The Colorado Department of Public Health and Environment today announced three additional human cases of bird flu among poultry farm workers, bringing the total number of confirmed human cases in the U.S. this year to 13.

The three new cases involved poultry farm workers who were killing infected chickens at a Weld County egg farm, the health department said.

The bird flu outbreak has infected poultry in nearly every U.S. state since early 2022. Since March, the virus has been circulating among dairy cattle and has infected more than 170 herds in 13 states, according to the U.S. Department of Agriculture.

Since April, four dairy farm workers and nine poultry farm workers have been infected with the virus in Colorado, Michigan and Texas.

Six workers culling infected chickens at a different Colorado chicken farm earlier this month were working without effective protective gear, according to the U.S. Centers for Disease Control and Protection.