MITCHELL, S.D. (MITCHELLNOW) Twenty-four companies are working to develop an avian flu vaccine for cattle, as the virus spreads among U.S. dairy herds.

Bird flu has infected 90 dairy herds across 12 states since late March, according to the U.S. Department of Agriculture (USDA). Three human cases, all dairy farm workers, have also been reported – two in Michigan and one in Texas.

In addition to the two dozen companies working at varying stages of vaccine development, the USDA is conducting its own preliminary research into a vaccine at its laboratory in Ames, Iowa.

The agency’s other efforts on bird flu include research into the possibility of respiratory spread of the virus between cows. The main risks to the spreading of bird flu on dairy farms appears to be the movement of people and equipment.