A highly pathogenic form of avian flu called H5N2 is spreading in much of the western and midwestern United States, the U.S. Department of Agriculture (USDA) said Wednesday.

Since mid-December 2014, cases in wild birds, turkeys and chicken have been reported in 16 states, including Arkansas, California, Iowa, Idaho, Kansas, Minnesota, Missouri and Montana.

David Swayne, director of the USDA Southeast Poultry Research Lab, told reporters that they were working on a potential vaccine that could be used to inoculate poultry from H5N2.

In the meantime, the U.S. Centers for Disease Control and Prevention (CDC) considered the risk to people from the current bird flu outbreak to be low.

Alicia Fry of the CDC National Center for Immunization and Respiratory Diseases, Influenza noted that the virus has "not caused infections in humans anywhere in the world."

"While we are cautiously optimistic that there will not be human cases, we must be prepared for that possibility," Fry added.