Five counties in Texas and New Mexico are being quarantined by USDA officials to prevent the spread of Exotic Newcastle Disease after a small flock of backyard chickens near El Paso, Texas tested positive for the highly contagious poultry virus.

The disease, which is harmless to humans and does not affect the safety of poultry meat or eggs, is almost always fatal to fowl.

“As of Wednesday evening, infection has been confirmed only in El Paso County,” Bob Hillman, the executive director of the Texas Animal Health Commission, was quoted by Reuters as saying. “However, the five counties quarantined in Texas and New Mexico are considered to be a trade area in which there is a significant movement of birds.”

Texas is the fourth US state to be affected by the six-month outbreak that has so far hit California, Arizona and Nevada.

California’s poultry industry has been hit hard since the disease was discovered there in October. The virus has since affected 22 commercial poultry farms in the state and led to 3.2 million birds being destroyed. Texas is a bigger US poultry producer and exporter than California.

Mexico, which is one of the main buyers of US poultry, has said it would extend its ban on poultry to include Texas. The country has already banned poultry from California, Nevada, Arizona, Idaho, Oregon and Utah.