The U.S. Department of Agriculture said Tuesday that the strain of bird flu currently circulating among dairy cattle appears to be only transmissible via mammary glands, or udders, rather than respiratory droplets, a finding that gives officials greater optimism about managing the risks facing dairy workers.

This finding could have significant implications for the spread of bird flu from dairy cattle to humans, with dairy workers being most at risk. So far, three people in the United States have confirmed cases of bird flu: One dairy worker in Texas and two in Michigan. 

“This is principally mediated by milk. This virus has an exquisite fondness for mammary tissue and is expressed in milk,” Eric Deeble, acting senior adviser for USDA’s bird flu response team, told reporters Tuesday. 

Public health officials have been vigilant since the spring with the discovery of the highly pathogenic bird flu strain H5N1 in dairy cattle, which has spread to 13 states as of June.

Deeble told reporters that although H5N1 circulates via respiratory droplets among poultry, none of the studies conducted so far demonstrate respiratory spread among cattle. The virus also does not appear to affect other tissues in an infected cow.