-- State officials have destroyed 100,000 infected chickens and quarantined poultry in three Southern California counties because of a potentially ruinous outbreak of a deadly farm disease.

The outbreak of Exotic Newcastle Disease, which is deadly to poultry but cannot be contracted by humans, was first detected in backyard flocks in October. This week, officials confirmed it had been discovered at a poultry farm near Riverside.

"Finding it in a commercial flock is a first in California since 1974," said U.S. Department of Agriculture spokesman Larry Hawkins. "It's not only serious because there is a direct threat to the poultry industry in California, but because it also brings about quarantines from our trading partners."

A statewide outbreak of the disease in the early 1970s threatened the entire U.S. poultry and egg supply and led authorities to destroy nearly 12 million chickens. It cost $56 million to eradicate the disease.

The California Department of Food and Agriculture and the U.S. Department of Agriculture have prohibited the movement of all poultry, poultry products and nesting materials from Los Angeles County and portions of Riverside and San Bernardino counties, Hawkins said.

Because the disease cannot be transmitted to humans, eggs are being sanitized and allowed to pass through the quarantine zone.

California is the nation's third-largest egg producer. More than 9 million of the state's 12 million egg-laying hens are in the quarantine zone.

The California Department of Food and Agriculture would not identify the commercial poultry farm where the outbreak was discovered.

"At this point, we have euthanized the flock, and they have been safely disposed of," spokeswoman Leticia Rico said. "The facility is being cleaned and disinfected."

A task force of state and federal agriculture officials and scientists has been monitoring the outbreak and advising commercial farms on security measures.