The USDA said yesterday [Thursday] that the highly contagious poultry virus, Exotic Newcastle Disease, has infected four more commercial poultry farms in California, the most cases reported in a single day since the outbreak began more than four months ago.

The disease, which is harmless to humans but fatal to poultry, has infected flocks in California, Nevada and Arizona and resulted in export bans from infected states to several foreign countries. The disease has now been discovered in a total of 12 commercial poultry farms in California.

Agriculture secretary Ann Veneman called the disease a “threat to the US poultry and bird industries.”

“It constitutes a real danger to the national economy and a potential serious burden on interstate and foreign commerce,” she was quoted as saying by Reuters.