Bird flu has infected more than 500 dairy herds in top milk state California, and more than 700 nationwide since March, according to the U.S. Department of Agriculture, raising concerns of the impact to farmers and the milk supply as well as the risk to human health from ongoing spread.

Nearly 60 people have contracted the virus since April, most of them workers on infected poultry and dairy farms, according to the U.S. Centers for Disease Control and Prevention. The symptoms have been mild and CDC officials have said the risk to the general public from bird flu is low.

The USDA first announced it would begin national milk testing in October after a push from industry and veterinary groups for stronger surveillance of the virus, but had not detailed the extent of the program or how it would be implemented.

The testing plan, which will involve monthly or weekly sample collection from bulk milk tanks and dairy processors, will be rolled out first in California, Colorado, Michigan, Mississippi, Pennsylvania, and Oregon, Vilsack said.

The latter three states have not reported bird flu infections in dairy cattle.

"It gives us the opportunity to get potentially a heads up about potentially new virus in areas of a state that currently has the virus, or new states," Vilsack said of the testing.

The agency will begin the testing on Dec. 16, Vilsack said.

The pace of testing will depend on whether virus is detected in initial sampling, said Rosemary Sifford, USDA's chief veterinarian.

The order requires farms and processors to make raw milk available for sampling, and farm owners whose animals test positive for the virus to tell the agency about animal movements and other information.

The USDA currently only requires testing of lactating dairy cattle moving across state lines, a requirement laid out in its only other federal bird flu order issued in April.

Both federal orders will remain in effect indefinitely, Vilsack said, adding that the testing plan was developed with industry groups, state officials and veterinarians.

California this week issued a recall of milk and cream products made by raw milk company Raw Farm after bird flu was detected in its retail products and bulk milk.

The Food and Drug Administration advises against consuming raw milk because it can contain pathogens, including bird flu, but says pasteurized dairy products are safe because the pasteurization process kills those pathogens.

The state has about 1,100 dairies, according to the California Milk Advisory Board, meaning nearly half have been infected with bird flu.