June 12 (Reuters) - Twenty-four companies are working to

develop an avian flu vaccine for cattle, as the virus spreads

among U.S. dairy herds, Agriculture Secretary Tom Vilsack told

Reuters on Wednesday.

Bird flu has infected 90 dairy herds across 12 states since

late March, according to the U.S. Department of Agriculture

(USDA). Three dairy farm workers also have been infected with

the virus, two in Michigan and one in Texas.

A vaccine could curb the risk of bird flu spreading to new

species and lessen potential economic losses for dairy farmers,

but may take years to develop.

Animal healthcare company Zoetis ( ZTS ) said it started

development of a vaccine for dairy cattle this spring. Merck

Animal Health said it is evaluating technologies and

strategies that would allow for timely responses to emerging

diseases, including bird flu in cattle.

In addition to the two dozen companies working at varying

stages of vaccine development, the USDA is conducting its own

preliminary research into a vaccine at its laboratory in Ames,

Iowa, Vilsack said in an interview.

The agency is looking for a vaccine candidate to test for

efficacy, he said.

"That could happen tomorrow, or it could take six months, or

it could take a year," Vilsack said.

The agency's other efforts on bird flu include research into

potential respiratory spread of the virus between cows and

providing support to farmers to increase biosecurity on farms.

While wild birds were a major vector for bringing bird flu

to poultry farms, the main risks to spread on dairy farms appear

to be the movement of people and equipment, he said.

"For dairy cows, it isn't about migratory birds, it's about

cows moving, it's about people, vehicles and equipment that may

have virus they don't even realize that they're carrying," he

said. "That's why the biosecurity becomes just incredibly,

incredibly important."

A pilot program for bulk milk testing will be rolled out "in

the very near future," Vilsack said. The program is meant to

expand testing for the virus while enabling healthy herds to

move across state lines without negative tests from each cow.

Michigan and Idaho are among states that have expressed

interest in the program, Vilsack said.