The Department of Agriculture says at least 18 outbreaks of Virulent Newcastle Disease have been confirmed in backyard chickens in Southern California. The outbreaks prompted the Animal and Plant Health Inspection Service to issue a warning to commercial poultry operations. The infected backyard chickens were found in Los Angeles and San Bernardino in May and June and officials are concerned it could spread across the country. In the USDA statement, the agency says the virus is not a food safety concern, and no human cases have ever occurred. The virus has not been found in commercial poultry in the United States since 2003. USDA says the virus is a contagious and is a fatal viral disease affecting the respiratory, nervous and digestive systems of birds and poultry. The disease is so strong that many birds and poultry die without showing any clinical signs. USDA says it is essential that back yard bird owners along with commercial operators follow good biosecurity practices to help protect their birds from infectious diseases.