONS

The U.S. Department of Agriculture (USDA) said it believes

unpasteurized milk is the primary vector for transmitting the

virus among cows, though officials do not know exactly how it

spreads.

To limit transmission in cattle, the USDA in late April

started requiring dairy cows to test negative before being

shipped across state lines.

The USDA will provide financial support to farmers whose

herds have not tested positive for avian flu for developing and

implementing biosecurity and testing measures, said senior

adviser Eric Deeble.

The agency will also retroactively compensate farmers for

lost milk due to infected herds, Deeble said.

"It's likely that there will be several cases that emanate

from exposure to infected cows and their milk amongst farm

workers," said Dr. Amesh Adalja, an infectious disease expert at

the Johns Hopkins Center for Health Security.

"The key thing is to make sure that testing is wide enough

to capture them."

Nirav Shah, CDC's principal deputy director, said the agency

received the Michigan patient sample on Tuesday and confirmed

the positive test result that evening. Investigators are looking

into whether the worker was wearing or was offered protective

equipment.

A nasal swab from the worker tested negative for influenza

in the state, but an eye swab was shipped to CDC and tested

positive for the bird flu virus, the agency said.

The CDC said it recommended isolation, not quarantine, for

close contacts of the person who tested positive for avian flu.

It added that close contacts of exposed workers will be offered

antiviral drugs.

The U.S. is in "active conversations" with mRNA vaccine

makers Pfizer ( PFE ) and Moderna ( MRNA ) on a potential

vaccine for humans, a U.S. Department of Health and Human

Services (HHS) official said.

Last week, the U.S. government started the process of

manufacturing about 4.8 million doses of vaccine matched to the

currently circulating strain of H5N1, a move that Dawn

O'Connell, assistant secretary for preparedness and response at

HHS said "further strengthens our preparedness posture."