Newcastle disease, a deadly avian infection that struck the state's poultry industry in the 1970s, has resurfaced in Southern California.

About 60 locations in Riverside, San Bernardino and Los Angeles counties have been quarantined and more than 5,600 backyard hens, roosters, show fowl and other birds have been killed to stop the spread of the disease, state Department of Food and Agriculture spokeswoman Leticia Rico said Monday.

Although the disease has yet to infect commercial flocks, producers and state agricultural officials worry it could spread quickly to egg farms.  Riverside County led the state last year in egg production at $56 million, while San Bernardino County's egg crop topped $26 million.

The disease cannot be transmitted to humans and poses no public health threat.

A statewide outbreak in the early 1970s threatened the entire U.S. poultry and egg supply. Efforts to stop the disease cost $56 million and involved destruction of nearly 12 million chickens.

Officials said the disease may have been brought into the area by fighting roosters or other birds brought illegally into the country, or by migratory birds.

The disease affects virtually all types of domestic and wild birds. It can be transmitted through feces, feed, cages, clothing or other materials and can become airborne.

Many egg producers are disinfecting workers' boots and the tires of all vehicles that enter their farms, limiting visitor access, or discouraging employees from visiting bird shows. Farmers are also taking steps to prevent wild birds from feeding or drinking on their property.