Oklahoma has detected bird flu in a herd of dairy cattle, the state's agriculture department said on Friday, making it the 13th U.S. state to find the virus.

The positive sample of highly pathogenic avian influenza (HPAI) was collected by a farm in April and recently submitted to the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) for testing. 

The herd has fully recovered and the farm has not reported any other cases of HPAI, the department said.

"The commercial milk supply remains safe due to both federal animal health requirements and pasteurization," the department said.

The U.S. confirmed the first case in a dairy herd on March 25 and a dairy worker in Texas tested positive on April 1, raising concerns about the virus' spread among animals and humans.

Other states that have reported infected herds are Wyoming, Iowa, Minnesota, Colorado, Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota and Texas.

U.S. dairy farmers are raising their defenses to try to contain the spread of bird flu by banning visitors, cutting down trees to discourage wild birds from landing and disinfecting vehicles coming onto their land.

"Our team has been in constant communication with Oklahoma dairies asking them to heighten their biosecurity practices. PPE (personal protective equipment) has also been made available to Oklahoma dairy farmers." Oklahoma State Veterinarian Dr. Rod Hall said in a statement.